Web Release Date: December 9,
The Two-State Issue in the Mixed-Valence Binuclear CuA Center in Cytochrome c Oxidase and N2O Reductase
and
Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305, and Division of Biology, The Scripps Research Institute, La Jolla, California 92037
Received October 23, 2006
Abstract:
For the CuA site in the protein, 
u* and 
u 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 u* and u which requires an energy gap between u* and u 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, u* and u, 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 u* and u states which are at very close energies in their respective equilibrium geometries. Alternatively, the EPR g-value analysis involves the u* ground state in the geometry with a short dCu-Cu where the u 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 u* 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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