Formation and characterization of an all-ferrous Rieske cluster and stabilization of the [2Fe-2S]0 core by protonation

The all-ferrous Rieske cluster, [2Fe-2S](0), has been produced in solution and characterized by protein-film voltammetry and UV-visible, EPR, and Mossbauer spectroscopies. The [2Fe-2S]() cluster, in the overexpressed soluble domain of the Rieske protein from the bovine cytochrome bc(1), complex, is formed at -0.73 V at pH 7. Therefore, at pH 7, the [2Fe-2S](1+/0) couple is 1.0 V below the [2Fe-2S](2+/1+) couple. The two cluster-bound ferrous irons are both high spin(S 2), and they are coupled antiferromagnetically (-J greater than or equal to 30 cm(-1), H -2JS1(.)S2) to give a diamagnetic (S = 0) ground state. The ability of the Rieske cluster to exist in three oxidation states (2+, 1+, and 0) without an accompanying coupled reaction, such as a conformational change or protonation, is highly unusual. However, uncoupled reduction to the [2Fe-2S](0) state occurs at pH > 9.8 only, and at high pH the intact cluster persists in solution for < 1 min. At pH < 9.8, the all-ferrous cluster is stabilized significantly by protonation. A combination of experimental data and calculations based on density functional theory suggests strongly that the proton binds to one of the cluster mu2-sulfides, consistent with observations that reduced [3Fe-4S] clusters are protonated also. The implications for our understanding of coupled reactions at iron-sulfur clusters and of the factors that determine the relative stabilities of their different oxidation states are discussed.