A detailed electronic structure description of the reduced blue copper active site has now been developed. To date, the Cu(I) 3d(10) oxidation state of this site has been generally inaccessible to the spectroscopic techniques commonly employed in the extensive studies of the open shell, oxidized blue copper active site. Photoelectron spectroscopy (PES) of imidazole, dimethyl sulfide, and methanethiolate bound to Cu(I) sites at single crystal surfaces has been used to define normal Cu(I) bonding to ligands relevant to the blue copper site. Variable photon energy PES has been used to assign valence band spectra, assess metal-ligand covalency, and probe specific orbital contributions to Cu(I) bonding. Self Consistent Field-Xa-Scattered Wave (SCF-X alpha-SW) molecular orbital calculations calibrated to the photoelectron spectra have been performed to quantitatively complement the experimental bonding descriptions. These calculations have been extended to the reduced blue copper active site in plastocyanin, the prototypical blue copper protein, to detail the electronic structure changes that occur relative to normal Cu(I) bonding and upon oxidation. It is found that the weakened axial interaction associated with the elongated Cu-thioether bond is compensated by a strong Cu-thiolate equatorial pi bond, which activates the cysteine residue as an effective superexchange pathway for electron transfer. The metal-ligand bonding at the reduced blue copper site is found to be dominated by ligand p --> Cu(I) 4p charge transfer. Upon oxidation new Cu 3d bonding contributions arise as a result of the hole created in the Cu(II) 3d(x)(2)-(2)(y) orbital. In particular, the thiolate S pi orbital exhibits significant overlap with the d(x)(2)-(2)(y) orbital, which leads to a considerable increase in the thiolate pi donor strength of the Cu-S(thiolate) bond. Ionization energies have been used to estimate the electronic structure contributions to the reduction potential. The long Cu-thioether axial bond present at the active site destabilizes the oxidized state and is therefore a key determining factor in the high reduction potentials generally observed for blue copper proteins. Linear coupling terms have been evaluated for the distortions of a blue copper site unconstrained by the protein backbone. The geometry changes which occur in the blue copper site upon oxidation are found to be consistent with the changes in the electronic structure. Therefore, the reduced Cu(I) geometry is not imposed on the oxidized site by the protein environment. Rather, the structural constraints due to the protein matrix are present in the reduced site, where the long Cu-thioether bond lowers the site symmetry and eliminates the electronic degeneracy of the ground state and, thus, the Jahn-Teller distortion that would normally occur upon oxidation. As a result the geometric changes are small, giving rise to a low Franck-Condon barrier to electron transfer.