Abstract
Cytochrome c oxidase (CcO) participates in cellular respiration by coupling the four-electron reduction O sub(2) arrow right H sub(2)O with transmembrane proton pumping. The Cu sub(A) center of CcO is the first destination in electron transfer (ET) from cytochrome c to the Cu sub(B)-heme a sub(3) pair, where O sub(2) reduction occurs. 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 and dicupric.