Calculations on the electronic structure and UV-visible spectrum of oxyhemocyanin

The ground-state electronic structure and UV-visible spectra of models for oxyhemocyanin are examined using the intermediate neglect of differential o verlap model and multireference configuration interaction. The experimental features are interpreted as excitations involving the d orbitals of the Cu atoms and the valence orbitals of the peroxide bridge through which they a re antiferromagnetically (AF) coupled. Our model, which replaces the histid ine residues with imidazoles, reproduces correctly the higher stability of the AF singlet state and the major spectroscopic features. We examine in de tail the geometry of the central part of the molecule, responsible for the activity, using the experimental electronic spectrum as a guide. We further examine the effect that deprotonation of the chelating imidazoles has on t he predicted spectroscopy. The central structure that we assumed to best re produce the spectroscopy is very similar to that obtained from BLYP density functional calculations. For comparison with other results, we also examin e a model compound in which NH3 replaces histidine. For both the imidazole- and the NH3-based models, the [Cu-2((mu-eta(2):eta(2)-O-2)](2+) and [Cu-2( eta-O)(2)](2+) isomers have been considered and the results compared with a vailable data. We conclude with the observation that the electronic structu re of these compounds does depend on the redox properties of the chelating nitrogen ligands.