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The stability of a peroxo-iron species in the second time scale when reacted with H2O2 is due to the E47A mutation in superoxide reductase (SOR) from Desulfoarculus baarsii. Further characterization on the basis of Mossbauer and spin Hamiltonian parameters shows that the peroxo-Fe(super +3) species is a high-spin side-on complex.

The variations of the electronic properties of the [2Fe-2S*] ferredoxins, from the cyanobacaterium Anabaena are investigated by density functional theory, depending on the redox-linked structural changes observed by X-ray diffraction at atomic resolution. Calculation indicates that the Glu94 carboxylate negative charge regulates the electron charge delocalization between the Ser47 CO group and the Phe65 aromatic ring, depending on the redox state.

An analytical expression of varDeltaP2-(Cu) for two bridged Cu(II) dimers is presented.

A (4Fe-4S)+ center with properties similar to reduced states in some ferredoxins, has been studied using proton ENDOR spectroscopy on gamma-irradiated single crystals of the synthetic model compound (Etsub4N)sub2-(Fesub4Ssub4(SCsub6Hsub4-o-OH)sub4). One iron atom was found to be five-coordinate, having an extra phenolic oxygen attached to its thiolate ligand. This represents an asymmetrical (4Fe-4S) cluster in the solid crystalline state.

An experimental EPR study on (4Fe-4S)super3+ and (4Fe-4S)+ centers with single crystals of a model compound with asymmetrical cubane-type clusters is presented. A theoretical model to interpret the g-tensors of such states observed in synthetic complexes and protein active sites is also reported. The analysis enables understanding of the reasons leading to the diversity of paramagnetic centres, and proposes a useful set of localizations.

Evaluation of redox potentials for 1Fe, 2Fe and 4Fe iron-sulfur clusters by density functional/Poisson-Boltzmann calculations reveals the significant contribution of both spin coupling and resonance delocalization to higher redox potential of 4Fe iron-sulfur clusters. Redox potential shift due to Heisenberg spin coupling is more negative for 2Fe clusters than for one iron clusters. Examination of the contribution of solvents to redox potential indicates the significant contribution of solvation for all redox couples and in all clusters.

EPR studies of the (Et4N)2(FeS4(SBenz)4) model compound reveal the presence of gamma irradiation-created (Fe4S4)3+ and (Fe4S4)+ paramagnetic species in single crystals. Analysis of the 9-tensors yields principal values that agree well with the corresponding geometric and electronic structure of model compounds. The EPR spectra for (Fe4S4)3+ species allow good interpretation of a high-potential protein's oxidized state.

The structure of a spore photoproduct of the dinucleoside TpT (SPTpT) is reexamined to determine the absolute configuration of the natural SP by combining NMR measurements and density functional theory calculations. The results have shown that this natural SPTpT compound is the dinucleotide 5'->3' with an R configuration at the carbon [C.sub.5a].

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.