Bio

Academic Appointments


Administrative Appointments


  • Professor of Photon Science, SSRL (2005 - Present)

Professional Education


  • Postdoc, California Inst. of Technology, Pasadena, CA, Bioinorganic (H. Gray) (1975)
  • Postdoc, University of Copenhagen (H.C. Ørsted Inst.), Denmark, Phys. Inorg. (C.Ballhausen) (1974)
  • Postdoc, Princeton University, Princeton, N.J., Chem. Phys. (D. McClure) (1973)
  • PhD, Princeton University, Princeton, N.J., Phys. Chem (1972)
  • M.S., Princeton University, Princeton, N.J, Phys. Chem (1970)
  • B.S., Rensselaer Polytechnic Institute, Troy, NY, Chemistry (1968)

Research & Scholarship

Current Research and Scholarly Interests


The fields of Physical-Inorganic and Bioinorganic Chemistry emphasizing the application of a wide variety of spectroscopic and computational methods to determine the electronic structure of transition metal complexes. Research is directed toward both high symmetry small molecule complexes to define in detail electronic structure contributions to chemical and physical properties, and metal ion active sites in catalysis to understand their unusual spectral features in terms of electronic and geometric structure and to evaluate these structural contributions to reactivity. The focus of many studies is on fundamental problems in Bioinorganic Chemistry. Areas of present interest include: 1) Electronic structure contributions to electron transfer in blue copper, CuA and iron sulfur sites; 2) O2 and N2O activation by Cu cluster active sites; 3) Structure/function correlations over non-heme iron enzymes; 4) Development of new spectroscopic and electronic structure methods in bioinorganic chemistry.

Teaching

2013-14 Courses


Graduate and Fellowship Programs


Publications

Journal Articles


  • Elucidation of the Fe(IV)=O intermediate in the catalytic cycle of the halogenase SyrB2 NATURE Wong, S. D., Srnec, M., Matthews, M. L., Liu, L. V., Kwak, Y., Park, K., Bell, C. B., Alp, E. E., Zhao, J., Yoda, Y., Kitao, S., Seto, M., Krebs, C., Bollinger, J. M., Solomon, E. I. 2013; 499 (7458): 320-?

    Abstract

    Mononuclear non-haem iron (NHFe) enzymes catalyse a broad range of oxidative reactions, including halogenation, hydroxylation, ring closure, desaturation and aromatic ring cleavage reactions. They are involved in a number of biological processes, including phenylalanine metabolism, the production of neurotransmitters, the hypoxic response and the biosynthesis of secondary metabolites. The reactive intermediate in the catalytic cycles of these enzymes is a high-spin S = 2 Fe(IV)=O species, which has been trapped for a number of NHFe enzymes, including the halogenase SyrB2 (syringomycin biosynthesis enzyme 2). Computational studies aimed at understanding the reactivity of this Fe(IV)=O intermediate are limited in applicability owing to the paucity of experimental knowledge about its geometric and electronic structure. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes involving Fe on the nature of the Fe(IV)=O active site. Here we present NRVS structural characterization of the reactive Fe(IV)=O intermediate of a NHFe enzyme, namely the halogenase SyrB2 from the bacterium Pseudomonas syringae pv. syringae. This intermediate reacts via an initial hydrogen-atom abstraction step, performing subsequent halogenation of the native substrate or hydroxylation of non-native substrates. A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate directs the orientation of the Fe(IV)=O intermediate, presenting specific frontier molecular orbitals that can activate either selective halogenation or hydroxylation.

    View details for DOI 10.1038/nature12304

    View details for Web of Science ID 000321910700032

    View details for PubMedID 23868262

  • Nuclear resonance vibrational spectroscopic and computational study of high-valent diiron complexes relevant to enzyme intermediates. Proceedings of the National Academy of Sciences of the United States of America Park, K., Bell, C. B., Liu, L. V., Wang, D., Xue, G., Kwak, Y., Wong, S. D., Light, K. M., Zhao, J., Alp, E. E., Yoda, Y., Saito, M., Kobayashi, Y., Ohta, T., Seto, M., Que, L., Solomon, E. I. 2013; 110 (16): 6275-6280

    Abstract

    High-valent intermediates of binuclear nonheme iron enzymes are structurally unknown despite their importance for understanding enzyme reactivity. Nuclear resonance vibrational spectroscopy combined with density functional theory calculations has been applied to structurally well-characterized high-valent mono- and di-oxo bridged binuclear Fe model complexes. Low-frequency vibrational modes of these high-valent diiron complexes involving Fe motion have been observed and assigned. These are independent of Fe oxidation state and show a strong dependence on spin state. It is important to note that they are sensitive to the nature of the Fe2 core bridges and provide the basis for interpreting parallel nuclear resonance vibrational spectroscopy data on the high-valent oxo intermediates in the binuclear nonheme iron enzymes.

    View details for DOI 10.1073/pnas.1304238110

    View details for PubMedID 23576760

  • Nuclear Resonance Vibrational Spectroscopy and DFT study of Peroxo-Bridged Biferric Complexes: Structural Insight into Peroxo Intermediates of Binuclear Non-heme Iron Enzymes ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Park, K., Tsugawa, T., Furutachi, H., Kwak, Y., Liu, L. V., Wong, S. D., Yoda, Y., Kobayashi, Y., Saito, M., Kurokuzu, M., Seto, M., Suzuki, M., Solomon, E. I. 2013; 52 (4): 1294-1298

    View details for DOI 10.1002/anie.201208240

    View details for Web of Science ID 000313719300042

    View details for PubMedID 23225363

  • Analysis of resonance Raman data on the blue copper site in pseudoazurin: Excited state pi and sigma charge transfer distortions and their relation to ground state reorganization energy JOURNAL OF INORGANIC BIOCHEMISTRY Hadt, R. G., Xie, X., Pauleta, S. R., Moura, I., Solomon, E. I. 2012; 115: 155-162

    Abstract

    The short Cu(2+)-S(Met) bond in pseudoazurin (PAz) results in the presence of two relatively intense S(p)(?) and S(p)(?) charge transfer (CT) transitions. This has enabled resonance Raman (rR) data to be obtained for each excited state. The rR data show very different intensity distribution patterns for the vibrations in the 300-500 cm(-1) region. Time-dependent density functional theory (TDDFT) calculations have been used to determine that the change in intensity distribution between the S(p)(?) and S(p)(?) excited states reflects the differential enhancement of S(Cys) backbone modes with Cu-S(Cys)-C(?) out-of-plane (oop) and in-plane (ip) bend character in their respective potential energy distributions (PEDs). The rR excited state distortions have been related to ground state reorganization energies (? s) and predict that, in addition to M-L stretches, the Cu-S(Cys)-C(?) oop bend needs to be considered. DFT calculations predict a large distortion in the Cu-S(Cys)-C(?) oop bending coordinate upon reduction of a blue copper (BC) site; however, this distortion is not present in the X-ray crystal structures of reduced BC sites. The lack of Cu-S(Cys)-C(?) oop distortion upon reduction corresponds to a previously unconsidered constraint on the thiolate ligand orientation in the reduced state of BC proteins and can be considered as a contribution to the entatic/rack nature of BC sites.

    View details for DOI 10.1016/j.jinorgbio.2012.03.006

    View details for Web of Science ID 000309990500021

    View details for PubMedID 22560510

  • pi-Frontier molecular orbitals in S=2 ferryl species and elucidation of their contributions to reactivity PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Srnec, M., Wong, S. D., England, J., Que, L., Solomon, E. I. 2012; 109 (36): 14326-14331

    Abstract

    S = 2 Fe(IV) ? O species are key intermediates in the catalysis of most nonheme iron enzymes. This article presents detailed spectroscopic and high-level computational studies on a structurally-defined S = 2 Fe(IV) ? O species that define its frontier molecular orbitals, which allow its high reactivity. Importantly, there are both ?- and ?-channels for reaction, and both are highly reactive because they develop dominant oxyl character at the transition state. These ?- and ?-channels have different orientation dependences defining how the same substrate can undergo different reactions (H-atom abstraction vs. electrophilic aromatic attack) with Fe(IV) ? O sites in different enzymes, and how different substrates can undergo different reactions (hydroxylation vs. halogenation) with an Fe(IV) ? O species in the same enzyme.

    View details for DOI 10.1073/pnas.1212693109

    View details for Web of Science ID 000308912600016

    View details for PubMedID 22908238

  • Structure/function correlations among coupled binuclear copper proteins through spectroscopic and reactivity studies of NspF PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Ginsbach, J. W., Kieber-Emmons, M. T., Nomoto, R., Noguchi, A., Ohnishi, Y., Solomon, E. I. 2012; 109 (27): 10793-10797

    Abstract

    The terminal step of 4-hydroxy-3-nitrosobenzamide biosynthesis in Streptomyces murayamaensis is performed by NspF, a mono-oxygenase that converts o-aminophenols to the corresponding nitroso product (hydroxyanilinase activity). Previous biochemical characterization of the resting form of NspF suggested that this enzyme belonged to the coupled binuclear copper enzyme (CBC) family. Another member of this enzyme family, tyrosinase, is able to mono-oxygenate monophenols (monophenolase activity) but not o-aminophenols. To gain insight into the unique reactivity of NspF, we have generated and characterized the oxy form of its active site. The observation of spectral features identical to those of oxy-tyrosinase indicates that oxy-NspF contains a Cu(2)O(2) core where peroxide is coordinated in a ?-?(2):?(2) mode, confirming that NspF is a CBC enzyme. This oxy form is found to react with monophenols, indicating that, like tyrosinase, NspF also possesses monophenolase activity. A comparison of the two electrophilic mechanisms for the monophenolase and hydroxyanilinase activity indicates a large geometric change between their respective transition states. The potential for specific interactions between the protein pocket and the substrate in each transition state is discussed within the context of the differential reactivity of this family of enzymes with equivalent ?-?(2):?(2) peroxy bridged coupled binuclear copper active sites.

    View details for DOI 10.1073/pnas.1208718109

    View details for Web of Science ID 000306641100022

    View details for PubMedID 22711806

  • Substrate and Metal Control of Barrier Heights for Oxo Transfer to Mo and W Bis-dithiolene Sites INORGANIC CHEMISTRY Tenderholt, A. L., Hodgson, K. O., Hedman, B., Holm, R. H., Solomon, E. I. 2012; 51 (6): 3436-3442

    Abstract

    Reaction coordinates for oxo transfer from the substrates Me(3)NO, Me(2)SO, and Me(3)PO to the biologically relevant Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)(2)](-) where mdt = 1,2-dimethyl-ethene-1,2-dithiolate(2-), and from Me(2)SO to the analogous W(IV) complex, have been calculated using density functional theory. In each case, the reaction first proceeds through a transition state (TS1) to an intermediate with substrate weakly bound, followed by a second transition state (TS2) around which breaking of the substrate X-O bond begins. By analyzing the energetic contributions to each barrier, it is shown that the nature of the substrate and metal determines which transition state controls the rate-determining step of the reaction.

    View details for DOI 10.1021/ic2020397

    View details for Web of Science ID 000301624500013

    View details for PubMedID 22372518

  • Spectroscopic Elucidation of a New Heme/Copper Dioxygen Structure Type: Implications for O center dot center dot center dot O Bond Rupture in Cytochrome c Oxidase ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Kieber-Emmons, M. T., Qayyum, M. F., Li, Y., Halime, Z., Hodgson, K. O., Hedman, B., Karlin, K. D., Solomon, E. I. 2012; 51 (1): 168-172

    View details for DOI 10.1002/anie.201104080

    View details for Web of Science ID 000298598500025

    View details for PubMedID 22095556

  • Electronic Structure of a Low-Spin Heme/Cu Peroxide Complex: Spin-State and Spin-Topology Contributions to Reactivity INORGANIC CHEMISTRY Kieber-Emmons, M. T., Li, Y., Halime, Z., Karlin, K. D., Solomon, E. I. 2011; 50 (22): 11777-11786

    Abstract

    This study details the electronic structure of the heme–peroxo–copper adduct {[(F8)Fe(DCHIm)]-O2-[Cu(AN)]}+ (LS(AN)) in which O2(2–) bridges the metals in a ?-1,2 or “end-on” configuration. LS(AN) is generated by addition of coordinating base to the parent complex {[(F8)Fe]-O2-[Cu(AN)]}+ (HS(AN)) in which the O2(2–) bridges the metals in an ?-?2:?2 or “side-on” mode. In addition to the structural change of the O2(2–) bridging geometry, coordination of the base changes the spin state of the heme fragment (from S = 5/2 in HS(AN) to S = 1/2 in LS(AN)) that results in an antiferromagnetically coupled diamagnetic ground state in LS(AN). The strong ligand field of the porphyrin modulates the high-spin to low-spin effect on Fe–peroxo bonding relative to nonheme complexes, which is important in the O–O bond cleavage process. On the basis of DFT calculations, the ground state of LS(AN) is dependent on the Fe–O–O–Cu dihedral angle, wherein acute angles (<~150°) yield an antiferromagnetically coupled electronic structure while more obtuse angles yield a ferromagnetic ground state. LS(AN) is diamagnetic and thus has an antiferromagnetically coupled ground state with a calculated Fe–O–O–Cu dihedral angle of 137°. The nature of the bonding in LS(AN) and the frontier molecular orbitals which lead to this magneto-structural correlation provide insight into possible spin topology contributions to O–O bond cleavage by cytochrome c oxidase.

    View details for DOI 10.1021/ic2018727

    View details for Web of Science ID 000296830400061

    View details for PubMedID 22007669

  • X-ray Absorption Spectroscopic and Computational Investigation of a Possible S center dot center dot center dot S Interaction in the [Cu3S2](3+) Core JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., Yang, L., Winikoff, S. G., Gagliardi, L., Cramer, C. J., Tolman, W. B., Solomon, E. I. 2011; 133 (43): 17180-17191

    Abstract

    The electronic structure of the [Cu(3)S(2)](3+) core of [(LCu)(3)(S)(2)](3+) (L = N,N,N',N'-tetramethyl-2R,3R-cyclohexanediamine) is investigated using a combination of Cu and S K-edge X-ray absorption spectroscopy and calculations at the density functional and multireference second-order perturbation levels of theory. The results show that the [Cu(3)S(2)](3+) core is best described as having all copper centers close to but more oxidized than Cu(2+), while the charge on the S(2) fragment is between that of a sulfide (S(2-)) and a subsulfide (S(2)(3-)) species. The [Cu(3)S(2)](3+) core thus is different from a previously described, analogous [Cu(3)O(2)](3+) core, which has a localized [(Cu(3+)Cu(2+)Cu(2+))(O(2-))(2)](3+) electronic structure. The difference in electronic structure between the two analogues is attributed to increased covalent overlap between the Cu 3d and S 3p orbitals and the increased radial distribution function of the S 3p orbital (relative to O 2p). These features result in donation of electron density from the S-S ?* to the Cu and result in some bonding interaction between the two S atoms at ~2.69 Å in [Cu(3)S(2)](3+), stabilizing a delocalized S = 1 ground state.

    View details for DOI 10.1021/ja111323m

    View details for Web of Science ID 000297380900021

    View details for PubMedID 21923178

  • Spectroscopic and Computational Studies of alpha-Keto Acid Binding to Dke1: Understanding the Role of the Facial Triad and the Reactivity of beta-Diketones JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Diebold, A. R., Straganz, G. D., Solomon, E. I. 2011; 133 (40): 15979-15991

    Abstract

    The O(2) activating mononuclear nonheme iron enzymes generally have a common facial triad (two histidine and one carboxylate (Asp or Glu) residue) ligating Fe(II) at the active site. Exceptions to this motif have recently been identified in nonheme enzymes, including a 3His triad in the diketone cleaving dioxygenase Dke1. This enzyme is used to explore the role of the facial triad in directing reactivity. A combination of spectroscopic studies (UV-vis absorption, MCD, and resonance Raman) and DFT calculations is used to define the nature of the binding of the ?-keto acid, 4-hydroxyphenlpyruvate (HPP), to the active site in Dke1 and the origin of the atypical cleavage (C2-C3 instead of C1-C2) pattern exhibited by this enzyme in the reaction of ?-keto acids with dioxygen. The reduced charge of the 3His triad induces ?-keto acid binding as the enolate dianion, rather than the keto monoanion, found for ?-keto acid binding to the 2His/1 carboxylate facial triad enzymes. The mechanistic insight from the reactivity of Dke1 with the ?-keto acid substrate is then extended to understand the reaction mechanism of this enzyme with its native substrate, acac. This study defines a key role for the 2His/1 carboxylate facial triad in ?-keto acid-dependent mononuclear nonheme iron enzymes in stabilizing the bound ?-keto acid as a monoanion for its decarboxylation to provide the two additional electrons required for O(2) activation.

    View details for DOI 10.1021/ja203005j

    View details for Web of Science ID 000296036700043

    View details for PubMedID 21870808

  • Copper dioxygen (bio) inorganic chemistry FARADAY DISCUSSIONS Solomon, E. I., Ginsbach, J. W., Heppner, D. E., Kieber-Emmons, M. T., Kjaergaard, C. H., Smeets, P. J., Tian, L., Woertink, J. S. 2011; 148: 11-39

    Abstract

    Cu/O2 intermediates in biological, homogeneous, and heterogeneous catalysts exhibit unique spectral features that reflect novel geometric and electronic structures that make significant contributions to reactivity. This review considers how the respective intermediate electronic structures overcome the spin-forbidden nature of O2 binding, activate O2 for electrophilic aromatic attack and H-atom abstraction, catalyze the 4 e- reduction of O2 to H2O, and discusses the role of exchange coupling between Cu ions in determining reactivity.

    View details for DOI 10.1039/c005500j

    View details for Web of Science ID 000285361500002

    View details for PubMedID 21322475

  • Nuclear Resonance Vibrational Spectroscopy on the Fe-IV=O S=2 Non-Heme Site in TMG(3)tren: Experimentally Calibrated Insights into Reactivity ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Wong, S. D., Bell, C. B., Liu, L. V., Kwak, Y., England, J., Alp, E. E., Zhao, J., Que, L., Solomon, E. I. 2011; 50 (14): 3215-3218

    View details for DOI 10.1002/anie.201007692

    View details for Web of Science ID 000288796600017

    View details for PubMedID 21370371

  • Definition of the intermediates and mechanism of the anticancer drug bleomycin using nuclear resonance vibrational spectroscopy and related methods PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Liu, L. V., Bell, C. B., Wong, S. D., Wilson, S. A., Kwak, Y., Chow, M. S., Zhao, J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 107 (52): 22419-22424

    Abstract

    Bleomycin (BLM) is a glycopeptide anticancer drug capable of effecting single- and double-strand DNA cleavage. The last detectable intermediate prior to DNA cleavage is a low spin Fe(III) peroxy level species, termed activated bleomycin (ABLM). DNA strand scission is initiated through the abstraction of the C-4' hydrogen atom of the deoxyribose sugar unit. Nuclear resonance vibrational spectroscopy (NRVS) aided by extended X-ray absorption fine structure spectroscopy and density functional theory (DFT) calculations are applied to define the natures of Fe(III)BLM and ABLM as (BLM)Fe(III)?OH and (BLM)Fe(III)(?(1)?OOH) species, respectively. The NRVS spectra of Fe(III)BLM and ABLM are strikingly different because in ABLM the ?Fe?O?O bending mode mixes with, and energetically splits, the doubly degenerate, intense O?Fe?N(ax) transaxial bends. DFT calculations of the reaction of ABLM with DNA, based on the species defined by the NRVS data, show that the direct H-atom abstraction by ABLM is thermodynamically favored over other proposed reaction pathways.

    View details for DOI 10.1073/pnas.1016323107

    View details for Web of Science ID 000285684200017

    View details for PubMedID 21149675

  • CD and MCD Spectroscopic Studies of the Two Dps Miniferritin Proteins from Bacillus anthracis: Role of O-2 and H2O2 Substrates in Reactivity of the Diiron Catalytic Centers BIOCHEMISTRY Schwartz, J. K., Liu, X. S., Tosha, T., Diebold, A., Theil, E. C., Solomon, E. I. 2010; 49 (49): 10516-10525

    Abstract

    DNA protection during starvation (Dps) proteins are miniferritins found in bacteria and archaea that provide protection from uncontrolled Fe(II)/O radical chemistry; thus the catalytic sites are targets for antibiotics against pathogens, such as anthrax. Ferritin protein cages synthesize ferric oxymineral from Fe(II) and O(2)/H(2)O(2), which accumulates in the large central cavity; for Dps, H(2)O(2) is the more common Fe(II) oxidant contrasting with eukaryotic maxiferritins that often prefer dioxygen. To better understand the differences in the catalytic sites of maxi- versus miniferritins, we used a combination of NIR circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD (VTVH MCD) to study Fe(II) binding to the catalytic sites of the two Bacillus anthracis miniferritins: one in which two Fe(II) react with O(2) exclusively (Dps1) and a second in which both O(2) or H(2)O(2) can react with two Fe(II) (Dps2). Both result in the formation of iron oxybiomineral. The data show a single 5- or 6-coordinate Fe(II) in the absence of oxidant; Fe(II) binding to Dps2 is 30× more stable than Dps1; and the lower limit of K(D) for binding a second Fe(II), in the absence of oxidant, is 2-3 orders of magnitude weaker than for the binding of the single Fe(II). The data fit an equilibrium model where binding of oxidant facilitates formation of the catalytic site, in sharp contrast to eukaryotic M-ferritins where the binuclear Fe(II) centers are preformed before binding of O(2). The two different binding sequences illustrate the mechanistic range possible for catalytic sites of the family of ferritins.

    View details for DOI 10.1021/bi101346c

    View details for Web of Science ID 000284975000017

    View details for PubMedID 21028901

  • Bis(mu-oxo) Dicopper(III) Species of the Simplest Peralkylated Diamine: Enhanced Reactivity toward Exogenous Substrates INORGANIC CHEMISTRY Kang, P., Bobyr, E., Dustman, J., Hodgson, K. O., Hedman, B., Solomon, E. I., Stack, T. D. 2010; 49 (23): 11030-11038

    Abstract

    N,N,N',N'-tetramethylethylenediamine (TMED), the simplest and most extensively used peralkylated diamine ligand, is conspicuously absent from those known to form a bis(?-oxo)dicopper(III) (O) species, [(TMED)(2)Cu(III)(2)(?(2)-O)(2)](2+), upon oxygenation of its Cu(I) complex. Presented here is the characterization of this O species and its reactivity toward exogenous substrates. Its formation is complicated both by the instability of the [(TMED)Cu(I)](1+) precursor and by competitive formation of a presumed mixed-valent trinuclear [(TMED)(3)Cu(III)Cu(II)(2)(?(3)-O)(2)](3+) (T) species. Under most reaction conditions, the T species dominates, yet, the O species can be formed preferentially (>80%) upon oxygenation of acetone solutions, if the copper concentration is low (<2 mM) and [(TMED)Cu(I)](1+) is prepared immediately before use. The experimental data of this simplest O species provide a benchmark by which to evaluate density functional theory (DFT) computational methods for geometry optimization and spectroscopic predictions. The enhanced thermal stability of [(TMED)(2)Cu(III)(2)(?(2)-O)(2)](2+) and its limited steric demands compared to other O species allows more efficient oxidation of exogenous substrates, including benzyl alcohol to benzaldehyde (80% yield), highlighting the importance of ligand structure to not only enhance the oxidant stability but also maintain accessibility to the nascent metal/O(2) oxidant.

    View details for DOI 10.1021/ic101515g

    View details for Web of Science ID 000284518800037

    View details for PubMedID 21028910

  • Synthesis, Structural, and Spectroscopic Characterization and Reactivities of Mononuclear Cobalt(III)-Peroxo Complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cho, J., Sarangi, R., Kang, H. Y., Lee, J. Y., Kubo, M., Ogura, T., Solomon, E. I., Nam, W. 2010; 132 (47): 16977-16986

    Abstract

    Metal-dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+), were synthesized by reacting [Co(12-TMC)(CH(3)CN)](2+) and [Co(13-TMC)(CH(3)CN)](2+), respectively, with H(2)O(2) in the presence of triethylamine. The mononuclear cobalt(III)-peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on ?(2) fashion. The O-O bond stretching frequency of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) was determined to be 902 cm(-1) by resonance Raman spectroscopy. The structural properties of the CoO(2) core in both complexes are nearly identical; the O-O bond distances of [Co(12-TMC)(O(2))](+) and [Co(13-TMC)(O(2))](+) were 1.4389(17) Å and 1.438(6) Å, respectively. The cobalt(III)-peroxo complexes showed reactivities in the oxidation of aldehydes and O(2)-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt-peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O(2))](+) > [Co(12-TMC)(O(2))](+). In the O(2)-transfer reactions, the cobalt(III)-peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)-peroxo complexes. The reactivity of the cobalt-peroxo complexes in O(2)-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O(2)-transfer reactions was the same as that observed in the aldehyde oxidation reactions.

    View details for DOI 10.1021/ja107177m

    View details for Web of Science ID 000284972400041

    View details for PubMedID 21062059

  • Oxygen Precursor to the Reactive Intermediate in Methanol Synthesis by Cu-ZSM-5 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Smeets, P. J., Hadt, R. G., Woertink, J. S., Vanelderen, P., Schoonheydt, R. A., Sels, B. F., Solomon, E. I. 2010; 132 (42): 14736-14738

    Abstract

    The reactive oxidizing species in the selective oxidation of methane to methanol in oxygen activated Cu-ZSM-5 was recently defined to be a bent mono(?-oxo)dicopper(II) species, [Cu(2)O](2+). In this communication we report the formation of an O(2)-precursor of this reactive site with an associated absorption band at 29,000 cm(-1). Laser excitation into this absorption feature yields a resonance Raman (rR) spectrum characterized by (18)O(2) isotope sensitive and insensitive vibrations, ?O-O and ?Cu-Cu, at 736 (?(18)O(2) = 41 cm(-1)) and 269 cm(-1), respectively. These define the precursor to be a ?-(?(2):?(2)) peroxo dicopper(II) species, [Cu(2)(O(2))](2+). rR experiments in combination with UV-vis absorption data show that this [Cu(2)(O(2))](2+) species transforms directly into the [Cu(2)O](2+) reactive site. Spectator Cu(+) sites in the zeolite ion-exchange sites provide the two electrons required to break the peroxo bond in the precursor. O(2)-TPD experiments with (18)O(2) show the incorporation of the second (18)O atom into the zeolite lattice in the transformation of [Cu(2)(O(2))](2+) into [Cu(2)O](2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5.

    View details for DOI 10.1021/ja106283u

    View details for Web of Science ID 000283403200016

    View details for PubMedID 20923156

  • Spectroscopic and Computational Studies of an End-on Bound Superoxo-Cu(II) Complex: Geometric and Electronic Factors That Determine the Ground State INORGANIC CHEMISTRY Woertink, J. S., Tian, L., Maiti, D., Lucas, H. R., Himes, R. A., Karlin, K. D., Neese, F., Wuertele, C., Holthausen, M. C., Bill, E., Sundermeyer, J., Schindler, S., Solomon, E. I. 2010; 49 (20): 9450-9459

    Abstract

    A variety of techniques including absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and resonance Raman (rR) spectroscopies are combined with density functional theory (DFT) calculations to elucidate the electronic structure of the end-on (?(1)) bound superoxo-Cu(II) complex [TMG(3)trenCuO(2)](+) (where TMG(3)tren is 1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine). The spectral features of [TMG(3)trenCuO(2)](+) are assigned, including the first definitive assignment of a superoxo intraligand transition in a metal-superoxo complex, and a detailed description of end-on superoxo-Cu(II) bonding is developed. The lack of overlap between the two magnetic orbitals of [TMG(3)trenCuO(2)](+) eliminates antiferromagnetic coupling between the copper(II) and the superoxide, while the significant superoxo ?*(?) character of the copper dz(2) orbital leads to its ferromagnetically coupled, triplet, ground state.

    View details for DOI 10.1021/ic101138u

    View details for Web of Science ID 000282783400045

    View details for PubMedID 20857998

  • Solvation Effects on S K-Edge XAS Spectra of Fe-S Proteins: Normal and Inverse Effects on WT and Mutant Rubredoxin JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sun, N., Dey, A., Xiao, Z., Wedd, A. G., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (36): 12639-12647

    Abstract

    S K-edge X-ray absorption spectroscopy (XAS) was performed on wild type Cp rubredoxin and its Cys --> Ser mutants in both solution and lyophilized forms. For wild type rubredoxin and for the mutants where an interior cysteine residue (C6 or C39) is substituted by serine, a normal solvent effect is observed, that is, the S covalency increases upon lyophilization. For the mutants where a solvent accessible surface cysteine residue is substituted by serine, the S covalency decreases upon lyophilization which is an inverse solvent effect. Density functional theory (DFT) calculations reproduce these experimental results and show that the normal solvent effect reflects the covalency decrease due to solvent H-bonding to the surface thiolates and that the inverse solvent effect results from the covalency compensation from the interior thiolates. With respect to the Cys --> Ser substitution, the S covalency decreases. Calculations indicate that the stronger bonding interaction of the alkoxide with the Fe relative to that of thiolate increases the energy of the Fe d orbitals and reduces their bonding interaction with the remaining cysteines. The solvent effects support a surface solvent tuning contribution to electron transfer, and the Cys --> Ser result provides an explanation for the change in properties of related iron-sulfur sites with this mutation.

    View details for DOI 10.1021/ja102807x

    View details for Web of Science ID 000282074200026

    View details for PubMedID 20726554

  • The Three-His Triad in Dke1: Comparisons to the Classical Facial Triad BIOCHEMISTRY Diebold, A. R., Neidig, M. L., Moran, G. R., Straganz, G. D., Solomon, E. I. 2010; 49 (32): 6945-6952

    Abstract

    The oxygen activating mononuclear non-heme ferrous enzymes catalyze a diverse range of chemistry yet typically maintain a common structural motif: two histidines and a carboxylate coordinating the iron center in a facial triad. A new Fe(II) coordinating triad has been observed in two enzymes, diketone-cleaving dioxygenase, Dke1, and cysteine dioxygenase (CDO), and is composed of three histidine residues. The effect of this three-His motif in Dke1 on the geometric and electronic structure of the Fe(II) center is explored via a combination of absorption, CD, MCD, and VTVH MCD spectroscopies and DFT calculations. This geometric and electronic structure of the three-His triad is compared to that of the classical (2-His-1-carboxylate) facial triad in the alpha-ketoglutarate (alphaKG)-dependent dioxygenases clavaminate synthase 2 (CS2) and hydroxyphenylpyruvate dioxygenase (HPPD). Comparison of the ligand fields at the Fe(II) shows little difference between the three-His and 2-His-1-carboxylate facial triad sites. Acetylacetone, the substrate for Dke1, will also bind to HPPD and is identified as a strong donor, similar to alphaKG. The major difference between the three-His and 2-His-1-carboxylate facial triad sites is in MLCT transitions observed for both types of triads and reflects their difference in charge. These studies provide insight into the effects of perturbation of the facial triad ligation of the non-heme ferrous enzymes on their geometric and electronic structure and their possible contributions to reactivity.

    View details for DOI 10.1021/bi100892w

    View details for Web of Science ID 000280668000014

    View details for PubMedID 20695531

  • Sulfur K-Edge X-ray Absorption Spectroscopy and Density Functional Calculations on Mo(IV) and Mo(VI)=O Bis-dithiolenes: Insights into the Mechanism of Oxo Transfer in DMSO Reductase and Related Functional Analogues JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Tenderholt, A. L., Wang, J., Szilagyi, R. K., Holm, R. H., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (24): 8359-8371

    Abstract

    Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of two Mo bis-dithiolene complexes, [Mo(OSi)(bdt)(2)](1-) and [MoO(OSi)(bdt)(2)](1-), where OSi = [OSiPh(2)(t)Bu](1-) and bdt = benzene-1,2-dithiolate(2-), that model the Mo(IV) and Mo(VI)=O states of the DMSO reductase family of molybdenum enzymes. These results show that the Mo(IV) complex undergoes metal-based oxidation unlike Mo tris-dithiolene complexes, indicating that the dithiolene ligands are behaving innocently. Experimentally validated calculations have been extended to model the oxo transfer reaction coordinate using dimethylsulfoxide (DMSO) as a substrate. The reaction proceeds through a transition state (TS1) to an intermediate with DMSO weakly bound, followed by a subsequent transition state (TS2) which is the largest barrier of the reaction. The factors that control the energies of these transition states, the nature of the oxo transfer process, and the role of the dithiolene ligand are discussed.

    View details for DOI 10.1021/ja910369c

    View details for Web of Science ID 000278905700034

    View details for PubMedID 20499905

  • Multireference Ab Initio Calculations of g tensors for Trinuclear Copper Clusters in Multicopper Oxidases JOURNAL OF PHYSICAL CHEMISTRY B Vancoillie, S., Chalupsky, J., Ryde, U., Solomon, E. I., Pierloot, K., Neese, F., Rulisek, L. 2010; 114 (22): 7692-7702

    Abstract

    EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins. In recent years, experimental EPR data have been complemented by theoretical calculations, which have become a standard tool of many quantum chemical packages. However, there have only been a few attempts to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions bridged by hydroxide or O(2-) anions and their ground states are antiferromagnetically coupled doublets. We demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors into terms arising from interactions of the ground state with the various excited states, the origin of the zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum chemical method that is applicable to systems of the size of our target systems.

    View details for DOI 10.1021/jp103098r

    View details for Web of Science ID 000278301000033

    View details for PubMedID 20469875

  • Systematic Perturbation of the Trinuclear Copper Cluster in the Multicopper Oxidases: The Role of Active Site Asymmetry in Its Reduction of O-2 to H2O JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Augustine, A. J., Kjaergaard, C., Qayyum, M., Ziegler, L., Kosman, D. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (17): 6057-6067

    Abstract

    The multicopper oxidase Fet3p catalyzes the four-electron reduction of dioxygen to water, coupled to the one-electron oxidation of four equivalents of substrate. To carry out this process, the enzyme utilizes four Cu atoms: a type 1, a type 2, and a coupled binuclear, type 3 site. Substrates are oxidized at the T1 Cu, which rapidly transfers electrons, 13 A away, to a trinuclear copper cluster composed of the T2 and T3 sites, where dioxygen is reduced to water in two sequential 2e(-) steps. This study focuses on two variants of Fet3p, H126Q and H483Q, that perturb the two T3 Cu's, T3alpha and T3beta, respectively. The variants have been isolated in both holo and type 1 depleted (T1D) forms, T1DT3alphaQ and T1DT3betaQ, and their trinuclear copper clusters have been characterized in their oxidized and reduced states. While the variants are only mildly perturbed relative to T1D in the resting oxidized state, in contrast to T1D they are both found to have lost a ligand in their reduced states. Importantly, T1DT3alphaQ reacts with O(2), but T1DT3betaQ does not. Thus loss of a ligand at T3beta, but not at T3alpha, turns off O(2) reactivity, indicating that T3beta and T2 are required for the 2e(-) reduction of O(2) to form the peroxide intermediate (PI), whereas T3alpha remains reduced. This is supported by the spectroscopic features of PI in T1DT3alphaQ, which are identical to T1D PI. This selective redox activity of one edge of the trinuclear cluster demonstrates its asymmetry in O(2) reactivity. The structural origin of this asymmetry between the T3alpha and T3beta is discussed, as is its contribution to reactivity.

    View details for DOI 10.1021/ja909143d

    View details for Web of Science ID 000277158500040

    View details for PubMedID 20377263

  • Fe L-Edge X-ray Absorption Spectroscopy Determination of Differential Orbital Covalency of Siderophore Model Compounds: Electronic Structure Contributions to High Stability Constants JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hocking, R. K., George, S. D., Raymond, K. N., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010; 132 (11): 4006-4015

    Abstract

    Most bacteria and fungi produce low-molecular-weight iron chelators called siderophores. Although different siderophore structures have been characterized, the iron-binding moieties often contain catecholate or hydroxamate groups. Siderophores function because of their extraordinarily high stability constants (K(STAB) = 10(30)-10(49)) and selectivity for Fe(III), yet the origin of these high stability constants has been difficult to quantify experimentally. Herein, we utilize Fe L-edge X-ray absorption spectroscopy to determine the differential orbital covalency (i.e., the differences in the mixing of the metal d-orbitals with ligand valence orbitals) of a series of siderophore model compounds. The results enable evaluation of the electronic structure contributions to their high stability constants in terms of sigma- and pi-donor covalent bonding, ionic bonding, and solvent effects. The results indicate substantial differences in the covalent contributions to stability constants of hydroxamate and catecholate complexes and show that increased sigma as well as pi bonding contributes to the high stability constants of catecholate complexes.

    View details for DOI 10.1021/ja9090098

    View details for Web of Science ID 000275868700061

    View details for PubMedID 20187651

  • Reaction Coordinate of Isopenicillin N Synthase: Oxidase versus Oxygenase Activity BIOCHEMISTRY Brown-Marshall, C. D., Diebold, A. R., Solomon, E. I. 2010; 49 (6): 1176-1182

    Abstract

    Isopenicillin N synthase (IPNS) can have both oxidase and oxygenase activity depending on the substrate. For the native substrate, ACV, oxidase activity exists; however, for the substrate analogue ACOV, which lacks an amide nitrogen, IPNS exhibits oxygenase activity. The potential energy surfaces for the O-O bond elongation and cleavage were calculated for three different reactions: homolytic cleavage via traditional Fenton chemistry, heterolytic cleavage, and nucleophilic attack. These surfaces show that the hydroperoxide-ferrous intermediate, formed by O(2)-activated H atom abstraction from the substrate, can exploit different reaction pathways and that interactions with the substrate govern the pathway. The hydrogen bonds from hydroperoxide to the amide nitrogen of ACV polarize the sigma* orbital of the peroxide toward the proximal oxygen, facilitating heterolytic cleavage. For the substrate analogue ACOV, this hydrogen bond is no longer present, leading to nucleophilic attack on the substrate intermediate C-S bond. After cleavage of the hydroperoxide, the two reaction pathways proceed with minimal barriers, resulting in the closure of the beta-lactam ring for the oxidase activity (ACV) or formation of the thiocarboxylate for oxygenase activity (ACOV).

    View details for DOI 10.1021/bi901772w

    View details for Web of Science ID 000274342000014

    View details for PubMedID 20078029

  • S K-Edge X-Ray Absorption Spectroscopy and Density Functional Theory Studies of High and Low Spin {FeNO}(7) Thiolate Complexes: Exchange Stabilization of Electron Delocalization in {FeNO}(7) and {FeO(2)}(8). Inorganic chemistry Sun, N., Liu, L. V., Dey, A., Villar-Acevedo, G., Kovacs, J. A., Darensbourg, M. Y., Hodgson, K. O., Hedman, B., Solomon, E. I. 2010

    Abstract

    S K-edge X-ray absorption spectroscopy (XAS) is a direct experimental probe of metal ion electronic structure as the pre-edge energy reflects its oxidation state, and the energy splitting pattern of the pre-edge transitions reflects its spin state. The combination of sulfur K-edge XAS and density functional theory (DFT) calculations indicates that the electronic structures of {FeNO}(7) (S = 3/2) (S(Me2)N(4)(tren)Fe(NO), complex I) and {FeNO}(7) (S = 1/2) ((bme-daco)Fe(NO), complex II) are Fe(III)(S = 5/2)-NO(-)(S = 1) and Fe(III)(S = 3/2)-NO(-)(S = 1), respectively. When an axial ligand is computationally added to complex II, the electronic structure becomes Fe(II)(S = 0)-NO•(S = 1/2). These studies demonstrate how the ligand field of the Fe center defines its spin state and thus changes the electron exchange, an important factor in determining the electron distribution over {FeNO}(7) and {FeO(2)}(8) sites.

