Nature of the Oxomolybdenum−Thiolate π-Bond: Implications for Mo−S Bonding in Sulfite Oxidase and Xanthine Oxidase

Second-Coordination-Sphere Effects Reveal Electronic Structure Differences between the Mitochondrial Amidoxime Reducing Component and Sulfite Oxidase

A combination of X-ray absorption and low-temperature electronic absorption spectroscopies has been used to probe the geometric and electronic structures of the human mitochondrial amidoxime reducing component enzyme (hmARC1) in the oxidized Mo(VI) and reduced Mo(IV) forms. Extended X-ray absorption fine structure analysis revealed that oxidized enzyme possesses a 5-coordinate [MoO2(SCys)(PDT)]- (PDT = pyranopterin dithiolene) active site with a cysteine coordinated to Mo. A 5-coordinate geometry is retained in the reduced state, with the equatorial oxo being protonated. Low-temperature electronic absorption spectroscopy of hmARC1 reveals a spectrum for the oxidized enzyme that is significantly different from what has been reported for sulfite oxidase family enzymes. Time-dependent density functional theory computations on oxidized and reduced hmARC1, and a small molecule analogue for hmARC1ox, have been used to assist us in making detailed band assignments and developing a greater understanding of enzyme electronic structure contributions to reactivity. Our understanding of the hmARCred HOMO and the LUMO of the benzamidoxime substrate reveal a potential π-bonding interaction between these redox orbitals, with two-electron occupation of the substrate LUMO along the reaction coordinate activating the O-N bond for cleavage and promoting oxygen atom transfer to the Mo site.

Resonance Raman spectroscopy of pyranopterin molybdenum enzymes

Resonance Raman spectroscopy (rR) is a powerful spectroscopic probe that is widely used for studying the geometric and electronic structure of metalloproteins. In this focused review, we detail how resonance Raman spectroscopy has contributed to a greater understanding of electronic structure, geometric structure, and the reaction mechanisms of pyranopterin molybdenum enzymes. The review focuses on the enzymes sulfite oxidase (SO), dimethyl sulfoxide reductase (DMSOR), xanthine oxidase (XO), and carbon monoxide dehydrogenase. Specifically, we highlight how Mo-Ooxo, Mo-Ssulfido, Mo-Sdithiolene, and dithiolene CC vibrational modes, isotope and heavy atom perturbations, resonance enhancement, and associated Raman studies of small molecule analogs have provided detailed insight into the nature of these metalloenzyme active sites.

Protonation and Non-Innocent Ligand Behavior in Pyranopterin Dithiolene Molybdenum Complexes

