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Manganese is an essential transition metal that, among other functions, can act independently of proteins to either defend against or promote oxidative stress and disease. The majority of cellular manganese exists as low molecular-weight [Mn.sup.2+] complexes, and the balance between opposing 'essential' and 'toxic' roles is thought to be governed by the nature of the ligands coordinating [Mn.sup.2+]. Until now, it has been impossible to determine manganese speciation within intact, viable cells, but we here report that this speciation can be probed through measurements of 1H and 31p electron-nuclear double resonance (ENDOR) signal intensities for intracellular [Mn.sup.2+]. Application of this approach to yeast (Saccharomyces cerevisiae) cells, and two pairs of yeast mutants genetically engineered to enhance or suppress the accumulation of manganese or phosphates, supports an in vivo role for the orthophosphate complex of [Mn.sup.2+] in resistance to oxidative stress, thereby corroborating in vitro studies that demonstrated superoxide dismutase activity for this species. ENDOR | phosphate | Saccharomyce cerevisiae | superoxide dismutase doi/ 10.1073/pnas.1009648107

Oxo-molybdenum reduction potentials can be greatly influenced by the nature of the dithiolate bound to the metal center and the origin of this behavior lies in the dithiolate-dependent differences in Mo-S covalency. This provides deeper insight into the nature of metal-dithiolate bonding interactions.

The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) is analyzed. [super 1]H and [super 2]H ENDOR spectra of xanthine oxidase(C[super 1,2][H.sub.2]O) in [H.sub.2]O/[D.sub.2]O buffer have shown that the active-site structure of xanthine oxidase contains a C[H.sub.2]O adduct of Mo(V) in the form of a four-membered ring with S and O linking the C to Mo and have ruled out a direct Mo-C bond.

The EPR spectra of the trigonally symmetric Mo(III) coordination compounds [HIPT[N.sub.3]NMoL (L = [N.sub.2], CO, N[H.sub.3], [HIPT[N.sub.3]N]Mo = [[(3,5-[(2,4,6-i-[Pr.sub.3][C.sub.6][H.sub.2]).sub.2][C.sub.6][H.sub.3]NC[H.sub.2]C[H.sub.2]).sub.3]N]Mo) and their temperature and solvent dependencies are interpreted within a formal etwo-orbitalE model that has reflected the ground-state configuration. The studies have provided new insights into the role of trigonal coordination, as imposed by the [[HIPT[N.sub.3]N].sup.3-] ligand, in activating the Mo ion for the binding and reduction of [N.sub.2].

The electronic structure of the unique desoxo Mo-bisdithiolene active sites found in dimethylsulfoxide reductase (DMSOR) enzyme family is evaluated by using symmetrized model compound analogues of the reduced DMSOR active site probed by electronic absorption and resonance Raman spectroscopies. The studies have provided an electronic-structure basis for understanding oxo-transfer and electron-transfer reactivity in members of the DMSOR enzyme family.

ENDOR analysis procedures are used for characterizing biomimetic complex for nitrogenase FeMo-co, an isolable mixed-valence [Fe.sup.II]-[Fe.sup.III] complex. The results have shown that the ions are not high spin and that the spin coupling coefficients of the coordinating cofactor iron ions in the intermediate are exceptionally small.

The effect of sterically encumbering ligands on the electronic structure of three para-substituted oxomolybdenum tetrathiolate complexes was determined using spectroscopy techniques. The spectra show that the observed reduction potential differences in these complexes are not a result of changes in the electronic structure of the [MoOS(sub 4)](super -) site.

The electronic structure of cis,trans-(L-N2S2)MoO(X) (where L-N2S2 =N,N`-bis(2-mercaptophenyl)-ethylenediamine and X=Cl, SCH(sub 2)C(sub 6)H(sub 5), SC(sub 6)H(sub 4)-OCH(sub 3), or SC(sub 6)H(sub 4)Ch(sub 3)) is probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum-thiolate bonding in complexes possessing three equational sulfur ligands.

The possible vibronic behaviors of [Mo][N.sub.2] are described. Additionally an EPR study is presented, which characterizes the electronic configuration and vibronic properties of this complex.