    View details for PubMedID 21158471

  • XAS and DFT Investigation of Mononuclear Cobalt(III) Peroxo Complexes: Electronic Control of the Geometric Structure in CoO(2) versus NiO(2) Systems. Inorganic chemistry Sarangi, R., Cho, J., Nam, W., Solomon, E. I. 2010

    Abstract

    The geometric and electronic structures of two mononuclear [(L)CoO(2)](+) complexes, [(12-TMC)CoO(2)](ClO(4)) (1) and [(14-TMC)CoO(2)](ClO(4)) (2), have been evaluated using Co K-edge X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) and correlated with density functional theory (DFT) calculations to evaluate the differences in the geometric and electronic structures due to changes in the TMC chelate ring size. Co K-edge XAS shows that both 1 and 2 are Co(III) species. Co K-edge EXAFS data show that both 1 and 2 are side-on O(2)-bound cobalt(III) peroxide complexes. A combination of EXAFS and DFT calculations reveals that while the constrained 12-TMC ring in 1 allows for side-on O(2) binding to the Co center with ease, the 14-TMC chelate in 2 has to undergo significant distortion of the ring to overcome steric hindrance posed by the four cis-methyl groups of the chelate to allow side-on O(2) binding to the Co center. The Ni analogue of 2, [(14-TMC)NiO(2)](+), has been shown to form an end-on-bound nickel(II) superoxide species. The electronic and geometric factors that determine the different electronic structures of 2 and [(14-TMC)NiO(2)](+) are evaluated using DFT calculations. The results show that while the sterics of the cis-14-TMC chelate contribute to the geometry of O(2) binding and result in an end-on-bound Ni(II)O(2)(-) complex in [(14-TMC)NiO(2)](+), the higher thermodynamic driving force for oxidation of Co(II) overcomes this steric constraint, resulting in stabilization of a side-on-bound Co(III)O(2)(2-) electronic structure in 2.

    View details for PubMedID 21142119

  • A [Cu2O](2+) core in Cu-ZSM-5, the active site in the oxidation of methane to methanol PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Woertink, J. S., Smeets, P. J., Groothaert, M. H., Vance, M. A., Sels, B. F., Schoonheydt, R. A., Solomon, E. I. 2009; 106 (45): 18908-18913

    Abstract

    Driven by the depletion of crude oil, the direct oxidation of methane to methanol has been of considerable interest. Promising low-temperature activity of an oxygen-activated zeolite, Cu-ZSM-5, has recently been reported in this selective oxidation and the active site in this reaction correlates with an absorption feature at 22,700 cm(-1). In the present study, this absorption band is used to selectively resonance enhance Raman vibrations of this active site. (18)O(2) labeling experiments allow definitive assignment of the observed vibrations and exclude all previously characterized copper-oxygen species for the active site. In combination with DFT and normal coordinate analysis calculations, the oxygen activated Cu core is uniquely defined as a bent mono-(mu-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu(2)O](2+) core, which is directed into the zeolite channel, upon approach of CH(4). This induces significant oxyl character into the bridging O atom leading to a low transition state energy consistent with experiment and explains why the bent mono-(mu-oxo)dicupric core is highly activated for H atom abstraction from CH(4). The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction.

    View details for DOI 10.1073/pnas.0910461106

    View details for Web of Science ID 000271637500009

    View details for PubMedID 19864626

  • A variable temperature spectroscopic study on Paracoccus pantotrophus pseudoazurin: Protein constraints on the blue Cu site JOURNAL OF INORGANIC BIOCHEMISTRY Xie, X., Hadt, R. G., Pauleta, S. R., Gonzalez, P. J., Un, S., Moura, I., Solomon, E. I. 2009; 103 (10): 1307-1313

    Abstract

    The blue or Type 1 (T1) copper site of Paracoccuspantotrophus pseudoazurin exhibits significant absorption intensity in both the 450 and 600 nm regions. These are sigma and pi S(Cys) to Cu(2+) charge transfer (CT) transitions. The temperature dependent absorption, EPR, and resonance Raman (rR) vibrations enhanced by these bands indicate that a single species is present at all temperatures. This contrasts the temperature dependent behavior of the T1 center in nitrite reductase [S. Ghosh, X. Xie, A. Dey, Y. Sun, C. Scholes, E. Solomon, Proc. Natl. Acad. Sci. 106 (2009) 4969-4974] which has a thioether ligand that is unconstrained by the protein. The lack of temperature dependence in the T1 site in pseudoazurin indicates the presence of a protein constraint similar to the blue Cu site in plastocyanin where the thioether ligand is constrained at 2.8 A. However, plastocyanin exhibits only pi CT. This spectral difference between pseudoazurin and plastocyanin reflects a coupled distortion of the site where the axial thioether in pseudoazurin is also constrained, but at a shorter Cu-S(Met) bond length. This leads to an increase in the Cu(2+)-S(Cys) bond length, and the site undergoes a partial tetragonal distortion in pseudoazurin. Thus, its ground state wavefunction has both sigma and pi character in the Cu(2+)-S(Cys) bond.

    View details for DOI 10.1016/j.jinorgbio.2009.04.012

    View details for Web of Science ID 000270795900003

    View details for PubMedID 19481814

  • Peroxo-Type Intermediates in Class I Ribonucleotide Reductase and Related Binuclear Non-Heme Iron Enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Jensen, K. P., Bell, C. B., Clay, M. D., Solomon, E. I. 2009; 131 (34): 12155-12171

    Abstract

    We have performed a systematic study of chemically possible peroxo-type intermediates occurring in the non-heme di-iron enzyme class Ia ribonucleotide reductase, using spectroscopically calibrated computational chemistry. Density functional computations of equilibrium structures, Fe-O and O-O stretch frequencies, Mossbauer isomer shifts, absorption spectra, J-coupling constants, electron affinities, and free energies of O(2) and proton or water binding are presented for a series of possible intermediates. The results enable structure-property correlations and a new rationale for the changes in carboxylate conformations occurring during the O(2) reaction of this class of non-heme iron enzymes. Our procedure identifies and characterizes various possible candidates for peroxo intermediates experimentally observed along the ribonucleotide reductase dioxygen activation reaction. The study explores how water or a proton can bind to the di-iron site of ribonucleotide reductase and facilitate changes that affect the electronic structure of the iron sites and activate the site for further reaction. Two potential reaction pathways are presented: one where water adds to Fe1 of the cis-mu-1,2 peroxo intermediate P causing opening of a bridging carboxylate to form intermediate P' that has an increased electron affinity and is activated for proton-coupled electron transfer to form the Fe(III)Fe(IV) intermediate X; and one that is more energetically favorable where the P to P' conversion involves addition of a proton to a terminal carboxylate ligand in the site which increases the electron affinity and triggers electron transfer to form X. Both pathways provide a mechanism for the activation of peroxy intermediates in binuclear non-heme iron enzymes for reactivity. The studies further show that water coordination can induce the conformational changes observed in crystal structures of the met state.

    View details for DOI 10.1021/ja809983g

    View details for Web of Science ID 000269379600046

    View details for PubMedID 19663382

  • Spectroscopy and Kinetics of Wild-Type and Mutant Tyrosine Hydroxylase: Mechanistic Insight into O-2 Activation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chow, M. S., Eser, B. E., Wilson, S. A., Hodgson, K. O., Hedman, B., Fitzpatrick, P. F., Solomon, E. I. 2009; 131 (22): 7685-7698

    Abstract

    Tyrosine hydroxylase (TH) is a pterin-dependent nonheme iron enzyme that catalyzes the hydroxylation of L-tyr to L-DOPA in the rate-limiting step of catecholamine neurotransmitter biosynthesis. We have previously shown that the Fe(II) site in phenylalanine hydroxylase (PAH) converts from six-coordinate (6C) to five-coordinate (5C) only when both substrate + cofactor are bound. However, steady-state kinetics indicate that TH has a different co-substrate binding sequence (pterin + O(2) + L-tyr) than PAH (L-phe + pterin + O(2)). Using X-ray absorption spectroscopy (XAS), and variable-temperature-variable-field magnetic circular dichroism (VTVH MCD) spectroscopy, we have investigated the geometric and electronic structure of the wild-type (WT) TH and two mutants, S395A and E332A, and their interactions with substrates. All three forms of TH undergo 6C --> 5C conversion with tyr + pterin, consistent with the general mechanistic strategy established for O(2)-activating nonheme iron enzymes. We have also applied single-turnover kinetic experiments with spectroscopic data to evaluate the mechanism of the O(2) and pterin reactions in TH. When the Fe(II) site is 6C, the two-electron reduction of O(2) to peroxide by Fe(II) and pterin is favored over individual one-electron reactions, demonstrating that both a 5C Fe(II) and a redox-active pterin are required for coupled O(2) reaction. When the Fe(II) is 5C, the O(2) reaction is accelerated by at least 2 orders of magnitude. Comparison of the kinetics of WT TH, which produces Fe(IV)=O + 4a-OH-pterin, and E332A TH, which does not, shows that the E332 residue plays an important role in directing the protonation of the bridged Fe(II)-OO-pterin intermediate in WT to productively form Fe(IV)=O, which is responsible for hydroxylating L-tyr to L-DOPA.

    View details for DOI 10.1021/ja810080c

    View details for Web of Science ID 000267177900058

    View details for PubMedID 19489646

  • S K-edge XAS and DFT Calculations on Cytochrome P450: Covalent and Ionic Contributions to the Cysteine-Fe Bond and Their Contribution to Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Jiang, Y., de Montellano, P. O., Hodgson, K. O., Hedman, B., Solomon, E. I. 2009; 131 (22): 7869-7878

    Abstract

    Experimental covalencies of the Fe-S bond for the resting low-spin and substrate-bound high-spin active site of cytochrome P450 are reported. DFT calculations on the active site indicate that one H-bonding interaction from the protein backbone is needed to reproduce the experimental values. The H-bonding to the thiolate from the backbone decreases the anisotropic pi covalency of the Fe-S bond lowering the barrier of free rotation of the exchangeable axial ligand, which is important for reactivity. The anionic axial thiolate ligand is calculated to lower the Fe(III/II) reduction potential of the active site by more than 1 V compared to a neutral imidazole ligand. About half of this derives from its covalent bonding and half from its electrostatic interaction with the oxidized Fe. This axial thiolate ligand increases the pK(a) of compound 0 (Fe(III)-hydroperoxo) favoring its protonation which promotes O-O bond heterolysis forming compound I. The reactivity of compound I is calculated to be relatively insensitive to the nature of the axial ligand due to opposing reduction potential and proton affinity contributions to the H-atom abstraction energy.

    View details for DOI 10.1021/ja901868q

    View details for Web of Science ID 000267177900077

    View details for PubMedID 19438234

  • Reaction Coordinate of a Functional Model of Tyrosinase: Spectroscopic and Computational Characterization JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 't Holt, B. T., Vance, M. A., Mirica, L. M., Heppner, D. E., Stack, T. D., Solomon, E. I. 2009; 131 (18): 6421-6438

    Abstract

    The mu-eta(2):eta(2)-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the bis-diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED) with O(2) is a functional and spectroscopic model of the coupled binuclear copper protein tyrosinase. This complex reacts with 2,4-di-tert-butylphenolate at low temperature to produce a mixture of the catechol and quinone products, which proceeds through three intermediates (A-C) that have been characterized. A, stabilized at 153 K, is characterized as a phenolate-bonded bis-mu-oxo dicopper(III) species, which proceeds at 193 K to B, presumably a catecholate-bridged coupled bis-copper(II) species via an electrophilic aromatic substitution mechanism wherein aromatic ring distortion is the rate-limiting step. Isotopic labeling shows that the oxygen inserted into the aromatic substrate during hydroxylation derives from dioxygen, and a late-stage ortho-H(+) transfer to an exogenous base is associated with C-O bond formation. Addition of a proton to B produces C, determined from resonance Raman spectra to be a Cu(II)-semiquinone complex. The formation of C (the oxidation of catecholate and reduction to Cu(I)) is governed by the protonation state of the distal bridging oxygen ligand of B. Parallels and contrasts are drawn between the spectroscopically and computationally supported mechanism of the DBED system, presented here, and the experimentally derived mechanism of the coupled binuclear copper protein tyrosinase.

    View details for DOI 10.1021/ja807898h

    View details for Web of Science ID 000265939200042

    View details for PubMedID 19368383

  • Geometric and electronic structure differences between the type 3 copper sites of the multicopper oxidases and hemocyanin/tyrosinase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yoon, J., Fujii, S., Solomon, E. I. 2009; 106 (16): 6585-6590

    Abstract

    The coupled binuclear "type 3" Cu sites are found in hemocyanin (Hc), tyrosinase (Tyr), and the multicopper oxidases (MCOs), such as laccase (Lc), and play vital roles in O(2) respiration. Although all type 3 Cu sites share the same ground state features, those of Hc/Tyr have very different ligand-binding properties relative to those of the MCOs. In particular, the type 3 Cu site in the MCOs (Lc(T3)) is a part of the trinuclear Cu cluster, and if the third (i.e., type 2) Cu is removed, the Lc(T3) site does not react with O(2). Density functional theory calculations indicate that O(2) binding in Hc is approximately 9 kcal mol(-1) more favorable than for Lc(T3). The difference is mostly found in the total energy difference of the deoxy states (approximately 7 kcal mol(-1)), where the stabilization of deoxy Lc(T3) derives from its long equilibrium Cu-Cu distance of approximately 5.5-6.5 A, relative to approximately 4.2 A in deoxy Hc/Tyr. The O(2) binding in Hc is driven by the electrostatic destabilization of the deoxy Hc site, in which the two Cu(I) centers are kept close together by the protein for facile 2-electron reduction of O(2). Alternatively, the lack of O(2) reactivity in Lc(T3) reflects the flexibility of the active site, capable of minimizing the electrostatic repulsion of the 2 Cu(I)s. Thus, the O(2) reactivity of the MCOs is intrinsic to the trinuclear Cu cluster, leading to different O(2) intermediates as required by its function of irreversible reduction of O(2) to H(2)O.

    View details for DOI 10.1073/pnas.0902127106

    View details for Web of Science ID 000265506800031

    View details for PubMedID 19346471

  • Thermodynamic equilibrium between blue and green copper sites and the role of the protein in controlling function PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Ghosh, S., Xie, X., Dey, A., Sun, Y., Scholes, C. P., Solomon, E. I. 2009; 106 (13): 4969-4974

    Abstract

    A combination of spectroscopies and density functional theory calculations indicate that there are large temperature-dependent absorption spectral changes present in green nitrite reductases (NiRs) due to a thermodynamic equilibrium between a green and a blue type 1 (T1) copper site. The axial methionine (Met) ligand is unconstrained in the oxidized NiRs, which results in an enthalpically favored (DeltaH approximately 4.6 kcal/mol) Met-bound green copper site at low temperatures, and an entropically favored (TDeltaS approximately 4.5 kcal/mol, at room temperature) Met-elongated blue copper site at elevated temperatures. In contrast to the NiRs, the classic blue copper sites in plastocyanin and azurin show no temperature-dependent behavior, indicating that a single species is present at all temperatures. For these blue copper proteins, the polypeptide matrix opposes the gain in entropy that would be associated with the loss of the weak axial Met ligand at physiological temperatures by constraining its coordination to copper. The potential energy surfaces of Met binding indicate that it stabilizes the oxidized state more than the reduced state. This provides a mechanism to tune down the reduction potential of blue copper sites by >200 mV.

    View details for DOI 10.1073/pnas.0900995106

    View details for Web of Science ID 000264790600005

    View details for PubMedID 19282479

  • Toluene and Ethylbenzene Aliphatic C-H Bond Oxidations Initiated by a Dicopper(II)-mu-1,2-Peroxo Complex JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lucas, H. R., Li, L., Sarjeant, A. A., Vance, M. A., Solomon, E. I., Karlin, K. D. 2009; 131 (9): 3230-3245

    Abstract

    With an anisole-containing polypyridylamine potential tetradentate ligand (O)L, a mu-1,2-peroxo-dicopper(II) complex [{(O)LCu(II)}(2)(O(2)(2-))](2+) forms from the reaction of the mononuclear compound [Cu(I)((O)L)(MeCN)]B(C(6)F(5))(4) ((O)LCu(I)) with O(2) in noncoordinating solvents at -80 degrees C. Thermal decay of this peroxo complex in the presence of toluene or ethylbenzene leads to rarely seen C-H activation chemistry; benzaldehyde and acetophenone/1-phenylethanol mixtures, respectively, are formed. Experiments with (18)O(2) confirm that the oxygen source in the products is molecular O(2) and deuterium labeling experiments indicate k(H)/k(D) = 7.5 +/- 1 for the toluene oxygenation. The O(2)-reaction of [Cu(I)((Bz)L)(CH(3)CN)](+) ((Bz)LCu(I)) leads to a dicopper(III)-bis-mu-oxo species [{(Bz)LCu(III)}(2)(mu-O(2-))(2)](2+) at -80 degrees C, and from such solutions, very similar toluene oxygenation chemistry occurs. Ligand (Bz)L is a tridentate chelate, possessing the same moiety found in (O)L, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{(O)LCu(II)}(2)(O(2)(2-))](2+) is discussed and likely mechanisms of reaction initiated by toluene H-atom abstraction chemistry are detailed. To confirm the structural formulations of the dioxygen-adducts, UV-vis and resonance Raman spectroscopic studies have been carried out and these results are reported and compared to previously described systems including [{Cu(II)((Py)L)}(2)(O(2))](2+) ((Py)L = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu(I), CO-binding properties (i.e., nu(C-O) values) along with electrochemical property comparisons, the relative donor abilities of (O)L, (Bz)L, and (Py)L are assessed.

    View details for DOI 10.1021/ja807081d

    View details for Web of Science ID 000264792400041

    View details for PubMedID 19216527

  • Fe L- and K-edge XAS of Low-Spin Ferric Corrole: Bonding and Reactivity Relative to Low-Spin Ferric Porphyrin INORGANIC CHEMISTRY Hocking, R. K., George, S. D., Gross, Z., Walker, F. A., Hodgson, K. O., Hedman, B., Solomon, E. I. 2009; 48 (4): 1678-1688

    Abstract

    Corrole is a tetrapyrrolic macrocycle that has one carbon atom less than a porphyrin. The ring contraction reduces the symmetry from D(4h) to C(2v), changes the electronic structure of the heterocycle, and leads to a smaller central cavity with three protons rather than the two of a porphyrin. The differences between ferric corroles and porphyrins lead to a number of differences in reactivity including increased axial ligand lability and a tendency to form 5-coordinate complexes. The electronic structure origin of these differences has been difficult to study experimentally as the dominant porphyrin/corrole pi --> pi* transitions obscure the electronic transitions of the metal. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., the differences in mixing of the metal d orbitals with the ligand valence orbitals) using a valence bond configuration interaction model. Herein, we apply this methodology, combined with a ligand field analysis of the Fe K pre-edge to a low-spin ferric corrole, and compare it to a low-spin ferric porphyrin. The experimental results combined with DFT calculations show that the contracted corrole is both a stronger sigma donor and a very anisotropic pi donor. These differences decrease the bonding interactions with axial ligands and contribute to the increased axial ligand lability and reactivity of ferric corroles relative to ferric porphyrins.

    View details for DOI 10.1021/ic802248t

    View details for Web of Science ID 000263227100051

    View details for PubMedID 19149467

  • Peroxo and oxo intermediates in mononuclear nonheme iron enzymes and related active sites CURRENT OPINION IN CHEMICAL BIOLOGY Solomon, E. I., Wong, S. D., Liu, L. V., Decker, A., Chow, M. S. 2009; 13 (1): 99-113

    Abstract

    Fe(III)OOH and Fe(IV)O intermediates have now been documented in a number of nonheme iron active sites. In this Current Opinion we use spectroscopy combined with electronic structure calculations to define the frontier molecular orbitals (FMOs) of these species and their contributions to reactivity. For the low-spin Fe(III)OOH species in activated bleomycin we show that the reactivity of this nonheme iron intermediate is very different from that of the analogous Compound 0 of cytochrome P450. For Fe(IV)O S=1 model species we experimentally define the electronic structure and its contribution to reactivity, and computationally evaluate how this would change for the Fe(IV)O S=2 intermediates found in nonheme iron enzymes.

    View details for DOI 10.1016/j.cbpa.2009.02.011

    View details for Web of Science ID 000266192400013

    View details for PubMedID 19278895

  • Spectroscopic and Computational Studies of Nitrite Reductase: Proton Induced Electron Transfer and Backbonding Contributions to Reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Dey, A., Sun, Y., Scholes, C. P., Solomon, E. I. 2009; 131 (1): 277-288

    Abstract

    A combination of spectroscopy and DFT calculations has been used to define the geometric and electronic structure of the nitrite bound type 2 (T2) copper site at high and low pH in nitrite reductase from Rhodobacter sphaeroides. At high pH there is no electron transfer from reduced type 1 (T1) to the nitrite bound T2 copper, while protonation triggers T1 --> T2 electron transfer and generation of NO. The DFT calculated reaction coordinate for the N-O bond cleavage in nitrite reduction by the reduced T2 copper suggests that the process is best described as proton transfer triggering electron transfer. Bidentate nitrite binding to copper is calculated to play a major role in activating the reductive cleavage of the nitrite bond through backbonding combined with stabilization of the (-)OH product by coordination to the Cu(2+).

    View details for DOI 10.1021/ja806873e

    View details for Web of Science ID 000262483100059

    View details for PubMedID 19053185

  • Spectroscopic Definition of the Biferrous and Biferric Sites in de Novo Designed Four-Helix Bundle DFsc Peptides: Implications for O-2 Reactivity of Binuclear Non-Heme Iron Enzymes BIOCHEMISTRY Bell, C. B., Calhoun, J. R., Bobyr, E., Wei, P., Hedman, B., Hodgson, K. O., DeGrado, W. F., Solomon, E. T. 2009; 48 (1): 59-73

    Abstract

    DFsc is a single chain de novo designed four-helix bundle peptide that mimics the core protein fold and primary ligand set of various binuclear non-heme iron enzymes. DFsc and the E11D, Y51L, and Y18F single amino acid variants have been studied using a combination of near-IR circular dichroism (CD), magnetic circular dichroism (MCD), variable temperature variable field MCD (VTVH MCD), and X-ray absorption (XAS) spectroscopies. The biferrous sites are all weakly antiferromagnetically coupled with mu-1,3 carboxylate bridges and one 4-coordinate and one 5-coordinate Fe, very similar to the active site of class I ribonucleotide reductase (R2) providing open coordination positions on both irons for dioxygen to bridge. From perturbations of the MCD and VTVH MCD the iron proximal to Y51 can be assigned as the 4-coordinate center, and XAS results show that Y51 is not bound to this iron in the reduced state. The two open coordination positions on one iron in the biferrous state would become occupied by dioxygen and Y51 along the O(2) reaction coordinate. Subsequent binding of Y51 functions as an internal spectral probe of the O(2) reaction and as a proton source that would promote loss of H(2)O(2). Coordination by a ligand that functions as a proton source could be a structural mechanism used by natural binuclear iron enzymes to drive their reactions past peroxo biferric level intermediates.

    View details for DOI 10.1021/bi8016087

    View details for Web of Science ID 000262265900008

    View details for PubMedID 19090676

  • Geometric Structure Determination of N694C Lipoxygenase: A Comparative Near-Edge X-Ray Absorption Spectroscopy and Extended X-Ray Absorption Fine Structure Study INORGANIC CHEMISTRY Sarangi, R., Hocking, R. K., Neidig, M. L., Benfatto, M., Holman, T. R., Solomon, E. I., Hodgson, K. O., Hedman, B. 2008; 47 (24): 11543-11550

    Abstract

    The mononuclear nonheme iron active site of N694C soybean lipoxygenase (sLO1) has been investigated in the resting ferrous form using a combination of Fe-K-pre-edge, near-edge (using the minuit X-ray absorption near-edge full multiple-scattering approach), and extended X-ray absorption fine structure (EXAFS) methods. The results indicate that the active site is six-coordinate (6C) with a large perturbation in the first-shell bond distances in comparison to the more ordered octahedral site in wild-type sLO1. Upon mutation of the asparagine to cysteine, the short Fe-O interaction with asparagine is replaced by a weak Fe-(H(2)O), which leads to a distorted 6C site with an effective 5C ligand field. In addition, it is shown that near-edge multiple scattering analysis can give important three-dimensional structural information, which usually cannot be accessed using EXAFS analysis. It is further shown that, relative to EXAFS, near-edge analysis is more sensitive to partial coordination numbers and can be potentially used as a tool for structure determination in a mixture of chemical species.

    View details for DOI 10.1021/ic800580f

    View details for Web of Science ID 000261510100016

    View details for PubMedID 18656914

  • Intermediates Involved in the Two Electron Reduction of NO to N2O by a Functional Synthetic Model of Heme Containing Bacterial NO Reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Collman, J. P., Dey, A., Yang, Y., Decreau, R. A., Ohta, T., Solomon, E. I. 2008; 130 (49): 16498-?

    View details for DOI 10.1021/ja807700n

    View details for Web of Science ID 000263320200028

    View details for PubMedID 19049449

  • Spectroscopic and Electronic Structure Studies of Phenolate Cu(II) Complexes: Phenolate Ring Orientation and Activation Related to Cofactor Biogenesis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Cirera, J., Vance, M. A., Ono, T., Fujisawa, K., Solomon, E. I. 2008; 130 (48): 16262-16273

    Abstract

    A combination of spectroscopies and DFT calculations have been used to define the electronic structures of two crystallographically defined Cu(II)-phenolate complexes. These complexes differ in the orientation of the phenolate ring which results in different bonding interactions of the phenolate donor orbitals with the Cu(II), which are reflected in the very different spectroscopic properties of the two complexes. These differences in electronic structures lead to significant differences in DFT calculated reactivities with oxygen. These calculations suggest that oxygen activation via a Cu(I) phenoxyl ligand-to-metal charge transfer complex is highly endergonic (>50 kcal/mol), hence an unlikely pathway. Rather, the two-electron oxidation of the phenolate forming a bridging Cu(II) peroxoquinone complex is more favorable (11.3 kcal/mol). The role of the oxidized metal in mediating this two-electron oxidation of the coordinated phenolate and its relevance to the biogenesis of the covalently bound topa quinone in amine oxidase are discussed.

    View details for DOI 10.1021/ja8044986

    View details for Web of Science ID 000263319800041

    View details for PubMedID 18998639

  • Circular Dichroism and Magnetic Circular Dichroism Studies of the Biferrous Site of the Class Ib Ribonucleotide Reductase from Bacillus cereus: Comparison to the Class Ia Enzymes BIOCHEMISTRY Tomter, A. B., Bell, C. B., Rohr, A. K., Andersson, K. K., Solomon, E. I. 2008; 47 (43): 11300-11309

    Abstract

    The rate limiting step in DNA biosynthesis is the reduction of ribonucleotides to form the corresponding deoxyribonucleotides. This reaction is catalyzed by ribonucleotide reductases (RNRs) and is an attractive target against rapidly proliferating pathogens. Class I RNRs are binuclear non-heme iron enzymes and can be further divided into subclasses. Class Ia is found in many organisms, including humans, while class Ib has only been found in bacteria, notably some pathogens. Both Bacillus anthracis and Bacillus cereus encode class Ib RNRs with over 98% sequence identity. The geometric and electronic structure of the B. cereus diiron containing subunit (R2F) has been characterized by a combination of circular dichroism, magnetic circular dichroism (MCD) and variable temperature variable field MCD and is compared to class Ia RNRs. While crystallography has given several possible descriptions for the class Ib RNR biferrous site, the spectroscopically defined active site contains a 4-coordinate and a 5-coordinate Fe(II), weakly antiferromagnetically coupled via mu-1,3-carboxylate bridges. Class Ia biferrous sites are also antiferromagnetically coupled 4-coordinate and 5-coordinate Fe(II), however quantitatively differ from class Ib in bridging carboxylate conformation and tyrosine radical positioning relative to the diiron site. Additionally, the iron binding affinity in B. cereus RNR R2F is greater than class Ia RNR and provides the pathogen with a competitive advantage relative to host in physiological, iron-limited environments. These structural differences have potential for the development of selective drugs.

    View details for DOI 10.1021/bi801212f

    View details for Web of Science ID 000260254500016

    View details for PubMedID 18831534

  • A functional nitric oxide reductase model PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Collman, J. P., Yang, Y., Dey, A., Decreau, R. A., Ghosh, S., Ohta, T., Solomon, E. I. 2008; 105 (41): 15660-15665

    Abstract

    A functional heme/nonheme nitric oxide reductase (NOR) model is presented. The fully reduced diiron compound reacts with two equivalents of NO leading to the formation of one equivalent of N(2)O and the bis-ferric product. NO binds to both heme Fe and nonheme Fe complexes forming individual ferrous nitrosyl species. The mixed-valence species with an oxidized heme and a reduced nonheme Fe(B) does not show NO reduction activity. These results are consistent with a so-called "trans" mechanism for the reduction of NO by bacterial NOR.

    View details for DOI 10.1073/pnas.0808606105

    View details for Web of Science ID 000260240900007

    View details for PubMedID 18838684

  • Further insights into the mechanism of the reaction of activated bleomycin with DNA PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chow, M. S., Liu, L. V., Solomon, E. I. 2008; 105 (36): 13241-13245

    Abstract

    Bleomycin (BLM) is a glycopeptide anticancer drug that effectively carries out single- and double-stranded DNA cleavage. Activated BLM (ABLM), a low-spin ferric-hydroperoxide, BLM-Fe(III)-OOH, is the last intermediate detected before DNA cleavage. We have previously shown through experiments and DFT calculations that both ABLM decay and reaction with H atom donors proceed via direct H atom abstraction. However, the rate of ABLM decay had been previously found, based on indirect methods, to be independent of the presence of DNA. In this study, we use a circular dichroism (CD) feature unique to ABLM to directly monitor the kinetics of ABLM reaction with a DNA oligonucleotide. Our results show that the ABLM + DNA reaction is appreciably faster, has a different kinetic isotope effect, and has a lower Arrhenius activation energy than does ABLM decay. In the ABLM reaction with DNA, the small normal k(H)/k(D) ratio is attributed to a secondary solvent effect through DFT vibrational analysis of reactant and transition state (TS) frequencies, and the lower E(a) is attributed to the weaker bond involved in the abstraction reaction (C-H for DNA and N-H for the decay in the absence of DNA). The DNA dependence of the ABLM reaction indicates that DNA is involved in the TS for ABLM decay and thus reacts directly with BLM-Fe(III)-OOH instead of its decay product.

    View details for DOI 10.1073/pnas.0806378105

    View details for Web of Science ID 000259251700014

    View details for PubMedID 18757754

  • CD and MCD studies of the effects of component B variant binding on the biferrous active site of methane monooxygenase BIOCHEMISTRY Mitic, N., Schwartz, J. K., Brazeau, B. J., Lipscomb, J. D., Solomon, E. I. 2008; 47 (32): 8386-8397

    Abstract

    The multicomponent soluble form of methane monooxygenase (sMMO) catalyzes the oxidation of methane through the activation of O 2 at a nonheme biferrous center in the hydroxylase component, MMOH. Reactivity is limited without binding of the sMMO effector protein, MMOB. Past studies show that mutations of specific MMOB surface residues cause large changes in the rates of individual steps in the MMOH reaction cycle. To define the structural and mechanistic bases for these observations, CD, MCD, and VTVH MCD spectroscopies coupled with ligand-field (LF) calculations are used to elucidate changes occurring near and at the MMOH biferrous cluster upon binding of MMOB and the MMOB variants. Perturbations to both the CD and MCD are observed upon binding wild-type MMOB and the MMOB variant that similarly increases O 2 reactivity. MMOB variants that do not greatly increase O 2 reactivity fail to cause one or both of these changes. LF calculations indicate that reorientation of the terminal glutamate on Fe2 reproduces the spectral perturbations in MCD. Although this structural change allows O 2 to bridge the diiron site and shifts the redox active orbitals for good overlap, it is not sufficient for enhanced O 2 reactivity of the enzyme. Binding of the T111Y-MMOB variant to MMOH induces the MCD, but not CD changes, and causes only a small increase in reactivity. Thus, both the geometric rearrangement at Fe2 (observed in MCD) coupled with a more global conformational change that may control O 2 access (probed by CD), induced by MMOB binding, are critical factors in the reactivity of sMMO.

    View details for DOI 10.1021/bi800818w

    View details for Web of Science ID 000258225600017

    View details for PubMedID 18627173

  • Oxygen reactivity of the biferrous site in the de novo designed four helix bundle peptide DFsc: Nature of the "intermediate" and reaction mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Calhoun, J. R., Bell, C. B., Smith, T. J., Thamann, T. J., DeGrado, W. F., Solomon, E. I. 2008; 130 (29): 9188-?

    Abstract

    The DFsc and DFscE11D de novo designed protein scaffolds support biomimetic diiron cofactor sites that react with dioxygen forming a 520 nm "intermediate" species with an apparent pseudo-first-order formation rate constant of 2.2 and 4.8 s-1, respectively. Resonance Raman spectroscopy shows that this absorption feature is due to a phenolate-to-ferric charge transfer transition arising from a single tyrosine residue coordinating terminally to one of the ferric ions in the site. Phenol coordination could provide a proton to promote rapid loss of a putative peroxo species.

    View details for DOI 10.1021/ja801657y

    View details for Web of Science ID 000257796500005

    View details for PubMedID 18572936

  • Spectroscopic definition of the ferroxidase site in M ferritin: Comparison of binuclear substrate vs cofactor active sites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schwartz, J. K., Liu, X. S., Tosha, T., Theil, E. C., Solomon, E. I. 2008; 130 (29): 9441-9450

    Abstract

    Maxi ferritins, 24 subunit protein nanocages, are essential in humans, plants, bacteria, and other animals for the concentration and storage of iron as hydrated ferric oxide, while minimizing free radical generation or use by pathogens. Formation of the precursors to these ferric oxides is catalyzed at a nonheme biferrous substrate site, which has some parallels with the cofactor sites in other biferrous enzymes. A combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD (VTVH MCD) has been used to probe Fe(II) binding to the substrate active site in frog M ferritin. These data determined that the active site within each subunit consists of two inequivalent five-coordinate (5C) ferrous centers that are weakly antiferromagnetically coupled, consistent with a mu-1,3 carboxylate bridge. The active site ligand set is unusual and likely includes a terminal water bound to each Fe(II) center. The Fe(II) ions bind to the active sites in a concerted manner, and cooperativity among the sites in each subunit is observed, potentially providing a mechanism for the control of ferritin iron loading. Differences in geometric and electronic structure--including a weak ligand field, availability of two water ligands at the biferrous substrate site, and the single carboxylate bridge in ferritin--coincide with the divergent reaction pathways observed between this substrate site and the previously studied cofactor active sites.

    View details for DOI 10.1021/ja801251q

    View details for Web of Science ID 000257796500058

    View details for PubMedID 18576633

  • Interaction of nitric oxide with a functional model of cytochrome c oxidase PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Collman, J. P., Dey, A., Decreau, R. A., Yang, Y., Hosseini, A., Solomon, E. I., Eberspacher, T. A. 2008; 105 (29): 9892-9896

    Abstract

    Cytochrome c oxidase (CcO) is a multimetallic enzyme that carries out the reduction of O2 to H2O and is essential to respiration, providing the energy that powers all aerobic organisms by generating heat and forming ATP. The oxygen-binding heme a(3) should be subject to fatal inhibition by chemicals that could compete with O2 binding. Near the CcO active site is another enzyme, NO synthase, which produces the gaseous hormone NO. NO can strongly bind to heme a(3), thus inhibiting respiration. However, this disaster does not occur. Using functional models for the CcO active site, we show how NO inhibition is avoided; in fact, it is found that NO can protect the respiratory enzyme from other inhibitors such as cyanide, a classic poison.

    View details for DOI 10.1073/pnas.0804257105

    View details for Web of Science ID 000257913200010

    View details for PubMedID 18632561

  • Electronic control of the "Bailar Twist" in formally d(0)-d(2) molybdenum tris(dithiolene) complexes: A sulfur K-edge X-ray absorption spectroscopy and density functional theory study INORGANIC CHEMISTRY Tenderholt, A. L., Szilagyi, R. K., Holm, R. H., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 47 (14): 6382-6392

    Abstract

    Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations have been used to determine the electronic structures of a series of Mo tris(dithiolene) complexes, [Mo(mdt)3](z) (where mdt = 1,2-dimethylethene-1,2-dithiolate(2-) and z = 2-, 1-, 0), with near trigonal-prismatic geometries (D3h symmetry). These results show that the formally Mo(IV), Mo(V), and Mo(VI) complexes actually have a (dz(2))(2) configuration, that is, remain effectively Mo(IV) despite oxidation. Comparisons with the XAS data of another set of Mo tris(dithiolene) complexes, [Mo(tbbdt)3](z) (where tbbdt = 3,5-ditert-butylbenzene-1,2-dithiolate(2-) and z = 1-, 0), show that both neutral complexes, [Mo(mdt)3] and [Mo(tbbdt)3], have similar electronic structures while the monoanions do not. Calculations reveal that the "Bailar twist" present in the crystal structure of [Mo(tbbdt)3](1-) (D3 symmetry) but not [Mo(mdt)3](1-) (D3h symmetry) is controlled by electronic factors which arise from bonding differences between the mdt and tbbdt ligands. In the former, configuration interaction between the Mo d(z(2)) and a deeper energy, occupied ligand orbital, which occurs in D3 symmetry, destabilizes the Mo d(z(2)) to above another ligand orbital which is half-occupied in the D3h [Mo(mdt)3](1-) complex. This leads to a metal d(1) configuration with no ligand holes (i.e., d(1)[L3](0h)) for [Mo(tbbdt)3](1-) rather than the metal d(2) configuration with one ligand hole (i.e., d(2)[L3](1h)) for [Mo(mdt)3](1-). Thus, the Bailar twist observed in some metal tris(dithiolene) complexes is the result of configuration interaction between metal and ligand orbitals and can be probed experimentally by S K-edge XAS.

