Spectroscopy of Mixed-Valence Cu_A-Type Centers:  Ligand-Field Control of Ground-State Properties Related to Electron Transfer

Using a combination of electronic spectroscopies, electronic structural descriptions have been developed for a series of binuclear Cu_A-type centers in Bacillus subtilis CcO and engineered into the blue copper proteins Pseudomonas aeruginosa azurin and Thiobacillus versutus amicyanin. Parallel descriptions are developed for two structurally characterized mixed-valence (MV) and homovalent (II,II) synthetic copper thiolate dimers. Assignment of the excited-state spectral features allows the electronic structures of Cu_A and the MV model to be understood and compared in relation to their copper coordination environments. These electronic structural descriptions are supported by SCF-Xα-SW MO calculations, which are used to test systematically the effects of major structural perturbations linking the MV model geometry to that of Cu_A. It is determined that both Cu−Cu compression and removal of the axial ligands are critical determinants of the orbital ground state in these dimers. The weakened axial interactions in Cu_A appear to parallel the mechanism for protein control of electron transfer (ET) function observed in blue copper centers. The major geometric and electronic features of Cu_A, including metal−ligand covalency, redox potentials, reorganization energies, valence delocalization, and the weakened axial bonding interactions, are discussed in relation to its ET function, and specific potential ET pathways are identified and compared.