Abstract
A theoretical study of Heisenberg exchange and double exchange effects in clusters with four and six iron ions has been performed for [Fe4 S3 O] m +, [Fe4 S4] m + (where m = 3, 2), and [Fe6 S6] n + (where n = 5, 4) ions as models of the Desulfovibrio vulgaris iron–sulfur centers. Assuming that the redox potential mostly depends on the Heisenberg spin coupling and the resonance delocalization, we performed an analysis of the reduction process for the [Fe4 S3 O] 3+/2+, [Fe4 S4] 3+/2+, and [Fe6 S6] 5+/4+ ions and showed that the redox potential can be calculated as a difference between average spin energies of the tetravalent and pentavalent double cubane superclusters. For the Heisenberg parameter of J1 = 20 cm-1, the redox potential amounts to about 0.03 V.It complies with close to zero experimental values of the redox potential.
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