The complex [TEA][Tp*MoIV(O)(S2BMOPP)] (1) [TEA = tetraethylammonium, Tp* = tris(3,5-dimethylpyrazolyl)hydroborate, and BMOPP = 6-(3-butynyl-2-methyl-2-ol)-2-pivaloyl pterin] is a structural analogue of the molybdenum cofactor common to all pyranopterin molybdenum enzymes because it possesses a pyranopterin-ene-1,2-dithiolate ligand (S2BMOPP) that exists primarily in the ring-closed pyrano structure as a resonance hybrid of ene-dithiolate and thione-thiolate forms. Compound 1, the protonated [Tp*MoIV(O)(S2BMOPP-H)] (1-H) and one-electron-oxidized [Tp*MoV(O)(S2BMOPP)] [1-Mo(5+)] species have been studied using a combination of electrochemistry, electronic absorption, and electron paramagnetic resonance (EPR) spectroscopy. Additional insight into the nature of these molecules has been derived from electronic structure computations. Differences in dithiolene C-S bond lengths correlate with relative contributions from both ene-dithiolate and thione-thiolate resonance structures. Upon protonation of 1 to form 1-H, large spectroscopic changes are observed with transitions assigned as Mo(xy) → pyranopterin metal-to-ligand charge transfer and dithiolene → pyranopterin intraligand charge transfer, respectively, and this underscores a dramatic change in electronic structure between 1 and 1-H. The changes in electronic structure that occur upon protonation of 1 are also reflected in a large >300 mV increase in the Mo(V/IV) redox potential for 1-H, resulting from the greater thione-thiolate resonance contribution and decreased charge donation that stabilize the Mo(IV) state in 1-H with respect to one-electron oxidation. EPR spin Hamiltonian parameters for one-electron-oxidized 1-Mo(5+) and uncyclized [Tp*MoV(O)(S2BDMPP)] [3-Mo(5+)] [BDMPP = 6-(3-butynyl-2,2-dimethyl)-2-pivaloyl pterin] are very similar to each other and to those of [Tp*MoVO(bdt)] (bdt = 1,2-ene-dithiolate). This indicates that the dithiolate form of the ligand dominates at the Mo(V) level, consistent with the demand for greater S → Mo charge donation and a corresponding increase in Mo-S covalency as the oxidation state of the metal is increased. Protonation of 1 represents a simple reaction that models how the transfer of a proton from neighboring acidic amino acid residues to the Mo cofactor at a nitrogen atom within the pyranopterin dithiolene (PDT) ligand in pyranopterin molybdenum enzymes can impact the electronic structure of the Mo-PDT unit. This work also illustrates how pyran ring-chain tautomerization drives changes in resonance contributions to the dithiolene chelate and may adjust the reduction potential of the Mo ion.

Iron and Zinc Porphyrin Linked MoO(dithiolene) Complexes in Relevance to Electron Transfer between Mo-cofactor and Cytochrome b5 in Sulfite Oxidase

Oxo-molybdenum (dithiolene) complex covalently linked individually to iron and zinc porphyrin have been synthesized to show an electron transfer between the two metal centres in relevance to electron transfer from...

Addressing Ligand-Based Redox in Molybdenum-Dependent Methionine Sulfoxide Reductase

A combination of pulsed EPR, CW EPR, and X-ray absorption spectroscopies has been employed to probe the geometric and electronic structure of the E. coli periplasmic molybdenum-dependent methionine sulfoxide reductase (MsrP). ¹⁷O and ¹H pulsed EPR spectra show that the as-isolated Mo(V) enzyme form does not possess an exchangeable H₂O/OH^- ligand bound to Mo as found in the sulfite oxidizing enzymes of the same family. The nature of the unusual CW EPR spectrum has been re-evaluated in light of new data on the MsrP-N45R variant and related small-molecule analogues of the active site. These data point to a novel "thiol-blocked" [(PDT)Mo^VO(S_Cys)(thiolate)]^- structure, which is supported by new EXAFS data. We discuss these new results in the context of ligand-based and metal-based redox chemistry in the enzymatic oxygen atom transfer reaction.

Tannic acid modified MoS2 nanosheet membranes with superior water flux and ion/dye rejection

Energy-efficient membranes are urgently needed for water desalination and separation due to ever-increasing demand for fresh water. However, it is extremely challenging to increase membrane water flux and simultaneously achieve high rejection rates of cations or organic dyes. Herein, we report a tannic acid (TA) assisted exfoliation method to fabricate TA-modified MoS₂ (TAMoS₂) nanosheets with high production yield (90 ± 5%). The TAMoS₂ nanosheets membranes show excellent non-swelling stability in water. It is found that a hybrid membrane with 1 wt% of TAMoS₂ in MoS₂ nanosheets demonstrates overall better performance than pure MoS₂ and TAMoS₂ membrane. Such a hybrid membrane with a thickness of 5 µm shows fast water flux at around 32 L m^-2 h^-1 (LMH) and >97% rejection of various cations under static diffusion mode. Under vacuum-driven filtration condition, the as-prepared hybrid membrane demonstrates ultrafast water flux of 15,000 ± 100 L/(m² h bar) and 99.87 ± 0.1% rejection of multiple model organic dyes. To the best of our knowledge, the above performances are superior to those of all MoS₂-based membranes reported previously in terms of water flux and ion/dye rejection. This work represents a leap forward towards the practical applications of 2D TAMoS₂ membranes in various engineering and environmental areas.

