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This is the X-ray structure of a 7-iron ferredoxin, [3Fe-4S][4Fe-4S], from a bacterium. These proteins have a distinctive red colour due to a sulfur to iron charge transfer transition (LMCT). The [Fe3S4(S-cys)3]-2/-3 cluster serves as an electron carrier.
4S4 cluster exists in two oxidation levels, an oxidized form [Fe4S4]2+2 (shown here) and a reduced form [Fe4S4]+.
The oxidized form is comprised of two Fe(III) and two Fe(II) and the reduced form, of one Fe(III) and three Fe(II). An unusually oxidized [Fe4S4]+3 state is found in the high-potential iron protein (HiPIP) (not shown). In the oxidized state shown here, [Fe4S4]2+2, electrons are delocalized over the cluster so that the average oxidation state of each iron is effectively 2.5.
7-Cys- (or -Cys-X5-Cys- in other proteins) is characteristic of a [3Fe-4S] cluster. The [3Fe-4S] cluster has the shape of a cube with one vertex missing. This cluster would become a [4Fe-4S] cluster if an Fe3+ were to add to the vacant site in between the three sulfide ligands (labeled) and a Cys were to coordinate. In this protein Cys11 is not in the correct orientation to do this. The redox potential of this center is probably also about -0.3 V.
4S4 cluster has a cubane-type structure which can be visualized as a dimer of two Fe2S2 units. The sulfido groups in Fe4S4 bridge three iron centers rather than the two in Fe2S2. The Fe-Fe separations are approximately 2.7 Å The redox potential of this center is about -0.3 V. However different Fe4S4 proteins can have potentials ranging from -0.4 V to +0.35 V depending on the accessibility of the three oxidation levels, the protein environment and the types of ligands attached to the Fe center (see the tour of Hydrogenase).