Geometric and electronic structure of oxyhemocyanin: spectral and chemical correlations to met apo, half met, met, and dimer active sites.

The chemical and spectral properties of a series of hemocyanin derivatives were systematically compared to provide insight into the geometric and electronic structure of the oxyhemocyanin active site. The binuclear copper site is characterized as two tetragonal Cu(II) atoms bridged by both an endogenous protein ligand and the exogenous ligand (i.e., peroxide), with the lack of an electron paramagnetic resonance signal being the result of antiferromagnetic exchange via the endogenous bridge. A transition dipole-vector coupling model is used to assign the unique absorption spectral properties of oxyhemocyanin: the bands at 570 and 486 nm are assigned as components of the peroxide pi v* to copper dx2-y2 charge transfer. The 345-nm band is one component of the pi sigma* leads to dx2-y2 charge transfer. The model also predicts an end-to-end bridging geometry for the peroxide in oxyhemocyanin.