Vibrational Probes of Molybdenum Cofactor-Protein Interactions in Xanthine Dehydrogenase

The pyranopterin dithiolene (PDT) ligand is an integral component of the molybdenum cofactor (Moco) found in all molybdoenzymes with the sole exception of nitrogenase. However, the roles of the PDT in catalysis are still unknown. The PDT is believed to be bound to the proteins by an extensive hydrogen-bonding network, and it has been suggested that these interactions may function to fine-tune Moco for electron- and atom-transfer reactivity in catalysis. Here, we use resonance Raman (rR) spectroscopy to probe Moco-protein interactions using heavy-atom congeners of lumazine, molecules that bind tightly to both wild-type xanthine dehydrogenase (wt-XDH) and its Q102G and Q197A variants following enzymatic hydroxylation to the corresponding violapterin product molecules. The resulting enzyme-product complexes possess intense near-IR absorption, allowing high-quality rR spectra to be collected on wt-XDH and the Q102G and Q197A variants. Small negative frequency shifts relative to wt-XDH are observed for the low-frequency Moco vibrations. These results are interpreted in the context of weak hydrogen-bonding and/or electrostatic interactions between Q102 and the -NH₂ terminus of the PDT, and between Q197 and the terminal oxo of the Mo≡O group. The Q102A, Q102G, Q197A, and Q197E variants do not appreciably affect the kinetic parameters k_red and k_red/K_D, indicating that a primary role for these glutamine residues is to stabilize and coordinate Moco in the active site of XO family enzymes but to not directly affect the catalytic throughput. Raman frequency shifts between wt-XDH and its Q102G variant suggest that the changes in the electron density at the Mo ion that accompany Mo oxidation during electron-transfer regeneration of the catalytically competent active site are manifest in distortions at the distant PDT amino terminus. This implies a primary role for the PDT as a conduit for facilitating enzymatic electron-transfer reactivity in xanthine oxidase family enzymes.

Large Ligand Folding Distortion in an Oxomolybdenum Donor-Acceptor Complex

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO(SPh)2((i)Pr2Dt(0)) (where (i)Pr2Dt(0) = N,N'-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° "envelope"-type fold of the five-membered dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ∼18 000 cm(-1) is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL'CT) transition that also possesses Mo(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo(x(2)-y(2)) orbital character is derived from a strong pseudo Jahn-Teller effect, which drives the large fold-angle distortion to yield a double-well potential in the electronic ground state.

Pulsed electron paramagnetic resonance spectroscopy of (33)S-labeled molybdenum cofactor in catalytically active bioengineered sulfite oxidase

Molybdenum enzymes contain at least one pyranopterin dithiolate (molybdopterin, MPT) moiety that coordinates Mo through two dithiolate (dithiolene) sulfur atoms. For sulfite oxidase (SO), hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of magnetic nuclei (I ≠ 0) near the Mo(V) (d(1)) center have been measured using high-resolution pulsed electron paramagnetic resonance (EPR) methods and interpreted with the help of density functional theory (DFT) calculations. These have provided important insights about the active site structure and the reaction mechanism of the enzyme. However, it has not been possible to use EPR to probe the dithiolene sulfurs directly since naturally abundant (32)S has no nuclear spin (I = 0). Here we describe direct incorporation of (33)S (I = 3/2), the only stable magnetic sulfur isotope, into MPT using controlled in vitro synthesis with purified proteins. The electron spin echo envelope modulation (ESEEM) spectra from (33)S-labeled MPT in this catalytically active SO variant are dominated by the "interdoublet" transition arising from the strong nuclear quadrupole interaction, as also occurs for the (33)S-labeled exchangeable equatorial sulfite ligand [ Klein, E. L., et al. Inorg. Chem. 2012 , 51 , 1408 - 1418 ]. The estimated experimental hfi and nqi parameters for (33)S (aiso = 3 MHz and e(2)Qq/h = 25 MHz) are in good agreement with those predicted by DFT. In addition, the DFT calculations show that the two (33)S atoms are indistinguishable by EPR and reveal a strong intermixing between their out-of-plane pz orbitals and the dxy orbital of Mo(V).

