Assignment of Soret MLCT band of reduced form of copper binuclear cluster in cytochrome c oxidase film

Abstract

Low concentration of dithionite results in the reduction of Cu-Cu binuclear and heme a active sites of the cytochrome c oxidase thin solid film immersed in the acidic phosphate buffer, but Fe-Cu binuclear center keeps in the oxidation state. It manifests as a negative peak at 426 nm and a positive one at ∼408 nm in the difference spectra induced by dithionite. The former implies decrease of the oxidized form of heme a center, that is, Fe a 3+ →Fe a 2+ · And the latter results from the contribution of metal-ligand charge transfer (MLCT) transition in the reduced binuclear Cu-Cu cluster, rather than from that of heme a center. This stronger Soret MLCT band must be helpful to overcoming the difficulty in distinguishing the weaker copper sign from the stronger one of iron when studying copper-iron protein.