The CuA site functions as an electron transfer intermediate in cytochrome c oxidase (CcO) and nitrous oxide reductase. X-ray crystal structures have shown that the CuA site is a binuclear copper site, bridged by two cysteinyl thiolate groups. Each copper ion is coordinated by a histidine side chain and a weaker axial ligand: a methionine side chain or a backbone carbonyl group. The ground state of the oxidized CuA site is strongly related to the singly occupied orbital, as the oxidized state of the CuA site is doublet. Spectroscopic studies have suggested the σu* ground state of the CuA site due to the direct CuCu interaction, facilitating a rapid electron transfer over long distances with low driving forces. In this study, to elucidate what factors are responsible for the stabilization of the σu* ground state of the CuA site, the electronic structures of the Cu2S2 core of the CuA have been studied, using the density functional theory (M06). Our computational results show that the ground state of the Cu2S2 core is the πu state even with a shorter CuCu distance, and that an addition of electrostatic and orbital interactions by ligand coordination stabilizes the σu* state rather than the πu state, because electrostatic repulsion and antibonding orbital interaction between the dσ* orbital of the CuCu dimer and the lone pairs of the coordinating His residues rise the orbital energy of the σu* orbital of the CuA site. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
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