Why Does the Active Form of Galactose Oxidase Possess a Diamagnetic Ground State?

The relative orientation of the two magnetic orbitals, the CuII d orbital and the half-occupied π orbital of the tyrosyl radical, is the key to answering the question in the title. The arrangement shown (CuII-O-C bond angle of about 130° and a dihedral angle of about 90° between the x,y plane of the CuII polyhedron and the tyrosyl ring plane) leads to an overlap of the orbitals, which results in a singlet ground state.