    View details for DOI 10.1021/ic800494h

    View details for Web of Science ID 000257642700037

    View details for PubMedID 18517189

  • Geometric and electronic structure studies of the binuclear nonheme ferrous active site of Toluene-4-monooxygenase: Parallels with methane monooxygenase and insight into the role of the effector proteins in O-2 activation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schwartz, J. K., Wei, P., Mitchell, K. H., Fox, B. G., Solomon, E. I. 2008; 130 (22): 7098-7109

    Abstract

    Multicomponent monooxygenases, which carry out a variety of highly specific hydroxylation reactions, are of great interest as potential biocatalysts in a number of applications. These proteins share many similarities in structure and show a marked increase in O2 reactivity upon addition of an effector component. In this study, circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD have been used to gain spectroscopic insight into the Fe(II)Fe(II) active site in the hydroxylase component of Toluene-4 monoxygenase (T4moH) and the complex of T4moH bound by its effector protein, T4moD. These results have been correlated to spectroscopic data and density functional theory (DFT) calculations on MmoH and its interaction with MmoB. Together, these data provide further insight into the geometric and electronic structure of these biferrous active sites and, in particular, the perturbation associated with component B/D binding. It is found that binding of the effector protein changes the geometry of one iron center and orientation of its redox active orbital to accommodate the binding of O2 in a bridged structure for efficient 2-electron transfer that can form a peroxo intermediate.

    View details for DOI 10.1021/ja800654d

    View details for Web of Science ID 000256301200046

    View details for PubMedID 18479085

  • Copper dioxygen adducts: Formation of bis(mu-oxo)dicopper(III) versus (mu-1,2)peroxodicopper(II) complexes with small changes in one pyridyl-ligand substituent INORGANIC CHEMISTRY Maiti, D., Woertink, J. S., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2008; 47 (9): 3787-3800

    Abstract

    The preference for the formation of a particular Cu 2O 2 isomer coming from (ligand)-Cu (I)/O 2 reactivity can be regulated with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X = N or C) the traditional Cu (I)/O 2 adduct forms a (mu-1,2)peroxodicopper(II) species ( A). However, when the substituent is the slightly bulkier XR 2 moiety {aryl or NR 2 (R not equal H)}, a bis(mu-oxo)dicopper(III) structure ( C) is favored. The reactivity of one of the bis(mu-oxo)dicopper(III) species, [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) (6tbp = (6- (t)Bu-phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene hydrocarbon oxygenation reactions were observed. Typical monooxygenase chemistry occurred: the benzaldehyde product includes an 18-O atom for toluene/ 7- (1) (8)O 2 reactivity, and a H-atom abstraction by 7-O 2 is apparent from study of its reactions with ArOH substrates, as well as the determination of k H/ k D approximately 7 in the toluene oxygenation (i.e., PhCH 3 vs PhCD 3 substrates). Proposed courses of reaction are presented, including the possible involvement of PhCH 2OO (*) and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior of 7-O 2 . External TMPA ligand exchange for copper in 7-O 2 and O-O bond (re)formation chemistry, along with the ability to protonate 7-O 2 and release of H 2O 2 indicate the presence of an equilibrium between [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) and a (mu-1,2)peroxodicopper(II) form.

    View details for DOI 10.1021/ic.702437c

    View details for Web of Science ID 000255380500044

    View details for PubMedID 18396862

  • Perturbations to the geometric and electronic structure of the CUA site: Factors that influence delocalization and their contributions to electron transfer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Xie, X., Gorelsky, S. I., Sarangi, R., Garner, D. K., Hwang, H. J., Hodgsont, K. O., Hedman, B., Lu, Y., Solornon, E. I. 2008; 130 (15): 5194-5205

    Abstract

    Using a combination of electronic spectroscopies and DFT calculations, the effect of pH perturbation on the geometric and electronic structure of the CuA site has been defined. Descriptions are developed for high pH (pH = 7) and low pH (pH = 4) forms of CuA azurin and its H120A mutant which address the discrepancies concerning the extent of delocalization indicated by multifrequency EPR and ENDOR data (J. Am. Chem. Soc. 2005, 127, 7274; Biophys. J. 2002, 82, 2758). Our resonance Raman and MCD spectra demonstrate that the low pH and H120A mutant forms are essentially identical and are the perturbed forms of the completely delocalized high pH CuA site. However, in going from high pH to low pH, a seven-line hyperfine coupling pattern associated with complete delocalization of the electron (S = 1/2) over two Cu coppers (I(Cu) = 3/2) changes into a four-line pattern reflecting apparent localization. DFT calculations show that the unpaired electron is delocalized in the low pH form and reveal that its four-line hyperfine pattern results from the large EPR spectral effects of approximately 1% 4s orbital contribution of one Cu to the ground-state spin wave function upon protonative loss of its His ligand. The contribution of the Cu-Cu interaction to electron delocalization in this low symmetry protein site is evaluated, and the possible functional significance of the pH-dependent transition in regulating proton-coupled electron transfer in cytochrome c oxidase is discussed.

    View details for DOI 10.1021/ja7102668

    View details for Web of Science ID 000254933000044

    View details for PubMedID 18348522

  • Spectroscopic and density functional theory studies of the blue-copper site in M121SeM and C112SeC azurin: Cu-Se versus Cu-S bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., Gorelsky, S. I., Basumallick, L., Hwang, H. J., Pratt, R. C., Stack, T. D., Lu, Y., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 130 (12): 3866-3877

    Abstract

    S K-edge X-ray absorption, UV-vis absorption, magnetic circular dichroism (MCD), and resonance Raman spectroscopies are used to investigate the electronic structure differences among WT, M121SeM, and C112SeC Pseudomonas aeruginosa (P.a) azurin. A comparison of S K-edge XAS of WT and M121SeM azurin and a CuII-thioether model complex shows that the 38% S character in the ground state wave function of the blue-copper (BC) sites solely reflects the Cu-SCys bond. Resonance Raman (rR) data on WT and C112SeC azurin give direct evidence for the kinematic coupling between the Cu-SCys stretch and the cysteine deformation modes in WT azurin, which leads to multiple features in the rR spectrum of the BC site. The UV-vis absorption and MCD data on WT, M121SeM, and C112SeC give very similar C0/D0 ratios, indicating that the C-term MCD intensity mechanism involves Cu-centered spin-orbit coupling (SOC). The spectroscopic data combined with density functional theory (DFT) calculations indicate that SCys and SeCys have similar covalent interactions with Cu at their respective bond lengths of 2.1 and 2.3 A. This reflects the similar electronegativites of S and Se in the thiolate/selenolate ligand fragment and explains the strong spectroscopic similarities between WT and C112SeC azurin.

    View details for DOI 10.1021/ja076495a

    View details for Web of Science ID 000254173600045

    View details for PubMedID 18314977

  • Spectroscopic studies of perturbed T1 Cu sites in the multicopper oxidases Saccharomyces cerevisiae Fet3p and Rhus vernicifera laccase: Allosteric coupling between the T1 and trinuclear Cu sites BIOCHEMISTRY Augustine, A. J., Kragh, M. E., Sarangi, R., Fujii, S., Liboiron, B. D., Stoj, C. S., Kosman, D. J., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 47 (7): 2036-2045

    Abstract

    The multicopper oxidases catalyze the 4e- reduction of O2 to H2O coupled to the 1e- oxidation of 4 equiv of substrate. This activity requires four Cu atoms, including T1, T2, and coupled binuclear T3 sites. The T2 and T3 sites form a trinuclear cluster (TNC) where O2 is reduced. The T1 is coupled to the TNC through a T1-Cys-His-T3 electron transfer (ET) pathway. In this study the two T3 Cu coordinating His residues which lie in this pathway in Fet3 have been mutated, H483Q, H483C, H485Q, and H485C, to study how perturbation at the TNC impacts the T1 Cu site. Spectroscopic methods, in particular resonance Raman (rR), show that the change from His to Gln to Cys increases the covalency of the T1 Cu-S Cys bond and decreases its redox potential. This study of T1-TNC interactions is then extended to Rhus vernicifera laccase where a number of well-defined species including the catalytically relevant native intermediate (NI) can be trapped for spectroscopic study. The T1 Cu-S covalency and potential do not change in these species relative to resting oxidized enzyme, but interestingly the differences in the structure of the TNC in these species do lead to changes in the T1 Cu rR spectrum. This helps to confirm that vibrations in the cysteine side chain of the T1 Cu site and the protein backbone couple to the Cu-S vibration. These changes in the side chain and backbone provide a possible mechanism for regulating intramolecular T1 to TNC ET in NI and partially reduced enzyme forms for efficient turnover.

    View details for DOI 10.1021/bi7020052

    View details for Web of Science ID 000253102000021

    View details for PubMedID 18197705

  • Near-IR MCD of the nonheme ferrous active site in naphthalene 1,2-dioxygenase: Correlation to crystallography and structural insight into the mechanism of Rieske dioxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ohta, T., Chakrabarty, S., Lipscomb, J. D., Solomon, E. I. 2008; 130 (5): 1601-1610

    Abstract

    Near-IR MCD and variable temperature, variable field (VTVH) MCD have been applied to naphthalene 1,2-dioxygenase (NDO) to describe the coordination geometry and electronic structure of the mononuclear nonheme ferrous catalytic site in the resting and substrate-bound forms with the Rieske 2Fe2S cluster oxidized and reduced. The structural results are correlated with the crystallographic studies of NDO and other related Rieske nonheme iron oxygenases to develop molecular level insights into the structure/function correlation for this class of enzymes. The MCD data for resting NDO with the Rieske center oxidized indicate the presence of a six-coordinate high-spin ferrous site with a weak axial ligand which becomes more tightly coordinated when the Rieske center is reduced. Binding of naphthalene to resting NDO (Rieske oxidized and reduced) converts the six-coordinate sites into five-coordinate (5c) sites with elimination of a water ligand. In the Rieske oxidized form the 5c sites are square pyramidal but transform to a 1:2 mixture of trigonal bipyramial/square pyramidal sites when the Rieske center is reduced. Thus the geometric and electronic structure of the catalytic site in the presence of substrate can be significantly affected by the redox state of the Rieske center. The catalytic ferrous site is primed for the O2 reaction when substrate is bound in the active site in the presence of the reduced Rieske site. These structural changes ensure that two electrons and the substrate are present before the binding and activation of O2, which avoids the uncontrolled formation and release of reactive oxygen species.

    View details for DOI 10.1021/ja074769o

    View details for Web of Science ID 000253100100031

    View details for PubMedID 18189388

  • X-ray absorption spectroscopic and theoretical studies on (L)(2)[Cu-2(S-2)n](2+) complexes: Disulfide versus disulfide(center dot 1-) bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., York, J. T., Helton, M. E., Fujisawa, K., Karlin, K. D., Tolman, W. B., Hodgson, K. O., Hedman, B., Solomon, E. I. 2008; 130 (2): 676-686

    Abstract

    Cu K-, Cu L-, and S K-edge X-ray absorption spectroscopic (XAS) data have been combined with density functional theory (DFT) calculations on [{(TMPA)Cu}2S2](ClO4)2 (1), [{Cu[HB(3,5-Pr(i)2pz)3]}2(S2)] (2), and [{(TMEDA)Cu}2(S2)2](OTf)2 (3) to obtain a quantitative description of their ground state wavefunctions. The Cu L-edge intensities give 63 and 37% Cu d-character in the ground state of 1 and 2, respectively, whereas the S K-pre-edge intensities reflect 20 and 48% S character in their ground states, respectively. These data indicate a more than 2-fold increase in the total disulfide bonding character in 2 relative to 1. The increase in the number of Cu-S bonds in 2 (mu-eta(2):eta(2) S2(2-) bridge) compared to 1 ((mu-eta(1):eta(1) S2(2-) bridge) dominantly determines the large increase in covalency and Cu-disulfide bond strength in 2. Cu K- and L- and S K-pre-edge energy positions directly demonstrate the Cu(II)/(S2(-))2 nature of 3. The two disulfide(*1-)'s in 3 undergo strong bonding interactions that destabilize the resultant filled antibonding pi* orbitals of the (S2(-))2 fragment relative to the Cu 3d levels. This leads to an inverted bonding scheme in 3 with dominantly ligand-based holes in its ground state, consistent with its description as a dicopper(II)-bis-disulfide(*1-) complex.

    View details for DOI 10.1021/ja0762745

    View details for Web of Science ID 000252292500063

    View details for PubMedID 18076173

  • Mixed valent sites in biological electron transfer CHEMICAL SOCIETY REVIEWS Solomon, E. I., Xie, X., Dey, A. 2008; 37 (4): 623-638

    Abstract

    Many of the active sites involved in electron transfer (ET) in biology have more than one metal and are mixed valent in at least one redox state. These include Cu(A), and the polynuclear Fe-S clusters which vary in their extent of delocalization. In this tutorial review the relative contributions to delocalization are evaluated using S K-edge X-ray absorption, magnetic circular dichroism and other spectroscopic methods. The role of intra-site delocalization in ET is considered.

    View details for DOI 10.1039/b714577m

    View details for Web of Science ID 000254315700001

    View details for PubMedID 18362972

  • A Combined NRVS and DFT Study of Fe-IV=O Model Complexes: A Diagnostic Method for the Elucidation of Non-Heme Iron Enzyme Intermediates ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Bell, C. B., Wong, S. D., Xiao, Y., Klinker, E. J., Tenderholt, A. L., Smith, M. C., Rohde, J., Que, L., Cramer, S. P., Solomon, E. I. 2008; 47 (47): 9071-9074

    View details for DOI 10.1002/anie.200803740

    View details for Web of Science ID 000261038700013

    View details for PubMedID 18925598

  • O-2 Reduction to H2O by the multicopper oxidases DALTON TRANSACTIONS Solomon, E. I., Augustine, A. J., Yoon, J. 2008: 3921-3932

    Abstract

    In nature the four electron reduction of O2 to H2O is carried out by Cytochrome c oxidase (CcO) and the multicopper oxidases (MCOs). In the former, Cytochrome c provides electrons for pumping protons to produce a gradient for ATP synthesis, while in the MCOs the function is the oxidation of substrates, either organic or metal ions. In the MCOs the reduction of O2 is carried out at a trinuclear Cu cluster (TNC). Oxygen intermediates have been trapped which exhibit unique spectroscopic features that reflect novel geometric and electronic structures. These intermediates have both intact and cleaved O-O bonds, allowing the reductive cleavage of the O-O bond to be studied in detail both experimentally and computationally. These studies show that the topology of the TNC provides a unique geometric and electronic structure particularly suited to carry out this key reaction in nature.

    View details for DOI 10.1039/b800799c

    View details for Web of Science ID 000257875200001

    View details for PubMedID 18648693

  • Spectroscopic and quantum chemical studies on low-spin Fe-IV=O complexes: Fe-O bonding and its contributions to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Decker, A., Rohde, J., Klinker, E. J., Wong, S. D., Que, L., Solomon, E. I. 2007; 129 (51): 15983-15996

    Abstract

    High-valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes and have been structurally defined in model systems. Variable-temperature magnetic circular dichroism (VT-MCD) spectroscopy has been used to evaluate the electronic structures and in particular the Fe-O bonds of three FeIV=O (S = 1) model complexes, [FeIV(O)(TMC)(NCMe)]2+, [FeIV(O)(TMC)(OC(O)CF3)]+, and [FeIV(O)(N4Py)]2+. These complexes are characterized by their strong and covalent Fe-O pi-bonds. The MCD spectra show a vibronic progression in the nonbonding --> pi* excited state, providing the Fe-O stretching frequency and the Fe-O bond length in this excited state and quantifying the pi-contribution to the total Fe-O bond. Correlation of these experimental data to reactivity shows that the [FeIV(O)(N4Py)]2+ complex, with the highest reactivity toward hydrogen-atom abstraction among the three, has the strongest Fe-O pi-bond. Density functional calculations were correlated to the data and support the experimental analysis. The strength and covalency of the Fe-O pi-bond result in high oxygen character in the important frontier molecular orbitals (FMOs) for this reaction, the unoccupied beta-spin d(xz/yz) orbitals, that activates these for electrophilic attack. An extension to biologically relevant FeIV=O (S = 2) enzyme intermediates shows that these can perform electrophilic attack reactions along the same mechanistic pathway (pi-FMO pathway) with similar reactivity but also have an additional reaction channel involving the unoccupied alpha-spin d(z2) orbital (sigma-FMO pathway). These studies experimentally probe the FMOs involved in the reactivity of FeIV=O (S = 1) model complexes resulting in a detailed understanding of the Fe-O bond and its contributions to reactivity.

    View details for DOI 10.1021/ja074900s

    View details for Web of Science ID 000251974000049

    View details for PubMedID 18052249

  • Solvent tuning of electrochemical potentials in the active sites of HiPIP versus ferredoxin SCIENCE Dey, A., Jenney, F. E., Adams, M. W., Babini, E., Takahashi, Y., Fukuyama, K., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 318 (5855): 1464-1468

    Abstract

    A persistent puzzle in the field of biological electron transfer is the conserved iron-sulfur cluster motif in both high potential iron-sulfur protein (HiPIP) and ferredoxin (Fd) active sites. Despite this structural similarity, HiPIPs react oxidatively at physiological potentials, whereas Fds are reduced. Sulfur K-edge x-ray absorption spectroscopy uncovers the substantial influence of hydration on this variation in reactivity. Fe-S covalency is much lower in natively hydrated Fd active sites than in HiPIPs but increases upon water removal; similarly, HiPIP covalency decreases when unfolding exposes an otherwise hydrophobically shielded active site to water. Studies on model compounds and accompanying density functional theory calculations support a correlation of Fe-S covalency with ease of oxidation and therefore suggest that hydration accounts for most of the difference between Fd and HiPIP reduction potentials.

    View details for DOI 10.1126/science.1147753

    View details for Web of Science ID 000251246100050

    View details for PubMedID 18048692

  • CD and MCD of CytC3 and taurine dioxygenase: Role of the facial triad in alpha-KG-dependent oxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neidig, M. L., Brown, C. D., Light, K. M., Fujimori, D. G., Nolan, E. M., Price, J. C., Barr, E. W., Bollinger, J. M., Krebs, C., Walsh, C. T., Solomon, E. I. 2007; 129 (46): 14224-14231

    Abstract

    The alpha-ketoglutarate (alpha-KG)-dependent oxygenases are a large and diverse class of mononuclear non-heme iron enzymes that require FeII, alpha-KG, and dioxygen for catalysis with the alpha-KG cosubstrate supplying the additional reducing equivalents for oxygen activation. While these systems exhibit a diverse array of reactivities (i.e., hydroxylation, desaturation, ring closure, etc.), they all share a common structural motif at the FeII active site, termed the 2-His-1-carboxylate facial triad. Recently, a new subclass of alpha-KG-dependent oxygenases has been identified that exhibits novel reactivity, the oxidative halogenation of unactivated carbon centers. These enzymes are also structurally unique in that they do not contain the standard facial triad, as a Cl- ligand is coordinated in place of the carboxylate. An FeII methodology involving CD, MCD, and VTVH MCD spectroscopies was applied to CytC3 to elucidate the active-site structural effects of this perturbation of the coordination sphere. A significant decrease in the affinity of FeII for apo-CytC3 was observed, supporting the necessity of the facial triad for iron coordination to form the resting site. In addition, interesting differences observed in the FeII/alpha-KG complex relative to the cognate complex in other alpha-KG-dependent oxygenases indicate the presence of a distorted 6C site with a weak water ligand. Combined with parallel studies of taurine dioxygenase and past studies of clavaminate synthase, these results define a role of the carboxylate ligand of the facial triad in stabilizing water coordination via a H-bonding interaction between the noncoordinating oxygen of the carboxylate and the coordinated water. These studies provide initial insight into the active-site features that favor chlorination by CytC3 over the hydroxylation reactions occurring in related enzymes.

    View details for DOI 10.1021/ja074557r

    View details for Web of Science ID 000251182000047

    View details for PubMedID 17967013

  • Substrate activation for O-2 reactions by oxidized metal centers in biology PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Pau, M. Y., Lipscomb, J. D., Solomon, E. 2007; 104 (47): 18355-18362

    Abstract

    The uncatalyzed reactions of O(2) (S = 1) with organic substrates (S = 0) are thermodynamically favorable but kinetically slow because they are spin-forbidden and the one-electron reduction potential of O(2) is unfavorable. In nature, many of these important O(2) reactions are catalyzed by metalloenzymes. In the case of mononuclear non-heme iron enzymes, either Fe(II) or Fe(III) can play the catalytic role in these spin-forbidden reactions. Whereas the ferrous enzymes activate O(2) directly for reaction, the ferric enzymes activate the substrate for O(2) attack. The enzyme-substrate complex of the ferric intradiol dioxygenases exhibits a low-energy catecholate to Fe(III) charge transfer transition that provides a mechanism by which both the Fe center and the catecholic substrate are activated for the reaction with O(2). In this Perspective, we evaluate how the coupling between this experimentally observed charge transfer and the change in geometry and ligand field of the oxidized metal center along the reaction coordinate can overcome the spin-forbidden nature of the O(2) reaction.

    View details for DOI 10.1073/pnas.0704191104

    View details for Web of Science ID 000251292500005

    View details for PubMedID 18003930

  • SK-Edge XAS and DFT calculations on square-planar NiII-thiolate complexes: Effects of active and passive H-bonding INORGANIC CHEMISTRY Dey, A., Green, K. N., Jenkins, R. M., Jeffrey, S. P., Darensbourg, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 46 (23): 9655-9660

    Abstract

    S K-edge XAS for a low-spin NiII-thiolate complex shows a 0.2 eV shift to higher pre-edge energy but no change in Ni-S bond covalency upon H-bonding. This is different from the H-bonding effect we observed in high-spin FeIII-thiolate complexes where there is a significant decrease in Fe-S bond covalency but no change in energy due to H-bonding (Dey, A.; Okamura, T.-A.; Ueyama, N.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2005, 127, 12046-12053). These differences were analyzed using DFT calculations, and the results indicate that two different types of H-bonding interactions are possible in metal-thiolate systems. In the high-spin FeIII-thiolate case, the H-bonding involves a thiolate donor orbital which is also involved in bonding with the metal (active), while in the low-spin NiII-thiolate, the orbital involved in H-bonding is nonbonding with respect to the M-S bonding (passive). The contributions of active and passive H-bonds to the reduction potential and Lewis acid properties of a metal center are evaluated.

    View details for DOI 10.1021/ic7006292

    View details for Web of Science ID 000250732000024

    View details for PubMedID 17949080

  • Sulfur K-edge XAS of W-V=O vs. Mo-V=O bis(dithiolene) complexes: Contributions of relativistic effects to electronic structure and reactivity of tungsten enzymes JOURNAL OF INORGANIC BIOCHEMISTRY Tenderholt, A. L., Szilagyi, R. K., Holm, R. H., Hodgson, K., Hedman, B., Solomon, E. 2007; 101 (11-12): 1594-1600

    Abstract

    Molybdenum- or tungsten-containing enzymes catalyze oxygen atom transfer reactions involved in carbon, sulfur, or nitrogen metabolism. It has been observed that reduction potentials and oxygen atom transfer rates are different for W relative to Mo enzymes and the isostructural Mo/W complexes. Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations on [Mo(V)O(bdt)(2)](-) and [W(V)O(bdt)(2)](-), where bdt=benzene-1,2-dithiolate(2-), have been used to determine that the energies of the half-filled redox-active orbital, and thus the reduction potentials and MO bond strengths, are different for these complexes due to relativistic effects in the W sites.

    View details for DOI 10.1016/j.jinorgbio.2007.07.011

    View details for Web of Science ID 000251523100008

    View details for PubMedID 17720249

  • Polarized X-ray absorption spectroscopy of single-crystal Mn(V) complexes relevant to the oxygen-evolving complex of photosystem II JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yano, J., Robblee, J., Pushkar, Y., Marcus, M. A., Bendix, J., Workman, J. M., Collins, T. J., Solomon, E. I., George, S. D., Yachandra, V. K. 2007; 129 (43): 12989-13000

    Abstract

    High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.

    View details for DOI 10.1021/ja071286b

    View details for Web of Science ID 000250818900032

    View details for PubMedID 17918832

  • Spectroscopic and kinetic studies of perturbed trinuclear copper clusters: The role of protons in reductive cleavage of the O-O bond in the multicopper oxidase Fet3p JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Augustine, A. J., Quintanar, L., Stoj, C. S., Kosman, D. J., Solomon, E. I. 2007; 129 (43): 13118-13126

    Abstract

    The multicopper oxidase Fet3p couples four 1e(-) oxidations of substrate to the 4e(-) reduction of O2 to H2O. Fet3p uses four Cu atoms to accomplish this reaction: the type 1, type 2, and coupled binuclear type 3 sites. The type 2 and type 3 sites together form a trinuclear Cu cluster (TNC) which is the site of O2 reduction. This study focuses on mutants of two residues, E487 and D94, which lie in the second coordination sphere of the TNC and defines the role that each plays in the structural integrity of the TNC, its reactivity with O2, and in the directional movement of protons during reductive cleavage of the O-O bond. The E487D, E487A, and D94E mutants have been studied in the holo and type 1 depleted (T1D) forms. Residue E487, located near the T3 center, is found to be responsible for donation of a proton during the reductive cleavage of the O-O bond in the peroxide intermediate and an inverse kinetic solvent isotope effect, which indicates that this proton is already transferred when the O-O bond is cleaved. Residue D94, near the T2 site, plays a key role in the reaction of the reduced TNC with O2 and drives electron transfer from the T2 Cu to cleave the O-O bond by deprotonating the T2 Cu water ligand. A mechanism is developed where these second sphere residues participate in the proton assisted reductive cleavage of the O-O bond at the TNC.

    View details for DOI 10.1021/ja073905m

    View details for Web of Science ID 000250818900046

    View details for PubMedID 17918838

  • Electronic structure of the peroxy intermediate and its correlation to the native intermediate in the multicopper oxidases: Insights into the reductive cleavage of the O-O bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoon, J., Solomon, E. I. 2007; 129 (43): 13127-13136

    Abstract

    The multicopper oxidases (MCOs) utilize a blue type 1 (T1) copper site and a trinuclear Cu cluster composed of a type 2 (T2) and a binuclear type 3 (T3) site that together catalyze the four-electron reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 proceeds via two sequential two-electron steps generating the peroxy intermediate (PI) and the native intermediate (NI). While a detailed description of the geometric and electronic structure of NI has been developed, this has been more elusive for PI largely due to the diamagnetic nature of its ground state. Density functional theory (DFT) calculations have been used to correlate to spectroscopic data to generate a description of the geometric and electronic structure of PI. A highly conserved carboxylate residue near the T2 site is found to play a critical role in stabilizing the PI structure, which induces oxidation of the T2 and one T3 Cu center and strong superexchange stabilization via the peroxide bridge, allowing irreversible binding of O2 at the trinuclear Cu site. Correlation of PI to NI is achieved using a two-dimensional potential energy surface generated to describe the catalytic two-electron reduction of the peroxide O-O bond by the MCOs. It is found that the reaction is thermodynamically driven by the relative stability of NI and the involvement of the simultaneous two-electron-transfer process. A low activation barrier (calculated approximately 5-6 kcal/mol and experimental approximately 3-5 kcal/mol) is produced by the triangular topology of the trinuclear Cu cluster site, as this symmetry provides good donor-acceptor frontier molecular orbital (FMO) overlap. Finally, the O-O bond cleavage in the trinuclear Cu cluster can be achieved via either a proton-assisted or a proton-unassisted process, allowing the MCOs to function over a wide range of pH. It is found that while the proton helps to stabilize the acceptor O22- sigma* orbital in the proton-assisted process for better donor-acceptor FMO overlap, the third oxidized Cu center in the trinuclear site assumes the role as a Lewis acid in the proton-unassisted process for similarly efficient O-O bond cleavage.

    View details for DOI 10.1021/ja073947a

    View details for Web of Science ID 000250818900047

    View details for PubMedID 17918839

  • Sulfur K-edge X-ray absorption Spectroscopy and density functional theory calculations on superoxide reductase: Role of the axial thiolate in reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Jenney, F. E., Adams, M. W., Johnson, M. K., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 129 (41): 12418-12431

    Abstract

    Superoxide reductase (SOR) is a non-heme iron enzyme that reduces superoxide to peroxide at a diffusion-controlled rate. Sulfur K-edge X-ray absorption spectroscopy (XAS) is used to investigate the ground-state electronic structure of the resting high-spin and CN- bound low-spin FeIII forms of the 1Fe SOR from Pyrococcus furiosus. A computational model with constrained imidazole rings (necessary for reproducing spin states), H-bonding interaction to the thiolate (necessary for reproducing Fe-S bond covalency of the high-spin and low-spin forms), and H-bonding to the exchangeable axial ligand (necessary to reproduce the ground state of the low-spin form) was developed and then used to investigate the enzymatic reaction mechanism. Reaction of the resting ferrous site with superoxide and protonation leading to a high-spin FeIII-OOH species and its subsequent protonation resulting in H2O2 release is calculated to be the most energetically favorable reaction pathway. Our results suggest that the thiolate acts as a covalent anionic ligand. Replacing the thiolate with a neutral noncovalent ligand makes protonation very endothermic and greatly raises the reduction potential. The covalent nature of the thiolate weakens the FeIII bond to the proximal oxygen of this hydroperoxo species, which raises its pKa by an additional 5 log units relative to the pKa of a primarily anionic ligand, facilitating its protonation. A comparison with cytochrome P450 indicates that the stronger equatorial ligand field from the porphyrin results in a low-spin FeIII-OOH species that would not be capable of efficient H2O2 release due to a spin-crossing barrier associated with formation of a high-spin 5C FeIII product. Additionally, the presence of the dianionic porphyrin pi ring in cytochrome P450 allows O-O heterolysis, forming an FeIV-oxo porphyrin radical species, which is calculated to be extremely unfavorable for the non-heme SOR ligand environment. Finally, the 5C FeIII site that results from the product release at the end of the O2- reduction cycle is calculated to be capable of reacting with a second O2-, resulting in superoxide dismutase (SOD) activity. However, in contrast to FeSOD, the 5C FeIII site of SOR, which is more positively charged, is calculated to have a high affinity for binding a sixth anionic ligand, which would inhibit its SOD activity.

    View details for DOI 10.1021/ja064167p

    View details for Web of Science ID 000250105500039

    View details for PubMedID 17887751

  • Resolution of the spectroscopy versus crystallography issue for NO intermediates of nitrite reductase from Rhodobacter sphaeroides JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Dey, A., Usov, O. M., Sun, Y., Grigoryants, V. M., Scholes, C. P., Solomon, E. I. 2007; 129 (34): 10310-?

    View details for DOI 10.1021/ja072841c

    View details for Web of Science ID 000249035200006

    View details for PubMedID 17685522

  • The two oxidized forms of the trinuclear Cu cluster in the multicopper oxidases and mechanism for the decay of the native intermediate PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Yoon, J., Liboiron, B. D., Sarangi, R., Hodgson, K. O., Hedman, B., Solomona, E. I. 2007; 104 (34): 13609-13614

    Abstract

    Multicopper oxidases (MCOs) catalyze the 4e(-) reduction of O(2) to H(2)O. The reaction of the fully reduced enzyme with O(2) generates the native intermediate (NI), which undergoes a slow decay to the resting enzyme in the absence of substrate. NI is a fully oxidized form, but its spectral features are very different from those of the resting form (also fully oxidized), because the type 2 and the coupled-binuclear type 3 Cu centers in the O(2)-reducing trinuclear Cu cluster site are isolated in the resting enzyme, whereas these are all bridged by a micro(3)-oxo ligand in NI. Notably, the one azide-bound NI (NI(Az)) exhibits spectral features very similar to those of NI, in which the micro(3)-oxo ligand in NI has been replaced by a micro(3)-bridged azide. Comparison of the spectral features of NI and NI(Az), combined with density functional theory (DFT) calculations, allows refinement of the NI structure. The decay of NI to the resting enzyme proceeds via successive proton-assisted steps, whereas the rate-limiting step involves structural rearrangement of the micro(3)-oxo-bridge from inside to outside the cluster. This phenomenon is consistent with the slow rate of NI decay that uncouples the resting enzyme from the catalytic cycle, leaving NI as the catalytically relevant fully oxidized form of the MCO active site. The all-bridged structure of NI would facilitate electron transfer to all three Cu centers of the trinuclear cluster for rapid proton-coupled reduction of NI to the fully reduced form for catalytic turnover.

    View details for Web of Science ID 000249064700017

    View details for PubMedID 17702865

  • Spectroscopic and electronic structure studies of intermediate X in ribonucleotide reductase R2 and two variants: A description of the Fe-IV-Oxo bond in the Fe-III-O-Fe-IV dimer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mitic, N., Clay, M. D., Saleh, L., Bollinger, J. M., Solomon, E. I. 2007; 129 (29): 9049-9065

    Abstract

    Spectroscopic and electronic structure studies of the class I Escherichia coli ribonucleotide reductase (RNR) intermediate X and three computationally derived model complexes are presented, compared, and evaluated to determine the electronic and geometric structure of the FeIII-FeIV active site of intermediate X. Rapid freeze-quench (RFQ) EPR, absorption, and MCD were used to trap intermediate X in R2 wild-type (WT) and two variants, W48A and Y122F/Y356F. RFQ-EPR spin quantitation was used to determine the relative contributions of intermediate X and radicals present, while RFQ-MCD was used to specifically probe the FeIII/FeIV active site, which displayed three FeIV d-d transitions between 16,700 and 22,600 cm(-1), two FeIV d-d spin-flip transitions between 23,500 and 24,300 cm(-1), and five oxo to FeIV and FeIII charge transfer (CT) transitions between 25,000 and 32,000 cm(-1). The FeIV d-d transitions were perturbed in the two variants, confirming that all three d-d transitions derive from the d-pi manifold. Furthermore, the FeIV d-pi splittings in the WT are too large to correlate with a bis-mu-oxo structure. The assignment of the FeIV d-d transitions in WT intermediate X best correlates with a bridged mu-oxo/mu-hydroxo [FeIII(mu-O)(mu-OH)FeIV] structure. The mu-oxo/mu-hydroxo core structure provides an important sigma/pi superexchange pathway, which is not present in the bis-mu-oxo structure, to promote facile electron transfer from Y122 to the remote FeIV through the bent oxo bridge, thereby generating the tyrosyl radical for catalysis.

    View details for DOI 10.1021/ja070909i

    View details for Web of Science ID 000248185500035

    View details for PubMedID 17602477

  • Copper(I) complex O-2-reactivity with a N3S thioether ligand: A copper-dioxygen adduct including sulfur ligation, ligand oxygenation, and comparisons with all nitrogen ligand analogues INORGANIC CHEMISTRY Lee, D., Hatcher, L. Q., Vance, M. A., Sarangi, R., Milligan, A. E., Sarjeant, A. A., Incarvito, C. D., Rheingold, A. L., Hodgson, K. O., Hedman, B., Solomon, E. I., Karlin, K. D. 2007; 46 (15): 6056-6068

    Abstract

    In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O(2) reactivity, the tetradentate ligands L(N3S) (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L(N3S')(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(L(N3S))]ClO(4) (1-ClO(4)), [CuI(L(N3S))]B(C(6)F(5))(4) (1-B(C(6)F(5))(4)), and [CuI(L(N3S'))]ClO(4) (2), were generated, and their redox properties, CO binding, and O(2)-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [CuI(TMPA)(MeCN)](+) and [Cu(I)(PMAP)](+) (TMPA = tris(2-pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine). X-ray structures of 1-B(C(6)F(5))(4), a dimer, and copper(II) complex [Cu(II)(L(N3S))(MeOH)](ClO(4))(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH(2)Cl(2), acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O(2) and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu(II)(L(N3S))}(2)(mu-1,2-O(2)(2-))](2+) (4)){lambda(max) = 530 (epsilon approximately 9200 M(-1) cm(-1)) and 605 nm (epsilon approximately 11,800 M(-1) cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {lambda(max) = 524 nm (epsilon = 11,300 M(-1) cm(-1)) and 615 nm (epsilon = 5800 M(-1) cm(-1))} and are ascribed to electronic structure variation due to coordination geometry changes with the L(N3S) ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {nu(O-O) = 817 cm(-1) (16-18O(2) Delta = 46 cm(-1)) and nu(Cu-O) = 545 cm(-1) (16-18O(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Angstrom}. Following a [Cu(I)(L(N3S))](+)/O(2) reaction and warming, the L(N3S) thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu(II)/Cu(I) redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O(2)-activation, and situations of biological stress involving methionine oxidation, is provided.

    View details for DOI 10.1021/ic700541k

    View details for Web of Science ID 000248011300034

    View details for PubMedID 17580938

  • Copper(I)/S-8 reversible reactions leading to an end-on bound dicopper(II) disulfide complex: Nucleophilic reactivity and analogies to copper-dioxygen chemistry JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Maiti, D., Woertink, J. S., Vance, M. A., Milligan, A. E., Sarjeant, A. A., Solomon, E. I., Karlin, K. D. 2007; 129 (28): 8882-8892

    Abstract

    Elemental sulfur (S8) reacts reversibly with the copper(I) complex [(TMPA')CuI](+) (1), where TMPA' is a TMPA (tris(2-pyridylmethyl)amine) analogue with a 6-CH2OCH3 substituent on one pyridyl ligand arm, affording a spectroscopically pure end-on bound disulfido-dicopper(II) complex [{(TMPA')Cu(II)}2(mu-1,2-S2(2-))](2+) (2) {nu(S-S) = 492 cm(-1); nu(Cu-S)sym = 309 cm(-1)}; by contrast, [(TMPA)Cu(I)(CH3CN)](+) (3)/S8 chemistry produces an equilibrium mixture of at least three complexes. The reaction of excess PPh3 with 2 leads to formal "release" of zerovalent sulfur and reduction of copper ion to give the corresponding complex [(TMPA')Cu(I)(PPh3)](+) (11) along with S=PPh3 as products. Dioxygen displaces the disulfur moiety from 2 to produce the end-on Cu2O2 complex, [{(TMPA')Cu(II)}2(mu-1,2-O2(2-)](2+) (9). Addition of the tetradentate ligand TMPA to 2 generates the apparently more thermodynamically stable [{(TMPA)Cu(II)}2(mu-1,2-S2(2-))](2+) (4) and expected mixture of other species. Bubbling 2 with CO leads to the formation of the carbonyl adduct [(TMPA')CuI(CO)](+) (8). Carbonylation/sulfur-release/CO-removal cycles can be repeated several times. Sulfur atom transfer from 2 also occurs in a near quantitative manner when it is treated with 2,6-dimethylphenyl isocyanide (ArNC), leading to the corresponding isothiocyanate (ArNCS) and [(TMPA')Cu(I)(CNAr)](+) (12). Complex 2 readily reacts with PhCH2Br: [{(TMPA')Cu(II)}2(mu-1,2-S(2)(2-)](2+) (2) + 2 PhCH2Br --> [{(TMPA')Cu(II)(Br)}2](2+) (6) + PhCH2SSCH2Ph. The unprecedented substrate reactivity studies reveal that end-on bound mu-1,2-disulfide-dicopper(II) complex 2 provides a nucleophilic S2(2-) moiety, in striking contrast to the electrophilic behavior of a recently described side-on bound mu-eta(2):eta(2)-disulfido-dicopper(II) complex, [{(N3)Cu(II)}(2)(mu-eta(2):eta(2)-S2(2-))](2+) (5) with tridentate N3 ligand. The investigation thus reveals striking analogies of copper/sulfur and copper/dioxygen chemistries, with regard to structure type formation and specific substrate reactivity patterns.