Spectroscopic and electronic structure studies probing covalency contributions to C-H bond activation and transition-state stabilization in xanthine oxidase

A detailed electron paramagnetic resonance (EPR) and computational study of a key paramagnetic form of xanthine oxidase (XO) has been performed and serves as a basis for developing a valence-bond description of C-H activation and transition-state (TS) stabilization along the reaction coordinate with aldehyde substrates. EPR spectra of aldehyde-inhibited XO have been analyzed in order to provide information regarding the relationship between the g, (95,97)Mo hyperfine (A(Mo)), and (13)C hyperfine (A(C)) tensors. Analysis of the EPR spectra has allowed for greater insight into the electronic origin of key delocalizations within the Mo-O(eq)-C fragment and how these contribute to C-H bond activation/cleavage and TS stabilization. A natural bond orbital analysis of the enzyme reaction coordinate with aldehyde substrates shows that both Mo═S π → C-H σ* (ΔE = 24.3 kcal mol(-1)) and C-H σ → Mo═S π* (ΔE = 20.0 kcal mol(-1)) back-donation are important in activating the substrate C-H bond for cleavage. Additional contributions to C-H activation derive from O(eq) lp → C-H σ* (lp = lone pair; ΔE = 8.2 kcal mol(-1)) and S lp → C-H σ* (ΔE = 13.2 kcal mol(-1)) stabilizing interactions. The O(eq)-donor ligand that derives from water is part of the Mo-O(eq)-C fragment probed in the EPR spectra of inhibited XO, and the observation of O(eq) lp → C-H σ* back-donation indicates a key role for O(eq) in activating the substrate C-H bond for cleavage. We also show that the O(eq) donor plays an even more important role in TS stabilization. We find that O(eq) → Mo + C charge transfer dominantly contributes to stabilization of the TS (ΔE = 89.5 kcal mol(-1)) and the Mo-O(eq)-C delocalization pathway reduces strong electronic repulsions that contribute to the classical TS energy barrier. The Mo-O(eq)-C delocalization at the TS allows for the TS to be described in valence-bond terms as a resonance hybrid of the reactant (R) and product (P) valence-bond wave functions.

Molybdenum enzymes in higher organisms

Recent progress in our understanding of the structural and catalytic properties of molybdenum-containing enzymes in eukaryotes is reviewed, along with aspects of the biosynthesis of the cofactor and its insertion into apoprotein.

Monooxomolybdenum(VI) complexes possessing olefinic dithiolene ligands: probing Mo-S covalency contributions to electron transfer in dimethyl sulfoxide reductase family molybdoenzymes

A monooxomolybdenum(VI) model complex for the oxidized active site in the DMSOR family of molybdoenzymes has been synthesized and structurally characterized. The compound was obtained from the desoxomolybdenum(IV) derivative by clean oxygen-atom transfer from an amine N-oxide in a manner similar to that observed in the enzyme. A combination of electronic absorption and resonance Raman spectroscopies, coupled with the results of bonding and excited-state calculations, has been used to provide strong support for a highly covalent Mo(d(xy))-S(dithiolene) pi*-bonding interaction in the molybdenum(VI) complex. It is proposed that the resulting Mo-S covalency facilitates electron-transfer regeneration of the catalytically competent DMSOR Mo(IV) active site.