A study of the low-energy ligand-to-metal charge transfer (LMCT) bands in (PPH(sub 4))-[MoO(bdt)(sub 2)], a small molecule paramagnetic analogue of the DMSOR active site is presented. The results show that a pseudo-sigma bonding interaction is a common electronic structure theme of oxo-Mo and oxo-W dithiolate centers in enzymes, providing an efficient way to couple the metal redox orbital into ET pathways involving the pyranopterin.

Manganese is an essential transition metal that, among other functions, can act independently of proteins to either defend against or promote oxidative stress and disease. The majority of cellular manganese exists as low molecular-weight Mn2+ complexes, and the balance between opposing "essential" and "toxic" roles is thought to be governed by the nature of the ligands coordinating Mn2+. Until now, it has been impossible to determine manganese speciation within intact, viable cells, but we here report that this speciation can be probed through measurements of 1H and 31P electron-nuclear double resonance (ENDOR) signal intensities for intracellular Mn2+. Application of this approach to yeast (Saccharomyces cerevisiae) cells, and two pairs of yeast mutants genetically engineered to enhance or suppress the accumulation of manganese or phosphates, supports an in vivo role for the orthophosphate complex of Mn2+ in resistance to oxidative stress, thereby corroborating in vitro studies that demonstrated superoxide dismutase activity for this species. [ABSTRACT FROM AUTHOR]

The trigonally symmetric Mo(III) coordination compounds [HIPTN3N]MoL (L = N2, CO, NH3; [HIPTN3N]Mo = [(3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2)3N]Mo) are low-spin d3 (S = ½) species that exhibit a doubly degenerate 2E ground state susceptible to a Jahn-Teller (JT) distortion. The EPR spectra of all three complexes and their temperature and solvent dependences are interpreted within a formal "two-orbital" model that reflects the ground-state configuration, describes the vibronic interactions that lead to the JT distortions, and addresses whether these complexes exhibit static or dynamic JT distortions. The electronic and vibronic properties of these complexes are then analyzed through ab initio quantum chemical computations. It is not possible to interpret the spectroscopic properties of the orbitally degenerate [HIPTN3N]MoL with DFT methods, so we have resorted to multi-reference wavefunction approaches, the entry level of which is the complete active space self-consistent field (CASSCF) method. Overall, the experimental and computational studies provide new insights into the role of trigonal coordination, as enforced by the [HIPTN3N]3- ligand, in activating the Mo ion for the binding and reduction of N2. [ABSTRACT FROM AUTHOR]

Superoxide reductase (SOR) and P450 enzymes contain similar [Fe(N)4(SCys)] active sites and, although they catalyze very different reactions, are proposed to involve analogous low-spin (hydro)peroxo- Fe(III) intermediates in their respective mechanisms that can be modeled by cyanide binding. The equatorial FeN4 ligation by four histidine ligands in CN-SOR and the heme in CN-P450cam is directly compared by 14N ENDOR, while the axial FeCN and FeS bonding is probed by 13C ENDOR of the cyanide ligand and ¹Hβ ENDOR measurements to determine the spin density delocalization onto the cysteine sulfur. There are small, but notable, differences in the bonding between Fe(III) and its ligands in the two enzymes. The ENDOR measurements are complemented by DFT computations that support the semiempirical equation used to compute spin densities on metal-coordinated cysteinyl and shed light on bonding changes as the FeCN linkage bends. They further indicate that H bonds to the cysteinyl thiolate sulfur ligand reduce the spin density on the sulfur in both active sites to a degree that exceeds the difference induced by the alternative sets of ‘in-plane’ nitrogen ligands. [ABSTRACT FROM AUTHOR]

