Models for ferredoxins: electronic structures of iron-sulfur clusters with one, two, and four iron atoms

We report results of Xα valence bond scattered wave (Xα-VB-SW) calculations for a variety of clusters that mimic the active sites in iron-sulfur proteins: Fe(SR)41-,2- (R = H, CH3), Fe2S2(SH)42-,3-, and Fe4S4(SCH3)42-,3-. Emphasis is placed on comparisons among the various clusters, including changes in the elctron distribution upon reduction, upon going from low spin to high spin, and upon changes in cluster geometry. We present estimates based on the calculations of Mössbauer isomer shifts and quadrupole splittings which lead to a consistent account of many experimental observations. The calculations predict the iron-(bridging sulfur) bonds to be stronger than those between iron and the terminal sulfur atoms; this has interesting structural and spectroscopic implications. To a good approximation, the oxidized 4-Fe complex can be viewed as two high-spin reduced 2-Fe clusters, despite the differences in geometry. Some of the implications of these results for structure-function problems in iron-sulfur proteins are discussed.