Spectroscopic characterization of YedY: the role of sulfur coordination in a Mo(V) sulfite oxidase family enzyme form

Electronic paramagnetic resonance (EPR), electronic absorption, and magnetic circular dichroism spectroscopies have been performed on YedY, a SUOX fold protein with a Mo domain that is remarkably similar to that found in chicken sulfite oxidase, Arabidopsis thaliana plant sulfite oxidase, and the bacterial sulfite dehydrogenase from Starkeya novella. Low-energy dithiolene --> Mo and cysteine thiolate --> Mo charge-transfer bands have been assigned for the first time in a Mo(V) form of a SUOX fold protein, and the spectroscopic data have been used to interpret the results of bonding calculations. The analysis shows that second coordination sphere effects modulate dithiolene and cysteine sulfur covalency contributions to the Mo bonding scheme. In particular, a more acute O(oxo)-Mo-S(Cys)-C dihedral angle results in increased cysteine thiolate S --> Mo charge transfer and a large g(1) in the EPR spectrum. The spectrosocopic results, coupled with the available structural data, indicate that these second coordination sphere effects may play key roles in modulating the active-site redox potential, facilitating hole superexchange pathways for electron transfer regeneration, and affecting the type of reactions catalyzed by sulfite oxidase family enzymes.

Sulfur K-edge spectroscopic investigation of second coordination sphere effects in oxomolybdenum-thiolates: relationship to molybdenum-cysteine covalency and electron transfer in sulfite oxidase

Second-coordination sphere effects such as hydrogen bonding and steric constraints that provide for specific geometric configurations play a critical role in tuning the electronic structure of metalloenzyme active sites and thus have a significant effect on their catalytic efficiency. Crystallographic characterization of vertebrate and plant sulfite oxidase (SO) suggests that an average O(oxo)-Mo-S(Cys)-C dihedral angle of approximately 77 degrees exists at the active site of these enzymes. This angle is slightly more acute (approximately 72 degrees) in the bacterial sulfite dehydrogenase (SDH) from Starkeya novella. Here we report the synthesis, crystallographic, and electronic structural characterization of Tp*MoO(mba) (where Tp* = (3,5-dimethyltrispyrazol-1-yl)borate; mba = 2-mercaptobenzyl alcohol), the first oxomolybdenum monothiolate to possess an O(ax)-Mo-S(thiolate)-C dihedral angle of approximately 90 degrees . Sulfur X-ray absorption spectroscopy clearly shows that O(ax)-Mo-S(thiolate)-C dihedral angles near 90 degrees effectively eliminate covalency contributions to the Mo(xy) redox orbital from the thiolate sulfur. Sulfur K-pre-edge X-ray absorption spectroscopy intensity ratios for the spin-allowed S(1s) --> Sv(p) + Mo(xy) and S(1s) --> Sv(p) + Mo(xz,yz) transitions have been calibrated by a direct comparison of theory with experiment to yield thiolate Sv(p) orbital contributions, c(j)(2), to the Mo(xy) redox orbital and the Mo(xz,yz) orbital set. Furthermore, these intensity ratios are related to a second coordination sphere structural parameter, the O(oxo)-Mo-S(thiolate)-C dihedral angle. The relationship between Mo-S(thiolate) and Mo-S(dithiolene) covalency in oxomolydenum systems is discussed, particularly with respect to electron-transfer regeneration in SO.

Synthesis, Structural Characterization, and Multifrequency Electron Paramagnetic Resonance Studies of Mononuclear Thiomolybdenyl Complexes