    View details for DOI 10.1021/ja071968z

    View details for Web of Science ID 000247966200043

    View details for PubMedID 17592845

  • O-2 and N2O activation by bi-, tri-, and tetranuclear Cu clusters in biology ACCOUNTS OF CHEMICAL RESEARCH Solomon, E. I., Sarangi, R., Woertink, J. S., Augustine, A. J., Yoon, J., Ghosh, S. 2007; 40 (7): 581-591

    Abstract

    Copper-cluster sites in biology exhibit unique spectroscopic features reflecting exchange coupling between oxidized Cu's and e (-) delocalization in mixed valent sites. These novel electronic structures play critical roles in O 2 binding and activation for electrophilic aromatic attack and H-atom abstraction, the 4e (-)/4H (+) reduction of O 2 to H 2O, and in the 2e (-)/2H (+) reduction of N 2O. These electronic structure/reactivity correlations are summarized below.

    View details for DOI 10.1021/ar600060t

    View details for Web of Science ID 000248074000014

    View details for PubMedID 17472331

  • Kinetic and spectroscopic studies of N694C lipoxygenase: A probe of the substrate activation mechanism of a nonheme ferric enzyme JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neidig, M. L., Wecksler, A. T., Schenk, G., Holman, T. R., Solomon, E. I. 2007; 129 (24): 7531-7537

    Abstract

    Lipoxygenases (LOs) comprise a class of substrate activating mononuclear nonheme iron enzymes which catalyze the hydroperoxidation of unsaturated fatty acids. A commonly proposed mechanism for LO catalysis involves H-atom abstraction by an FeIII-OH- site, best described as a proton coupled electron transfer (PCET) process, followed by direct reaction of O2 with the resulting substrate radical to yield product. An alternative mechanism that has also been discussed involves the abstraction of a proton from the substrate by the FeIII-OH leading to a sigma-organoiron intermediate, where the subsequent sigma bond insertion of dioxygen into the C-Fe bond completes the reaction. H-atom abstraction is favored by a high E(o) of the FeII/FeIII couple and high pK(a) of water bound to the ferrous state, while an organoiron mechanism would be favored by a low E(o) (to keep the site oxidized) and a high pK(a) of water bound to the ferric state (to deprotonate the substrate). A first coordination sphere mutant of soybean LO (N694C) has been prepared and characterized by near-infrared circular dichroism (CD) and variable-temperature, variable-field (VTVH) magnetic circular dichroism (MCD) spectroscopies (FeII site), as well as UV/vis absorption, UV/vis CD, and electron paramagnetic resonance (EPR) spectroscopies (FeIII site). These studies suggest that N694C has a lowered E degrees of the FeII/FeIII couple and a raised pKa of water bound to the ferric site relative to wild type soybean lipoxygenase-1 (WT sLO-1) which would favor the organoiron mechanism. However, the observation in N694C of a significant deuterium isotope effect, anaerobic reduction of iron by substrate, and a substantial decrease in k(cat) (approximately 3000-fold) support H-atom abstraction as the relevant substrate-activation mechanism in sLO-1.

    View details for DOI 10.1021/ja068503d

    View details for Web of Science ID 000247240500022

    View details for PubMedID 17523638

  • VTVH-MCD and DFT studies of thiolate bonding to {FeNO}(7)/{FeO2}(8) complexes of isopenicillin N synthase: Substrate determination of oxidase versus oxygenase activity in nonheme Fe enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Brown, C. D., Neidig, M. L., Neibergall, M. B., Lipscomb, J. D., Solomon, E. I. 2007; 129 (23): 7427-7438

    Abstract

    Isopenicillin N synthase (IPNS) is a unique mononuclear nonheme Fe enzyme that catalyzes the four-electron oxidative double ring closure of its substrate ACV. A combination of spectroscopic techniques including EPR, absorbance, circular dichroism (CD), magnetic CD, and variable-temperature, variable-field MCD (VTVH-MCD) were used to evaluate the geometric and electronic structure of the [FeNO]7 complex of IPNS coordinated with the ACV thiolate ligand. Density Function Theory (DFT) calculations correlated to the spectroscopic data were used to generate an experimentally calibrated bonding description of the Fe-IPNS-ACV-NO complex. New spectroscopic features introduced by the binding of the ACV thiolate at 13 100 and 19 800 cm-1 are assigned as the NO pi*(ip) --> Fe dx2-y2 and S pi--> Fe dx2-y2 charge transfer (CT) transitions, respectively. Configuration interaction mixes S CT character into the NO pi*(ip) --> Fe dx2-y2 CT transition, which is observed experimentally from the VTVH-MCD data from this transition. Calculations on the hypothetical {FeO2}8 complex of Fe-IPNS-ACV reveal that the configuration interaction present in the [FeNO]7 complex results in an unoccupied frontier molecular orbital (FMO) with correct orientation and distal O character for H-atom abstraction from the ACV substrate. The energetics of NO/O2 binding to Fe-IPNS-ACV were evaluated and demonstrate that charge donation from the ACV thiolate ligand renders the formation of the FeIII-superoxide complex energetically favorable, driving the reaction at the Fe center. This single center reaction allows IPNS to avoid the O2 bridged binding generally invoked in other nonheme Fe enzymes that leads to oxygen insertion (i.e., oxygenase function) and determines the oxidase activity of IPNS.

    View details for DOI 10.1021/ja071364v

    View details for Web of Science ID 000247072300053

    View details for PubMedID 17506560

  • Sulfur K-edge XAS and DFT studies on Ni-II complexes with oxidized thiolate ligands: Implications for the roles of oxidized thiolates in the active sites of Fe and Co nitrile hydratase INORGANIC CHEMISTRY Dey, A., Jeffrey, S. P., Darensbourg, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 46 (12): 4989-4996

    Abstract

    S K-edge X-ray absorption spectroscopy data on a series of NiII complexes with thiolate (RS-) and oxidized thiolate (RSO2-) ligands are used to quantify Ni-S bond covalency and its change upon ligand oxidation. Analyses of these results using geometry-optimized density functional theory (DFT) calculations suggest that the Ni-S sigma bonds do not weaken on ligand oxidation. Molecular orbital analysis indicates that these oxidized thiolate ligands use filled high-lying S-O pi* orbitals for strong sigma donation. However, the RSO2- ligands are poor pi donors, as the orbital required for pi interaction is used in the S-O sigma-bond formation. The oxidation of the thiolate reduces the repulsion between electrons in the filled Ni t2 orbital and the thiolate out-of-plane pi-donor orbital leading to shorter Ni-S bond length relative to that of the thiolate donor. The insights obtained from these results are relevant to the active sites of Fe- and Co-type nitrile hydratases (Nhase) that also have oxidized thiolate ligands. DFT calculations on models of the active site indicate that whereas the oxidation of these thiolates has a major effect in the axial ligand-binding affinity of the Fe-type Nhase (where there is both sigma and pi donation from the S ligands), it has only a limited effect on the sixth-ligand-binding affinity of the Co-type Nhases (where there is only sigma donation). These oxidized residues may also play a role in substrate binding and proton shuttling at the active site.

    View details for DOI 10.1021/ic070244l

    View details for Web of Science ID 000246907800034

    View details for PubMedID 17500514

  • Intramolecular single-turnover reaction in a cytochrome c oxidase model bearing a Tyr244 mimic JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Collman, J. P., Decreau, R. A., Yan, Y., Yoon, J., Solomon, E. I. 2007; 129 (18): 5794-?

    View details for DOI 10.1021/ja0690969

    View details for Web of Science ID 000246180200005

    View details for PubMedID 17429972

  • Spectroscopic, computational, and kinetic studies of the mu(4)-sulfide-bridged tetranuclear Cu-Z cluster in N2O reductase: pH effect on the edge ligand and its contribution to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Gorelsky, S. I., George, S. D., Chan, J. M., Cabrito, I., Dooley, D. M., Moura, J. J., Moura, I., Solomon, E. I. 2007; 129 (13): 3955-3965

    Abstract

    A combination of spectroscopy and density functional theory (DFT) calculations has been used to evaluate the pH effect at the CuZ site in Pseudomonas nautica (Pn) nitrous oxide reductase (N2OR) and Achromobacter cycloclastes (Ac) N2OR and its relevance to catalysis. Absorption, magnetic circular dichroism, and electron paramagnetic resonance with sulfur K-edge X-ray absorption spectra of the enzymes at high and low pH show minor changes. However, resonance Raman (rR) spectroscopy of PnN2OR at high pH shows that the 415 cm-1 Cu-S vibration (observed at low pH) shifts to higher frequency, loses intensity, and obtains a 9 cm-1 18O shift, implying significant Cu-O character, demonstrating the presence of a OH- ligand at the CuICuIV edge. From DFT calculations, protonation of either the OH- to H2O or the mu4-S2- to mu4-SH- would produce large spectral changes which are not observed. Alternatively, DFT calculations including a lysine residue at an H-bonding distance from the CuICuIV edge ligand show that the position of the OH- ligand depends on the protonation state of the lysine. This would change the coupling of the Cu-(OH) stretch with the Cu-S stretch, as observed in the rR spectrum. Thus, the observed pH effect (pKa approximately 9.2) likely reflects protonation equilibrium of the lysine residue, which would both raise E degrees and provide a proton for lowering the barrier for the N-O cleavage and for reduction of the [Cu4S(im)7OH]2+ to the fully reduced 4CuI active form for turnover.

    View details for DOI 10.1021/ja066059e

    View details for Web of Science ID 000245241600048

    View details for PubMedID 17352474

  • Sulfur K-edge X-ray absorption spectroscopy as a probe of ligand-metal bond covalency: Metal vs ligand oxidation in copper and nickel dithiolene complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., George, S. D., Rudd, D. J., Szilagyi, R. K., Ribas, X., Rovira, C., Almeida, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 129 (8): 2316-2326

    Abstract

    A combination of Cu L-edge and S K-edge X-ray absorption data and density functional theory (DFT) calculations has been correlated with 33S electron paramagnetic resonance superhyperfine results to obtain the dipole integral (Is) for the S 1s-->3p transition for the dithiolene ligand maleonitriledithiolate (MNT) in (TBA)2[Cu(MNT)2] (TBA= tetra-n-butylammonium). The results have been combined with the Is of sulfide derived from XPS studies to experimentally obtain a relation between the S 1s-->4p transition energy (which reflects the charge on the S atom, QSmol) and the dipole integral over a large range of QSmol. The results show that, for high charges on S, Is can vary from the previously reported Is values, calculated using data over a limited range of QSmol. A combination of S K-edge and Cu K- and L-edge X-ray absorption data and DFT calculations has been used to investigate the one-electron oxidation of [Cu(MNT)2]2- and [Ni(MNT)2]2-. The conversion of [Cu(MNT)2]2- to [Cu(MNT)2]- results in a large change in the charge on the Cu atom in the molecule (QCumol) and is consistent with a metal-based oxidation. This is accompanied by extensive charge donation from the ligands to compensate the high charge on the Cu in [Cu(MNT)2]- based on the increased S K-edge and decreased Cu L-edge intensity, respectively. In contrast, the oxidation of [Ni(MNT)2]2- to [Ni(MNT)2]- results in a small change in QNimol, indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, psiSOMO (singly occupied molecular orbital), with predominant ligand character.

    View details for DOI 10.1021/ja0665949

    View details for Web of Science ID 000244330800033

    View details for PubMedID 17269767

  • Spectroscopic and electronic structure study of the enzyme-substrate complex of intradiol dioxygenases: Substrate activation by a high-spin ferric non-heme iron site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Pau, M. Y., Davis, M. I., Orville, A. M., Lipscomb, J. D., Solomon, E. I. 2007; 129 (7): 1944-1958

    Abstract

    Various mechanisms have been proposed for the initial O(2) attack in intradiol dioxygenases based on different electronic descriptions of the enzyme-substrate (ES) complex. We have examined the geometric and electronic structure of the high-spin ferric ES complex of protocatechuate 3,4-dioxygenase (3,4-PCD) with UV/visible absorption, circular dichroism (CD), magnetic CD (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. The experimental data were coupled with DFT and INDO/S-CI calculations, and an experimentally calibrated bonding description was obtained. The broad absorption spectrum for the ES complex in the 6000-31000 cm(-1) region was resolved into at least five individual transitions, assigned as ligand-to-metal charge transfer (LMCT) from the protocatechuate (PCA) substrate and Tyr408. From our DFT calculations, all five LMCT transitions originate from the PCA and Tyr piop orbitals to the ferric dpi orbitals. The strong pi covalent donor interactions dominate the bonding in the ES complex. Using hypothetical Ga(3+)-catecholate/semiquinone complexes as references, 3,4-PCD-PCA was found to be best described as a highly covalent Fe(3+)-catecholate complex. The covalency is distributed unevenly among the four PCA valence orbitals, with the strongest interaction between the piop-sym and Fe dxz orbitals. This strong pi interaction, as reflected in the lowest energy PCA-to-Fe(3+) LMCT transition, is responsible for substrate activation for the O(2) reaction of intradiol dioxygenases. This involves a multi-electron-transfer (one beta and two alpha) mechanism, with Fe3+ acting as a buffer for the spin-forbidden two-electron redox process between PCA and O(2) in the formation of the peroxy-bridged ESO2 intermediate. The Fe ligand field overcomes the spin-forbidden nature of the triplet O(2) reaction, which potentially results in an intermediate spin state (S = 3/2) on the Fe(3+) center which is stabilized by a change in coordination along the reaction coordinate.

    View details for DOI 10.1021/ja065671x

    View details for Web of Science ID 000244206400042

    View details for PubMedID 17256852

  • Fe L-edge x-ray absorption spectroscopy of low-spin heme relative to non-heme Fe complexes: Delocalization of Fe d-electrons into the porphyrin ligand JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hocking, R. K., Wasinger, E. C., Yan, Y., deGroot, F. M., Walker, F. A., Hodgson, K. O., Hedman, B., Solomon, E. I. 2007; 129 (1): 113-125

    Abstract

    Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin pi-->pi(*) transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P-->Fe sigma and pi-donation and Fe-->P pi back-bonding. We find that pi-donation to Fe(III) is much larger than pi back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.

    View details for DOI 10.1021/ja065627h

    View details for Web of Science ID 000243195100032

    View details for PubMedID 17199290

  • The two-state issue in the mixed-valence binuclear Cu-A center in cytochrome c oxidase and N2O reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gorelsky, S. I., Xie, X., Chen, Y., Fee, J. A., Solomon, E. I. 2006; 128 (51): 16452-16453

    Abstract

    For the CuA site in the protein, sigmau* and piu are the ground and lowest energy excited-states, respectively. EPR data on CuA proteins show a low g parallel value of 2.19 which derives from spin-orbital coupling between sigmau* and piu which requires an energy gap between sigmau* and piu of 3000-4500 cm-1. On the other hand, from paramagnetic NMR studies, it has been observed that the first excited-state is thermally accessible and the energy gap between the ground state and the thermally accessible state is approximately 350 cm-1. This study addressed this apparent discrepancy and evaluated the roles of the two electronic states, sigmau* and piu, in electron transfer (ET) of CuA. The potential energy surface calculations show that both NMR and EPR results are consistent with the electronic/geometric structure of CuA. The anti-Curie behavior observed in paramagnetic NMR studies of CuA results from the thermal equilibrium between the sigmau* and piu states which are at very close energies in their respective equilibrium geometries. Alternatively, the EPR g-value analysis involves the sigmau* ground state in the geometry with a short dCu-Cu where the piu state is a Frank-Condon excited-state with the energy of 3200 cm-1. The protein environment plays a role in maintaining CuA in the sigmau* state as a lowest-energy state with the lowest reorganization energy and high-covalent coupling to the Cys and His ligands for efficient intra- and intermolecular ET with a low-driving force.

    View details for DOI 10.1021/ja067583i

    View details for Web of Science ID 000242941600020

    View details for PubMedID 17177365

  • Spectroscopic and electronic structure studies of the role of active site interactions in the decarboxylation reaction of alpha-keto acid-dependent dioxygenases JOURNAL OF INORGANIC BIOCHEMISTRY Neidig, M. L., Brown, C. D., Kavana, M., Choroba, O. W., Spencer, J. B., Moran, G. R., Solomon, E. I. 2006; 100 (12): 2108-2116

    Abstract

    The alpha-ketoglutate (alpha-KG)-dependent dioxygenases are a large class of mononuclear non-heme iron enzymes that require Fe(II), alpha-KG and dioxygen for catalysis, with the alpha-KG cosubstrate supplying the two additional electrons required for dioxygen activation. A sub-class of these enzymes exists in which the alpha-keto acid is covalently attached to the substrate, including (4-hydroxy)mandelate synthase (HmaS) and (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) which utilize the same substrate but exhibit two different general reactivities (H-atom abstraction and electrophilic attack). Previous kinetic studies of Streptomyces avermitilis HPPD have shown that the substrate analog phenylpyruvate (PPA), which only differs from the normal substrate (4-hydroxyphenyl)pyruvate (HPP) by the absence of a para-hydroxyl group on the aromatic ring, does not induce a reaction with dioxygen. While an Fe(IV)O intermediate is proposed to be the reactive species in converting substrate to product, the key step utilizing O(2) to generate this species is the decarboxylation of the alpha-keto acid. It has been generally proposed that the two requirements for decarboxylation are bidentate coordination of the alpha-keto acid to Fe(II) and the presence of a 5C Fe(II) site for the O(2) reaction. Circular dichroism and magnetic circular dichroism studies have been performed and indicate that both enzyme complexes with PPA are similar with bidentate alpha-KG coordination and a 5C Fe(II) site. However, kinetic studies indicate that while HmaS reacts with PPA in a coupled reaction similar to the reaction with HPP, HPPD reacts with PPA in an uncoupled reaction at an approximately 10(5)-fold decreased rate compared to the reaction with HPP. A key difference is spectroscopically observed in the n-->pi( *) transition of the HPPD/Fe(II)/PPA complex which, based upon correlation to density functional theory calculations, is suggested to result from H-bonding between a nearby residue and the carboxylate group of the alpha-keto acid. Such an interaction would disfavor the decarboxylation reaction by stabilizing electron density on the carboxylate group such that the oxidative cleavage to yield CO(2) is disfavored.

    View details for DOI 10.1016/j.jinorgbio.2006.08.021

    View details for Web of Science ID 000242919600023

    View details for PubMedID 17070917

  • Circular dichroism and magnetic circular dichroism studies of the active site of p53R2 from human and mouse: Iron binding and nature of the biferrous site relative to other ribonucleotide reductases BIOCHEMISTRY Wei, P., Tomter, A. B., Rohr, A. K., Andersson, K. K., Solomon, E. I. 2006; 45 (47): 14043-14051

    Abstract

    Ribonucleotide reductases (RNR) catalyze the rate-limiting step in the synthesis of deoxyribonucleotides from the corresponding ribonucleotides in the synthesis of DNA. Class I RNR has two subunits: R1 with the substrate binding and active site and R2 with a stable tyrosyl radical and diiron cluster. Biferrous R2 reacts with oxygen to form the tyrosyl radical needed for enzymatic activity. A novel R2 form, p53R2, is a 351-amino acid protein induced by the "tumor suppressor gene" p53. p53R2 has been studied using a combination of circular dichroism, magnetic circular dichroism, variable-temperature variable-field MCD, and EPR spectroscopies. The active site of biferrous p53R2 in both the human (hp53R2) and mouse (mp53R2) forms is found to have one five-coordinate and one four-coordinate iron, which are weakly antiferromagnetically coupled through mu-1,3-carboxylate bridges. These spectroscopic data are very similar to those of Escherichia coli R2, and mouse R2, with a stronger resemblance to data of the former. Titrations of apo-hp53R2 and apo-mp53R2 with Fe(II) were pursued for the purpose of comparing their metal binding affinities to those of other R2s. Both p53R2s were found to have a high affinity for Fe(II), which is different from that of mouse R2 and may reflect differences in the regulation of enzymatic activity, as p53R2 is mainly triggered during DNA repair. The difference in ferrous affinity between mammalian R2 and p53R2 suggests the possibility of specific inhibition of DNA precursor synthesis during cell division.

    View details for DOI 10.1021/bi061127p

    View details for Web of Science ID 000242179100012

    View details for PubMedID 17115699

  • Spectroscopic methods in bioinorganic chemistry: Blue to green to red copper sites INORGANIC CHEMISTRY Solomon, E. I. 2006; 45 (20): 8012-8025

    Abstract

    A wide variety of spectroscopic methods are now available that provide complimentary insights into the electronic structures of transition-metal complexes. Combined with calculations, these define key bonding interactions, enable the evaluation of reaction coordinates, and determine the origins of unique spectroscopic features/electronic structures that can activate metal centers for catalysis. This presentation will summarize the contributions of a range of spectroscopic methods combined with calculations in elucidating the electronic structure of an active site using the blue copper site as an example. The contribution of electronic structure to electron-transfer reactivity will be considered in terms of anisotropic covalency, electron-transfer pathways, reorganization energy, and protein contributions to the geometric and electronic structures of blue-copper-related active sites.

    View details for DOI 10.1021/ic060450d

    View details for Web of Science ID 000240711500010

    View details for PubMedID 16999398

  • Metal-thiolate bonds in bioinorganic chemistry JOURNAL OF COMPUTATIONAL CHEMISTRY Solomon, E. I., Gorelsky, S. I., Dey, A. 2006; 27 (12): 1415-1428

    Abstract

    Metal-thiolate active sites play major roles in bioinorganic chemistry. The M--S(thiolate) bonds can be very covalent, and involve different orbital interactions. Spectroscopic features of these active sites (intense, low-energy charge transfer transitions) reflect the high covalency of the M--S(thiolate) bonds. The energy of the metal-thiolate bond is fairly insensitive to its ionic/covalent and pi/sigma nature as increasing M--S covalency reduces the charge distribution, hence the ionic term, and these contributions can compensate. Thus, trends observed in stability constants (i.e., the Irving-Williams series) mostly reflect the dominantly ionic contribution to bonding of the innocent ligand being replaced by the thiolate. Due to high effective nuclear charges of the Cu(II) and Fe(III) ions, the cupric- and ferric-thiolate bonds are very covalent, with the former having strong pi and the latter having more sigma character. For the blue copper site, the high pi covalency couples the metal ion into the protein for rapid directional long range electron transfer. For rubredoxins, because the redox active molecular orbital is pi in nature, electron transfer tends to be more localized in the vicinity of the active site. Although the energy of hydrogen bonding of the protein environment to the thiolate ligands tends to be fairly small, H-bonding can significantly affect the covalency of the metal-thiolate bond and contribute to redox tuning by the protein environment.

    View details for DOI 10.1002/jcc.20451

    View details for Web of Science ID 000239072600015

    View details for PubMedID 16807974

  • Spectroscopic and electronic structure studies of aromatic electrophilic attack and hydrogen-atom abstraction by non-heme iron enzymes PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Neidig, M. L., Decker, A., Choroba, O. W., Huang, F., Kavana, M., Moran, G. R., Spencer, J. B., Solomon, E. I. 2006; 103 (35): 12966-12973

    Abstract

    (4-Hydroxy)mandelate synthase (HmaS) and (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) are two alpha-keto acid dependent mononuclear non-heme iron enzymes that use the same substrate, (4-hydroxyphenyl)pyruvate, but exhibit two different general reactivities. HmaS performs hydrogen-atom abstraction to yield benzylic hydroxylated product (S)-(4-hydroxy)mandelate, whereas HPPD utilizes an electrophilic attack mechanism that results in aromatic hydroxylated product homogentisate. These enzymes provide a unique opportunity to directly evaluate the similarities and differences in the reaction pathways used for these two reactivities. An Fe(II) methodology using CD, magnetic CD, and variable-temperature, variable-field magnetic CD spectroscopies was applied to HmaS and compared with that for HPPD to evaluate the factors that affect substrate interactions at the active site and to correlate these to the different reactivities exhibited by HmaS and HPPD to the same substrate. Combined with density functional theory calculations, we found that HmaS and HPPD have similar substrate-bound complexes and that the role of the protein pocket in determining the different reactivities exhibited by these enzymes (hydrogen-atom abstraction vs. aromatic electrophilic attack) is to properly orient the substrate, allowing for ligand field geometric changes along the reaction coordinate. Elongation of the Fe(IV) O bond in the transition state leads to dominant Fe(III) O(*-) character, which significantly contributes to the reactivity with either the aromatic pi-system or the C H sigma-bond.

    View details for DOI 10.1073/pnas.0605067103

    View details for Web of Science ID 000240380800006

    View details for PubMedID 16920789

  • Fe L-edge XAS studies of K-4[Fe(CN)(6)] and K-3[Fe(CN)(6)]: A direct probe of back-bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Hocking, R. K., Wasinger, E. C., de Groot, F. M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2006; 128 (32): 10442-10451

    Abstract

    Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand pi orbitals due to metal-to-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of pi back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.

    View details for DOI 10.1021/ja061802i

    View details for Web of Science ID 000239618700027

    View details for PubMedID 16895409

  • X-ray absorption spectroscopy and density functional theory studies of [(H(3)buea)Fe-III-X](n-) (X = S2-, O2-, OH-): Comparison of bonding and hydrogen bonding in oxo and sulfido complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Hocking, R. K., Larsen, P., Borovik, A. S., Hodgson, K. O., Hedman, B., Solomon, E. I. 2006; 128 (30): 9825-9833

    Abstract

    Iron L-edge, iron K-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [Fe(III)H(3)buea(X)](n-) (X = S(2-), O(2-), OH(-)). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metal-ligand bonding and to evaluate the effects of H-bonding in Fe(III)(-)O vs Fe(III)(-)S complexes. It was found that the Fe(III)(-)O bond, while less covalent, is stronger than the Fe(III)(-)S bond. This dominantly reflects the larger ionic contribution to the Fe(III)(-)O bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the Fe-O bond. These results were extended to consider an Fe(IV)(-)O complex with the same ligand environment. It was found that hydrogen bonding to Fe(IV)(-)O is less energetically favorable than that to Fe(III)(-)O, which reflects the highly covalent nature of the Fe(IV)(-)O bond.

    View details for DOI 10.1021/ja061618x

    View details for Web of Science ID 000239278600060

    View details for PubMedID 16866539

  • X-ray absorption edge spectroscopy and computational studies on LCuO2 species: Superoxide-Cu-II versus peroxide-Cu-III bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Sarangi, R., Aboelella, N., Fujisawa, K., Tolman, W. B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2006; 128 (25): 8286-8296

    Abstract

    The geometric and electronic structures of two mononuclear CuO2 complexes, [Cu(O2){HB(3-Ad-5-(i)Prpz)3}] (1) and [Cu(O2)(beta-diketiminate)] (2), have been evaluated using Cu K- and L-edge X-ray absorption spectroscopy (XAS) studies in combination with valence bond configuration interaction (VBCI) simulations and spin-unrestricted broken symmetry density functional theory (DFT) calculations. Cu K- and L-edge XAS data indicate the Cu(II) and Cu(III) nature of 1 and 2, respectively. The total integrated intensity under the L-edges shows that the 's in 1 and 2 contain 20% and 28% Cu character, respectively, indicative of very covalent ground states in both complexes, although more so in 1. Two-state VBCI simulations also indicate that the ground state in 2 has more Cu (/3d8) character. DFT calculations show that the in both complexes is dominated by O2(n-) character, although the O2(n-) character is higher in 1. It is shown that the ligand L plays an important role in modulating Cu-O2 bonding in these LCuO2 systems and tunes the ground states of 1 and 2 to have dominant Cu(II)-superoxide-like and Cu(III)-peroxide-like character, respectively. The contributions of ligand field (LF) and the charge on the absorbing atom in the molecule (Q(mol)M) to L- and K-edge energy shifts are evaluated using DFT and time-dependent DFT calculations. It is found that LF makes a dominant contribution to the edge energy shift, while the effect of Q(mol)M is minor. The charge on the Cu in the Cu(III) complex is found to be similar to that in Cu(II) complexes, which indicates a much stronger interaction with the ligand, leading to extensive charge transfer.

    View details for DOI 10.1021/ja0615223

    View details for Web of Science ID 000238418000045

    View details for PubMedID 16787093

  • Metal and ligand K-edge XAS of titanium-TEMPO complexes: Determination of oxidation states and insights into Ti-O bond homolysis INORGANIC CHEMISTRY George, S. D., Huang, K., Waymouth, R. M., Solomon, E. I. 2006; 45 (11): 4468-4477

    Abstract

    Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.

    View details for DOI 10.1021/ic060402t

    View details for Web of Science ID 000237690700029

    View details for PubMedID 16711697

  • Direct hydrogen-atom abstraction by activated bleomycin: An experimental and computational study JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Decker, A., Chow, M. S., Kemsley, J. N., Lehnert, N., Solomon, E. I. 2006; 128 (14): 4719-4733

    Abstract

    Bleomycin (BLM), a glycopeptide antibiotic chemotherapy agent, is capable of single- and double-strand DNA damage. Activated bleomycin (ABLM), a low-spin Fe(III)-OOH complex, is the last intermediate detected prior to DNA cleavage following hydrogen-atom abstraction from the C-4' of a deoxyribose sugar moiety. The mechanism of this C-H bond cleavage reaction and the nature of the active oxidizing species are still open issues. We have used kinetic measurements in combination with density functional calculations to study the reactivity of ABLM and the mechanism of the initial attack on DNA. Circular dichroism spectroscopy was used to directly monitor the kinetics of the ABLM reaction. These experiments yield a deuterium isotope effect, kH/kD approximately 3 for ABLM decay, indicating the involvement of a hydrogen atom in the rate-determining step. H-atom donors with relatively weak X-H bonds accelerate the reaction rate, establishing that ABLM is capable of hydrogen-atom abstraction. Density functional calculations were used to evaluate the two-dimensional potential energy surface for the direct hydrogen-atom abstraction reaction of the deoxyribose 4'-H by ABLM. The calculations confirm that ABLM is thermodynamically and kinetically competent for H-atom abstraction. The activation and reaction energies for this pathway are favored over both homolytic and heterolytic O-O bond cleavage. Direct H-atom abstraction by ABLM would generate a reactive Fe(IV)=O species, which would be capable of a second DNA strand cleavage, as observed in vivo. This study provides experimental and theoretical evidence for direct H-atom abstraction by ABLM and proposes an attractive mechanism for the role of ABLM in double-strand cleavage.

    View details for DOI 10.1021/ja057378n

    View details for Web of Science ID 000236770300063

    View details for PubMedID 16594709

  • Spectroscopy and electronic structures of mono- and binuclear high-valent non-heme iron-oxo systems JOURNAL OF INORGANIC BIOCHEMISTRY Decker, A., Clay, M. D., Solomon, E. I. 2006; 100 (4): 697-706

    Abstract

    High-valent iron-oxo intermediates are known or believed to be key oxidizing species in the catalytic mechanisms of many mononuclear and binuclear non-heme iron enzymes. So far only limited experimental data on their electronic structures are available. In this study we extend knowledge from the experimentally well characterized mononuclear Fe(IV)=O (S=1) biomimetic model system to computational insight into the spectroscopy and electronic structures of mono-and binuclear high-valent iron-oxo enzyme intermediates. In the mononuclear Fe(IV)=O complexes, we predict the spectroscopy and energies of the electronic transitions to be very different for the S=1 and S=2 spin states, but the iron-oxo bonding for both spin states to be very similar. A comparison of the S=2 mono- and binuclear high-valent iron-sites predicts similar electronic transitions. However, the bent iron-oxo bridge and interactions with the second iron-center in the dimer shift the transitions to higher energies and splits the d(xz/yz) orbital set. These electronic structure and TD-DFT results provide a basis for understanding the spectroscopy and electronic structures of high-valent intermediates in mono- and binuclear non-heme iron enzymes.

    View details for DOI 10.1016/j.jinorgbio.2006.01.013

    View details for Web of Science ID 000237829000025

    View details for PubMedID 16510189

  • mu-eta(2):eta(2)-Peroxodicopper(II) complex with a secondary diamine ligand: A functional model of tyrosinase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mirica, L. M., Rudd, D. J., Vance, M. A., SOLOMON, E. I., Hodgson, K. O., Hedman, B., Stack, T. D. 2006; 128 (8): 2654-2665

    Abstract

    The activation of dioxygen (O(2)) by Cu(I) complexes is an important process in biological systems and industrial applications. In tyrosinase, a binuclear copper enzyme, a mu-eta(2):eta(2)-peroxodicopper(II) species is accepted generally to be the active oxidant. Reported here is the characterization and reactivity of a mu-eta(2):eta(2)-peroxodicopper(II) complex synthesized by reacting the Cu(I) complex of the secondary diamine ligand N,N'-di-tert-butyl-ethylenediamine (DBED), [(DBED)Cu(MeCN)](X) (1.X, X = CF(3)SO(3)(-), CH(3)SO(3)(-), SbF(6)(-), BF(4)(-)), with O(2) at 193 K to give [[Cu(DBED)](2)(O(2))](X)(2) (2.X(2)). The UV-vis and resonance Raman spectroscopic features of 2 vary with the counteranion employed yet are invariant with change of solvent. These results implicate an intimate interaction of the counteranions with the Cu(2)O(2) core. Such interactions are supported further by extended X-ray absorption fine structure (EXAFS) analyses of solutions that reveal weak copper-counteranion interactions. The accessibility of the Cu(2)O(2) core to exogenous ligands such as these counteranions is manifest further in the reactivity of 2 with externally added substrates. Most notable is the hydroxylation reactivity with phenolates to give catechol and quinone products. Thus the strategy of using simple bidentate ligands at low temperatures provides not only spectroscopic models of tyrosinase but also functional models.

    View details for DOI 10.1021/ja056740v

    View details for Web of Science ID 000235787200041

    View details for PubMedID 16492052

  • Sulfur K-Edge XAS and DFT calculations on nitrile hydratase: Geometric and electronic structure of the non-heme iron active site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., CHOW, M., Taniguchi, K., Lugo-Mas, P., Davin, S., Maeda, M., Kovacs, J. A., Odaka, M., Hodgson, K. O., Hedman, B., Solomon, E. I. 2006; 128 (2): 533-541

    Abstract

    The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS(-))-, sulfenate (RSO(-))-, and sulfinate (RSO(2)(-))-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO(-) species changes upon protonation as the S-O bond is elongated (by approximately 0.1 A). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe(III) in the active site of NHase as CysS(-), CysSOH, and CysSO(2)(-) both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z(eff) of the Fe and reveals that the Fe in [FeNO](6) NHase species has a Z(eff) very similar to that of its photolyzed Fe(III) counterpart. DFT calculations reveal that this results from the strong pi back-bonding into the pi antibonding orbital of NO, which shifts significant charge from the formally t(2)(6) low-spin metal to the coordinated NO.

    View details for DOI 10.1021/ja0549695

    View details for Web of Science ID 000234814900038

    View details for PubMedID 16402841

  • Mechanism of N2O reduction by the mu(4)-S tetranuclear Cu-z cluster of nitrous oxide reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gorelsky, S. I., Ghosh, S., Solomon, E. I. 2006; 128 (1): 278-290

    Abstract

    Reaction thermodynamics and potential energy surfaces are calculated using density functional theory to investigate the mechanism of the reductive cleavage of the N-O bond by the mu(4)-sulfide-bridged tetranuclear Cu(Z) site of nitrous oxide reductase. The Cu(Z) cluster provides an exogenous ligand-binding site, and, in its fully reduced 4Cu(I) state, the cluster turns off binding of stronger donor ligands while enabling the formation of the Cu(Z)-N(2)O complex through enhanced Cu(Z) --> N(2)O back-donation. The two copper atoms (Cu(I) and Cu(IV)) at the ligand-binding site of the cluster play a crucial role in the enzymatic function, as these atoms are directly involved in bridged N(2)O binding, bending the ligand to a configuration that resembles the transition state (TS) and contributing the two electrons for N(2)O reduction. The other atoms of the Cu(Z) cluster are required for extensive back-bonding with minimal sigma ligand-to-metal donation for the N(2)O activation. The low reaction barrier (18 kcal mol(-)(1)) of the direct cleavage of the N-O bond in the Cu(Z)-N(2)O complex is due to the stabilization of the TS by a strong Cu(IV)(2+)-O(-) bond. Due to the charge transfer from the Cu(Z) cluster to the N(2)O ligand, noncovalent interactions with the protein environment stabilize the polar TS and reduce the activation energy to an extent dependent on the strength of proton donor. After the N-O bond cleavage, the catalytic cycle consists of a sequence of alternating protonation/one-electron reduction steps which return the Cu(Z) cluster to the fully reduced (4Cu(I)) state for future turnover.

    View details for DOI 10.1021/ja055856o

    View details for Web of Science ID 000234547700067

    View details for PubMedID 16390158

  • Oxygen binding of water-soluble cobalt porphyrins in aqueous solution INORGANIC CHEMISTRY Collman, J. P., Yan, Y. L., Eberspacher, T., Xie, X. J., SOLOMON, E. I. 2005; 44 (26): 9628-9630

    Abstract

    Water-soluble cobalt porphyrin 1Co and imidazole ligand 2 were synthesized. 1Co binds dioxygen in the presence of imidazole ligand 2 in aqueous solution. The formation of the oxygen adduct 2-1Co(O(2)) was studied using UV-vis and EPR spectroscopy. The impact of pH on the kinetic stability of the oxygen adduct was examined.

    View details for DOI 10.1021/ic0516717

    View details for Web of Science ID 000234192300011

    View details for PubMedID 16363827

  • MXAN analysis of the XANES energy region of a mononuclear copper complex: Applications to bioinorganic systems INORGANIC CHEMISTRY Sarangi, R., Benfatto, M., Hayakawa, K., Bubacco, L., SOLOMON, E. I., Hodgson, K. O., Hedman, B. 2005; 44 (26): 9652-9659

    Abstract

    The near edge XAS spectra of the mononuclear copper complex [Cu(TMPA)(OH(2))](ClO(4))(2) (1) have been simulated using the multiple scattering edge simulation package MXAN (or Minuit XANes). These simulations, which employ the muffin-tin (MT) approximation, have been compared to simulations generated using the finite-difference method (FDM) to evaluate the effect of MT corrections. The sensitivity of the MXAN method was tested using structural models that included several different variations on the bond angles and bond distances for the first-shell atoms of 1. The sensitivity to small structural changes was also evaluated by comparing MXAN simulations of 1 and of structurally modified [Cu(TMPA)(L)](n)(+) complexes [where L = -O-(F(8)TPP)Fe(III), -F, -OPO(2)(O-p-nitrophenyl)Zn(II)(TMPA), and -NCMe] to the experimental data. The accuracy of the bond distances obtained from the MXAN simulations was then examined by comparison to the metrics of the crystal structures. The results show that MXAN can successfully extract geometric information from the edge structure of an XAS spectrum. The systematic application of MXAN to 1 indicates that this approach is sensitive to small structural changes in the molecule that are manifested in the XAS edge spectrum. These results represent the first step toward the application of this methodology to bioinorganic and biological systems.

