AN ASED-MO CALCULATION OF A SIDE-ON BOUND DIMER MODEL OF OXYHAEMOCYANIN

The molecular orbitals of a non-planar side-on bound mu-eta2:eta2-peroxodi-(diamminecopper(II))2+ model of the active site of oxyhaemocyanin have been calculated. The sigma(O-O) orbital, the two sigma(Cu-O) orbitals, a bonding and a non-bonding pi(v) orbital were assigned to -4.693, -4.215, - 5.183, -4.849 and -1.505 eV, respectively. The absorption bands which were observed in the absorption or CD spectra of oxyhaemocyanin appear to be transitions from molecular orbitals with large 3d (Cu) character to the LUMO orbital, which has ca 30.4% 3d(xy)(Cu) and 55.6% 2p(x)(O) character. The oscillator strengths of the transitions allowed in the C2v point group were calculated. The singlet-triplet separation corresponds to the difference in energy of the asymmetric and symmetric combinations of the 3d(xy)(Cu) atomic orbitals and is large (2.060 eV). These molecular orbitals have B2 and A1 symmetry, respectively, and, therefore, the electronic transition is allowed in the model compound. The singlet-triplet transition may not be spin-forbidden in all molecules.