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A phenomenological model aimed at rationalizing variations in both average g-tensor values and effective exchange coupling constants [J.sub.eff] is derived in order to describe the great variety of magnetic properties displayed by reduced [2Fe-2S] clusters in proteins. The crystallographic structures of wild-type and mutant As ferrodoxins (Fd's) are used for describing a distribution of spin states.

The synthesis, X-ray structure, magnetic properties and successive preliminary density functional theory analysis of azido-bridged binuclear [Cu.sup.II] complex [[Cu.sub.2]([[[mu].sub.1.1-][N.sub.3]).sub.2][(im-2py).sub.2][([N.sub.3]).sub.2] is reported. A large metal-nitroxide ferromagnetic exchange coupling was noticed whose magnitude compares well with that reported for closely related complexes.

Ni-containing carbon monoxide dehydrogenases (CODH), present in many anaerobic microorganisms, catalyze the reversible oxidation of CO to CO2 at the so-called C-cluster. This atypical active site is composed of a [NiFe3S4] cluster and a single unusual iron ion called ferrous component II or Feu that is bridged to the cluster via one sulfide ion. After additional refinement of recently published high-resolution structures of COOHx-, OHx-, and CN-bound CODH from Carboxydothermus hydrogenoformans (Jeoung and Dobbek Science 2007, 318, 1461-1464; J. Am. Chem. Soc. 2009, 131, 9922-9923), we have used computational methods on the predominant resulting structures to investigate the spectroscopically well-characterized catalytic intermediates, Cred1 and the two-electron more-reduced Cred2. Several models were geometry-optimized for both states using hybrid quantum mechanical/molecular mechanical potentials. The comparison of calculated Mössbauer parameters of these active site models with experimental data allows us to propose that the Cred1 state has a Feu-Ni2+ bridging hydroxide ligand and the Cred2 state has a hydride terminally bound to Ni2+. Using our combined structural and theoretical data, we put forward a revised version of an earlier proposal for the catalytic cycle of Ni-containing CODH (Volbeda and Fontecilla-Camps Dalton Trans. 2005, 21, 3443-3450) that agrees with available spectroscopic and structural data. This mechanism involves an abnormal CO2 insertion into the Ni2+-H- bond. [ABSTRACT FROM AUTHOR]

We relied on the density functional theory (DFT) to study the electronic structure of the [2Fe-2S*](SH)4 model of the active site of 2Fe ferredoxins and other proteins containing reduced [2Fe-2S*] clusters. The two (Fe3+-Fe2+S-H) dihedral angles Ω1 and Ω2 defined for the two ligands on the ferrous side were allowed to vary, while the two other (Fe2+-Fe3+-S-H) angles Ω3 and Ω4 on the ferric side were kept constant. The Landé (g), magnetic hypeffine, and quadrupole tensors for two geometries, C2 (Ω1 = Ω2) and C2 (Ω1 = -Ω2), were calculated. To apply our model to the actual proteins, we listed all of the crystallographic structures available for the [2Fe-2S*] systems. A classification of these proteins, based on the four dihedral angles {Ωi}I=1-4, separates them into three main classes. The main structural feature of the first class (Ω1 ≈ Ω2), with an average dihedral angle Ω av = (Ω1 + Ω2)/2 comprised between 115° and 150°, corresponds to a local ferrous C2 geometry (rather than C2v, as previously assumed by Bertrand and Gayda: Biochim. Biophys. Acta 1979, 579, 107). We then established a direct correlation between the three principal g values and Ωav. It is the first time that such a link has been made between the spectroscopic and structural parameters, a link, moreover, fully rationalized by our DFT calculations. We finally point out the basic differences between our C2 results with those of the C2v phenomenological model proposed in the late 1970s by Bertrand and Gayda. [ABSTRACT FROM AUTHOR]

Ni-containing carbon monoxide dehydrogenases (CODH), present in many anaerobic microorganisms, catalyze the reversible oxidation of CO to CO(2) at the so-called C-cluster. This atypical active site is composed of a [NiFe(3)S(4)] cluster and a single unusual iron ion called ferrous component II or Fe(u) that is bridged to the cluster via one sulfide ion. After additional refinement of recently published high-resolution structures of COOH(x)-, OH(x)-, and CN-bound CODH from Carboxydothermus hydrogenoformans (Jeoung and Dobbek Science 2007, 318, 1461-1464; J. Am. Chem. Soc. 2009, 131, 9922-9923), we have used computational methods on the predominant resulting structures to investigate the spectroscopically well-characterized catalytic intermediates, C(red1) and the two-electron more-reduced C(red2). Several models were geometry-optimized for both states using hybrid quantum mechanical/molecular mechanical potentials. The comparison of calculated Mössbauer parameters of these active site models with experimental data allows us to propose that the C(red1) state has a Fe(u)-Ni(2+) bridging hydroxide ligand and the C(red2) state has a hydride terminally bound to Ni(2+). Using our combined structural and theoretical data, we put forward a revised version of an earlier proposal for the catalytic cycle of Ni-containing CODH (Volbeda and Fontecilla-Camps Dalton Trans. 2005, 21, 3443-3450) that agrees with available spectroscopic and structural data. This mechanism involves an abnormal CO(2) insertion into the Ni(2+)-H(-) bond., (© 2011 American Chemical Society)