    View details for DOI 10.1021/ic050703n

    View details for Web of Science ID 000234192300017

    View details for PubMedID 16363833

  • Spectroscopic and computational studies of NTBC bound to the non-heme iron enzyme (4-hydroxyphenyl)pyruvate dioxygenase: Active site contributions to drug inhibition BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Neidig, M. L., Decker, A., Kavana, M., Moran, G. R., Solomon, E. I. 2005; 338 (1): 206-214

    Abstract

    (4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is an alpha-keto-acid-dependent dioxygenase which catalyzes the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate as part of tyrosine catabolism. While several di- and tri-ketone alkaloids are known as inhibitors of HPPD and used commercially as herbicides, one such inhibitor, [2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione (NTBC), has also been used therapeutically to treat type I tyrosinemia and alkaptonuria in humans. To gain further insight into the mechanism of inhibition by NTBC, a combination of CD/MCD spectroscopy and DFT calculations of HPPD/Fe(II)/NTBC has been performed to evaluate the contribution of the Fe(II)-NTBC bonding interaction to the high affinity of this drug for the enzyme. The results indicate that the bonding of NTBC to Fe(II) is very similar to that for HPP, both involving similar pi-backbonding interactions between NTBC/HPP and Fe(II). Combined with the result that the calculated binding energy of NTBC is, in fact, approximately 3 kcal/mol less than that for HPP, the bidentate coordination of NTBC to Fe(II) is not solely responsible for its extremely high affinity for the enzyme. Thus, the pi-stacking interactions between the aromatic rings of NTBC and two phenyalanine residues, as observed in the crystallography of the HPPD/Fe(II)/NTBC complex, appear to be responsible for the observed high affinity of drug binding.

    View details for DOI 10.1016/j.bbrc.2005.08.242

    View details for Web of Science ID 000233296700030

    View details for PubMedID 16197918

  • Spectroscopic and computational studies of the de novo designed protein DF2t: Correlation to the biferrous active site of ribonucleotide reductase and factors that affect O-2 reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wei, P. P., Skulan, A. J., Wade, H., DeGrado, W. F., Solomon, E. I. 2005; 127 (46): 16098-16106

    Abstract

    DF2t, a de novo designed protein that mimics the active-site structure of many non-heme biferrous enzymes, has been studied using a combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD. The active site of DF2t is found to have one five-coordinate iron and one four-coordinate iron, which are weakly antiferromagnetically coupled through a mu-1,3 carboxylate bridge. These results bear a strong resemblance to the spectra of Escherichia coli ribonucleotide reductase (R2), and density functional theory calculations were conducted on the W48F/D84E R2 mutant in order to determine the energetics of formation of a monodentate end-on-bound O2 to one iron in the binuclear site. The mu-1,3 carboxylate bridges found in O2-activating enzymes lack efficient superexchange pathways for the second electron transfer (i.e., the OH/oxo bridge in hemerythrin), and simulations of the binding of O2 in a monodentate end-on manner revealed that the bridging carboxylate ligands do not appear capable of transferring an electron to O2 from the remote Fe. Comparison of the results from previous studies of the mu-1,2 biferric-peroxo structure, which bridges both irons, finds that the end-on superoxide mixed-valent species is considerably higher in energy than the bridging peroxo-diferric species. Thus, one of the differences between O2-activating and O2-binding proteins appears to be the ability of O2 to bridge both Fe centers to generate a peroxo intermediate capable of further reactivity.

    View details for DOI 10.1021/ja053661a

    View details for Web of Science ID 000233445900033

    View details for PubMedID 16287296

  • Sulfur K-edge XAS and DFT calculations on [Fe4S4](2+) clusters: Effects of H-bonding and structural distortion on covalency and spin topology INORGANIC CHEMISTRY Dey, A., Roche, C. L., Walters, M. A., Hodgson, K. O., Hedman, B., Solomon, E. I. 2005; 44 (23): 8349-8354

    Abstract

    Sulfur K-edge X-ray absorption spectroscopy of a hydrogen-bonded elongated [Fe4S4]2+ cube is reported. The data show that this synthetic cube is less covalent than a normal compressed cube with no hydrogen bonding. DFT calculations reveal that the observed difference in electronic structure has significant contributions from both the cluster distortion and from hydrogen bonding. The elongated and compressed Fe4S4 structures are found to have different spin topologies (i.e., orientation of the delocalized Fe2S2 subclusters which are antiferromagnetically coupled to each other). It is suggested that the H-bonding interaction with the counterion does not contribute to the cluster elongation. A magneto-structural correlation is developed for the Fe4S4 cube that is used to identify the redox-active Fe2S2 subclusters in active sites of HiPIP and ferredoxin proteins involving these clusters.

    View details for DOI 10.1021/ic050981m

    View details for Web of Science ID 000233180600029

    View details for PubMedID 16270973

  • Ground-state electronic and magnetic properties of a mu(3)-Oxo-bridged trinuclear Cu(II) complex: Correlation to the native intermediate of the multicopper oxidases INORGANIC CHEMISTRY Yoon, J., Solomon, E. I. 2005; 44 (22): 8076-8086

    Abstract

    The ground-state electronic and magnetic properties of one of the possible structures of the trinuclear Cu(II) site in the native intermediate (NI) of the multicopper oxidases, the mu(3)-oxo-bridged structure, are evaluated using the C(3)-symmetric Cu(3)(II) complex, mu(3)O. mu(3)O is unique in that no ligand, other than the oxo, contributes to the exchange coupling. However, mu(3)O has a ferromagnetic ground state, inconsistent with that of NI. Therefore, two perturbations have been considered: protonation of the mu(3)-oxo ligand and relaxation of the mu(3)-oxo ligand into the Cu(3) plane. Notably, when the oxo ligand is sufficiently close to the Cu(3) plane (<0.3 Angstroms), the ground state of mu(3)O becomes antiferromagnetic and can be correlated to that of NI. In addition, the ferromagnetic (4)A ground state of mu(3)O is found from variable-temperature EPR to undergo a zero-field splitting (ZFS) of 2D = -5.0 cm(-1), which derives from the second-order anisotropic exchange. This allows evaluation of the sigma-to-pi excited-state exchange pathways and provides experimental evidence that the orbitally degenerate (2)E ground state of the antiferromagnetic mu(3)O would also undergo a ZFS by the first-order antisymmetric exchange that has the same physical origin as the anisotropic exchange. The important contribution of the mu(3)-oxo bridge to the ground-to-ground and ground-to-excited-state superexchange pathways that are responsible for the isotropic, antisymmetric, and anisotropic exchanges are discussed.

    View details for DOI 10.1021/ic0507870

    View details for Web of Science ID 000232898800046

    View details for PubMedID 16241158

  • Spectroscopic and electronic structure studies of the trinuclear Cu cluster active site of the multicopper oxidase laccase: Nature of its coordination unsaturation JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Quintanar, L., Yoon, J. J., Aznar, C. P., Palmer, A. E., Andersson, K. K., Britt, R. D., Solomon, E. I. 2005; 127 (40): 13832-13845

    Abstract

    Laccase is a multicopper oxidase that contains four Cu ions, one type 1 (T1), one type 2 (T2), and a coupled binuclear type 3 Cu pair (T3). The T2 and T3 centers form a trinuclear Cu cluster that is the active site for O2 reduction to H2O. A combination of spectroscopic and DFT studies on a derivative where the T1 Cu has been replaced by a spectroscopically innocent Hg2+ ion has led to a detailed geometric and electronic structure description of the resting trinuclear Cu cluster, complementing crystallographic results. The nature of the T2 Cu ligation has been elucidated; this site is three-coordinate with two histidines and a hydroxide over its functional pH range (stabilized by a large inductive effect, cluster charge, and a hydrogen-bonding network). Both the T2 and T3 Cu centers have open coordination positions oriented toward the center of the cluster. DFT calculations show that the negative protein pocket (four conserved Asp/Glu residues within 12 A) and the dielectric of the protein play important roles in the electrostatic stability and integrity of the highly charged, coordinatively unsaturated trinuclear cupric cluster. These tune the ligand binding properties of the cluster, leading to its high affinity for fluoride and its coordination unsaturation in aqueous media, which play a key role in its O2 reactivity.

    View details for DOI 10.1021/ja0421405

    View details for Web of Science ID 000232413300038

    View details for PubMedID 16201804

  • Variable-temperature, variable-field magnetic circular dichroism studies of tris-hydroxy- and mu(3)-oxo-bridged trinuclear Cu(II) complexes: Evaluation of proposed structures of the native intermediate of the multicopper oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoon, J., Mirica, L. M., Stack, T. D., Solomon, E. I. 2005; 127 (39): 13680-13693

    Abstract

    Multicopper oxidases catalyze the 4e- reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 produces the native intermediate (NI) that consists of four oxidized Cu centers, three of which form a trinuclear cluster site, all bridged by the product of full O2 reduction. The most characteristic feature of NI is the intense magnetic circular dichroism pseudo-A feature (a pair of temperature-dependent C-terms with opposite signs) associated with O --> Cu(II) ligand-to-metal charge transfer (LMCT) that derives from the strong Cu-O bonds in the trinuclear site. In this study, the two most plausible Cu-O structures of the trinuclear site, the tris-mu2-hydroxy-bridged and the mu3-oxo-bridged structures, are evaluated through spectroscopic and electronic structure studies on relevant model complexes, TrisOH and mu3O. It is found that the two components of a pseudo-A-term for TrisOH are associated with LMCT to the same Cu that are coupled by a metal-centered excited-state spin-orbit coupling (SOC), whereas for mu3O they are associated with LMCT to different Cu centers that are coupled by oxo-centered excited state SOC. Based on this analysis of the two candidate models, only the mu3-oxo-bridged structure is consistent with the spectroscopic properties of NI. The Cu-O sigma-bonds in the mu3-oxo-bridged structure would provide the thermodynamic driving force for the 4e- reduction of O2 and would allow the facile electron transfer to all Cu centers in the trinuclear cluster that is consistent with its involvement in the catalytic cycle.

    View details for DOI 10.1021/ja0525152

    View details for Web of Science ID 000232257100058

    View details for PubMedID 16190734

  • Geometric and electronic structure of the heme-peroxo-copper complex [(F8TPP)Fe-III-(O-2(2-))-Cu-II(TMPA)](CIO4) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY del Rio, D., Sarangi, R., Chufan, E. E., Karlin, K. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2005; 127 (34): 11969-11978

    Abstract

    The geometric and electronic structure of the untethered heme-peroxo-copper model complex [(F(8)TPP)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](ClO(4)) (1) has been investigated using Cu and Fe K-edge EXAFS spectroscopy and density functional theory calculations in order to describe its geometric and electronic structure. The Fe and Cu K-edge EXAFS data were fit with a Cu...Fe distance of approximately 3.72 A. Spin-unrestricted DFT calculations for the S(T) = 2 spin state were performed on [(P)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](+) as a model of 1. The peroxo unit is bound end-on to the copper, and side-on to the high-spin iron, for an overall mu-eta(1):eta(2) coordination mode. The calculated Cu...Fe distance is approximately 0.3 A longer than that observed experimentally. Reoptimization of [(P)Fe(III)-(O(2)(2)(-))-Cu(II)(TMPA)](+) with a 3.7 A Cu...Fe constrained distance results in a similar energy and structure that retains the overall mu-eta(1):eta(2)-peroxo coordination mode. The primary bonding interaction between the copper and the peroxide involves electron donation into the half-occupied Cu d(z)2 orbital from the peroxide pi(sigma) orbital. In the case of the Fe(III)-peroxide eta(2) bond, the two major components arise from the donor interactions of the peroxide pi*(sigma) and pi*(v) orbitals with the Fe d(xz) and d(xy) orbitals, which give rise to sigma and delta bonds, respectively. The pi*(sigma) interaction with both the half-occupied d(z)2 orbital on the copper (eta(1)) and the d(xz) orbital on the iron (eta(2)), provides an effective superexchange pathway for strong antiferromagnetic coupling between the metal centers.

    View details for DOI 10.1021/ja043374r

    View details for Web of Science ID 000231605900044

    View details for PubMedID 16117536

  • Sulfur K-edge XAS and DFT calculations on P450 model complexes: Effects of hydrogen bonding on electronic structure and redox potentials JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Okamura, T., Ueyama, N., Hedman, B., Hodgson, K. O., Solomon, E. I. 2005; 127 (34): 12046-12053

    Abstract

    Hydrogen bonding (H-bonding) is generally thought to play an important role in tuning the electronic structure and reactivity of metal-sulfur sites in proteins. To develop a quantitative understanding of this effect, S K-edge X-ray absorption spectroscopy (XAS) has been employed to directly probe ligand-metal bond covalency, where it has been found that protein active sites are significantly less covalent than their related model complexes. Sulfur K-edge XAS data are reported here on a series of P450 model complexes with increasing H-bonding to the ligated thiolate from its substituent. The XAS spectroscopic results show a dramatic decrease in preedge intensity. DFT calculations reproduce these effects and show that the observed changes are in fact solely due to H-bonding and not from the inductive effect of the substituent on the thiolate. These calculations also indicate that the H-bonding interaction in these systems is mainly dipolar in nature. The -2.5 kcal/mol energy of the H-bonding interaction was small relative to the large change in ligand-metal bond covalency (30%) observed in the data. A bond decomposition analysis of the total energy is developed to correlate the preedge intensity change to the change in Fe-S bonding interaction on H-bonding. This effect is greater for the reduced than the oxidized state, leading to a 260 mV increase in the redox potential. A simple model shows that E degrees should vary approximately linearly with the covalency of the Fe-S bond in the oxidized state, which can be determined directly from S K-edge XAS.

    View details for DOI 10.1021/ja0519031

    View details for Web of Science ID 000231605900053

    View details for PubMedID 16117545

  • Spectroscopic and DFT investigation of [M{HB(3,5-(i)Pr(2)pz)(3)}(SC6F5)] (M = Mn, Fe, Co, Ni, Cu, and Zn) model complexes: Periodic trends in metal-thiolate bonding INORGANIC CHEMISTRY Gorelsky, S. I., Basumallick, L., Vura-Weis, J., Sarangi, R., Hodgson, K. O., Hedman, B., Fujisawa, K., Solomon, E. I. 2005; 44 (14): 4947-4960

    Abstract

    A series of metal-varied [ML(SC6F5)] model complexes (where L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate and M = Mn, Fe, Co, Ni, Cu, and Zn) related to blue copper proteins has been studied by a combination of absorption, MCD, resonance Raman, and S K-edge X-ray absorption spectroscopies. Density functional calculations have been used to characterize these complexes and calculate their spectra. The observed variations in geometry, spectra, and bond energies are interpreted in terms of changes in the nature of metal-ligand bonding interactions. The metal 3d-ligand orbital interaction, which contributes to covalent bonding in these complexes, becomes stronger going from Mn(II) to Co(II) (the sigma contribution) and to Cu(II) (the pi contribution). This change in the covalency results from the increased effective nuclear charge of the metal atom in going from Mn(II) to Zn(II) and the change in the 3d orbital populations (d5-->d10). Ionic bonding also plays an important role in determining the overall strength of the ML(+)-SC6F5(-) interaction. However, there is a compensating effect: as the covalent contribution to the metal-ligand bonding increases, the ionic contribution decreases. These results provide insight into the Irving-Williams series, where it is found that the bonding of the ligand being replaced by the thiolate makes a major contribution to the observed order of the stability constants over the series of metal ions.

    View details for DOI 10.1021/ic050371m

    View details for Web of Science ID 000230555400014

    View details for PubMedID 15998022

  • Tyrosinase reactivity in a model complex: An alternative hydroxylation mechanism SCIENCE Mirica, L. M., Vance, M., Rudd, D. J., Hedman, B., Hodgson, K. O., Solomon, E. I., Stack, T. D. 2005; 308 (5730): 1890-1892

    Abstract

    The binuclear copper enzyme tyrosinase activates O2 to form a mu-eta2:eta2-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic mu-eta2:eta2-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymatic active oxidant, is reported to rapidly hydroxylate phenolates at -80 degrees C. Upon phenolate addition at extreme temperature in solution (-120 degrees C), a reactive intermediate consistent with a bis-mu-oxodicopper(III)-phenolate complex, with the O-O bond fully cleaved, is observed experimentally. The subsequent hydroxylation step has the hallmarks of an electrophilic aromatic substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.

    View details for DOI 10.1126/science.1112081

    View details for Web of Science ID 000230120000034

    View details for PubMedID 15976297

  • Role of aspartate 94 in the decay of the peroxide intermediate in the multicopper oxidase Fet3p BIOCHEMISTRY Quintanar, L., Stoj, C., Wang, T. P., Kosman, D. J., Solomon, E. J. 2005; 44 (16): 6081-6091

    Abstract

    Fet3p is a multicopper oxidase that contains four Cu ions: one type 1, one type 2, and a coupled binuclear type 3 site. The type 2 and type 3 centers form a trinuclear cluster that is the active site for O(2) reduction to H(2)O. When the type 1 Cu is depleted (C484S mutation), the reaction of the reduced trinuclear cluster with O(2) generates a peroxide intermediate. Kinetic studies of the decay of the peroxide intermediate suggest that a carboxyl residue (D94 in Fet3p) assists the reductive cleavage of the O-O bond at low pH. Mutations at the D94 residue (D94A, D94N, and D94E) have been studied to evaluate its role in the decay of the peroxide intermediate. Spectroscopic studies show that the D94 mutations affect the geometric and electronic structure of the trinuclear cluster in a way that is consistent with the hydrogen bond connectivity of D94. While the D94E mutation does not affect the initial reaction of the cluster with O(2), the D94A mutation causes larger structural changes that render the trinuclear cluster unreactive toward O(2), demonstrating a structural role for the D94 residue. The decay of the peroxide intermediate is markedly affected by the D94E mutation, confirming the involvement of D94 in this reaction. The D94 residue appears to activate a proton of the type 2 Cu(+)-bound water for participation in the transition state. These studies provide new insight into the role of D94 and proton involvement in the reductive cleavage of the O-O bond.

    View details for DOI 10.1021/bi047379c

    View details for Web of Science ID 000228678900014

    View details for PubMedID 15835897

  • Dioxygen activation by copper, heme and non-heme iron enzymes: comparison of electronic structures and reactivities CURRENT OPINION IN CHEMICAL BIOLOGY Decker, A., Solomon, E. I. 2005; 9 (2): 152-163

    Abstract

    Enzymes containing heme, non-heme iron and copper active sites play important roles in the activation of dioxygen for substrate oxidation. One key reaction step is CH bond cleavage through H-atom abstraction. On the basis of the ligand environment and the redox properties of the metal, these enzymes employ different methods of dioxygen activation. Heme enzymes are able to stabilize the very reactive iron(IV)-oxo porphyrin-radical intermediate. This is generally not accessible for non-heme iron systems, which can instead use low-spin ferric-hydroperoxo and iron(IV)-oxo species as reactive oxidants. Copper enzymes employ still a different strategy and achieve H-atom abstraction potentially through a superoxo intermediate. This review compares and contrasts the electronic structures and reactivities of these various oxygen intermediates.

    View details for DOI 10.1016/j,cbpa.2005.02.012

    View details for Web of Science ID 000228607700009

    View details for PubMedID 15811799

  • Spectroscopy of non-heme iron thiolate complexes: Insight into the electronic structure of the low-spin active site of nitrile hydratase INORGANIC CHEMISTRY Kennepohl, P., Neese, F., Schweitzer, D., Jackson, H. L., Kovacs, J. A., Solomon, E. I. 2005; 44 (6): 1826-1836

    Abstract

    Detailed spectroscopic and computational studies of the low-spin iron complexes [Fe(III)(S2(Me2)N3 (Pr,Pr))(N3)] (1) and [Fe(III)(S2(Me2)N3 (Pr,Pr))]1+ (2) were performed to investigate the unique electronic features of these species and their relation to the low-spin ferric active sites of nitrile hydratases. Low-temperature UV/vis/NIR and MCD spectra of 1 and 2 reflect electronic structures that are dominated by antibonding interactions of the Fe 3d manifold and the equatorial thiolate S 3p orbitals. The six-coordinate complex 1 exhibits a low-energy S(pi) --> Fe 3d(xy) (approximately 13,000 cm(-1)) charge-transfer transition that results predominantly from the low energy of the singly occupied Fe 3d(xy) orbital, due to pure pi interactions between this acceptor orbital and both thiolate donor ligands in the equatorial plane. The 3d(pi) --> 3d(sigma) ligand-field transitions in this species occur at higher energies (>15,000 cm(-1)), reflecting its near-octahedral symmetry. The Fe 3d(xz,yz) --> Fe 3d(xy) (d(pi) --> d(pi)) transition occurs in the near-IR and probes the Fe(III)-S pi-donor bond; this transition reveals vibronic structure that reflects the strength of this bond (nu(e) approximately 340 cm(-1)). In contrast, the ligand-field transitions of the five-coordinate complex 2 are generally at low energy, and the S(pi) --> Fe charge-transfer transitions occur at much higher energies relative to those in 1. This reflects changes in thiolate bonding in the equatorial plane involving the Fe 3d(xy) and Fe 3d(x2-y2) orbitals. The spectroscopic data lead to a simple bonding model that focuses on the sigma and pi interactions between the ferric ion and the equatorial thiolate ligands, which depend on the S-Fe-S bond angle in each of the complexes. These electronic descriptions provide insight into the unusual S = 1/2 ground spin state of these complexes: the orientation of the thiolate ligands in these complexes restricts their pi-donor interactions to the equatorial plane and enforces a low-spin state. These anisotropic orbital considerations provide some intriguing insights into the possible electronic interactions at the active site of nitrile hydratases and form the foundation for further studies into these low-spin ferric enzymes.

    View details for DOI 10.1021/ic0487068

    View details for Web of Science ID 000227764700020

    View details for PubMedID 15762709

  • Spectroscopic and density functional studies of the red copper site in nitrosocyanin: Role of the protein in determining active site geometric and electronic structure JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Basumallick, L., Sarangi, R., George, S. D., Elmore, B., Hooper, A. B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2005; 127 (10): 3531-3544

    Abstract

    The electronic structure of the red copper site in nitrosocyanin is defined relative to that of the well understood blue copper site of plastocyanin by using low-temperature absorption, circular dichroism, magnetic circular dichroism, resonance Raman, EPR and X-ray absorption spectroscopies, combined with DFT calculations. These studies indicate that the principal electronic structure change in the red copper site is the sigma rather than the pi donor interaction of the cysteine sulfur with the Cu 3d(x2-y2) redox active molecular orbital (RAMO). Further, MCD data show that there is an increase in ligand field strength due to an increase in coordination number, whereas resonance Raman spectra indicate a weaker Cu-S bond. The latter is supported by the S K-edge data, which demonstrate a less covalent thiolate interaction with the RAMO of nitrosocyanin at 20% relative to plastocyanin at 38%. EXAFS results give a longer Cu-S(Cys) bond distance in nitrosocyanin (2.28 A) compared to plastocyanin (2.08 A) and also show a large change in structure with reduction of the red copper site. The red copper site is the only presently known blue copper-related site with an exogenous water coordinated to the copper. Density functional calculations reproduce the experimental properties and are used to determine the specific protein structure contributions to exogenous ligand binding in red copper. The relative orientation of the CuNNS and the CuSC(beta) planes (determined by the protein sequence) is found to be key in generating an exchangeable coordination position at the red copper active site. The exogenous water ligation at the red copper active site greatly increases the reorganization energy (by approximately 1.0 eV) relative to that of the blue copper protein site, making the red site unfavorable for fast outer-sphere electron transfer, while providing an exchangeable coordination position for inner-sphere electron transfer.

    View details for DOI 10.1021/ja044412+

    View details for Web of Science ID 000227627800062

    View details for PubMedID 15755175

  • Preface forum: "Functional insight from physical methods on metalloenzymes" INORGANIC CHEMISTRY SOLOMON, E. I. 2005; 44 (4): 723-726

    View details for DOI 10.1021/ic040127f

    View details for Web of Science ID 000227172200001

    View details for PubMedID 15859241

  • Metal and ligand K-Edge XAS of organotitanium complexes: Metal 4p and 3d contributions to pre-edge intensity and their contributions to bonding JOURNAL OF THE AMERICAN CHEMICAL SOCIETY George, S. D., Brant, P., Solomon, E. I. 2005; 127 (2): 667-674

    Abstract

    Titanium cyclopentadienyl (Cp) complexes play important roles as homogeneous polymerization catalysts and have recently received attention as potential anticancer agents. To systematically probe the contribution of the Cp to bonding in organotitanium complexes, Ti K-edge XAS has been applied to TiCl(4) and then to the mono- and bis-Cp complexes, TiCpCl(3) and TiCp(2)Cl(2). Ti K-edge XAS is used as a direct probe of metal 3d-4p mixing and provides insight into the contribution of the Cp to bonding. These data are complimented by Cl K-edge XAS data, which provide a direct probe of the effect of the Cp on the bonding to the spectator chloride ligand. The experimental results are correlated to DFT calculations. A model for metal 3d-4p mixing is proposed, which is based on covalent interactions with the ligands and demonstrates that metal K-pre-edge intensities may be used as a measure of ligand-metal covalency in molecular Ti(IV) systems in noncentrosymmetric environments.

    View details for DOI 10.1021/ja044827v

    View details for Web of Science ID 000226324500049

    View details for PubMedID 15643891

  • Structure-function correlations in oxygen activating non-heme iron enzymes CHEMICAL COMMUNICATIONS Neidig, M. L., Solomon, E. I. 2005: 5843-5863

    Abstract

    A large group of mononuclear non-heme iron enzymes exist which activate dioxygen to catalyze key biochemical transformations, including many of medical, pharmaceutical and environmental significance. These enzymes utilize high-spin Fe(II) active sites and additional reducing equivalents from cofactors or substrates to react with O2 to yield iron-oxygen intermediates competent to transform substrate to product. While Fe(II) sites have been difficult to study due to the lack of dominant spectroscopic features, a spectroscopic methodology has been developed which allows the elucidation of the geometric and electronic structures of these active sites and provides molecular level insight into the mechanisms of catalysis. This review provides a summary of this methodology with emphasis on its application to the determination of important active site structure-function correlations in mononuclear non-heme iron enzymes. These studies provide key insight into the mechanisms of oxygen activation, active site features that contribute to differences in reactivity and, combined with theoretical calculations and model studies, the nature of oxygen intermediates active in catalysis.

    View details for DOI 10.1039/b510233m

    View details for Web of Science ID 000233775600004

    View details for PubMedID 16317455

  • Comparison of Fe-IV = O heme and non-heme species: Electronic structures, bonding, and reactivities ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Decker, A., Solomon, E. I. 2005; 44 (15): 2252-2255

    View details for DOI 10.1002/anie.200462182

    View details for Web of Science ID 000228415900016

    View details for PubMedID 15719352

  • Ligand K-edge X-ray absorption spectroscopy and DFT calculations on [Fe3S4](0,+) clusters: Delocalization, redox, and effect of the protein environment JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Glaser, T., Moura, J. J., HOLM, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2004; 126 (51): 16868-16878

    Abstract

    Ligand K-edge XAS of an [Fe3S4]0 model complex is reported. The pre-edge can be resolved into contributions from the mu(2)S(sulfide), mu(3)S(sulfide), and S(thiolate) ligands. The average ligand-metal bond covalencies obtained from these pre-edges are further distributed between Fe(3+) and Fe(2.5+) components using DFT calculations. The bridging ligand covalency in the [Fe2S2]+ subsite of the [Fe3S4]0 cluster is found to be significantly lower than its value in a reduced [Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2]+ site (-146 cm(-1) vs -360 cm(-1), respectively). This decrease in J, along with estimates of the double exchange parameter B and vibronic coupling parameter lambda2/k(-), leads to an S = 2 delocalized ground state in the [Fe3S4]0 cluster. The S K-edge XAS of the protein ferredoxin II (Fd II) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same oxidation state, which correlates with the number of H-bonding interactions to specific sulfur ligands present in the active site. The changes in ligand-metal bond covalencies upon redox compared with DFT calculations indicate that the redox reaction involves a two-electron change (one-electron ionization plus a spin change of a second electron) with significant electronic relaxation. The presence of the redox inactive Fe(3+) center is found to decrease the barrier of the redox process in the [Fe3S4] cluster due to its strong antiferromagnetic coupling with the redox active Fe2S2 subsite.

    View details for DOI 10.1021/ja0466208

    View details for Web of Science ID 000225910400045

    View details for PubMedID 15612726

  • Spectroscopic demonstration of a large antisymmetric exchange contribution to the spin-frustrated ground state of a D-3 symmetric hydroxy-bridged trinuclear Cu(II) complex: Ground-to-excited state superexchange pathways JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Yoon, J., Mirica, L. M., Stack, T. D., Solomon, E. I. 2004; 126 (39): 12586-12595

    Abstract

    The magnetic and electronic properties of a spin-frustrated ground state of an antiferromagnetically coupled 3-fold symmetric trinuclear copper complex (TrisOH) is investigated using a combination of variable-temperature variable-field magnetic circular dichroism (VTVH MCD) and powder/single-crystal EPR. Direct evidence for a low-lying excited S = (1)/(2) state from the zero-field split ground (2)E state is provided by the nonlinear dependence of the MCD intensity on 1/T and the nesting of the VTVH MCD isotherms. A consistent zero-field splitting (Delta) value of approximately 65 cm(-1) is obtained from both approaches. In addition, the strong angular dependence of the single-crystal EPR spectrum, with effective g-values from 2.32 down to an unprecedented 1.2, requires in-state spin-orbit coupling of the (2)E state via antisymmetric exchange. The observable EPR intensities also require lowering of the symmetry of the trimer structure, likely reflecting a magnetic Jahn-Teller effect. Thus, the Delta of the ground (2)E state is shown to be governed by the competing effects of antisymmetric exchange (G = 36.0 +/- 0.8 cm(-1)) and symmetry lowering (delta = 17.5 +/- 5.0 cm(-1)). G and delta have opposite effects on the spin distribution over the three metal sites where the former tends to delocalize and the latter tends to localize the spin of the S(tot) = (1)/(2) ground state on one metal center. The combined effects lead to partial delocalization, reflected by the observed EPR parallel hyperfine splitting of 74 x 10(-4) cm(-1). The origin of the large G value derives from the efficient superexchange pathway available between the ground d(x2-y2) and excited d(xy) orbitals of adjacent Cu sites, via strong sigma-type bonds with the in-plane p-orbitals of the bridging hydroxy ligands. This study provides significant insight into the orbital origin of the spin Hamiltonian parameters of a spin-frustrated ground state of a trigonal copper cluster.

    View details for DOI 10.1021/ja046380w

    View details for Web of Science ID 000224219900078

    View details for PubMedID 15453791

  • O-2 activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chen, P., Solomon, E. I. 2004; 101 (36): 13105-13110

    Abstract

    Binuclear Cu proteins play vital roles in O(2) binding and activation in biology and can be classified into coupled and noncoupled binuclear sites based on the magnetic interaction between the two Cu centers. Coupled binuclear Cu proteins include hemocyanin, tyrosinase, and catechol oxidase. These proteins have two Cu centers strongly magnetically coupled through direct bridging ligands that provide a mechanism for the 2-electron reduction of O(2) to a mu-eta(2):eta(2) side-on peroxide bridged Cu(II)(2)(O(2)(2-)) species. This side-on bridged peroxo-Cu(II)(2) species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine alpha-hydroxylating monooxygenase and dopamine beta-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O(2) at a single Cu center to generate a reactive Cu(II)/O(2) species for H-atom abstraction from the C-H bond of substrates. O(2) intermediates in the coupled binuclear Cu enzymes can be trapped and studied spectroscopically. Possible intermediates in noncoupled binuclear Cu proteins can be defined through correlation to mononuclear Cu(II)/O(2) model complexes. The different intermediates in these two classes of binuclear Cu proteins exhibit different reactivities that correlate with their different electronic structures and exchange coupling interactions between the binuclear Cu centers. These studies provide insight into the role of exchange coupling between the Cu centers in their reaction mechanisms.

    View details for DOI 10.1073/pnas.0402114101

    View details for Web of Science ID 000223799100003

    View details for PubMedID 15340147

  • Nature of the peroxo intermediate of the W48F/D84E ribonucleotide reductase variant: Implications for O-2 activation by binuclear non-heme iron enzymes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Skulan, A. J., Brunold, T. C., Baldwin, J., Saleh, L., Bollinger, J. M., SOLOMON, E. I. 2004; 126 (28): 8842-8855

    Abstract

    Analysis of the spectroscopic signatures of the R2-W48F/D84E biferric peroxo intermediate identifies a cis mu-1,2 peroxo coordination geometry. DFT geometry optimizations on both R2-W48F/D84E and R2-wild-type peroxo intermediate models including constraints imposed by the protein also identify the cis mu-1,2 peroxo geometry as the most stable peroxo intermediate structure. This study provides significant insight into the electronic structure and reactivity of the R2-W48F/D84E peroxo intermediate, structurally related cis mu-1,2 peroxo model complexes, and other enzymatic biferric peroxo intermediates.

    View details for DOI 10.1021/ja049105a

    View details for Web of Science ID 000222704700061

    View details for PubMedID 15250738

  • Ligand K-Edge X-ray absorption spectroscopy of [Fe4S4](1+,2+,3+) clusters: Changes in bonding and electronic relaxation upon redox JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dey, A., Glaser, T., Couture, M. M., Eltis, L. D., HOLM, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2004; 126 (26): 8320-8328

    Abstract

    Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe(4)S(4)](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe(4)S(4)](1+/2+) (ferredoxin) and the [Fe(4)S(4)](2+/3+) (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed.

    View details for Web of Science ID 000222405400058

    View details for PubMedID 15225075

  • Ferrous binding to the multicopper oxidases Saccharomyces cerevisiae Fet3p and human ceruloplasmin: Contributions to ferroxidase activity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Quintanar, L., Gebhard, M., Wang, T. P., Kosman, D. J., Solomon, E. I. 2004; 126 (21): 6579-6589

    Abstract

    The multicopper oxidases are a family of enzymes that couple the reduction of O(2) to H(2)O with the oxidation of a range of substrates. Saccharomyces cerevisiae Fet3p and human ceruloplasmin (hCp) are members of this family that exhibit ferroxidase activity. Their high specificity for Fe(II) has been attributed to the existence of a binding site for iron. In this study, mutations at the E185 and Y354 residues, which are putative ligands for iron in Fet3p, have been generated and characterized. The effects of these mutations on the electronic structure of the T1 Cu site have been assessed, and the reactivities of this site toward 1,4-hydroquinone (a weak binding substrate) and Fe(II) have been evaluated and interpreted in terms of the semiclassical Marcus theory for electron transfer. The electronic and geometric structure of the Fe(II) substrate bound to Fet3p and hCp has been studied for the first time, using variable-temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopy. The iron binding sites in Fet3p and hCp appear to be very similar in nature, and their contributions to the ferroxidase activity of these proteins have been analyzed. It is found that these iron binding sites play a major role in tuning the reduction potential of iron to provide a large driving force for the ferroxidase reaction, while still supporting the delivery of the Fe(III) product to the acceptor protein. Finally, the analysis of possible electron-transfer (ET) pathways from the protein-bound Fe(II) to the T1 Cu site indicates that the E185 residue not only plays a role in iron binding, but also provides the dominant ET pathway to the T1 Cu site.

    View details for DOI 10.1021/ja049220t

    View details for Web of Science ID 000221671400038

    View details for PubMedID 15161286

  • Photoelectron spectroscopic and electronic structure studies of CH2O bonding and reactivity on ZnO surfaces: Steps in the methanol synthesis reaction INORGANIC CHEMISTRY Jones, P. M., May, J. A., Reitz, J. B., Solomon, E. I. 2004; 43 (11): 3349-3370

    Abstract

    Adsorption of CH(2)O on ZnO(0001) has been investigated using XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and density functional theory (DFT) calculations. CH(2)O is chemisorbed on the (0001) surface at 130 K. Its C1s XPS peak position at 292.7 eV and NEXAFS sigma shape resonance at 302.6 eV are consistent with an eta(1) bound surface geometry. Geometry optimized DFT calculations also indicate that CH(2)O is bound to the Zn(II) site in an eta(1) configuration through its oxygen atom. The variable-energy PES of the eta(1) bound CH(2)O/ZnO(0001) complex exhibits four valence band features at 21.2, 16.4, 13.8, and 10.7 eV below the vacuum level providing an experimental and theoretical description of this surface interaction. Annealing the ZnO(0001)/CH(2)O surface complex to 220 K decomposes the chemisorbed CH(2)O, producing formyl (291.5 eV), methoxide (290.2 eV), and formate (293.6 eV) intermediates. Thus this reaction coordinate involves the conversion of an oxygen bound formaldehyde to a carbon bound formyl species on ZnO(0001). Only formate is formed on the ZnO(100) surface. DFT is used to explore surface intermediates and the transition state in the methanol synthesis reaction (MSR). The bonding interactions of H(2), CO, CH(3)O(-), HCO(-), and trans-HCOH to the ZnO(0001) surface are elucidated using geometry optimization. H(2) was found to be heterolytically cleaved on the ZnO(0001) surface, and carbon monoxide, formyl, and methoxide are calculated to be eta(1) bound. These results are consistent with observed metal oxide surface reactivity where heterolytic bond cleavage is dominant. The oxygen atom in the bound formyl was found to be activated for attack by a proton. This results in the planar eta(1) bound trans-HCOH surface species. The transition state in the gas phase rearrangement of trans-HCOH to formaldehyde was calculated to have a barrier of 31 kcal/mol. The correlation diagram for this rearrangement in the gas phase indicates that configuration interaction at the crossing of two levels helps to lower the barrier. A transition state calculation was also performed for this rearrangement on the ZnO(0001) surface. The surface transition state geometry is significantly different than the gas phase. The surface geometry is no longer planar (23 degrees dihedral angle) and is displaced parallel to the surface. Interaction with the Zn(II) site at the crossing of surface bound CH(2)O and trans-HCOH levels further lowers the barrier to rearrangement relative to gas phase by 9 kcal/mol. The rearrangement of trans-HCOH (carbon bound) to CH(2)O (oxygen bound) on ZnO(0001) was calculated to be the overall barrier of the MSR reaction.

    View details for DOI 10.1021/ic035252q

    View details for Web of Science ID 000221684900009

    View details for PubMedID 15154797

  • Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Spectroscopic definition of the resting sites and the putative Cu-M(II)-OOH intermediate BIOCHEMISTRY Chen, P., Bell, J., EIPPER, B. A., Solomon, E. I. 2004; 43 (19): 5735-5747

    Abstract

    Spectroscopic methods, density functional calculations, and ligand field analyses are combined to define the geometric models and electronic structure descriptions of the Cu(M) and Cu(H) sites in the oxidized form of the noncoupled binuclear copper protein peptidylglycine alpha-hydroxylating monooxygenase (PHM). The Cu(M) site has a square pyramidal geometry with a long axial Cu-methionine bond and two histidines, H(2)O, and OH(-) as equatorial ligands. The Cu(H) site has a slightly D(2)(d) distorted square planar geometry with three histidines and H(2)O ligands. The structurally inequivalent Cu(M) and Cu(H) sites do not exhibit measurable differences in optical and electron paramagnetic resonance (EPR) spectra, which result from their similar ligand field transition energies and ground-state Cu covalencies. The additional axial methionine ligand interaction and associated square pyramidal distortion of the Cu(M) site have the opposite effect of the strong equatorial OH(-) donor ligand on the Cu d orbital splitting pattern relative to the Cu(H) site leading to similar ligand field transition energies for both sites. The small molecule NO(2)(-) binds in different coordination modes to the Cu(M) and Cu(H) site because of differences in their exchangeable coordination positions resulting in these Cu(II) sites being spectroscopically distinguishable. Azide binding to PHM is used as a spectroscopic and electronic structure analogue to OOH(-) binding to provide a starting point for developing a geometric and electronic structural model for the putative Cu(II)(M)-OOH intermediate in the H-atom abstraction reaction of PHM. Possible electronic structure contributions of the Cu(II)(M)-OOH intermediate to reactivity are considered by correlation to the well-studied L3Cu(II)-OOH model complex (L3 = [HB[3-tBu-5-iPrpz](3)]). The Met-S ligand of the Cu(M) site is found to contribute to the stabilization of the Cu(II)(M)-oxyl species, which would be a product of Cu(II)(M)-OOH H-atom abstraction reaction. This Met-S contribution could have a significant effect on the energetics of a H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate.

