Electronic structure contributions to function in bioinorganic chemistry: The blue copper active site

The blue copper site has a unique electronic structure which contributes to its electron transfer function and is reflected in unique spectral properties. The HOMO in the oxidized blue Cu site exhibits high anisotripic covalency which enhances intra- and inter-protein electron transfer rates. The electronic structure of the reduced d(10) blue Cu centers is developed using a combination of variable energy photoelectron spectroscopy and density functional calculations. These studies establish the change in electronic structure that occurs upon oxidation of the blue Cu center and permit an evaluation of whether the reduced geometry is imposed on the oxidized site by the protein (i.e. the entatic/rack state). Studies of the perturbed blue Cu site in nitrite reductase demonstrate that a tetragonal distortion is the origin of the perturbed spectral features of this site. This distortion derives from a Jahn-Teller force along an similar to epsilon(u) mode not present in the classic blue copper proteins and reflects differences in protein contributions to active site structure.