Reaction of Tp*MoVSCl2 with a variety of phenols and thiols in the presence of triethylamine produces mononuclear, thiomolybdenyl complexes Tp*MoVSX2 [Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate; X = 2-(ethylthio)phenolate (etp), 2-(n-propyl)phenolate (pp), phenolate; X2 = benzene-1,2-dithiolate (bdt), 4-methylbenzene-1,2-dithiolate (tdt), benzene-1,2-diolate (cat)]. The complexes have been characterized by microanalysis, mass spectrometry, IR, EPR, and UV-visible spectroscopic data, and X-ray crystallography (for the etp, pp, bdt, and cat derivatives). The mononuclear, six-coordinate, distorted-octahedral Mo centers are coordinated by terminal sulfido (MoS = 2.123(1)-2.1368(8) A), tridentate facial Tp*, and monodentate or bidentate O/S-donor ligands. Multifrequency (S-, X-, Q-band) EPR spectra of the complexes and selected molybdenyl analogues were acquired at 130 K and 295 K and yielded a spin Hamiltonian of Cs symmetry or lower, with gzz < gyy < gxx < ge and Az'z' > Ax'x' approximately Ay'y', and a noncoincidence angle in the range of beta = 24-39 degrees . Multifrequency EPR, especially at S-band, was found to be particularly valuable in the unambiguous assignment of the spin Hamiltonian parameters in these low-symmetry complexes. The weaker pi-donor terminal sulfido ligand yields a smaller SOMO-LUMO gap and reduced g-values for the thiomolybdenyl complexes compared with molybdenyl analogues, supporting existing crystallographic and EPR data for an apically coordinated oxo group in the active site of xanthine oxidase.

Understanding the origin of metal-sulfur vibrations in an oxo-molybdenum dithiolene complex: relevance to sulfite oxidase

X-ray crystallography and resonance Raman (rR) spectroscopy have been used to further characterize (Tp*)MoO(qdt) (Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate and qdt is 2,3-quinoxalinedithiolene), which represents an important benchmark oxomolybdenum mono-dithiolene model system relevant to various pyranopterin Mo enzyme active sites, including sulfite oxidase. The compound (Tp*)MoO(qdt) crystallizes in the triclinic space group, P1, where a = 9.8424 (7) A, b = 11.2323 (8) A, c = 11.9408 (8) A, alpha = 92.7560 (10) degrees, beta = 98.9530 (10) degrees, and gamma = 104.1680 (10) degrees. The (Tp*)MoO(qdt) molecule exhibits the distorted six-coordinate geometry characteristic of related oxo-Mo(V) systems possessing a single coordinated dithiolene ligand. The first coordination sphere bond lengths and angles in (Tp*)MoO(qdt) are very similar to the corresponding structural parameters for (Tp*)MoO(bdt) (bdt is 1,2-benzenedithiolene). The relatively small inner-sphere structural variations observed between (Tp*)MoO(qdt) and (Tp*)MoO(bdt) strongly suggest that geometric effects are not a major contributor to the significant electronic structural differences reported for these two oxo-Mo(V) dithiolenes. Therefore, the large differences observed in the reduction potential and first ionization energy between the two molecules appear to derive primarily from differences in the effective nuclear charges of their respective sulfur donors. However, a subtle perturbation to Mo-S bonding is implied by the nonplanarity of the dithiolene chelate ring, which is defined by the fold angle. This angular distortion (theta = 29.5 degrees in (Tp*)MoO(qdt); 21.3 degrees in (Tp*)MoO(bdt)) observed between the MoS2 and S-C=C-S planes may contribute to the electronic structure of these oxo-Mo dithiolene systems by controlling the extent of S p-Mo d orbital overlap. In enzymes, the fold angle may be dynamically modulated by the pyranopterin, thereby functioning as a transducer of vibrational energy associated with protein conformational changes directly to the active site via changes in the fold angle. This process could effectively mediate charge redistribution at the active site during the course of atom- and electron-transfer processes. The rR spectrum shows bands at 348 and 407 cm(-1). From frequency analysis of the normal modes of the model, [(NH3)3MoO(qdt)]1+, using the Gaussian03 suite of programs, these bands are assigned as mixed-mode Mo-S vibrations of the five-membered Mo-ditholene core structure. Raman spectroscopy has also provided additional evidence for an in-plane pseudo-sigma dithiolene S-Mo d(xy) covalent bonding interaction in (Tp*)MoO(qdt) and related oxo-Mo-dithiolenes that has implications for electron-transfer regeneration of the active site in sulfite oxidase involving the pyranopterin dithiolene.