    View details for DOI 10.1021/bi0362830

    View details for Web of Science ID 000221365600018

    View details for PubMedID 15134448

  • Spectroscopic and quantum chemical characterization of the electronic structure and bonding in a non-heme FeIV[double bond]O complex. Journal of the American Chemical Society Decker, A., Rohde, J., Que, L., Solomon, E. I. 2004; 126 (17): 5378-5379

    Abstract

    High valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes involving the binding and activation of dioxygen. Using variable temperature magnetic circular dichroism (VT MCD) spectroscopy and experimentally calibrated density functional calculations, we are able to present the first detailed description of the electronic structure of a non-heme FeIV=O S = 1 complex. These studies define the nature of the FeIV=O bond and present the basis for understanding high-valent oxygen intermediates in non-heme iron enzymes.

    View details for PubMedID 15113207

  • Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Reaction mechanism and role of the noncoupled nature of the active site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Solomon, E. I. 2004; 126 (15): 4991-5000

    Abstract

    Reaction thermodynamics and potential energy surfaces are calculated using density functional methods to investigate possible reactive Cu/O(2) species for H-atom abstraction in peptidylglycine alpha-hydroxylating monooxygenase (PHM), which has a noncoupled binuclear Cu active site. Two possible mononuclear Cu/O(2) species have been evaluated, the 2-electron reduced Cu(II)(M)-OOH intermediate and the 1-electron reduced side-on Cu(II)(M)-superoxo intermediate, which could form with comparable thermodynamics at the catalytic Cu(M) site. The substrate H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate is found to be thermodynamically accessible due to the contribution of the methionine ligand, but with a high activation barrier ( approximately 37 kcal/mol, at a 3.0-A active site/substrate distance), arguing against the Cu(II)(M)-OOH species as the reactive Cu/O(2) intermediate in PHM. In contrast, H-atom abstraction from substrate by the side-on Cu(II)(M)-superoxo intermediate is a nearly isoenergetic process with a low reaction barrier at a comparable active site/substrate distance ( approximately 14 kcal/mol), suggesting that side-on Cu(II)(M)-superoxo is the reactive species in PHM. The differential reactivities of the Cu(II)(M)-OOH and Cu(II)(M)-superoxo species correlate to their different frontier molecular orbitals involved in the H-atom abstraction reaction. After the H-atom abstraction, a reasonable pathway for substrate hydroxylation involves a "water-assisted" direct OH transfer to the substrate radical, which generates a high-energy Cu(II)(M)-oxyl species. This provides the necessary driving force for intramolecular electron transfer from the Cu(H) site to complete the reaction in PHM. The differential reactivity pattern between the Cu(II)(M)-OOH and Cu(II)(M)-superoxo intermediates provides insight into the role of the noncoupled nature of PHM and dopamine beta-monooxygenase active sites, as compared to the coupled binuclear Cu active sites in hemocyanin, tyrosinase, and catechol oxidase, in O(2) activation.

    View details for DOI 10.1021/ja031564g

    View details for Web of Science ID 000220849900049

    View details for PubMedID 15080705

  • CD and MCD studies of the non-heme ferrous active site in (4-hydroxyphenyl)pyruvate dioxygenase: Correlation between oxygen activation in the extradiol and alpha-KG-dependent dioxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Neidig, M. L., Kavana, M., Moran, G. R., Solomon, E. I. 2004; 126 (14): 4486-4487

    Abstract

    (4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is an unusual alpha-keto acid-dependent non-heme iron dioxygenase as it incorporates both atoms of dioxygen into a single substrate, paralleling the extradiol dioxygenases. CD/MCD studies of the catalytically active ferrous site and its interaction with substrate reveal a geometic and electronic structure and mechanistic approach to oxygen activation which bridges those of the alpha-KG-dependent and the extradiol dioxygenases.

    View details for DOI 10.1021/ja0316521

    View details for Web of Science ID 000220752300013

    View details for PubMedID 15070344

  • Electronic and spectroscopic studies of the non-heme reduced binuclear iron sites of two ribonucleotide reductase variants: Comparison to reduced methane monooxygenase and contributions to O-2 reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wei, P. P., Skulan, A. J., Mitic, N., Yang, Y. S., Saleh, L., Bollinger, J. M., Solomon, E. I. 2004; 126 (12): 3777-3788

    Abstract

    Circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD have been used to probe the biferrous active site of two variants of ribonucleotide reductase. The aspartate to glutamate substitution (R2-D84E) at the binuclear iron site modifies the endogenous ligand set of ribonucleotide reductase to match that of the binuclear center in the hydroxylase component of methane monooxygenase (MMOH). The crystal structure of chemically reduced R2-D84E suggests that the active-site structure parallels that of MMOH. However, CD, MCD, and VTVH MCD data combined with spin-Hamiltonian analysis of reduced R2-D84E indicate a different coordination environment relative to reduced MMOH, with no mu-(1,1)(eta(1),eta(2)) carboxylate bridge. To further understand the variations in geometry of the active site, which lead to differences in reactivity, density functional theory (DFT) calculations have been carried out to identify active-site structures for R2-wt and R2-D84E consistent with these spectroscopic data. The effects of varying the ligand set, positions of bound and free waters, and additional protein constraints on the geometry and energy of the binuclear site of both R2-wt and variant R2s are also explored to identify the contributions to their structural differences and their relation to reduced MMOH.

    View details for DOI 10.1021/ja0374731

    View details for Web of Science ID 000220440400038

    View details for PubMedID 15038731

  • S K-edge X-ray absorption spectroscopic investigation of the Ni-containing superoxide dismutase active site: New structural insight into the mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Szilagyi, R. K., Bryngelson, P. A., Maroney, M. J., Hedman, B., Hodgson, K. O., Solomon, E. I. 2004; 126 (10): 3018-3019

    Abstract

    Superoxide dismutases protect cells from the toxic effects of reactive oxygen species derived from superoxide. Nickel-containing superoxide dismutases (NiSOD), found in Streptomyces species and in cyanobacteria, are distinct from Mn-, Fe-, or Cu/Zn-containing SODs in amino acid sequence and metal ligand environment. Sulfur K-edge X-ray absorption spectroscopic investigations were carried out for a series of mono- and binuclear Ni model compounds with varying sulfur ligation, and for oxidized and reduced NiSOD to elucidate the types of Ni-S interactions found in the two oxidation states. The S K-edge XAS spectra clearly indicate the presence of Ni(III)-bound terminal thiolate in the oxidized enzyme and the absence of such coordination to Ni(II) in the peroxide-reduced enzyme. This striking change in the S ligation for Ni with redox suggests that, upon peroxide reduction, an electron is transferred to the Ni(III) site and the terminal thiolate becomes protonated, providing an efficient mechanism for proton-coupled electron transfer.

    View details for DOI 10.1021/ja039106v

    View details for Web of Science ID 000220192000007

    View details for PubMedID 15012109

  • Electronic structures of metal sites in proteins and models: Contributions to function in blue copper proteins CHEMICAL REVIEWS SOLOMON, E. I., Szilagyi, R. K., George, S. D., Basumallick, L. 2004; 104 (2): 419-458

    View details for DOI 10.1021/cr0206317

    View details for Web of Science ID 000188934400005

    View details for PubMedID 14871131

  • Comparison between the geometric and electronic structures and reactivities of {FeNO}(7) and {FeO2}(8) complexes: A density functional theory study JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Schenk, G., Pau, M. Y., Solomon, E. I. 2004; 126 (2): 505-515

    Abstract

    In a previous study, we analyzed the electronic structure of S = 3/2 [FeNO](7) model complexes [Brown et al. J. Am. Chem. Soc. 1995, 117, 715-732]. The combined spectroscopic data and SCF-X alpha-SW electronic structure calculations are best described in terms of Fe(III) (S = 5/2) antiferromagnetically coupled to NO(-) (S = 1). Many nitrosyl derivatives of non-heme iron enzymes have spectroscopic properties similar to those of these model complexes. These NO derivatives can serve as stable analogues of highly labile oxygen intermediates. It is thus essential to establish a reliable density functional theory (DFT) methodology for the geometry and energetics of [FeNO](7) complexes, based on detailed experimental data. This methodology can then be extended to the study of [FeO(2)](8) complexes, followed by investigations into the reaction mechanisms of non-heme iron enzymes. Here, we have used the model complex Fe(Me(3)TACN)(NO)(N(3))(2) as an experimental marker and determined that a pure density functional BP86 with 10% hybrid character and a mixed triple-zeta/double-zeta basis set lead to agreement between experimental and computational data. This methodology is then applied to optimize the hypothetical Fe(Me(3)TACN)(O(2))(N(3))(2) complex, where the NO moiety is replaced by O(2). The main geometric differences are an elongated Fe[bond]O(2) and a steeper Fe[bond]O[bond]O angle in the [FeO(2)](8) complex. The electronic structure of [FeO(2)](8) corresponds to Fe(III) (S = 5/2) antiferromagnetically coupled to O(2)(-) (S = 1/2), and, consistent with the extended bond length, the [FeO(2)](8) unit has only one Fe(III)-O(2)(-) bonding interaction, while the [FeNO](7) unit has both sigma and pi type Fe(III)-NO(-) bonds. This is in agreement with experiment as NO forms a more stable Fe(III)-NO(-) adduct relative to O(2)(-). Although NO is, in fact, harder to reduce, the resultant NO(-) species forms a more stable bond to Fe(III) relative to O(2)(-) due to the different bonding interactions.

    View details for DOI 10.1021/ja036715u

    View details for Web of Science ID 000188197800043

    View details for PubMedID 14719948

  • N2O reduction by the mu(4)-sulfide-bridged tetranuclear Cu-Z cluster active site ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Chen, P., Gorelsky, S. I., Ghosh, S., Solomon, E. I. 2004; 43 (32): 4132-4140

    Abstract

    Nitrous oxide (N2O) reduction is a chemical challenge both in the selective oxidation of organic substrates by N2O and in the removal of N2O as a green-house gas. The reduction of N2O is thermodynamically favorable but kinetically inert, and requires activating transition-metal centers. In biological systems, N2O reduction is the last step in the denitrification process of the bacterial nitrogen cycle and is accomplished by the enzyme nitrous oxide reductase, whose active site consists of a micro4-sulfide-bridged tetranuclear CuZ cluster which has many unusual spectroscopic features. Recent studies have developed a detailed electronic-structure description of the resting CuZ cluster, determined its catalytically relevant state, and provided insight into the role of this tetranuclear copper cluster in N2O activation and reduction.

    View details for DOI 10.1002/anie.200301734

    View details for Web of Science ID 000223505600004

    View details for PubMedID 15307074

  • Activation of N2O reduction by the fully reduced mu(4)-sulfide bridged tetranuclear Cu-Z cluster in nitrous oxide reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ghosh, S., Gorelsky, S. I., Chen, P., Cabrito, I., Moura, J. J., Moura, I., Solomon, E. I. 2003; 125 (51): 15708-15709

    Abstract

    The tetranuclear CuZ cluster catalyzes the two-electron reduction of N2O to N2 and H2O in the enzyme nitrous oxide reductase. This study shows that the fully reduced 4CuI form of the cluster correlates with the catalytic activity of the enzyme. This is the first demonstration that the S = 1/2 form of CuZ can be further reduced. Complementary DFT calculations support the experimental findings and demonstrate that N2O binding in a bent mu-1,3-bridging mode to the 4CuI form is most efficient due to strong back-bonding from two reduced copper atoms. This back-donation activates N2O for electrophilic attack by a proton.

    View details for DOI 10.1021/ja038344n

    View details for Web of Science ID 000187436200012

    View details for PubMedID 14677937

  • Spectroscopic studies of the Met182Thr mutant of nitrite reductase: Role of the axial ligand in the geometric and electronic structure of blue and green copper sites JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Basumallick, L., Szilagyi, R. K., Zhao, Y. W., Shapleigh, J. P., Scholes, C. P., SOLOMON, E. I. 2003; 125 (48): 14784-14792

    Abstract

    A combination of spectroscopic methods and density functional calculations has been used to describe the electronic structure of the axial mutant (Met182Thr) of Rhodobacter sphaeroides nitrite reductase in which the axial methionine has been changed to a threonine. This mutation results in a dramatic change in the geometric and electronic structure of the copper site. The electronic absorption data imply that the type 1 site in the mutant is like a typical blue copper site in contrast to the wild-type site, which is green. Similar ligand field strength in the mutant and the wild type (from MCD spectra) explains the similar EPR parameters for very different electronic structures. Resonance Raman shows that the Cu-S(Cys) bond is stronger in the mutant relative to the wild type. From a combination of absorption, CD, MCD, and EPR data, the loss of the strong axial thioether (present in the wild-type site) results in an increase of the equatorial thiolate-Cu interaction and the site becomes less tetragonal. Spectroscopically calibrated density functional calculations were used to provide additional insight into the role of the axial ligand. The calculations reproduce well the experimental ground-state bonding and the changes in going from a green to a blue site along this coupled distortion coordinate. Geometry optimizations at the weak and strong axial ligand limits show that the bonding of the axial thioether is the key factor in determining the structure of the ground state. A comparison of plastocyanin (blue), wild-type nitrite reductase (green), and the Met182Thr mutant (blue) sites enables evaluation of the role of the axial ligand in the geometric and electronic structure of type 1 copper sites, which can affect the electron-transfer properties of these sites.

    View details for DOI 10.1021/ja037232t

    View details for Web of Science ID 000186834500045

    View details for PubMedID 14640653

  • L-edge X-ray absorption spectroscopy of non-heme iron sites: Experimental determination of differential orbital covalency JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Wasinger, E. C., de Groot, F. M., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (42): 12894-12906

    Abstract

    X-ray absorption spectroscopy has been utilized to obtain the L-edge multiplet spectra for a series of non-heme ferric and ferrous complexes. Using these data, a methodology for determining the total covalency and the differential orbital covalency (DOC), that is, differences in covalency in the different symmetry sets of the d orbitals, has been developed. The integrated L-edge intensity is proportional to the number of one-electron transition pathways to the unoccupied molecular orbitals as well as to the covalency of the iron site, which reduces the total L-edge intensity and redistributes intensity, producing shake-up satellites. Furthermore, differential orbital covalency leads to differences in intensity for the different symmetry sets of orbitals and, thus, further modifies the experimental spectra. The ligand field multiplet model commonly used to simulate L-edge spectra does not adequately reproduce the spectral features, especially the charge transfer satellites. The inclusion of charge transfer states with differences in covalency gives excellent fits to the data and experimental estimates of the different contributions of charge transfer shake-up pathways to the t(2g) and e(g) symmetry orbitals. The resulting experimentally determined DOC is compared to values calculated from density functional theory and used to understand chemical trends in high- and low-spin ferrous and ferric complexes with different covalent environments. The utility of this method toward problems in bioinorganic chemistry is discussed.

    View details for DOI 10.1021/ja034634s

    View details for Web of Science ID 000185990300050

    View details for PubMedID 14558838

  • EPR spectroscopy of [Fe2O2(5-Et-3-TPA)(2)](3+): Electronic origin of the unique spin-hamiltonian parameters of the (Fe2O2)-O-III,IV diamond core INORGANIC CHEMISTRY Skulan, A. J., Hanson, M. A., Hsu, H. F., Dong, Y. H., Que, L., SOLOMON, E. I. 2003; 42 (20): 6489-6496

    Abstract

    The electronic origins of the magnetic signatures of [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3), where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, were investigated by density functional calculations. These signatures consist of a near-axial EPR spectrum, anisotropic superhyperfine broadening upon (17)O substitution in the Fe(2)O(2) core, and an unusually large, positive zero-field splitting parameter, D = 38 +/- 3 cm(-1). Density functional calculations identify the anisotropic (17)O superhyperfine broadening to be due to a preponderance of oxo 2p density perpendicular to the plane of the Fe(2)O(2) core in the three singly occupied molecular orbitals of the S = (3)/(2) ground state. The near-axial g-matrix arises from DeltaS = 0 spin-orbit mixing between the singly and doubly occupied d(pi) orbitals of the iron d-manifold. The large D is due to DeltaS = +/-1 spin-orbit mixing with low-lying d(pi) excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe(2)O(2) core, and define the low-lying valence orbitals responsible for reactivity.

    View details for DOI 10.1021/ic034170z

    View details for Web of Science ID 000185697300041

    View details for PubMedID 14514326

  • Spectroscopic investigation of stellacyanin mutants: Axial ligand interactions at the blue copper site JOURNAL OF THE AMERICAN CHEMICAL SOCIETY George, S. D., Basumallick, L., Szilagyi, R. K., Randall, D. W., Hill, M. G., Nersissian, A. M., Valentine, J. S., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (37): 11314-11328

    Abstract

    Detailed electronic and geometric structural descriptions of the blue copper sites in wild-type (WT) stellacyanin and its Q99M and Q99L axial mutants have been obtained using a combination of XAS, resonance Raman, MCD, EPR, and DFT calculations. The results show that the origin of the short Cu-S(Cys) bond in blue copper proteins is the weakened axial interaction, which leads to a shorter (based on EXAFS results) and more covalent (based on S K-edge XAS) Cu-S bond. XAS pre-edge energies show that the effective nuclear charge on the copper increases going from O(Gln) to S(Met) to no axial (Leu) ligand, indicating that the weakened axial ligand is not fully compensated for by the increased donation from the thiolate. This is further supported by EPR results. MCD data show that the decreased axial interaction leads to an increase in the equatorial ligand field, indicating that the site acquires a more trigonally distorted tetrahedral structure. These geometric and electronic structural changes, which result from weakening the bonding interaction of the axial ligand, allow the site to maintain efficient electron transfer (high H(DA) and low reorganization energy), while modulating the redox potential of the site to the biologically relevant range. These spectroscopic studies are complemented by DFT calculations to obtain insight into the factors that allow stellacyanin to maintain a trigonally distorted tetrahedral structure with a relatively strong axial Cu(II)-oxygen bond.

    View details for DOI 10.1021/ja035802j

    View details for Web of Science ID 000185341800053

    View details for PubMedID 16220954

  • Spectroscopic and electronic structure studies of 2,3-dihydroxybiphenyl 1,2-dioxygenase: O-2 reactivity of the non-heme ferrous site in extradiol dioxygenases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Davis, M. I., Wasinger, E. C., Decker, A., Pau, M. Y., Vaillancourt, F. H., Bolin, J. T., Eltis, L. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (37): 11214-11227

    Abstract

    The extradiol dioxygenase, 2,3-dihydroxybiphenyl 1,2-dioxygenase (DHBD, EC 1.13.11.39), has been studied using magnetic circular dichroism (MCD), variable-temperature variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies, which are analogous to methods used in earlier studies on the extradiol dioxygenase catechol 2,3-dioxygenase [Mabrouk et al. J. Am. Chem Soc. 1991, 113, 4053-4061]. For DHBD, the spectroscopic data can be correlated to the results of crystallography and with the results from density functional calculations to obtain detailed geometric and electronic structure descriptions of the resting and substrate (DHB) bound forms of the enzyme. The geometry of the active site of the resting enzyme, square pyramidal with a strong Fe-glutamate bond in the equatorial plane, localizes the redox active orbital in an orientation appropriate for O(2) binding. However, the O(2) reaction is not favorable, as it would produce a ferric superoxide intermediate with a weak Fe-O bond. Substrate binding leads to a new square pyramidal structure with the strong Fe-glutamate bond in the axial direction as indicated by a decrease in the (5)E(g) and increase in the (5)T(2g) splitting. Electronic structure calculations provide insight into the relative lack of dioxygen reactivity for the resting enzyme and its activation upon substrate binding.

    View details for DOI 10.1021/ja029746i

    View details for Web of Science ID 000185341800039

    View details for PubMedID 16220940

  • Rapid-freeze-quench magnetic circular dichroism of intermediate X in ribonucleotide reductase: New structural insight JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mitic, N., Saleh, L., Schenk, G., Bollinger, J. M., Solomon, E. I. 2003; 125 (37): 11200-11201

    Abstract

    To elucidate the electronic structure of intermediate X in the oxygen activation reaction of the R2 subunit of ribonucleotide reductase, a protocol has been developed to perform magnetic circular dichroism (MCD) on a rapid-freeze-quench, strain free optical sample. RFQ-MCD data have been collected on intermediate X in the double mutant of R2, Y122/Y356F. While X has been reported to exhibit a broad absorption band at 365 nm, there are at least 10 electronic transitions observed at low-temperature MCD. From C0/D0 ratios, the transitions of X can be divided into three regions: 16 000-22 000 cm-1 region involving spin-allowed ligand field transitions of the Fe(IV), 23 000-24 000 cm-1 region of spin-forbidden, spin-flip transitions on the Fe(IV), and the charge transfer (CT) region from 26 000 to 32 000 cm-1. The C0/D0 ratios from d --> d and CT transitions strongly support significant Fe(IV) character coupled into the paramagnetic center. Ligand field (spin-allowed d --> d region) analysis allows the bis-mu-oxo and mu-oxo plus other monoanionic bridge possibilities for the structure of intermediate X to be distinguished, providing new insight into the molecular mechanism of the cluster formation in R2.

    View details for DOI 10.1021/ja036556e

    View details for Web of Science ID 000185341800032

    View details for PubMedID 16220933

  • Spectroscopic studies of the interaction of ferrous bleomycin with DNA JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kemsley, J. N., Zaleski, K. L., Chow, M. S., Decker, A., Shishova, E. Y., Wasinger, E. C., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (36): 10810-10821

    Abstract

    Bleomycin is an antibiotic used in cancer chemotherapy for its ability to achieve both single- and double-strand cleavage of DNA through abstraction of the deoxyribose C4'-H. Magnetic circular dichroism (MCD) and X-ray absorption (XAS) spectroscopies have been used to study the interaction of the biologically relevant FeIIBLM complex with DNA. Calf thymus DNA was used as the substrate as well as short oligonucleotides, including one with a preferred 5'-G-pyrimidine-3' cleavage site [d(GGAAGCTTCC)2] and one without [d(GGAAATTTCC)2]. DNA binding to FeIIBLM significantly perturbs the FeII active site, resulting in a change in intensity ratio of the d d transitions and a decrease in excited-state orbital splitting (5Eg). Although this effect is somewhat dependent on length and composition of the oligonucleotide, it is not correlated to the presence of a 5'-G-pyrimidine-3' cleavage site. No effect is observed on the charge-transfer transitions, indicating that the H-bonding recognition between the pyrimidine and guanine base does not perturb Fe-pyrimidine backbonding. Azide binding studies indicate that FeIIBLM bound to either oligomer has the same affinity for N3-. Parallel studies of BLM structural derivatives indicate that FeIIiso-PEPLM, in which the carbamoyl group is shifted on the mannose sugar, forms the same DNA-bound species as FeIIBLM. In contrast, FeIIDP-PEPLM, in which the -aminoalanine group is absent, forms a new species upon DNA binding. These data are consistent with a model in which the primary amine from the -aminoalanine is an FeII ligand and the mannose carbamoyl provides either a ligand to the FeII or significant second-sphere effects on the FeII site; intercalation of the bithiazole tail into the double helix likely brings the metal-bound complex close enough to the DNA to create steric interactions that remove the sugar groups from interaction with the FeII. The fact that the FeII active site is perturbed regardless of DNA sequence is consistent with the fact that cleavage is observed for both 5'-GC-3' and nonspecific oligomers and indicates that different reaction coordinates may be active, depending on orientation of the deoxyribose C4'-H.

    View details for DOI 10.1021/ja034579n

    View details for Web of Science ID 000185154300021

    View details for PubMedID 12952460

  • Description of the ground state wave functions of Ni dithiolenes using sulfur K-edge X-ray absorption spectroscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Szilagyi, R. K., Lim, B. S., Glaser, T., Holm, R. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 2003; 125 (30): 9158-9169

    Abstract

    The pterin-dithiolene cofactor is an essential component of the catalytic sites of all molybdoenzymes except nitrogenase. Understanding its bonding to transition metals allows for development of electronic structure/function correlations in catalysis. The electronic structure description for a series of bis(dithiolene) complexes ([NiL(2)](Z)(), L = 1,2-Me(2)C(2)S(2); Z = 2-, 1-, 0) using sulfur XAS provides the basis for extension to the biologically relevant metal-containing dithiolenes. The transition dipole integral has been developed for the dithiolene sulfur through correlation of XAS pre-edge energy positions of sulfide-, thiolate-, and enedithiolate-S. The ground state wave functions of all three NiL(2) complexes have more than 50% S character experimentally demonstrating the noninnocent behavior of the dithiolene ligand. The S K-edge experimental results are correlated with spin-unrestricted, broken-symmetry density functional calculations. These show only limited spin polarization in the neutral complex and delocalized, ligand based ground states for the mono- and dianionic complexes. These XAS and DFT results are correlated with other spectroscopic features and provide insight into reactivity.

    View details for DOI 10.1021/ja029806k

    View details for Web of Science ID 000184364500049

    View details for PubMedID 15369373

  • Spectroscopic characterization of the Leu513His variant of fungal laccase: Effect of increased axial ligand interaction on the geometric and electronic structure of the type 1 Cu site INORGANIC CHEMISTRY Palmer, A. E., Szilagyi, R. K., Cherry, J. R., Jones, A., Xu, F., Solomon, E. I. 2003; 42 (13): 4006-4017

    Abstract

    A variety of spectroscopic techniques, combined with density functional calculations, are used to describe the electronic structure of the Leu513His variant of the type 1 Cu site in Myceliophthora thermophila laccase. This mutation changes the type 1 Cu from a blue to a green site. Electron paramagnetic resonance (EPR), optical absorption, circular dichroism, and magnetic circular dichroism (MCD) spectroscopies reveal that, relative to the trigonal planar blue type 1 Cu site in wild-type fungal laccase, the covalency and the ligand field strength at the Leu513His green type 1 Cu center decrease. Additionally, there is a significant reorientation of the d(x)()()2(-)(y)()()2( )singly occupied MO, such that the overlap with the Cys sulfur valence orbital changes from pi to sigma. A density functional study in which internal coordinates are systematically altered reveals that these changes are due to the increased strength of the axial ligand (none to His), leading to a tetragonal distortion and elongation of the equatorial Cu-ligand bonds. These calculations provide insight into the experimental differences in the EPR parameters, charge-transfer absorption spectrum, and ligand-field MCD spectrum between the axial-His variant and blue Cu centers (plastocyanin and the type 1 site in fungal laccase). There are also significant differences between the green site in the Leu513His variant and other naturally occurring, green type 1 Cu sites such as in nitrite reductase, which have short axial Cu-S(Met) bonds. The large difference in EPR parameters between these green type 1 sites derives from a change in ligand field excitation energies observed by MCD, which reflects a decrease in ligand field strength. This is associated with different steric interactions of a His vs an axial Met ligand in a tetragonally distorted type 1 site. Changes in the electronic structure of the Cu site correlate with the difference in reactivity of the green His variant relative to blue wild-type fungal laccase.

    View details for DOI 10.1021/ic026099n

    View details for Web of Science ID 000183945100009

    View details for PubMedID 12817956

  • Spectroscopic characterization of soybean lipoxygenase-1 mutants: the role of second coordination sphere residues in the regulation of enzyme activity BIOCHEMISTRY Schenk, G., Neidig, M. L., Zhou, J., Holman, T. R., SOLOMON, E. I. 2003; 42 (24): 7294-7302

    Abstract

    Lipoxygenases are non-heme iron enzymes, which catalyze the stereo- and regiospecific hydroperoxidation of unsaturated fatty acids. Spectroscopic studies on soybean lipoxygenase have shown that the ferrous form of the enzyme is a mixture of five- and six-coordinate species (40 and 60%, respectively). Addition of substrate leads to a purely six-coordinate form. A series of mutations in the second coordination sphere (Q697E, Q697N, Q495A, and Q495E) were generated, and the structures of the mutants were solved by crystallography [Tomchick et al. (2001) Biochemistry 40, 7509-7517]. While this study clearly showed the contribution of H-bond interactions between the first and the second coordination spheres in catalysis, no correlation with the coordination environment of the Fe(II) was observed. A recent study using density-functional theory [Lehnert and Solomon (2002) J. Biol. Inorg. Chem. 8, 294-305] indicated that coordination flexibility, involving the Asn694 ligand, is regulated via H-bond interactions. In this paper, we investigate the solution structures of the second coordination sphere mutants using CD and MCD spectroscopy since these techniques are more sensitive indicators of the first coordination sphere ligation of Fe(II) systems. Our data demonstrate that the iron coordination environment directly relates to activity, with the mutations that have the ability to form a five-coordinate/six-coordinate mixture being more active. We propose that the H-bond between the weak Asn694 ligand and the Gln697 plays a key role in the modulation of the coordination flexibility of Asn694, and thus, is crucial for the regulation of enzyme reactivity.

    View details for DOI 10.1021/bi027380g

    View details for Web of Science ID 000183624800004

    View details for PubMedID 12809485

  • Spectroscopic study of [Fe2O2(5-Et-3-TPA)(2)](3+): Nature of the Fe2O2 diamond core and its possible relevance to high-valent binuclear non-heme enzyme intermediates JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Skulan, A. J., Hanson, M. A., Hsu, H. F., Que, L., Solomon, E. I. 2003; 125 (24): 7344-7356

    Abstract

    The spectroscopic properties and electronic structure of an Fe(2)(III,IV) bis-mu-oxo complex, [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3) where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe(2)O(2) diamond core. Low-temperature magnetic circular dichroism (MCD) allows the two features in the broad absorption envelope (4000-30000 cm(-)(1)) to be resolved into 13 transitions. Their C/D ratios and transition polarizations from variable temperature-variable field MCD saturation behavior indicate that these divide into three types of electronic transitions; t(2) --> t(2) involving excitations between metal-based orbitals with pi Fe-O overlap (4000-10000 cm(-)(1)), t(2)/t(2) --> e involving excitations to metal-based orbitals with sigma Fe-O overlap (12500-17000 cm(-)(1)) and LMCT (17000-30000 cm(-)(1)) and allows transition assignments and calibration of density functional calculations. Resonance Raman profiles show the C(2)(h)() geometric distortion of the Fe(2)O(2) core results in different stretching force constants for adjacent Fe-O bonds (k(str)(Fe-O(long)) = 1.66 and k(str)(Fe-O(short)) = 2.72 mdyn/A) and a small ( approximately 20%) difference in bond strength between adjacent Fe-O bonds. The three singly occupied pi-metal-based orbitals form strong superexchange pathways which lead to the valence delocalization and the S = (3)/(2) ground state. These orbitals are key to the observed reactivity of this complex as they overlap with the substrate C-H bonding orbital in the best trajectory for hydrogen atom abstraction. The electronic structure implications of these results for the high-valent enzyme intermediates X and Q are discussed.

    View details for DOI 10.1021/ja021137n

    View details for Web of Science ID 000183503500044

    View details for PubMedID 12797809

  • Spectroscopic evidence for a heme-superoxide/Cu(I) intermediate in a functional model of cytochrome C oxidase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Collman, J. P., Sunderland, C. J., Berg, K. E., Vance, M. A., SOLOMON, E. I. 2003; 125 (22): 6648-6649

    Abstract

    A superstructured tetraphenylporphyrin with a covalently attached proximal imidazole axial base and three distal imidazole pickets has been developed as a model for the active site of terminal oxidases such as cytochrome c oxidase. The oxygen adduct of the Fe-only heme (at low temperature) has a diamagnetic NMR and is EPR silent, which taken together with a resonance Raman oxygen isotope sensitive band (nuFe-O) at 575/554 cm-1 (16O2/18O2) indicates formation of a six-coordinate heme-superoxide complex. Unexpectedly, the Fe/Cu complex, where the copper is in a trisimidazole environment approximately 5 A above the heme plane, displays similar characteristics: a diamagnetic NMR, EPR silence, and nuFe-O at 570/544 cm-1. This indicates the dioxygen adduct of this Fe/Cu system is also a superoxide. This contrasts with previously characterized partially reduced dioxygen intermediates of binuclear heme/copper complexes that form Fe/Cu mu-peroxo complexes.

    View details for DOI 10.1021/ja034382v

    View details for Web of Science ID 000183314000025

    View details for PubMedID 12769571

  • Spectroscopy and bonding in side-on and end-on Cu-2(S-2) cores: Comparison to peroxide analogues JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Fujisawa, K., Helton, M. E., Karlin, K. D., SOLOMON, E. I. 2003; 125 (21): 6394-6408

    Abstract

    Spectroscopic methods combined with density functional calculations were used to study the disulfide-Cu(II) bonding interactions in the side-on micro -eta(2):eta(2)-bridged Cu(2)(S(2)) complex, [[Cu(II)[HB(3,5-Pr(i)(2)pz)(3)]](2)(S(2))], and the end-on trans- micro -1,2-bridged Cu(2)(S(2)) complex, [[Cu(II)(TMPA)](2)(S(2))](2+), in correlation to their peroxide structural analogues. Resonance Raman shows weaker S-S bonds and stronger Cu-S bonds in the disulfide complexes relative to the O-O and Cu-O bonds in the peroxide analogues. The weaker S-S bonds come from the more limited interaction between the S 3p orbitals relative to that of the O 2s/p hybrid orbitals. The stronger Cu-S bonds result from the more covalent Cu-disulfide interactions relative to the Cu-peroxide interactions. This is consistent with the higher energy of the disulfide valence level relative to that of the peroxide. The ground states of the side-on Cu(2)(S(2))/Cu(2)(O(2)) complexes are more covalent than those of the end-on Cu(2)(S(2))/Cu(2)(O(2)) complexes. This derives from the larger sigma-donor interactions in the side-on micro -eta(2):eta(2) structure, which has four Cu-disulfide/peroxide bonds, relative to the end-on trans- micro -1,2 structure, which forms two bonds to the Cu. The larger disulfide/peroxide sigma-donor interactions in the side-on complexes are reflected in their more intense higher energy disulfide/peroxide to Cu charge transfer transitions in the absorption spectra. The large ground-state covalencies of the side-on complexes result in significant nuclear distortions in the ligand-to-metal charge transfer excited states, which give rise to the strong resonance Raman enhancements of the metal-ligand and intraligand vibrations. Particularly, the large covalency of the Cu-disulfide interaction in the side-on Cu(2)(S(2)) complex leads to a different rR enhancement profile, relative to the peroxide analogues, reflecting a S-S bond distortion in the opposite directions in the disulfide/peroxide pi(sigma) to Cu charge transfer excited states. A ligand sigma back-bonding interaction exists only in the side-on complexes, and there is more sigma mixing in the side-on Cu(2)(S(2)) complex than in the side-on Cu(2)(O(2)) complex. This sigma back-bonding is shown to significantly weaken the S-S/O-O bond relative to that of the analogous end-on complex, leading to the low nu(S)(-)(S)/nu(O)(-)(O) vibrational frequencies observed in the resonance Raman spectra of the side-on complexes.

    View details for DOI 10.1021/ja0214678

    View details for Web of Science ID 000183031800025

    View details for PubMedID 12785779

  • Spectroscopic and kinetic studies of PKU-inducing mutants of phenylalanine hydroxylase: Arg158Gln and Glu280Lys JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Kemsley, J. N., Wasinger, E. C., Datta, S., Mitic, N., Acharya, T., Hedman, B., Caradonna, J. P., Hodgson, K. O., SOLOMON, E. I. 2003; 125 (19): 5677-5686

    Abstract

    Phenylalanine hydroxylase (PAH) is a tetrahydrobiopterin-dependent, nonheme iron enzyme that catalyzes the hydroxylation of L-Phe to L-Tyr in the rate-limiting step of phenylalanine catabolism. This reaction is tightly coupled in the wild-type enzyme to oxidation of the tetrahydropterin cofactor. Dysfunction of PAH activity in humans leads to the disease phenylketonuria (PKU). We have investigated two PKU-inducing mutants, Arg158Gln and Glu280Lys, using kinetic methods, magnetic circular dichrosim (MCD) spectroscopy, and X-ray absorption spectroscopy (XAS). Analysis of the products produced by the mutant enzymes shows that although both oxidize pterin at more than twice the rate of wild-type enzyme, these reactions are only approximately 20% coupled to production of L-Tyr. Previous MCD and XAS studies had demonstrated that the resting Fe(II) site is six-coordinate in the wild-type enzyme and converts to a five-coordinate site when both L-Phe and reduced pterin are present in the active site. Although the Arg158Gln mutant forms the five-coordinate site when both cosubstrates are bound, the Fe(II) site of the Glu280Lys mutant remains six-coordinate. These results provide insight into the PAH reaction and disease mechanism at a molecular level, indicating that the first step of the mechanism is formation of a peroxy-pterin species, which subsequently reacts with the Fe(II) site if the pterin is properly oriented for formation of an Fe-OO-pterin bridge and an open coordination position is available on the Fe(II).

    View details for DOI 10.1021/ja029106f

    View details for Web of Science ID 000182769400039

    View details for PubMedID 12733906

  • Resonance Raman investigation of equatorial ligand donor effects on the Cu2O22+ core in end-on and side-on mu-peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Henson, M. J., Vance, M. A., Zhang, C. X., Liang, H. C., Karlin, K. D., SOLOMON, E. I. 2003; 125 (17): 5186-5192

    Abstract

    The effect of endogenous donor strength on Cu(2)O(2) bonds was studied by electronically perturbing [[(R-TMPA)Cu(II)]](2)(O(2))](2+) and [[(R-MePY2)Cu](2)(O(2))](2+) (R = H, MeO, Me(2)N), which form the end-on mu-1,2 bound peroxide and an equilibrium mixture of side-on peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) isomers, respectively. For [[(R-TMPA)Cu(II)](2)(O(2))](2+), nu(O-O) shifts from 827 to 822 to 812 cm(-1) and nu(Cu)(-)(O(sym)) shifts from 561 to 557 to 551 cm(-1), respectively, as R- varies from H to MeO to Me(2)N. Thus, increasing the N-donor strength to the copper decreases peroxide pi(sigma) donation to the copper, weakening the Cu-O and O-O bonds. A decrease in nu(Cu-O) of the bis-mu-oxo-dicopper(III) complex was also observed with increasing N-donor strength for the R-MePY2 ligand system. However, no change was observed for nu(O-O) of the side-on peroxo. This is attributed to a reduced charge donation from the peroxide pi(sigma) orbital with increased N-donor strength, which increases the negative charge on the peroxide and adversely affects the back-bonding from the Cu to the peroxide sigma orbital. However, an increase in the bis-mu-oxo-dicopper(III) isomer relative to side-on peroxo-dicopper(II) species is observed for R-MePY2 with R = H < MeO < Me(2)N. This effect is attributed to the thermodynamic stabilization of the bis-mu-oxo-dicopper(III) isomer relative to the side-on peroxo-dicopper(II) isomer by strong donor ligands. Thus, the side-on peroxo-dicopper(II)/bis-mu-oxo-dicopper(III) equilibrium can be controlled by electronic as well as steric effects.

