Ground-state electronic and magnetic properties of a μ3-Oxo-bridged trinuclear Cu(II) complex:: Correlation to the native intermediate of the multicopper oxidases

The ground-state electronic and magnetic properties of one of the possible structures of the trinuclear Cull site in the native intermediate (NI) of the multicopper oxidases, the mu(3)-oxo-bridged structure, are evaluated using the C-3-symmetric Cu3(11) complex, mu O-3, mu O-3 is unique in that no ligand, other than the oxo, contributes to the exchange coupling. However, mu O-3 has a ferromagnetic ground state, inconsistent with that of NI. Therefore, two perturbations have been considered: protonation of the mu(3)-Oxo ligand and relaxation of the mu(3)-OXO ligand into the Cu-3 plane. Notably, when the oxo ligand is sufficiently close to the Cu-3 plane (<0.3 A), the ground state Of mu O-3 becomes antiferromagnetic and can be correlated to that of NI. In addition, the ferromagnetiC (4)A ground state of mu O-3 is found from variable-temperature EPR to undergo a zero-field splitting (ZFS) of 2D = -5.0 cm(-1), which derives from the second-order anisotropic exchange. This allows evaluation of the sigma-to-pi T excited-state exchange pathways and provides experimental evidence that the orbitally degenerate E-2 ground state of the antiferromagnetiC mu O-3 would also undergo a ZFS by the first-order antisymmetric exchange that has the same physical origin as the anisotropic exchange. The important contribution of the mu(3)-oxo bridge to the ground-to-g round and ground-to-excited-state superexchange pathways that are responsible for the isotropic, antisymmetric, and anisotropic exchanges are discussed.