Ground-state electronic and magnetic properties of a mu3-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 Cu(II) site in the native intermediate (NI) of the multicopper oxidases, the mu(3)-oxo-bridged structure, are evaluated using the C(3)-symmetric Cu(3)(II) complex, mu(3)O. mu(3)O is unique in that no ligand, other than the oxo, contributes to the exchange coupling. However, mu(3)O 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 Angstroms), the ground state of mu(3)O becomes antiferromagnetic and can be correlated to that of NI. In addition, the ferromagnetic (4)A ground state of mu(3)O 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 excited-state exchange pathways and provides experimental evidence that the orbitally degenerate (2)E ground state of the antiferromagnetic mu(3)O 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-ground and ground-to-excited-state superexchange pathways that are responsible for the isotropic, antisymmetric, and anisotropic exchanges are discussed.