    View details for DOI 10.1021/ja0276366

    View details for Web of Science ID 000182491700042

    View details for PubMedID 12708870

  • Non-heme iron enzymes: Contrasts to heme catalysis PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA SOLOMON, E. I., Decker, A., Lehnert, N. 2003; 100 (7): 3589-3594

    Abstract

    Non-heme iron enzymes catalyze a wide range of O(2) reactions, paralleling those of heme systems. Non-heme iron active sites are, however, much more difficult to study because they do not exhibit the intense spectral features characteristic of the porphyrin ligand. A spectroscopic methodology was developed that provides significant mechanistic insight into the reactivity of non-heme ferrous active sites. These studies reveal a general mechanistic strategy used by these enzymes and differences in substrate and cofactor interactions dependent on their requirement for activation by iron. Contributions to O(2) activation have been elucidated for non-heme relative to heme ligand sets, and major differences in reactivity are defined with respect to the heterolytic and homolytic cleavage of O-O bonds.

    View details for DOI 10.1073/pnas.0336792100

    View details for Web of Science ID 000182058400014

    View details for PubMedID 12598659

  • Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 3. Kinetics of electron transfer INORGANIC CHEMISTRY Kennepohl, P., SOLOMON, E. I. 2003; 42 (3): 696-708

    Abstract

    The kinetics of electron transfer for rubredoxins are examined using density functional methods to determine the electronic structure characteristics that influence and allow for fast electron self-exchange in these electron-transport proteins. Potential energy surfaces for [FeX(4)](2-,1-) models confirm that the inner-sphere reorganization energy is inherently small for tetrathiolates ( approximately 0.1 eV), as evidenced by the only small changes in the equilibrium Fe-S bond distance during redox (Deltar(redox) approximately 0.05 A). It is concluded that electronic relaxation and covalency in the reduced state allow for this small in this case relative to other redox couples, such as the tetrachloride. Using a large computational model to include the protein medium surrounding the [Fe(SCys)(4)](2-,1-) active site in Desulfovibrio vulgaris Rubredoxin, the electronic coupling matrix element for electron self-exchange is defined for direct active-site contact (H0(DA)). Simple Beratan-Onuchic model is used to extend coupling over the complete surface of the protein to provide an understanding of probable electron-transfer pathways. Regions of similar coupling properties are grouped together to define a surface coupling map, which reveals that very efficient self-exchange occurs only within 4 sigma-bonds of the active site. Longer-range electron transfer cannot support the fast rates of electron self-exchange observed experimentally. Pathways directly through the two surface cysteinate ligands dominate, but surface-accessible amides hydrogen-bonded to the cysteinates also contribute significantly to the rate of electron self-exchange.

    View details for DOI 10.1021/ic0303320

    View details for Web of Science ID 000180870300012

    View details for PubMedID 12562183

  • Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 1. Photoelectron spectroscopic determination of electronic relaxation INORGANIC CHEMISTRY Kennepohl, P., SOLOMON, E. I. 2003; 42 (3): 679-688

    Abstract

    Electronic relaxation, the change in molecular electronic structure as a response to oxidation, is investigated in [FeX(4)](2)(-)(,1)(-) (X = Cl, SR) model complexes. Photoelectron spectroscopy, in conjunction with density functional methods, is used to define and evaluate the core and valence electronic relaxation upon ionization of [FeX(4)](2)(-). The presence of intense yet formally forbidden charge-transfer satellite peaks in the PES data is a direct reflection of electronic relaxation. The phenomenon is evaluated as a function of charge redistribution at the metal center (Deltaq(rlx)) resulting from changes in the electronic structure. This charge redistribution is calculated from experimental core and valence PES data using a valence bond configuration interaction (VBCI) model. It is found that electronic relaxation is very large for both core (Fe 2p) and valence (Fe 3d) ionization processes and that it is greater in [Fe(SR)(4)](2)(-) than in [FeCl(4)](2)(-). Similar results are obtained from DFT calculations. The results suggest that, although the lowest-energy valence ionization (from the redox-active molecular orbital) is metal-based, electronic relaxation causes a dramatic redistribution of electron density ( approximately 0.7?) from the ligands to the metal center corresponding to a generalized increase in covalency over all M-L bonds. The more covalent tetrathiolate achieves a larger Deltaq(rlx) because the LMCT states responsible for relaxation are significantly lower in energy than those in the tetrachloride. The large observed electronic relaxation can make significant contributions to the thermodynamics and kinetics of electron transfer in inorganic systems.

    View details for DOI 10.1021/ic020330f

    View details for Web of Science ID 000180870300010

    View details for PubMedID 12562181

  • Electronic structure contributions to electron-transfer reactivity in iron-sulfur active sites: 2. Reduction potentials INORGANIC CHEMISTRY Kennepohl, P., SOLOMON, E. I. 2003; 42 (3): 689-695

    Abstract

    This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl(4)](2)(-)(,1)(-) and [Fe(SR)(4)](2)(-)(,1)(-) (DeltaE(0) approximately 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (DeltaI(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to DeltaI(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to DeltaI(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I(vert) relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge () of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe)( )(3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases and significantly lowers its redox potential.

    View details for DOI 10.1021/ic0203318

    View details for Web of Science ID 000180870300011

    View details for PubMedID 12562182

  • Density-functional investigation on the mechanism of H-atom abstraction by lipoxygenase JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Lehnert, N., Solomon, E. I. 2003; 8 (3): 294-305

    Abstract

    Using experimentally calibrated density functional calculations on models of the active site of soybean lipoxygenase 1 (SLO-1), insight has been obtained into the coordination flexibility of the iron active site and its molecular mechanism of catalysis. The ferrous form of SLO-1 shows a variation in coordination number in solution that is related to a weakly coordinating Asn694 ligand. From the calculations it is determined that the weak Fe-O(694) bond associated with this coordination flexibility is due to a sideways tilted geometry of Asn694 that is imposed on the site by the protein. Release of this constraint (by altering the hydrogen bonding network) leads to a pure six-coordinate site. In contrast, the ferric form of the enzyme stays five-coordinate. In this case, deprotonation of a coordinated water gives a strong hydroxo donor in the cis position to Asn694, weakening the Fe-O(694) bond. Hence, Asn694 is a stronger ligand to the reduced relative to the oxidized site. Using these experimentally calibrated models, the reaction energy for H-atom transfer in SLO-1 has been calculated to be about -18 kcal/mol. The observed change in coordination number going from five-coordinate in ferric to six-coordinate in ferrous SLO-1 increases the reduction potential of the iron active site. Hence, the protein adjusts the active site for optimal reactivity. Analysis of the electronic structure along the reaction coordinate shows that the H-atom transfer in SLO-1 actually corresponds to a proton-coupled electron transfer (PCET). The transferred electron does not localize on the proton, but tunnels directly from the substrate to the ferric active site in a concerted proton tunneling-electron tunneling (PTET) process. The covalently linked Fe-O-H-C bridge in the transition state lowers the energy barrier and provides an efficient superexchange pathway for this tunneling. The thermal barrier for the PTET process is estimated from the calculations to be about +15 kcal/mol including zero-point energy corrections. This corresponds to a thermal reaction rate of k(therm) approximately 1 s(-1). In comparison, the rate of proton tunneling can be as high as 2 x 10(9) s(-1) under these conditions.

    View details for DOI 10.1007/s00775-002-0415-6

    View details for Web of Science ID 000181365200007

    View details for PubMedID 12589565

  • Spectroscopic studies of the effect of ligand donor strength on the Fe-NO bond in intradiol dioxygenases INORGANIC CHEMISTRY Wasinger, E. C., Davis, M. I., Pau, M. Y., Orville, A. M., Zaleski, J. M., Hedman, B., Lipscomb, J. D., Hodgson, K. O., SOLOMON, E. I. 2003; 42 (2): 365-376

    Abstract

    The geometric and electronic structure of NO bound to reduced protocatechuate 3,4-dioxygenase and its substrate (3,4-dihydroxybenzoate, PCA) complex have been examined by X-ray absorption (XAS), UV-vis absorption (Abs), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The results are compared to those previously published on model complexes described as [FeNO]7 systems in which an S = 5/2 ferric center is antiferromagnetically coupled to an S = 1 NO-. XAS pre-edge analysis indicates that the Fe-NO units in FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] lack the greatly increased pre-edge intensity representative of most [FeNO]7 model sites. Furthermore, from extended X-ray absorption fine structure (EXAFS) analysis, the FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] active sites are shown to have an Fe-NO distance of at least 1.91 A, approximately 0.2 A greater than those found in the model complexes. The weakened Fe-NO bond is consistent with the overall lengthening of the bond lengths and the fact that VTVH MCD data show that NO(-)-->FeIII CT transitions are no longer polarized along the z-axis of the zero-field splitting tensor. The weaker Fe-NO bond derives from the strong donor interaction of the endogenous phenolate and substrate catecholate ligands, which is observed from the increased intensity in the CT region relative to that of [FeNO]7 model complexes, and from the shift in XAS edge position to lower energy. As NO is an analogue of O2, the effect of endogenous ligand donor strength on the Fe-NO bond has important implications with respect to O2 activation by non-heme iron enzymes.

    View details for DOI 10.1021/ic025906f

    View details for Web of Science ID 000180594400017

    View details for PubMedID 12693216

  • Electronic structure and reactivity of high-spin iron-alkyl- and -pterinperoxo complexes INORGANIC CHEMISTRY Lehnert, N., Fujisawa, K., SOLOMON, E. I. 2003; 42 (2): 469-481

    Abstract

    The spectroscopic properties and electronic structure of the four-coordinate high-spin [FeIII(L3)(OOtBu)]+ complex (1; L3 = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate; tBu = tert-butyl) are investigated and compared to the six-coordinated high-spin [Fe(6-Me3TPA)(OHx)(OOtBu)]x+ system (TPA = tris(2-pyridylmethyl)amine, x = 1 or 2) studied earlier [Lehnert, N.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 12802-12816]. Complex 1 is characterized by Raman features at 889 and 830 cm-1 which are assigned to the O-O stretch (mixed with the symmetric C-C stretch) and a band at 625 cm-1 that corresponds to nu(Fe-O). The UV-vis spectrum shows a charge-transfer (CT) transition at 510 nm from the alkylperoxo pi v* (v = vertical to C-O-O plane) to a d orbital of Fe(III). A second CT is identified from MCD at 370 nm that is assigned to a transition from pi h* (h = horizontal to C-O-O plane) to an Fe(III) d orbital. For the TPA complex the pi v* CT is at 560 nm while the pi h* CT is to higher energy than 250 nm. These spectroscopic differences between four- and six-coordinate Fe(III)-OOR complexes are interpreted on the basis of their different ligand fields. In addition, the electronic structure of Fe-OOPtn complexes with the biologically relevant pterinperoxo ligand are investigated. Substitution of the tert-butyl group in 1 by pterin leads to the corresponding Fe(III)-OOPtn species (2), which shows a stronger electron donation from the peroxide to Fe(III) than 1. This is related to the lower ionization potential of pterin. Reduction of 2 by one electron leads to the Fe(II)-OOPtn complex (3), which is relevant as a model for potential intermediates in pterin-dependent hydroxylases. However, in the four-coordinate ligand field of 3, the additional electron is located in a nonbonding d orbital of iron. Hence, the pterinperoxo ligand is not activated for heterolytic cleavage of the O-O bond in this system. This is also evident from the calculated reaction energies that are endothermic by at least 20 kcal/mol.

    View details for DOI 10.1021/ic020496g

    View details for Web of Science ID 000180594400030

    View details for PubMedID 12693229

  • Spectroscopic and electronic structure studies of the diamagnetic side-on Cu-II-superoxo complex Cu(O-2)[HB(3-R-5-(i)Prpz)(3)]: Antiferromagnetic coupling versus covalent delocalization JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Root, D. E., Campochiaro, C., Fujisawa, K., SOLOMON, E. I. 2003; 125 (2): 466-474

    Abstract

    Magnetic, vibrational, and optical techniques are combined with density functional calculations to elucidate the electronic structure of the diamagnetic mononuclear side-on CuII-superoxo complex. The electronic nature of its lowest singlet/triplet states and the ground-state diamagnetism are explored. The triplet state is found to involve the interaction between the Cu xy and the superoxide pi v * orbitals, which are orthogonal to each other. The singlet ground state involves the interaction between the Cu xy and the in-plane superoxide pi v * orbitals, which have a large overlap and thus strong bonding. The ground-state singlet/triplet states are therefore fundamentally different in orbital origin and not appropriately described by an exchange model. The ground-state singlet is highly delocalized with no spin polarization.

    View details for DOI 10.1021/ja020969i

    View details for Web of Science ID 000180311800038

    View details for PubMedID 12517160

  • Distal metal effects in cobalt porphyrins related to CcO INORGANIC CHEMISTRY Collman, J. P., Berg, K. E., Sunderland, C. J., Aukauloo, A., Vance, M. A., SOLOMON, E. I. 2002; 41 (25): 6583-6596

    Abstract

    Cobalt(II) porphyrins were studied to determine the influence of distal site metalation and superstructure upon dioxygen reactivity in active site models of cytochrome c oxidase (CcO). Monometallic, Co(II)(P) complexes when ligated by an axial imidazole react with dioxygen to form reversible Co-superoxide adducts, which were characterized by EPR and resonance Raman (RR). Unexpectedly, certain Co porphyrins with Cu(I) metalated imidazole pickets do not form mu-peroxo Co(III)/Cu(II) products even though the calculated intermetallic distance suggests this is possible. Instead, cobalt-porphyrin-superoxide complexes are obtained with the distal copper remaining as Cu(I). Moreover, distal metals (Cu(I) or Zn(II)) greatly enhance the stability of the dioxygen adduct, such that Co superoxides of bimetallic complexes demonstrate minimal reversibility. The "trapping" of dioxygen by a second metal is attributed to structural and electrostatic changes within the distal pocket upon metalation. EPR evidence suggests that the terminal oxygen in these bimetallic Co-superoxide systems is H-bonded to the NH of an imidazole picket amide linker, which may contribute to enthalpic stabilization of the dioxygen adduct. Stabilization of the dioxygen adduct in these bimetallic systems suggests one possible role for the distal copper in the Fe/Cu bimetallic active site of terminal oxidases, which form a heme-superoxide/copper(I) adduct upon oxygenation.

    View details for DOI 10.1021/ic020395i

    View details for Web of Science ID 000179797700009

    View details for PubMedID 12470053

  • Electronic structure and reactivity of low-spin Fe(III)-hydroperoxo complexes: Comparison to activated bleomycin JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lehnert, N., Neese, F., Ho, R. Y., Que, L., Solomon, E. I. 2002; 124 (36): 10810-10822

    Abstract

    The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-hydroperoxo complex [Fe(N4Py)(OOH)](2+) (1, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) are investigated in comparison to those of activated bleomycin (ABLM). Complex 1 is characterized by Raman features at 632 (Fe-O stretch) and 790 cm(-1) (O-O stretch), corresponding to a strong Fe-O bond (force constant 3.62 mdyn/A) and a weak O-O bond (3.05 mdyn/A). The UV-vis spectrum of 1 shows a broad absorption band around 550 nm that is assigned to a charge-transfer transition from the hydroperoxo to a t(2g) d orbital of Fe(III) using resonance Raman and MCD spectroscopies and density functional (DFT) calculations. Compared to low-spin [Fe(TPA)(OH(x))(OO(t)Bu)](x+)(TPA = tris(2-pyridylmethyl)amine, x = 1 or 2), an overall similar Fe-OOR bonding results for low-spin Fe(III)-alkylperoxo and -hydroperoxo species. Correspondingly, both systems show similar reactivities and undergo homolytic cleavage of the O-O bond. From the DFT calculations, this reaction is more endothermic for 1 due to the reduced stabilization of the .OH radical compared to .O(t)Bu and the absence of the hydroxo ligand that helps to stabilize the resulting Fe(IV)=O species. In contrast, ABLM has a somewhat different electronic structure where no pi donor bond between the hydroperoxo ligand and iron(III) is present [Neese, F.; Zaleski, J. M.; Loeb-Zaleski, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11703]. Possible reaction pathways for ABLM are discussed in relation to known experimental results.

    View details for DOI 10.1021/ja012621d

    View details for Web of Science ID 000177872200039

    View details for PubMedID 12207537

  • Spectroscopic and electronic structure studies of the mu(4)-sulfide bridged tetranuclear Cu-z cluster in N2O reductase: Molecular insight into the catalytic mechanism JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., Cabrito, I., Moura, J. J., Moura, I., Solomon, E. I. 2002; 124 (35): 10497-10507

    Abstract

    Spectroscopic methods combined with density functional calculations are used to develop a detailed bonding description of the mu(4)-sulfide bridged tetranuclear Cu(Z) cluster in N(2)O reductase. The ground state of Cu(Z) has the 1Cu(II)/3Cu(I) configuration. The single electron hole dominantly resides on one Cu atom (Cu(I)) and partially delocalizes onto a second Cu atom (Cu(II)) via a Cu(I)-S-Cu(II) sigma/sigma superexchange pathway which is manifested by a Cu(II) --> Cu(I) intervalence transfer transition in absorption. The observed excited-state spectral features of Cu(Z) are dominated by the S --> Cu(I) charge-transfer transitions and Cu(I) based d-d transitions. The intensity pattern of individual S --> Cu(I) charge-transfer transitions reflects different bonding interactions of the sulfur valence orbitals with the four Cu's in the Cu(Z) cluster, which are consistent with the individual Cu-S force constants obtained from a normal coordinate analysis of the Cu(Z) resonance Raman frequencies and profiles. The Cu(I) d orbital splitting pattern correlates with its distorted T-shaped ligand field geometry and accounts for the observed low g( parallel ) value of Cu(Z) in EPR. The dominantly localized electronic structure description of the Cu(Z) site results from interactions of Cu(II) with the two additional Cu's of the cluster (Cu(III)/Cu(IV)), where the Cu-Cu electrostatic interactions lead to hole localization with no metal-metal bonding. The substrate binding edge of Cu(Z) has a dominantly oxidized Cu(I) and a dominantly reduced Cu(IV). The electronic structure description of Cu(Z) provides a strategy to overcome the reaction barrier of N(2)O reduction at this Cu(I)/Cu(IV) edge by simultaneous two-electron transfer to N(2)O in a bridged binding mode. One electron can be donated directly from Cu(IV) and the other from Cu(II) through the Cu(II)-S-Cu(I) sigma/sigma superexchange pathway. A frontier orbital scheme provides molecular insight into the catalytic mechanism of N(2)O reduction by the Cu(Z) cluster.

    View details for DOI 10.1021/ja0205028

    View details for Web of Science ID 000177881300047

    View details for PubMedID 12197752

  • A stabilized mu-eta(2):eta(2) peroxodicopper(II) complex with a secondary diamine ligand and its tyrosinase-like reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Mirica, L. M., Vance, M., Rudd, D. J., Hedman, B., Hodgson, K. O., SOLOMON, E. I., Stack, T. D. 2002; 124 (32): 9332-9333

    Abstract

    The activation of dioxygen (O(2)) by Cu(I) complexes is an ubiquitous process in biology and industrial applications. In tyrosinase, a binuclear copper enzyme, a mu-eta(2):eta(2)-peroxodicopper(II) species is generally accepted to be the active oxidant. Reported here is the characterization and reactivity of a stable mu-eta(2):eta(2)-peroxodicopper(II) complex at -80 degrees C using a secondary diamine ligand, N,N'-di-tert-butyl-ethylenediamine (DBED). The spectroscopic characteristics of this complex (UV-vis, resonance Raman) prove to be strongly dependent on the counteranion employed and not on the solvent, suggesting an intimate interaction of the counteranions with the Cu-O(2) cores. This interaction is also supported by solution EXAFS data. This new complex exhibits hydroxylation reactivity by converting phenolates to catechols, proving to be a functional model of tyrosinase. Additional interest in this Cu/O(2) species results from the use of Cu(I)-DBED as a polymerization catalyst of phenols to polyphenylene oxide (PPO) with O(2) as the terminal oxidant.

    View details for DOI 10.1021/ja026905p

    View details for Web of Science ID 000177358600005

    View details for PubMedID 12167002

  • Nature of the intermediate formed in the reduction of O-2 to H2O at the trinuclear copper cluster active site in native laccase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lee, S. K., George, S. D., Antholine, W. E., Hedman, B., Hodgson, K. O., Solomon, E. I. 2002; 124 (21): 6180-6193

    Abstract

    The multicopper oxidases contain at least four copper atoms and catalyze the four-electron reduction of O(2) to H(2)O at a trinuclear copper cluster. An intermediate, termed native intermediate, has been trapped by a rapid freeze-quench technique from Rhus vernicifera laccase when the fully reduced form reacts with dioxygen. This intermediate had been described as an oxygen-radical bound to the trinuclear copper cluster with one Cu site reduced. XAS, however, shows that all copper atoms are oxidized in this intermediate. A combination of EXAFS, multifrequency EPR, and VTVH MCD has been used to understand how this fully oxidized trinuclear Cu cluster relates to the fully oxidized resting form of the enzyme. It is determined that in the native intermediate all copper atoms of the cluster are bridged by the product of full O(2) reduction. In contrast, the resting form has one copper atom of the cluster (the T2 Cu) magnetically isolated from the others. The native intermediate decays to the resting oxidized form with a rate that is too slow to be in the catalytic cycle. Thus, the native intermediate appears to be the catalytically relevant fully oxidized form of the enzyme, and its role in catalysis is considered.

    View details for DOI 10.1021/ja0114052

    View details for Web of Science ID 000175781600044

    View details for PubMedID 12022853

  • Spectroscopic characterization and O-2 reactivity of the trinuclear Cu cluster of mutants of the multicopper oxidase Fet3p BIOCHEMISTRY Palmer, A. E., Quintanar, L., Severance, S., Wang, T. P., Kosman, D. J., Solomon, E. I. 2002; 41 (20): 6438-6448

    Abstract

    Fet3p is a multicopper oxidase that uses four copper ions (one type 1, one type 2, and one type 3 binuclear site) to couple substrate oxidation to the reduction of O(2) to H(2)O. The type 1 Cu site shuttles electrons between the substrate and the type 2/type 3 Cu sites which form a trinuclear Cu cluster that is the active site for O(2) reduction. This study extends the spectroscopic and reactivity studies that have been conducted with type 1-substituted Hg (T1Hg) laccase to Fet3p and a mutant of Fet3p in which the trinuclear Cu cluster is perturbed. To examine the reaction between the trinuclear Cu cluster and O(2), the type 1 Cu Cys(484) was mutated to Ser, resulting in a type 1-depleted (T1D) form of the enzyme. Additional His to Gln mutations were made at the trinuclear cluster to further probe specific contributions to reactivity. One of these mutants (His(126)Gln) produces the first stable but perturbed trinuclear Cu cluster (T1DT3' Fet3p). Spectroscopic characterization (absorption, circular dichroism, magnetic circular dichroism, and electron paramagnetic resonance) of the resting trinuclear sites in T1D and T1DT3' Fet3p reveal that the His(126)Gln mutation changes the electronic structure of both the type 3 and type 2 Cu sites. The trinuclear clusters in T1D and T1DT3' Fet3p react with O(2) to produce peroxide intermediates analogous to that observed in T1Hg laccase. Spectroscopic data on the peroxide intermediates in the three forms provide further insight into the structure of this intermediate. In T1D Fet3p, the decay of this peroxide intermediate is pH-dependent, and the rate of decay is 10-fold higher at low pH. In T1DT3' Fet3p, the decay of the peroxide intermediate is pH-independent and is slow at all pH's. This change in the pH dependence provides new insight into the mechanism of intermediate decay involving reductive cleavage of the O-O bond.

    View details for DOI 10.1021/bi011979j

    View details for Web of Science ID 000175651400027

    View details for PubMedID 12009907

  • X-ray absorption spectroscopic investigation of the resting ferrous and cosubstrate-bound active sites of phenylalanine hydroxylase BIOCHEMISTRY Wasinger, E. C., Mitic, N., Hedman, B., Caradonna, J., SOLOMON, E. I., Hodgson, K. O. 2002; 41 (20): 6211-6217

    Abstract

    Previous studies of ferrous wild-type phenylalanine hydroxylase, [Fe(2+)]PAH(T)[], have shown the active site to be a six-coordinate distorted octahedral site. After the substrate and cofactor bind to the enzyme ([Fe(2+)]PAH(R)[L-Phe,5-deaza-6-MPH(4)]), the active site converts to a five-coordinate square pyramidal structure in which the identity of the missing ligand had not been previously determined. X-ray absorption spectroscopy (XAS) at the Fe K-edge further supports this coordination number change with the binding of both cosubstrates to the enzyme, and determines this to be due to the loss of a water ligand.

    View details for DOI 10.1021/bi0121510

    View details for Web of Science ID 000175651400001

    View details for PubMedID 12009881

  • Spectroscopic studies of 1-aminocyclopropane-1-carboxylic acid oxidase: Molecular mechanism and CO2 activation in the biosynthesis of ethylene JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhou, J., Rocklin, A. M., Lipscomb, J. D., Que, L., Solomon, E. I. 2002; 124 (17): 4602-4609

    Abstract

    1-Aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO) catalyzes the last step in the biosynthesis of the gaseous plant hormone ethylene, which is involved in development, including germination, fruit ripening, and senescence. ACCO is a mononuclear non-heme ferrous enzyme that couples the oxidation of the cosubstrate ascorbate to the oxidation of substrate ACC by dioxygen. In addition to substrate and cosubstrate, ACCO requires the activator CO(2) for continuous turnover. NIR circular dichroism and magnetic circular dichroism spectroscopies have been used to probe the geometric and electronic structure of the ferrous active site in ACCO to obtain molecular-level insight into its catalytic mechanism. Resting ACCO/Fe(II) is coordinatively saturated (six-coordinate). In the presence of CO(2), one ferrous ligand is displaced to yield a five-coordinate site only when both the substrate ACC and cosubstrate ascorbate are bound to the enzyme. The open coordination position allows rapid O(2) activation for the oxidation of both substrates. In the absence of CO(2), ACC binding alone converts the site to five-coordinate, which would react with O(2) in the absence of ascorbate and quickly deactivate the enzyme. These studies show that ACCO employs a general strategy similar to other non-heme iron enzymes in terms of opening iron coordination sites at the appropriate time in the reaction cycle and define the role of CO(2) as stabilizing the six-coordinate ACCO/Fe(II)/ACC complex, thus preventing the uncoupled reaction that inactivates the enzyme.

    View details for DOI 10.1021/ja017250f

    View details for Web of Science ID 000175227600027

    View details for PubMedID 11971707

  • Electronic structure and its relation to function in copper proteins CURRENT OPINION IN CHEMICAL BIOLOGY Szilagyi, R. K., Solomon, E. I. 2002; 6 (2): 250-258

    Abstract

    Spectroscopic and theoretical investigations of the geometric and electronic structures of mononuclear and binuclear copper sites in proteins help in understanding the contributions of these proteins to biological electron transfer. Spectroscopically calibrated density functional theory calculations, which give reasonable bonding descriptions in both ground- and excited-states, define the role of the protein in determining the geometric and electronic structure of the active site.

    View details for Web of Science ID 000174821700020

    View details for PubMedID 12039012

  • Electronic structure description of the mu(4)-sulfide bridged tetranuclear Cu-z center in N2O reductase JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, P., George, S. D., Cabrito, I., Antholine, W. E., Moura, J. J., Moura, I., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2002; 124 (5): 744-745

    Abstract

    Spectroscopy coupled with density functional calculations has been used to define the spin state, oxidation states, spin distribution, and ground state wave function of the mu4-sulfide bridged tetranuclear CuZ cluster of nitrous oxide reductase. Initial insight into the electronic contribution to N2O reduction is developed, which involves a sigma superexchange pathway through the bridging sulfide.

    View details for DOI 10.1021/ja0169623

    View details for Web of Science ID 000173628900010

    View details for PubMedID 11817937

  • Frontier molecular orbital analysis of Cu-n-O-2 reactivity JOURNAL OF INORGANIC BIOCHEMISTRY Chen, P., Solomon, E. I. 2002; 88 (3-4): 368-374

    Abstract

    Frontier molecular orbital (FMO) theory coupled with density functional calculations has been applied to investigate the chemical reactivity of three key bioinorganic Cu(n)-O(2) complexes, the mononuclear end-on hydroperoxo-Cu(II), the side-on bridged mu-eta(2):eta(2)-O(2)(2-) Cu(II)(2) dimer and the bis-mu-oxo Cu(III)(2) dimer. Two acceptor orbitals (sigma* and pi*) of each complex and two types of donating substrates (sigma-substrate, phosphine; pi-substrate, alkylbenzene) are considered in the electrophilic attack mechanism. The angular dependences of different reaction pathways are determined using FMO theory and the angular overlap model. Including steric effects, the sigma*/sigma and pi*/pi pathways are found more reactive than the corresponding cross sigma*/pi and pi*/sigma pathways which have poor donor-acceptor orbital overlaps in the sterically constrained substrate access region.

    View details for Web of Science ID 000175319500018

    View details for PubMedID 11897352

  • Spectroscopic and electronic structure studies of protocatechuate 3,4-dioxygenase: Nature of tyrosinate-Fe(III) bonds and their contribution to reactivity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Davis, M. I., Orville, A. M., Neese, F., Zaleski, J. M., Lipscomb, J. D., SOLOMON, E. I. 2002; 124 (4): 602-614

    Abstract

    The geometric and electronic structure of the high-spin ferric active site of protocatechuate 3,4-dioxygenase (3,4-PCD) has been examined by absorption (Abs), circular dichroism (CD), magnetic CD (MCD), and variable-temperature-variable-field (VTVH) MCD spectroscopies. Density functional (DFT) and INDO/S-CI molecular orbital calculations provide complementary insight into the electronic structure of 3,4-PCD and allow an experimentally calibrated bonding scheme to be developed. Abs, CD, and MCD indicate that there are at least seven transitions below 35 000 cm(-1) which arise from tyrosinate ligand-to-metal-charge transfer (LMCT) transitions. VTVH MCD spectroscopy gives the polarizations of these LMCT bands in the principal axis system of the D-tensor, which is oriented relative to the molecular structure from the INDO/S-CI calculations. Three transitions are associated with the equatorial tyrosinate and four with the axial tyrosinate. This large number of transitions per tyrosinate is due to the pi and importantly the sigma overlap of the two tyrosinate valence orbitals with the metal d orbitals and is governed by the Fe-O-C angle and the Fe-O-C-C dihedral angles. The previously reported crystal structure indicates that the Fe-O-C angles are 133 degrees and 148 degrees for the equatorial and axial tyrosinate, respectively. Each tyrosinate has transitions at different energies with different intensities, which correlate with differences in geometry that reflect pseudo-sigma bonding to the Fe(III) and relate to reactivity. These factors reflect the metal-ligand bond strength and indicate that the axial tyrosinate-Fe(III) bond is weaker than the equatorial tyrosinate-Fe(III) bond. Furthermore, it is found that the differences in geometry, and hence electronic structure, are imposed by the protein. The consequences to catalysis are significant because the axial tyrosinate has been shown to dissociate upon substrate binding and the equatorial tyrosinate in the enzyme-substrate complex is thought to influence asymmetric binding of the chelated substrate moiety via a strong trans influence which activates the substrate for reaction with O2.

    View details for DOI 10.1021/ja011945z

    View details for Web of Science ID 000173456800022

    View details for PubMedID 11804491

  • X-ray absorption spectroscopic investigation of Fe(II)-peplomycin and peplomycin derivatives: the effect of axial ligation on Fe-pyrimidine back-bonding JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Wasinger, E. C., Zaleski, K. L., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2002; 7 (1-2): 157-164

    Abstract

    X-ray absorption spectroscopy (XAS) is used to study ferrous complexes of a bleomycin (BLM) congener, peplomycin (PEP), and two of its derivatives, iso-peplomycin (ISO) and depyruvamide peplomycin (DP), in which potential axial ligands have been perturbed and removed, respectively. Application of extended X-ray absorption fine structure analysis shows an elongation of the short-distance component of the first coordination sphere in DP and ISO relative to PEP. The XAS pre-edge intensity concomitantly decreases with increased axial perturbation. The short-distance component of PEP is correlated to the Fe-pyrimidine bond and is related to the amount of pi-back-bonding. Thus, the XAS analysis of these complexes provides structural information relevant to their differences in O2 reactivity.

    View details for DOI 10.1007/s007750100283

    View details for Web of Science ID 000173024100018

    View details for PubMedID 11862552

  • Electronic structure of high-spin iron(III)-alkylperoxo complexes and its relation to low-spin analogues: Reaction coordinate of O-O bond homolysis JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lehnert, N., Ho, R. Y., Que, L., SOLOMON, E. I. 2001; 123 (51): 12802-12816

    Abstract

    The spectroscopic properties of the high-spin Fe(III)-alkylperoxo model complex [Fe(6-Me(3)TPA)(OH(x))(OO(t)Bu)](x)(+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are defined and related to density functional calculations of corresponding models in order to determine the electronic structure and reactivity of this system. The Raman spectra of 1 show four peaks at 876, 842, 637, and 469 cm(-1) that are assigned with the help of normal coordinate analysis, and corresponding force constants have been determined to be 3.55 mdyn/A for the O-O and 2.87 mdyn/A for the Fe-O bond. Complex 1 has a broad absorption feature around 560 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t(2g) d orbital of Fe(III) with the help of resonance Raman profiles and MCD spectroscopy. An additional contribution to the Fe-O bond arises from a sigma interaction between and an e(g) d orbital of iron. The electronic structure of 1 is compared to the related low-spin model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x)(+) and the reaction coordinate for O-O homolysis is explored for both the low-spin and the high-spin Fe(III)-alkylperoxo systems. Importantly, there is a barrier for homolytic cleavage of the O-O bond on the high-spin potential energy surface that is not present for the low-spin complex, which is therefore nicely set up for O-O homolysis. This is reflected by the electronic structure of the low-spin complex having a strong Fe-O and a weak O-O bond due to a strong Fe-O sigma interaction. In addition, the reaction coordinate of the Fe-O homolysis has been investigated, which is a possible decay pathway for the high-spin system, but which is thermodynamically unfavorable for the low-spin complex.

    View details for DOI 10.1021/ja011450+

    View details for Web of Science ID 000172939700008

    View details for PubMedID 11749538

  • A new Cu(II) side-on peroxo model clarifies the assignment of the oxyhemocyanin Raman spectrum INORGANIC CHEMISTRY Henson, M. J., Mahadevan, V., Stack, T. D., SOLOMON, E. I. 2001; 40 (20): 5068-?

    View details for Web of Science ID 000171177500007

    View details for PubMedID 11559060

  • Spectroscopic properties and electronic structure of low-spin Fe(III)-alkylperoxo complexes: Homolytic cleavage of the O-O bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Lehnert, N., Ho, R. Y., Que, L., SOLOMON, E. I. 2001; 123 (34): 8271-8290

    Abstract

    The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-alkylperoxo model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O-O stretch (796 cm(-1)), the Fe-O stretch (696 cm(-)(1)), and a combined O-C-C/C-C-C bending mode (490 cm(-1)) that is mixed with upsilon(FeO). The corresponding force constants have been determined to be 2.92 mdyn/A for the O-O bond which is small and 3.53 mdyn/A for the Fe-O bond which is large. Complex 1 is characterized by a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo pi*(upsilon) to a t(2g) d orbital of Fe(III). This metal-ligand pi bond is probed by MCD and resonance Raman spectroscopies which show that the CT state is mixed with a ligand field state (t(2g) --> e(g)) by configuration interaction. This gives rise to two intense transitions under the broad 600 nm envelope with CT character which are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Raman profiles of the 796, 696, and 490 cm(-1) vibrations. Additional contributions to the Fe-O bond arise from sigma interactions between mainly O-O bonding donor orbitals of the alkylperoxo ligand and an e(g) d orbital of Fe(III), which explains the observed O-O and Fe-O force constants. The observed homolytic cleavage of the O-O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. The O-O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)=O species formed is highly stabilized (for spin states S = 1 and 2) by two strong pi and a strong sigma bond between Fe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting the O-O bond. In comparison, Cu(II)-alkylperoxo complexes studied before [Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 10177] are much less suited for O-O bond homolysis, because the resulting Cu(III)=O species is less stable. This difference in metal-oxo intermediate stability enables the O-O homolysis in the case of iron but directs the copper complex toward alternative reaction channels.

    View details for DOI 10.1021/ja010165n

    View details for Web of Science ID 000170730000013

    View details for PubMedID 11516278

  • Spectroscopic studies of substrate interactions with clavaminate synthase 2, a multifunctional alpha-KG-dependent non-heme iron enzyme: Correlation with mechanisms and reactivities JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Zhou, J., Kelly, W. L., Bachmann, B. O., Gunsior, M., TOWNSEND, C. A., SOLOMON, E. I. 2001; 123 (30): 7388-7398

    Abstract

    Using a single ferrous active site, clavaminate synthase 2 (CS2) activates O(2) and catalyzes the hydroxylation of deoxyguanidinoproclavaminic acid (DGPC), the oxidative ring closure of proclavaminic acid (PC), and the desaturation of dihydroclavaminic acid (and a substrate analogue, deoxyproclavaminic acid (DPC)), each coupled to the oxidative decarboxylation of cosubstrate, alpha-ketoglutarate (alpha-KG). CS2 can also catalyze an uncoupled decarboxylation of alpha-KG both in the absence and in the presence of substrate, which results in enzyme deactivation. Resting CS2/Fe(II) has a six-coordinate Fe(II) site, and alpha-KG binds to the iron in a bidentate mode. The active site becomes five-coordinate only when both substrate and alpha-KG are bound, the latter still in a bidentate mode. Absorption, CD, MCD, and VTVH MCD studies of the interaction of CS2 with DGPC, PC, and DPC provide significant molecular level insight into the structure/function correlations of this multifunctional enzyme. There are varying amounts of six-coordinate ferrous species in the substrate complexes, which correlate to the uncoupled reaction. Five-coordinate ferrous species with similar geometric and electronic structures are present for all three substrate/alpha-KG complexes. Coordinative unsaturation of the Fe(II) in the presence of both cosubstrate and substrate appears to be critical for the coupling of the oxidative decarboxylation of alpha-KG to the different substrate oxidation reactions. In addition to the substrate orientation relative to the open coordination position on the iron site, it is hypothesized that the enzyme can affect the nature of the reactivity by further regulating the binding energy of the water to the ferrous species in the enzyme/succinate/product complex.

