Electronic structure of a Rieske iron-sulfur complex and the calculation of its reduction potential

Abstract

The electronic structure of a Rieske iron-sulfur complex of spinach chloroplast has been investigated by the CNDO/2 method. The crystallographic coordinates were obtained from the Protein Data Bank at Brookhaven National Laboratory. The protein chain was truncated in such a way that the entity Fe2S2 along with its four ligands was separated from the rest of the chain. The separated species retains the essential structural features of the Rieske iron-sulfur complex. It consists of the basic Fe2S2 unit, two histidine residues, and two cysteine residues such that each Fe atom is tetrahedrally coordinated. The orbital angular momenta of the iron atoms are found to be fully quenched. The HOMOs and the first few LUMOs are basically pi type orbitals of the Fe2S2 cluster with predominantly large contributions from the sulfur orbitals. The reduction potential calculated for the soluble Rieske iron-sulfur fragment in an aqueous solution at pH 7 is 0.370 V at 25 degrees C, which is in excellent agreement with the experimentally determined midpoint potential, 0.375 V. The calculated potential for the Rieske cluster in the condensed phase of thylakoid is 0.252 V, which agrees with the placement of the Rieske complex in the Z-scheme of photosynthesis in green plants.