THE DIFFERENT [FE4S4](3+) AND [FE4S4](+) SPECIES CREATED BY GAMMA-IRRADIATION IN SINGLE-CRYSTALS OF THE (ET(4)N)(2)[FE4S4(SBENZ)(4)] MODEL-COMPOUND - THEIR EPR DESCRIPTION AND THEIR BIOLOGICAL SIGNIFICANCE

Extensive EPR studies of single crystals of the [(C2H5)(4)N]2[Fe4S4(SCH2C6H5)(4)] compound and of its partially and fully deuterated counterparts show that many different [Fe4S4](n+) (n = 1 or 3) paramagnetic species with S = 1/2 are created in these crystals after irradiation by gamma rays. Five different [Fe4S4](3+) species are identified: they correspond to the oxidized state of the high-potential iron-sulfur proteins. Moreover, two different [Fe4S4](+) species are also identified which correspond to the reduced state of the ferredoxins. All these species have been characterized by their g-tensors. They have principal values in good agreement with the values corresponding to these states in the proteins. Concerning their principal directions, one feature has been found common to all the centers observed: the principal direction V ($) over bar associated with their greatest principal value g(1) is near the direction of the common perpendicular to the directions of the mixed-valence pair of iron atoms and of the ferric or the ferrous pair. The comparisons of these tl directions made between the different centers have permitted us to give an explanation of their number and their diversity which are due to the different possibilities that the mixed-valence pair Fe-2 (5+)-Fe-2 5+ has to be localized on two particular atoms of the four iron atoms of the cubane cluster. The results obtained on the [Fe4S4](3+) state suggest a new interpretation of the composite EPR spectrum of the oxidized high-potential Chromatium vinosum protein. Since the diverse components of this spectrum follow rather closely in g-values and saturation behaviors those of the [Fe4S4](3+) states observed in the irradiated crystals, it is likely that these diverse components must also be due to different localizations of the mixed-valence pair on the irons of the active site.