    View details for DOI 10.1021/ja004025+

    View details for Web of Science ID 000170111600024

    View details for PubMedID 11472170

  • Invited award contribution for ACS Award in Inorganic Chemistry. Geometric and electronic structure contributions to function in bioinorganic chemistry: active sites in non-heme iron enzymes. Inorganic chemistry Solomon, E. I. 2001; 40 (15): 3656-3669

    Abstract

    Spectroscopy has played a major role in the definition of structure/function correlations in bioinorganic chemistry. The importance of spectroscopy combined with electronic structure calculations is clearly demonstrated by the non-heme iron enzymes. Many members of this large class of enzymes activate dioxygen using a ferrous active site that has generally been difficult to study with most spectroscopic methods. A new spectroscopic methodology has been developed utilizing variable temperature, variable field magnetic circular dichroism, which enables one to obtain detailed insight into the geometric and electronic structure of the non-heme ferrous active site and probe its reaction mechanism on a molecular level. This spectroscopic methodology is presented and applied to a number of key mononuclear non-heme iron enzymes leading to a general mechanistic strategy for O2 activation. These studies are then extended to consider the new features present in the binuclear non-heme iron enzymes and applied to understand (1) the mechanism of the two electron/coupled proton transfer to dioxygen binding to a single iron center in hemerythrin and (2) structure/function correlations over the oxygen-activating enzymes stearoyl-ACP Delta9-desaturase, ribonucleotide reductase, and methane monooxygenase. Electronic structure/reactivity correlations for O2 activation by non-heme relative to heme iron enzymes will also be developed.

    View details for PubMedID 11442362

  • Decay of the peroxide intermediate in laccase: Reductive cleavage of the O-O bond JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Palmer, A. E., Lee, S. K., SOLOMON, E. I. 2001; 123 (27): 6591-6599

    Abstract

    Laccase is a multicopper oxidase that contains four Cu ions, one type 1, one type 2, and a coupled binuclear type 3 Cu pair. The type 2 and type 3 centers form a trinuclear Cu cluster that is the active site for O(2) reduction to H(2)O. To examine the reaction between the type 2/type 3 trinuclear cluster and dioxygen, the type 1 Cu was removed and replaced with Hg(2+), producing the T1Hg derivative. When reduced T1Hg laccase is reacted with dioxygen, a peroxide intermediate (P) is formed. The present study examines the kinetics and mechanism of formation and decay of P in T1HgLc. The formation of P was found to be independent of pH and did not involve a kinetic solvent isotope effect, indicating that no proton is involved in the rate-determining step of formation of P. Alternatively, pH and isotope studies on the decay of P revealed that a proton enhances the rate of decay by 10-fold at low pH. This process shows an inverse k(H)/k(D) kinetic solvent isotope effect and involves protonation of a nearby residue that assists in catalysis, rather than direct protonation of the peroxide. Decay of P also involves a significant oxygen isotope effect (k(16)O(2)/k(18)O(2)) of 1.11 +/- 0.05, indicating that reductive cleavage of the O-O bond is the rate-determining step in the decay of P. The activation energy for this process was found to be approximately 9.0 kcal/mol. The exceptionally slow rate of decay of P is explained by the fact that this process involves a 1e(-) reductive cleavage of the O-O bond and there is a large Franck-Condon barrier associated with this process. Alternatively, the 2e(-) reductive cleavage of the O-O bond has a much larger driving force which minimizes this barrier and accelerates the rate of this reaction by approximately 10(7) in the native enzyme. This large difference in rate for the 2e(-) versus 1e(-) process supports a molecular mechanism for multicopper oxidases in which O(2) is reduced to H(2)O in two 2e(-) steps.

    View details for Web of Science ID 000169835300014

    View details for PubMedID 11439045

  • A quantitative description of the ground-state wave function of Cu-A by X-ray absorption spectroscopy: Comparison to plastocyanin and relevance to electron transfer JOURNAL OF THE AMERICAN CHEMICAL SOCIETY George, S. D., Metz, M., Szilagyi, R. K., Wang, H. X., Cramer, S. P., Lu, Y., Tolman, W. B., Hedman, B., Hodgson, K. O., Solomon, E. I. 2001; 123 (24): 5757-5767

    Abstract

    To evaluate the importance of the electronic structure of Cu(A) to its electron-transfer (ET) function, a quantitative description of the ground-state wave function of the mixed-valence (MV) binuclear Cu(A) center engineered into Pseudomonas aeruginosa azurin has been developed, using a combination of S K-edge and Cu L-edge X-ray absorption spectroscopies (XAS). Parallel descriptions have been developed for a binuclear thiolate-bridged MV reference model complex ([(L(i)(PrdacoS)Cu)(2)](+)) and a homovalent (II,II) analogue ([L(i)(Pr2tacnS)Cu)(2)](2+), where L(i)(PrdacoS) and L(i)(Pr2tacnS) are macrocyclic ligands with attached thiolates that bridge the Cu ions. Previous studies have qualitatively defined the ground-state wave function of Cu(A) in terms of ligand field effects on the orbital orientation and the presence of a metal--metal bond. The studies presented here provide further evidence for a direct Cu--Cu interaction and, importantly, experimentally quantify the covalency of the ground-state wave function. The experimental results are further supported by DFT calculations. The nature of the ground-state wave function of Cu(A) is compared to that of the well-defined blue copper site in plastocyanin, and the importance of this wave function to the lower reorganization energy and ET function of Cu(A) is discussed. This wave function incorporates anisotropic covalency into the intra- and intermolecular ET pathways in cytochrome c oxidase. Thus, the high covalency of the Cys--Cu bond allows a path through this ligand to become competitive with a shorter His path in the intramolecular ET from Cu(A) to heme a and is particularly important for activating the intermolecular ET path from heme c to Cu(A).

    View details for DOI 10.1021/ja004109i

    View details for Web of Science ID 000169338400020

    View details for PubMedID 11403610

  • Spectroscopy and reactivity of the type 1 copper site in Fet3p from Saccharomyces cerevisiae: Correlation of structure with reactivity in the multicopper oxidases JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Machonkin, T. E., Quintanar, L., Palmer, A. E., Hassett, R., Severance, S., Kosman, D. J., SOLOMON, E. I. 2001; 123 (23): 5507-5517

    Abstract

    Fet3p is a multicopper oxidase recently isolated from the yeast, Saccharomyces cerevisiae. Fet3p is functionally homologous to ceruloplasmin (Cp) in that both are ferroxidases. However, by sequence homology Fet3p is more similar to fungal laccase, and both contain a type 1 Cu site that lacks the axial methionine ligand present in the functional type 1 sites of Cp. To determine the contribution of the electronic structure of the type 1 Cu site of Fet3p to the ferroxidase mechanism, we have examined the absorption, circular dichroism, magnetic circular dichroism, electron paramagnetic resonance, and resonance Raman spectra of wild-type Fet3p and type 1 and type 2 Cu-depleted mutants. The spectroscopic features of the type 1 Cu site of Fet3p are nearly identical to those of fungal laccase, indicating a very similar three-coordinate geometry. We have also examined the reactivity of the type 1 Cu site by means of redox titrations and stopped-flow kinetics. From poised potential redox titrations, the E degrees of the type 1 Cu site is 427 mV, which is low for a three-coordinate type 1 Cu site. The kinetics of reduction of the type 1 Cu sites of four different multicopper oxidases with two different substrates were compared. The type 1 site of a plant laccase (Rhus vernicifera) is reduced moderately slowly by both Fe(II) and a bulky organic substrate, 1,4-hydroquinone (with 6 equiv of substrate, k(obs) = 0.029 and 0.013 s(-)(1), respectively). On the other hand, the type 1 site of a fungal laccase (Coprinus cinereus) is reduced very rapidly by both substrates (k(obs) > 23 s(-)(1)). In contrast, both Fet3p and Cp are rapidly reduced by Fe(II) (k(obs) > 23 s(-)(1)), but only very slowly by 1,4-hydroquinone (10- and 100-fold more slowly than plant laccase, respectively). Semiclassical theory is used to analyze the origin of these differences in reactivity in terms of type 1 Cu site accessibility to specific substrates.

    View details for Web of Science ID 000169176300018

    View details for PubMedID 11389633

  • Sulfur K-edge X-ray absorption spectroscopy of 2Fe-2S ferredoxin: Covalency of the oxidized and reduced 2Fe forms and comparison to model complexes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Anxolabehere-Mallart, E., Glaser, T., Frank, P., Aliverti, A., Zanetti, G., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2001; 123 (23): 5444-5452

    Abstract

    Ligand K-edge X-ray absorption spectroscopy (XAS) provides a direct experimental probe of ligand-metal bonding. In previous studies, this method has been applied to mononuclear Fe-S and binuclear 2Fe-2S model compounds as well as to rubredoxins and the Rieske protein. These studies are now extended to the oxidized and reduced forms of ferredoxin I from spinach. Because of its high instability, the mixed-valence state was generated electrochemically in the protein matrix, and ligand K-edge absorption spectra were recorded using an XAS spectroelectrochemical cell. The experimental setup is described. The XAS edge data are analyzed to independently determine the covalencies of the iron-sulfide and -thiolate bonds. The results are compared with those obtained previously for the Rieske protein and for 2Fe-2S model compounds. It is found that the sulfide covalency is significantly lower in oxidized FdI compared to that of the oxidized model complex. This decrease is interpreted in terms of H bonding present in the protein, and its contribution to the reduction potential E degrees is estimated. Further, a significant increase in covalency for the Fe(III)-sulfide bond and a decrease of the Fe(II)-sulfide bond are observed in the reduced Fe(III)Fe(II) mixed-valence species compared to those of the Fe(III)Fe(III) homovalent site. This demonstrates that, upon reduction, the sulfide interactions with the ferrous site decrease, allowing greater charge donation to the remaining ferric center. That is the dominant change in electronic structure of the Fe(2)S(2)RS(4) center upon reduction and can contribute to the redox properties of this active site.

    View details for Web of Science ID 000169176300010

    View details for PubMedID 11389625

  • Dioxygen binding to deoxyhemocyanin: Electronic structure and mechanism of the spin-forbidden two-electron reduction of O-2 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Metz, M., SOLOMON, E. I. 2001; 123 (21): 4938-4950

    Abstract

    Spectroscopically calibrated DFT is used to investigate the reaction coordinate of O(2) binding to Hemocyanin (Hc). A reaction path is calculated in which O(2) approaches the binuclear copper site with increasing metal-ligand overlap, which switches the coordination mode from end-on eta(1)-eta(1), to mu-eta(1):eta(2), then to butterfly, and finally to the planar [Cu(2)(mu-eta(2):eta(2)O(2))] structure. Analysis of the electronic structures during O(2) binding reveals that simultaneous two-electron transfer (ET) takes place. At early stages of O(2) binding the energy difference between the triplet and the singlet state is reduced by charge transfer (CT), which delocalizes the unpaired electrons and thus lowers the exchange stabilization onto the separated copper centers. The electron spins on the copper(II) ions are initially ferromagnetically coupled due to close to orthogonal magnetic orbital pathways through the dioxygen bridging ligand, and a change in the structure of the Cu(2)O(2) core turns on the superexchange coupling between the coppers. This favors the singlet state over the triplet state enabling intersystem crossing. Comparison with mononuclear model complexes indicates that the protein matrix holds the two copper(I) centers in close proximity, which enthalpically and entropically favors O(2) binding due to destabilization of the reduced binuclear site. This also allows regulation of the enthalpy by the change of the Cu--Cu distance in deoxyHc, which provides an explanation for the O(2) binding cooperativity in Hc. These results are compared to our earlier studies of Hemerythrin (Hr) and a common theme emerges where the spin forbiddeness of O(2) binding is overcome through delocalization of unpaired electrons onto the metal centers and the superexchange coupling of the metal centers via a ligand bridge.

    View details for Web of Science ID 000168914400009

    View details for PubMedID 11457321

  • Protein effects on the electronic structure of the [Fe4S4](2+) cluster in ferredoxin and HiPIP JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Glaser, T., Bertini, I., Moura, J. J., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2001; 123 (20): 4859-4860

    View details for DOI 10.1021/ja0155940

    View details for Web of Science ID 000168912000033

    View details for PubMedID 11457306

  • Recent advances in bioinorganic spectroscopy CURRENT OPINION IN CHEMICAL BIOLOGY Lehnert, N., George, S. D., Solomon, E. I. 2001; 5 (2): 176-187

    Abstract

    Spectroscopic methods covering many energy regions together provide complementary insight into metalloenzyme active sites. These methods probe geometric and electronic structure and define these contributions to reactivity. Two recent advances--determination of the polarizations of electronic transitions in solution using magnetic circular dichroism, electron paramagnetic resonance and quantum chemistry, and experimental estimation of covalency using metal L-edges and ligand K-edges--are particularly important.

    View details for Web of Science ID 000168200800012

    View details for PubMedID 11282345

  • SK-edge X-ray absorption studies of tetranuclear iron-sulfur clusters: mu-sulfide bonding and its contribution to electron delocalization JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Glaser, T., Rose, K., Shadle, S. E., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2001; 123 (3): 442-454

    Abstract

    X-ray absorption spectroscopy (XAS) at the sulfur ( approximately 2470 eV) and chlorine ( approximately 2822 eV) K-edges has been applied to a series of 4Fe-4S model complexes. These are compared to 2Fe-2S model complexes to obtain insight into the localized ground state in the mixed-valence dimer versus the delocalized ground state in the mixed-valence tetramer. The preedges of hypothetical delocalized mixed-valence dimers [Fe(2)S(2)](+) are estimated using trends from experimental data and density functional calculations, for comparison to the delocalized mixed-valence tetramer [Fe(4)S(4)](2+). The differences between these two mixed-valence sites are due to the change of the sulfide-bridging mode from micro(2) to micro(3). The terminal chloride and thiolate ligands are used as spectator ligands for the electron density of the iron center. From the intensity of the preedge, the covalency of the terminal ligands is found to increase in the tetramer as compared to the dimer. This is associated with a higher effective nuclear charge on the iron in the tetramer (derived from the energies of the preedge). The micro(3)-bridging sulfide in the tetramer has a reduced covalency per bond (39%) as compared to the micro(2)-bridging sulfide in the dimer (51%). A simple perturbation model is used to derive a quadratic dependence of the superexchange coupling constant J on the covalency of the metal ions with the bridging ligands. This relationship is used to estimate the superexchange contribution in the tetramer (J = -156 cm(-)(1)) as compared to the mixed-valence dimer (J = -360 cm(-)(1)). These results, combined with estimates for the double exchange and the vibronic coupling contributions of the dimer sub-site of the tetramer, lead to a delocalized S(t) = (9)/(2) spin ground state for the mixed-valence dimer in the tetramer. Thus, the decrease in the covalency, hence the superexchange pathway associated with changing the bridging mode of the sulfides from micro(2) to micro(3) on going from the dimer to the tetramer, significantly contributes to the delocalization of the excess electron over the dimer sub-site in the tetramer.

    View details for DOI 10.1021/ja002183v

    View details for Web of Science ID 000166698000011

    View details for PubMedID 11456546

  • Oxygen Binding, Activation, and Reduction to Water by Copper Proteins. Angewandte Chemie (International ed. in English) Solomon, E. I., Chen, P., Metz, M., Lee, S. K., Palmer, A. E. 2001; 40 (24): 4570-4590

    Abstract

    Copper active sites play a major role in biological and abiological dioxygen activation. Oxygen intermediates have been studied in detail for the proteins and enzymes involved in reversible O(2) binding (hemocyanin), activation (tyrosinase), and four-electron reduction to water (multicopper oxidases). These oxygen intermediates exhibit unique spectroscopic features indicative of new geometric and electronic structures involved in oxygen activation. The spectroscopic and quantum-mechanical study of these intermediates has defined geometric- and electronic-structure/function correlations, and developed detailed reaction coordinates for the reversible binding of O(2), hydroxylation, and H-atom abstraction from different substrates, and the reductive cleavage of the O-O bond in the formation water.

    View details for PubMedID 12404359

  • Ligand K-edge X-ray absorption spectroscopy: A direct probe of ligand-metal covalency ACCOUNTS OF CHEMICAL RESEARCH Glaser, T., Hedman, B., Hodgson, K. O., SOLOMON, E. I. 2000; 33 (12): 859-868

    Abstract

    Ligand K-edge X-ray absorption spectroscopy (XAS) is a new experimental probe of the covalency of a metal-ligand bond. The intensity of the ligand pre-edge feature is proportional to the mixing of ligand orbitals into the metal d orbitals. The methodology to determine covalencies in one-electron (hole) and many-electron systems is described and demonstrated for a series of metal tetrachlorides [MCl(4)](n)(-), metal tetrathiolates [M(SR)(4)](n)(-), and dimeric iron-sulfur (Fe-S) clusters [Fe(2)S(2)(SR)(4)](2-). It is then applied to blue Cu proteins, the Cu(A) site, hydrogen bonding in Fe-S clusters, and the delocalization behavior in [2Fe-2S] vs [4Fe-4S] clusters. The covalencies determined in these studies provide important electronic structure insight into function.

    View details for Web of Science ID 000166180700006

    View details for PubMedID 11123885

  • Electronic structure contributions to electron transfer in blue Cu and Cu-A JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY Randall, D. W., Gamelin, D. R., LaCroix, L. B., Solomon, E. I. 2000; 5 (1): 16-29

    Abstract

    The experimentally determined electronic structures of mononuclear blue Cu and binuclear Cu(A) centers are summarized and their relation to intra- and inter-protein electron transfer (ET) kinetics are described. Specific contributions of the electronic structures of these two broad classes of Cu ET proteins to H(AB), lambda, and deltaE degrees are discussed. Also, the role of the protein structure in determining key geometric features which define the electronic structures of the metal sites in these proteins is considered.

    View details for Web of Science ID 000085954900002

    View details for PubMedID 10766432

  • Geometric and electronic structure/function correlations in non-heme iron enzymes. Chemical reviews Solomon, E. I., Brunold, T. C., Davis, M. I., Kemsley, J. N., Lee, S. K., Lehnert, N., Neese, F., Skulan, A. J., Yang, Y. S., Zhou, J. 2000; 100 (1): 235-350

    View details for PubMedID 11749238

  • Investigation of the anomalous spectroscopic features of the copper sites in chicken ceruloplasmin: Comparison to human ceruloplasmin BIOCHEMISTRY Machonkin, T. E., Musci, G., Zhang, H. H., di Patti, M. C., Calabrese, L., Hedman, B., Hodgson, K. O., Solomon, E. I. 1999; 38 (34): 11093-11102

    Abstract

    Chicken ceruloplasmin has been previously reported to display a number of key differences relative to human ceruloplasmin: a lower copper content and a lack of a type 2 copper signal by electron paramagnetic resonance (EPR) spectroscopy. We have studied the copper sites of chicken ceruloplasmin in order to probe the origin of these differences, focusing on two forms of the enzyme: "resting" (as isolated by a fast, one-step procedure) and "peroxide-oxidized". From X-ray absorption, EPR, and UV/visible absorption spectroscopies, we have shown that all of the copper sites are oxidized in peroxide-oxidized chicken ceruloplasmin and that none of the type 1 copper sites display the EPR features typical for type 1 copper sites that lack an axial methionine. In the resting form, the type 2 copper center is reduced. Upon oxidation, it does not appear in the EPR spectrum at 77 K, but it can be observed by using magnetic susceptibility, EPR at approximately 8 K, and magnetic circular dichroism spectroscopy. It displays unusually fast relaxation, indicative of coupling with the adjacent type 3 copper pair of the trinuclear copper cluster. From reductive titrations, we have found that the reduction potential of the type 2 center is higher than those of the other copper sites, thus explaining why it is reduced in the resting form. These results provide new insight into the nature of the additional type 1 copper sites and the redox distribution among copper sites in the different ceruloplasmins relative to other multicopper oxidases.

    View details for Web of Science ID 000082342600021

    View details for PubMedID 10460165

  • MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S >/= (1)/(2). Applications to S = (1)/(2) and S = (5)/(2). Inorganic chemistry Neese, F., Solomon, E. I. 1999; 38 (8): 1847-1865

    Abstract

    The magnetic circular dichroism (MCD) properties of a spin-allowed transition from an orbitally nondegenerate ground state manifold A to an orbitally nondegenerate excited state manifold J in the presence of spin-orbit coupling (SOC) are derived for any S >/= (1)/(2). Three physically distinct mechanisms are identified that lead to MCD intensity and depend on SOC between excited states which leads to a sum rule and SOC between the ground state and other excited states that leads to deviations from the sum rule. The model is valid for any symmetry of the magnetic coupling tensors and arbitrary transition polarizations. The S = (1)/(2) case is analytically solved, and the determination of linear polarizations from MCD saturation magnetization data is discussed. For all mechanisms the MCD intensity is proportional to the spin-expectation values of the ground state sublevels which are conveniently generated from a spin-Hamiltonian (SH). For Kramers systems with large zero-field splittings (ZFSs) this allows the contribution from each Kramers doublet to the total MCD intensity to be related through their effective g-values, therefore significantly reducing the number of parameters required to analyze experimental data. The behavior of high-spin systems is discussed in the limits of weak, intermediate, and strong ZFS relative to the Zeeman energy. The model remains valid in the important case of intermediate ZFS where the ground state sublevels may cross as a function of applied magnetic field and there are significant off-axis contributions to the MCD intensity due to a change of the electron spin quantization axis. The model permits calculation of MCD C-term signs from molecular wave functions, and explicit expressions are derived in terms of MOs for S = (1)/(2) and S = (5)/(2). Two examples from the literature are analyzed to demonstrate how the C-term signs can be evaluated by a graphical method that gives insight into their physical origin.

    View details for PubMedID 11670957

  • X-ray Absorption Spectra of the Oxidized and Reduced Forms of C112D Azurin from Pseudomonas aeruginosa. Inorganic chemistry DeBeer, S., Kiser, C. N., Mines, G. A., Richards, J. H., Gray, H. B., Solomon, E. I., Hedman, B., Hodgson, K. O. 1999; 38 (3): 433-438

    Abstract

    The oxidized and reduced forms of a mutant of Pseudomonas aeruginosa azurin, in which the Cys112 has been replaced by an aspartate, have been studied by X-ray absorption spectroscopy. It is well established that the characteristic approximately 600 nm absorption feature of blue copper proteins is due to the S(Cys112) 3ppi --> Cu 3d(x)()()2(-)(y)()()2 charge-transfer transition. While other mutagenesis studies have involved the creation of an artificial blue copper site, the present work involves a mutant in which the native blue copper site has been destroyed, thus serving as a direct probe of the importance of the copper-thiolate bond to the spectroscopy, active site structure, and electron-transfer function of azurin. Of particular interest is the dramatic decrease in electron-transfer rates, both electron self-exchange (k(ese) approximately 10(5) M(-)(1) s(-)(1) wild-type azurin vs k(ese) approximately 20 M(-)(1) s(-)(1) C112D azurin) and intramolecular electron transfer to ruthenium-labeled sites (k(et) approximately 10(6) s(-)(1) wild-type azurin vs k(et)

    View details for PubMedID 11673945

  • Spectroscopic Investigation of Reduced Protocatechuate 3,4-Dioxygenase: Charge-Induced Alterations in the Active Site Iron Coordination Environment. Inorganic chemistry Davis, M. I., Wasinger, E. C., Westre, T. E., Zaleski, J. M., Orville, A. M., Lipscomb, J. D., Hedman, B., Hodgson, K. O., Solomon, E. I. 1999; 38 (16): 3676-3683

    Abstract

    Chemical reduction of the mononuclear ferric active site in the bacterial intradiol cleaving catecholic dioxygenase protocatechuate 3,4-dioxygenase (3,4-PCD, Brevibacterium fuscum) produces a high-spin ferrous center. We have applied circular dichroism (CD), magnetic circular dichroism (MCD), variable-temperature-variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies to investigate the geometric and electronic structure of the reduced iron center. Excited-state ligand field CD and MCD data indicate that the site is six-coordinate where the (5)E(g) excited-state splitting is 2033 cm(-)(1). VTVH MCD analysis of the ground state indicates that the site has negative zero-field splitting with a small rhombic splitting of the lowest doublet (delta = 1.6 +/- 0.3 cm(-)(1)). XAS pre-edge analysis also indicates a six-coordinate site while EXAFS analysis provides accurate bond lengths. Since previous spectroscopic analysis and the crystal structure of oxidized 3,4-PCD indicate a five-coordinate ferric active site, the results presented here show that the coordination number increases upon reduction. This is attributed to the coordination of a second solvent ligand. The coordination number increase relative to the oxidized site also appears to be associated with a large decrease in the ligand donor strength in the reduced enzyme due to protonation of the original hydroxide ligand.

    View details for PubMedID 11671125

  • Relationship between the Dipole Strength of Ligand Pre-Edge Transitions and Metal-Ligand Covalency. Inorganic chemistry Neese, F., Hedman, B., Hodgson, K. O., Solomon, E. I. 1999; 38 (21): 4854-4860

    Abstract

    The electric dipole contributions to the observed pre-edge intensities in ligand K-edge X-ray absorption (XAS) spectra are analyzed in terms of covalent-bonding contributions between the metal and ligand for a prototype system with one hole in the d shell. One- and two-center contributions to the intensity are identified. By direct evaluation of the integrals involved in the intensity expression, the two-center terms are shown to be at least 1 order of magnitude smaller than the one-center terms and can be ignored to a reasonable approximation. The one-center terms reflect the amount of ligand character in the partially occupied metal-based MOs and are proportional to the intrinsic transition moment of a ligand-centered 1s --> np transition. The final intensity does not contain terms proportional to the square of the metal-ligand distance as might have been expected on the basis of the analogy between ligand K-edge and ligand-to-metal charge transfer (LMCT) transitions that both formally lead to transfer of electron density from the ligand to the metal. This is due to the fact that the transition density is completely localized on the ligand in the case of a ligand K-edge transition but is delocalized over the metal and the ligand in the case of a LMCT transition. The effective nuclear charge dependence of the one-center transition moment integral was studied by Hartree-Fock level calculations and was found to be small. Electronic relaxation effects were considered and found to be small from a Hartree-Fock calculation on a cupric chloride model.

    View details for PubMedID 11671216

  • Spectroscopic and magnetic studies of human ceruloplasmin: Identification of a redox-inactive reduced Type 1 copper site BIOCHEMISTRY Machonkin, T. E., Zhang, H. H., Hedman, B., Hodgson, K. O., Solomon, E. I. 1998; 37 (26): 9570-9578

    Abstract

    Ceruloplasmin is unique among the multicopper oxidases in that in addition to the usual copper stoichiometry of one Type 1 copper site and a Type 2/Type 3 trinuclear copper cluster, it contains two other Type 1 sites. This assignment of copper sites, based on copper quantitation, sequence alignment, and crystallography, is difficult to reconcile with the observed spectroscopy. Furthermore, some chemical or spectroscopic differences in ceruloplasmin have been reported depending on the method of purification. We have studied the resting (as isolated by a fast, one-step procedure) and peroxide-oxidized forms of human ceruloplasmin. Using a combination of X-ray absorption spectroscopy, a chemical assay, magnetic susceptibility, electron paramagnetic resonance spectroscopy, and absorption spectroscopy, we have determined that peroxide-oxidized ceruloplasmin contains one permanently reduced Type 1 site. This site is shown to have a reduction potential of approximately 1.0 V. Thus, one of the additional Type 1 sites in ceruloplasmin cannot be catalytically relevant in the form of the enzyme studied. Furthermore, the resting form of the enzyme contains an additional reducing equivalent, which is distributed among the remaining five copper sites as expected from their relative potentials. This may indicate that the resting form of ceruloplasmin in plasma under aerobic conditions is a four-electron oxidized form, which is consistent with its function in the four-electron reduction of dioxygen to water.

    View details for Web of Science ID 000074585100040

    View details for PubMedID 9649340

  • Calculation of Zero-Field Splittings, g-Values, and the Relativistic Nephelauxetic Effect in Transition Metal Complexes. Application to High-Spin Ferric Complexes. Inorganic chemistry Neese, F., Solomon, E. I. 1998; 37 (26): 6568-6582

    Abstract

    Equations are derived and discussed that allow the computation of zero-field splitting (ZFS) tensors in transition metal complexes for any value of the ground-state total spin S. An effective Hamiltonian technique is used and the calculation is carried to second order for orbitally nondegenerate ground states. The theory includes contributions from excited states of spin S and S +/- 1. This makes the theory more general than earlier treatments. Explicit equations are derived for the case where all states are well described by single-determinantal wave functions, for example restricted open shell Hartree-Fock (HF) and spin-polarized HF or density functional (DFT) calculation schemes. Matrix elements are evaluated for many electron wave functions that result from a molecular orbital (MO) treatment including configuration interaction (CI). A computational implementation in terms of bonded functions is outlined. The problem of ZFS in high-spin ferric complexes is treated at some length, and contributions due to low-symmetry distortions, anisotropic covalency, charge-transfer states, and ligand spin-orbit coupling are discussed. ROHF-INDO/S-CI results are presented for FeCl(4)(-) and used to evaluate the importance of the various terms. Finally, contributions to the experimentally observed reduction of the metal spin-orbit coupling constants (the relativistic nephelauxetic effect) are discussed. B3LYP and Hartree-Fock calculations for FeCl(4)(-) are used to characterize the change of the iron 3d radial function upon complex formation. It is found that the iron 3d radial distribution function is significantly expanded and that the expansion is anisotropic. This is interpreted as a combination of reduction in effective charge on the metal 3d electrons (central field covalence) together with expansive promotion effects that are a necessary consequence of chemical bond formation. The (3d) values that are important in the interpretation of magnetic data are up to 15% reduced from their free-ion value before any metal-ligand orbital mixing (symmetry-restricted covalency) is taken into account. Thus the use of free-ion values for spin-orbit coupling and related constants in the analysis of experimental data leads to values for MO coefficients that overestimate the metal-ligand covalency.

    View details for PubMedID 11670788

  • Effect of Protonation on Peroxo-Copper Bonding: Spectroscopic and Electronic Structure Study of [Cu(2)((UN-O-)(OOH)](2+). Inorganic chemistry Root, D. E., Mahroof-Tahir, M., Karlin, K. D., Solomon, E. I. 1998; 37 (19): 4838-4848

    Abstract

    Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.

    View details for PubMedID 11670647

  • New insights from spectroscopy into the structure/function relationships of lipoxygenases CHEMISTRY & BIOLOGY SOLOMON, E. I., Zhou, J., Neese, F., Pavel, E. G. 1997; 4 (11): 795-808

    Abstract

    Spectroscopic properties of the redox-active iron in the active site of plant and mammalian lipoxygenases can now be combined with recent crystal structure determinations to obtain new insights into lipoxygenase reaction mechanisms.

    View details for Web of Science ID A1997YJ93500003

    View details for PubMedID 9384534

  • Trinuclear intermediate in the copper-mediated reduction of O-2: Four electrons from three coppers SCIENCE Cole, A. P., Root, D. E., Mukherjee, P., SOLOMON, E. I., Stack, T. D. 1996; 273 (5283): 1848-1850

    Abstract

    The reaction of metal complexes with dioxygen (O2) generally proceeds in 1:1, 21, or 41 (metal:O2) stoichiometry. A discrete, structurally characterized 31 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O2 bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O2 at the trinuclear active sites of multicopper oxidases is discussed.

    View details for Web of Science ID A1996VJ71300041

    View details for PubMedID 8791587

  • Structural and Functional Aspects of Metal Sites in Biology. Chemical reviews Holm, R. H., Kennepohl, P., Solomon, E. I. 1996; 96 (7): 2239-2314

    View details for PubMedID 11848828

  • Multicopper Oxidases and Oxygenases. Chemical reviews Solomon, E. I., Sundaram, U. M., Machonkin, T. E. 1996; 96 (7): 2563-2606

    View details for PubMedID 11848837

  • Excited-State Distortions and Electron Delocalization in Mixed-Valence Dimers: Vibronic Analysis of the Near-IR Absorption and Resonance Raman Profiles of [Fe(2)(OH)(3)(tmtacn)(2)](2+). Inorganic chemistry Gamelin, D. R., Bominaar, E. L., Mathonière, C., Kirk, M. L., Wieghardt, K., Girerd, J. J., Solomon, E. I. 1996; 35 (15): 4323-4335

    Abstract

    The near-IR transition associated with valence delocalization in the class III mixed-valence dimer [Fe(2)(OH)(3)(tmtacn)(2)](2+) is studied using variable-temperature (VT) electronic absorption and resonance Raman (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption band leads to dominant resonance Raman enhancement of totally-symmetric [Fe(2)(OH)(3)](2+) core vibrational modes at 316 and 124 cm(-)(1), descriptions of which are calculated from a normal coordinate analysis. Vibronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provides a description of the geometric distortions accompanying this excitation. The excited-state distortion is dominated by expansion of the [Fe(2)(OH)(3)](2+) core along the Fe.Fe axis, reflecting the significant Fe-Fe sigma --> sigma character of this transition. The ground-state sigma-interaction between the two metals has been identified as the orbital pathway for valence delocalization, and the sigma --> sigma distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, H(AB), associated with this pathway. The dominant role of totally-symmetric nuclear coordinates in the absorption and RR spectroscopies of [Fe(2)(OH)(3)(tmtacn)(2)](2+) is also discussed in relation to the Q(-) vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q(-) coordinate to the absorption and RR spectra of [Fe(2)(OH)(3)(tmtacn)(2)](2+) are small relative to those of the totally-symmetric coordinates due to the inefficient change-in-curvature mechanism by which the Q(-) coordinate gains intensity, compared to the efficient excited-state displacement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q(-) coordinate over other totally-symmetric coordinates observed in intervalence transfer (IT) absorption and RR spectroscopies of class II mixed-valence complexes.

    View details for PubMedID 11666647

  • BIOINORGANIC SPECTROSCOPY BIOCHEMICAL SPECTROSCOPY SOLOMON, E. I., Kirk, M. L., Gamelin, D. R., PULVER, S. 1995; 246: 71-110

    View details for Web of Science ID A1995BC90V00005

    View details for PubMedID 7752944

  • Magnetic circular dichroism studies of exogenous ligand and substrate binding to the non-heme ferrous active site in phthalate dioxygenase. Chemistry & biology Pavel, E. G., Martins, L. J., ELLIS, W. R., SOLOMON, E. I. 1994; 1 (3): 173-183

    Abstract

    Mononuclear non-heme iron centers are found in the active sites of a variety of enzymes that require molecular oxygen for catalysis. The mononuclear non-heme iron is believed to be the active site for catalysis, and is presumed to bind and activate molecular oxygen. The mechanism of this reaction is not understood. Phthalate dioxygenase is one such enzyme. Because it also contains a second iron site, the Rieske site, it is difficult to obtain information on the structure of the active site. We therefore used magnetic circular dichroism (MCD) spectroscopy to probe the mononuclear, non-heme Fe2+ site in this biodegradative enzyme.The MCD spectrum of the resting enzyme shows features indicative of one six-coordinate Fe2+ site; substrate binding converts the site to two different five-coordinate species, opening up a coordination position for O2 binding. MCD spectra of the corresponding apoenzyme have been subtracted to account for temperature-independent contributions from the Rieske site. Azide binds both to the resting enzyme to produce a new six-coordinate species, showing that one of the ferrous ligands is exchangeable, and also to the enzyme-substrate complex to form a ternary species. The low azide binding constant for the substrate-enzyme species relative to the resting enzyme indicates steric interaction and close proximity between exogenous ligand and the substrate.We have been able to provide some detailed structural insight into exogenous ligand and substrate binding to the non-heme Fe2+ site, even in the presence of the enzyme's [2Fe-2S] Rieske center. Further mechanistic studies are now required to maximize the molecular-level detail available from these spectroscopic studies.

    View details for PubMedID 9383387

  • ELECTRONIC-STRUCTURE CONTRIBUTIONS TO FUNCTION IN BIOINORGANIC CHEMISTRY SCIENCE SOLOMON, E. I., LOWERY, M. D. 1993; 259 (5101): 1575-1581

    Abstract

    Many metalloenzymes exhibit distinctive spectral features that are now becoming well understood. These reflect active site electronic structures that can make significant contributions to catalysis. Copper proteins provide well-characterized examples in which the unusual electronic structures of their active sites contribute to rapid, long-range electron transfer reactivity, oxygen binding and activation, and the multielectron reduction of dioxygen to water.

    View details for Web of Science ID A1993KR54300029

    View details for PubMedID 8384374

  • ELECTRONIC ABSORPTION-SPECTROSCOPY OF COPPER PROTEINS METALLOBIOCHEMISTRY, PART C SOLOMON, E. I., LOWERY, M. D., LaCroix, L. B., Root, D. E. 1993; 226: 1-33

    Abstract

    We have seen from the previous discussion that absorption spectral studies in the ligand field region probe the energy splittings of the d orbitals and that this relates to the geometry of the metal center. The energies and intensities of ligand-to-metal charge transfer transitions sensitively probe bonding interactions of the ligand with the metal center. Charge transfer transitions can be used both qualitatively to observe ligand binding to a metal center, owing to the requirement of orbital overlap for significant charge transfer intensity, and quantitatively to define the electron donor ability of that ligand and experimentally evaluate the results of electronic structure calculations. Studies of the intensities of peaks at the ligand K edge can define the covalent interaction of the ligand with the metal valence orbitals, whereas copper K-edge spectroscopy is a powerful probe of metal ion oxidation state and the ligand field geometry of d10 cuprous sites that are inaccessible through other spectroscopic methods. Absorption spectral studies in all regions are strongly complemented by CD, variable temperature MCD, and single-crystal polarized absorption spectroscopies, which should also be pursued whenever possible to obtain detailed electronic structural insight of relevance to catalysis.

    View details for Web of Science ID A1993BZ58Z00001

    View details for PubMedID 8277862

  • Coupled binuclear copper proteins: catalytic mechanisms and structure-reactivity correlations. Progress in clinical and biological research SOLOMON, E. I. 1988; 274: 309-329

    View details for PubMedID 3136462

  • ELECTRON-PARAMAGNETIC RESONANCE STUDIES OF THE TUNGSTEN-CONTAINING FORMATE DEHYDROGENASE FROM CLOSTRIDIUM-THERMOACETICUM BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Deaton, J. C., SOLOMON, E. I., Watt, G. D., WETHERBEE, P. J., DURFOR, C. N. 1987; 149 (2): 424-430

    Abstract

    The redox centers in the tungsten-containing formate dehydrogenase from Clostridium thermoaceticum were examined by potentiometric titration and electron paramagnetic resonance spectroscopy. At low temperature two overlapping iron-sulfur signals which correlated with enzymatic activity were observed with formal potentials near -400 mV vs. SHE. Based on their temperature dependences, one signal is assigned to a reduced Fe2S2 cluster and one to a reduced Fe4S4 cluster. Quantitation of signal intensity suggests two Fe2S2 and two Fe4S4 clusters per formate dehydrogenase molecule. Another signal (g = 2.101, 1.980, 1.950) present in low concentrations at more negative potentials was observable up to 200 degrees K and is not attributed to any iron-sulfur cluster. The possible origin of this signal is analyzed using ligand field theory, and the redox behavior is considered with respect to possible ligation at the active site.

    View details for Web of Science ID A1987L261800016

    View details for PubMedID 2827642

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