The Two-State Issue in the Mixed-Valence Binuclear CuA Center in Cytochrome c Oxidase and N2O Reductase
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Abstract
For the CuA site in the protein, σu* and πu are the ground and lowest energy excited-states, respectively. EPR data on CuA proteins show a low g∥ value of 2.19 which derives from spin-orbital coupling between σu* and πu which requires an energy gap between σu* and πu 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 ∼350 cm-1. This study addressed this apparent discrepancy and evaluated the roles of the two electronic states, σu* and πu, 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 σu* and πu states which are at very close energies in their respective equilibrium geometries. Alternatively, the EPR g-value analysis involves the σu* ground state in the geometry with a short dCu-Cu where the πu 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 σu* 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.
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