Oxomolybdenum tetrathiolates with sterically encumbering ligands: modeling the effect of a protein matrix on electronic structure and reduction potentials

The effect of sterically encumbering ligands on the electronic structure of oxomolybdenum tetrathiolate complexes was determined using a combination of electronic absorption and magnetic circular dichroism spectroscopies, complimented by DFT bonding calculations, to understand geometric and electronic structure contributions to reduction potentials. These complexes are rudimentary models for a redox-active metalloenzyme active site in a protein matrix and allow for detailed spectroscopic probing of specific oxomolybdenum-thiolate interactions that are directly relevant to Mo-S(cysteine) bonding in pyranopterin molybdenum enzymes. Data are presented for three para-substituted oxomolybdenum tetrathiolate complexes ([PPh4][MoO(p-SPhCONHCH3)4], [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2CN)3)4], and [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2COOCH2CH3)3)4]). The Mo(V/IV) reduction potentials of the complexes in DMF are -1213, -1251, and -1247 mV, respectively. The remarkably similar electronic absorption and magnetic circular dichroism spectra of these complexes establish that the observed reduction potential differences are not a result of significant changes in the electronic structure of the [MoOS4]- cores as a function of the larger ligand size. We provide evidence that these reduction potential differences result from the driving force for a substantial reorganization of the O-Mo-S-C dihedral angle upon reduction, which decreases electron donation from the thiolate sulfurs to the reduced molybdenum center. The energy barrier to favorable O-Mo-S-C geometries results in a reorganizational energy increase, relative to [MoO(SPh)4](-/2-), that correlates with ligand size. The inherent flexible nature of oxomolybdenum-thiolate bonds indicate that thiolate ligand geometry, which controls Mo-S covalency, could affect the redox processes of monooxomolybdenum centers in pyranopterin molybdenum enzymes.

Structural analysis of missense mutations causing isolated sulfite oxidase deficiency

The physiologically essential oxidation of sulfite to sulfate is catalyzed by the molybdoheme enzyme, sulfite oxidase. Deficiencies of this enzyme in human patients lead to severe neurological symptoms, which usually result in death in early childhood. Up to date eleven missense mutations in the gene encoding sulfite oxidase have been identified from sulfite oxidase deficient patients. The structural characterization of these mutants is now possible after the chicken sulfite oxidase gene has been synthesized chemically and due to the high homology to the human enzyme it provides a good model of human sulfite oxidase. This review focuses on the possible effects of the sulfite oxidase deficiency causing mutations based on our new structures of recombinant chicken sulfite oxidase.

Determination of the g-Tensors and Their Orientations for cis,trans-(L-N2S2)MoVOX (X = Cl, SCH2Ph) by Single-Crystal EPR Spectroscopy and Molecular Orbital Calculations

A single-crystal study of cis,trans-(L-N2S2)MoVOCl (1) doped into cis,trans-(N2S2)MoVIO2 (3) has enabled the g-tensor of 1 and its orientation with respect to the molecular structure to be determined. The EPR parameters (g1, 2.004; g2, 1.960; g3, 1.946; A1, 71.7 x 10(-4) cm(-1); A2, 11.7 x 10(-4) cm(-1); A3, 32.0 x 10(-4) cm(-1)) of cis,trans-(L-N2S2)MoVOCl [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine] mimic those of the low-pH form of sulfite oxidase and the "very rapid" species of xanthine oxidase. The principal axis that corresponds to g1 is rotated approximately 10 degrees from the Mo[triple bond]O vector, while the principal axis that corresponds to g3 is located in the equatorial plane and approximately 38 degrees from the Mo-Cl vector. Independent theoretical calculations of the g-tensor of 1 were performed using two types of techniques: (1) the spectroscopically parametrized intermediate neglect of differential overlap technique (INDO/S) combined with single-excitation configuration interaction (CIS); (2) a scalar relativistic DFT (BP86 and B3LYP functionals) treatment using the zeroth order regular approximation to relativistic effects (ZORA) in combination with recently developed accurate multicenter mean field spin-orbit operators (RI-SOMF) and the estimation of solvent effects using dielectric continuum theory at the conductor-like screening model (COSMO) level. The excellent agreement between experiment and theory, as well as the high consistency between the INDO/S and BP86/ZORA results, provides a sound basis for analysis of the calculated orientation of the g-tensor for cis,trans-(L-N2S2)MoVO(SCH2Ph) (2), for which single-crystal EPR data are not available but which contains three equatorial sulfur donor atoms, as occurs in sulfite oxidase and xanthine oxidase. The implications of these results for the EPR spectra of the Mo(V) centers of enzymes are discussed.

