The two-state issue in the mixed-valence binuclear CuA center in cytochrome C oxidase and N2O reductase.

For the CuA site in the protein, sigmau* and piu are the ground and lowest energy excited-states, respectively. EPR data on CuA proteins show a low g parallel value of 2.19 which derives from spin-orbital coupling between sigmau* and piu which requires an energy gap between sigmau* and piu of 3000-4500 cm-1. On the other hand, from paramagnetic NMR studies, it has been observed that the first excited-state is thermally accessible and the energy gap between the ground state and the thermally accessible state is approximately 350 cm-1. This study addressed this apparent discrepancy and evaluated the roles of the two electronic states, sigmau* and piu, in electron transfer (ET) of CuA. The potential energy surface calculations show that both NMR and EPR results are consistent with the electronic/geometric structure of CuA. The anti-Curie behavior observed in paramagnetic NMR studies of CuA results from the thermal equilibrium between the sigmau* and piu states which are at very close energies in their respective equilibrium geometries. Alternatively, the EPR g-value analysis involves the sigmau* ground state in the geometry with a short dCu-Cu where the piu state is a Frank-Condon excited-state with the energy of 3200 cm-1. The protein environment plays a role in maintaining CuA in the sigmau* state as a lowest-energy state with the lowest reorganization energy and high-covalent coupling to the Cys and His ligands for efficient intra- and intermolecular ET with a low-driving force.