The electronic structure of cis, trans-(L-N2S2)MoO(X) (where L-N2S2 = N,N'-dimethyl-N,N'-bis(2-mercaptophenyl)ethylenediamine and X = Cl, SCH2C6H5, SC6H4-OCH3, or SC6H4CF3) has been probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum-thiolate bonding in complexes possessing three equatorial sulfur ligands. One of the phenyl mercaptide sulfur donors of the tetradentate L-N2S2 chelating ligand, denoted S180, coordinates to molybdenum in the equatorial plane such that the O≡Mo-S180-Cphenyl dihedral angle is ∼180°, resulting in a highly covalent &pi-bonding interaction between an S180 p orbital and the molybdenum dxy orbital. This highly covalent bonding scheme is the origin of an intense low-energy S → Mo dxy bonding-to-antibonding LMCT transition (Emax ∼16 000 cm-1, ∈ ∼ 4000 M-1 cm-1). Spectroscopically calibrated bonding calculations performed at the DFT level of theory reveal that S180 contributes ∼22% to the HOMO, which is predominantly a π antibonding molecular orbital between Mo dxy and the S180 p orbital oriented in the same plane. The second sulfur donor of the L-N2S2 ligand is essentially nonbonding with Mo dxy due to an O≡Mo-S-Cphenyl dihedral angle of ∼90°. Because the formal Mo dxy orbital is the electroactive or redox orbital, these Mo dxy-S 3p interactions are important with respect to defining key covalency contributions to the reduction potential in monooxomolybdenum thiolates, including the one- and two-electron reduced forms of sulfite oxidase. Interestingly, the highly covalent Mo-S180 π bonding interaction observed in these complexes is analogous to the well-known Cu-SCys π bond in type 1 blue copper proteins, which display electronic absorption and resonance Raman spectra that are remarkably similar to these monooxomolybdenum thiolate complexes Finally, the presence of a covalent Mo-S π interaction oriented orthogonal to the MO ≡ O bond is discussed with respect to electron-transfer regeneration in sulfite oxidase and MO=Ssulfido bonding in xanthine oxidase. [ABSTRACT FROM AUTHOR]

Dinitrogen is reduced to ammonia by the molybdenum complex of L = [HIPTN3N]3- [Mo; HIPT = 3,5-(2,4,6-iPr3C6H2)2C6H3]. The mechanism by which this occurs involves the stepwise addition of proton/electron pairs, but how the first pair converts MoN2 to MoN-NH remains uncertain. The first proton of reduction might bind either at Nβ of N2 or at one of the three amido nitrogen (Nam) ligands. Treatment of MoCO with [2,4,6-Me3C5H3N]BAr′4 [Ar′ = 2,3-(CF3)2C6H3] in the absence of reductant generates HMoCO+, whose electron paramagnetic resonance spectrum has greatly reduced g anisotropy relative to MoCO. 2H Mims pulsed electron nuclear double-resonance spectroscopy of 2HMoCO+ shows a signal that simulations show to have a hyperfine tensor with an isotropic coupling, aiso(2H) = -0.22 MHz, and a roughly dipolar anisotropic interaction, T(2H) = [-0.48, -0.93, 1.42] MHz. The simulations show that the deuteron is bound to Nam, near the Mo equatorial plane, not along the normal, and at a distance of 2.6 Å from Mo, which is nearly identical with the (Nam)2H+-Mo distance predicted by density functional theory computations. [ABSTRACT FROM AUTHOR]

The article presents an electron paramagnetic resonance (EPR) study of low-spin [d3; S = ½], Jahn-Teller-active, dinitrogen complex of molybdenum trisamidoamine (TAA) in the U.S. TAA complex [Mo] = Mo[HIPTN3N] is important for its ability to catalyze the reduction of N2 to NH3. Furthermore, PR study of an [Mo]L complex establishes that [Mo]N2 exhibits the 1s [e]3 electronic configuration.

The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) has been studied for four decades, yet it has not proven possible to distinguish unequivocally among the several structures proposed for this form. The uniquely large isotropic hyperfine coupling for 13C from CH2O led to the intriguing suggestion of a direct Mo-C bond for the active site of I. This suggestion was supported by the recent crystal structures of glycoland glycerol-inhibited forms of aldehyde oxidoreductase, a member of the xanthine oxidase family. 1H and 2H ENDOR spectra of I(C1,2H2O) in H2O/D2O buffer now have unambiguously revealed that the active-site structure of I contains a CH2O adduct of Mo(V) in the form of a four-membered ring with S and 0 linking the C to Mo and have ruled out a direct Mo-C bond. Density functional theory computations are consistent with this conclusion. We interpret the large 13C coupling as resulting from a `transannular hyperfine interaction". [ABSTRACT FROM AUTHOR]