AN ELECTRONIC STRUCTURAL COMPARISON OF COPPER PEROXIDE COMPLEXES OF RELEVANCE TO HEMOCYANIN AND TYROSINASE ACTIVE-SITES

Self-consistent field-X-alpha-scattered wave (SCF-X-alpha-SW) calculations have been performed on a series of copper(II) peroxide structures to evaluate the effects of peroxide bridging geometry on the electronic structure and bonding. The series of structures investigated allows comparison of a set of copper-peroxide monomers and dimers with different binding geometries but with similar ligand environments. The Cu-O and O-O bonding interactions, magnetic exchange interactions, and excited-state transition energies are calculated and compared to experimental properties of structurally related analogues. The primary bonding interaction in all structures involves the in-plane peroxide PI*-sigma orbital bonding with the Cu d ground-state orbitals. The side-on dimer has an additional bonding interaction where the peroxide-sigma* orbital acts as a pi-acceptor, weakening the O-O bond, consistent with the observed low stretching frequency. The energy difference between the in-plane peroxide-PI*-sigma and the out-of-plane peroxide-PI*-v orbitals is related to the strength of the copper-peroxide bond and is a measure of the stability of the copper-peroxide adduct. The PI*-v-PI*-sigma splitting of the side-on dimer is significantly larger than other geometries and is consistent with the observed absorption spectrum. Finally, the electronic structure origin of proposed reaction mechanisms is discussed, and models of relevance to oxyhemocyanin and oxytyrosinase are evaluated.