Influence of Copper−Sulfur Covalency and Copper−Copper Bonding on Valence Delocalization and Electron Transfer in the Cu A Site of Cytochrome c Oxidase

In this communication, we explore the contributions of Cu-ligand and Cu-Cu bonding to valence delocalization and ET in the Cu{sub A} site of CcO. Sulfur K-edge X-ray absorption spectroscopy (XAS) provides the first direct experimental probe of copper-sulfur covalency in the half-occupied highest-energy molecular orbital (HOMO) of Cu{sub A} and two structurally-defined dithiolate-bridged models, delocalized mixed-valence 1 and dicupric 2. The Cu coordination environment of each system in shown in chart 1. Electronic absorption (Abs) and magnetic circular dichroism (MCD) spectroscopies are used to measure the total inter-ion electronic coupling in Cu{sub A} and 1 through identification of the class III mixed-valence {Psi} {yields} {Psi}{sup *} transitions. The results from these techniques combine to define the pathways for delocalization in Cu{sub A} and 1 and to describe the individual Cu-S and Cu-Cu contributions to this process. The S K-edge data additionally reveal significant anisotropic Cu-ligand covalency, permitting comparison of competing N- and S-based ET pathways to and from the Cu{sub A} site. 20 refs., 2 figs.