Synthesis, Structure Determination, and Spectroscopic/Computational Characterization of a Series of Fe(II)−Thiolate Model Complexes: Implications for Fe−S Bonding in Superoxide Reductases

A combined synthetic/spectroscopic/computational approach has been employed to prepare and characterize a series of Fe(II)-thiolate complexes that model the square-pyramidal [Fe(II)(N(His))(4)(S(Cys))] structure of the reduced active site of superoxide reductases (SORs), a class of enzymes that detoxify superoxide in air-sensitive organisms. The high-spin (S = 2) Fe(II) complexes [(Me(4)cyclam)Fe(SC(6)H(4)-p-OMe)]OTf (2) and [FeL]PF(6) (3) (where Me(4)cyclam = 1,4,8,11-tetramethylcyclam and L is the pentadentate monoanion of 1-thioethyl-4,8,11-trimethylcyclam) were synthesized and subjected to structural, magnetic, and electrochemical characterization. X-ray crystallographic studies confirm that 2 and 3 possess an N(4)S donor set similar to that found for the SOR active site and reveal molecular geometries intermediate between square pyramidal and trigonal bipyramidal for both complexes. Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies were utilized, in conjunction with density functional theory (DFT) and semiemperical INDO/S-CI calculations, to probe the ground and excited states of complexes 2 and 3, as well as the previously reported Fe(II) SOR model [(L(8)py(2))Fe(SC(6)H(4)-p-Me)]BF(4) (1) (where L(8)py(2) is a tetradentate pyridyl-appended diazacyclooctane macrocycle). These studies allow for a detailed interpretation of the S-->Fe(II) charge transfer transitions observed in the absorption and MCD spectra of complexes 1-3 and provide significant insights into the nature of Fe(II)-S bonding in complexes with axial thiolate ligation. Of the three models investigated, complex 3 exhibits an absorption spectrum that is particularly similar to the one reported for the reduced SOR enzyme (SOR(red)), suggesting that this model accurately mimics key elements of the electronic structure of the enzyme active site; namely, highly covalent Fe-S pi- and sigma-interactions. These spectral similarities are shown to arise from the fact that 3 contains an alkyl thiolate tethered to the equatorial cyclam ring, resulting in a thiolate orientation that is very similar to the one adopted by the Cys residue in the SOR(red) active site. Possible implications of our results with respect to the electronic structure and reactivity of SOR(red) are discussed.

Modulation of Molybdenum-Centered Redox Potentials and Electron-Transfer Rates by Sulfur versus Oxygen Ligation

Temperature-dependent measurements of potential, E degrees', and electron-transfer rate constant, k(s,h), are reported for electrochemical reduction (in 0.3 M TBAPF(6)/CH(3)CN) of a series of oxomolybdenum(V) complexes, [(Tp)MoO(X,Y)], where Tp = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E degrees' values shift in the positive direction, and k(s,h) values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as DeltaH= -R partial differential[ln(k(s,h))]/ partial differential(1/T) and corrected for an outer-shell component by the mean spherical approximation, is approximately 10 kJ mol(-1) larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of Mo(V/IV) electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo d(xy) redox orbital.