Very recently, highly efficient biomimetic models of the mononuclear copper enzyme galactose oxidase were synthesized which are able to reproduce the structural, spectroscopic, and functional properties of the native system exceptionally well. We have characterized an inactive and an active form of one of these biomimetic compds. using unrestricted dynamical d. functional calcns. The peculiar nonsquare planar O2N2-coordination geometry of the copper ion in the catalytically inactive (EPR-active) form induces a complex energy-level diagram that cannot be related to crystal-field models: The highest occupied orbitals are located on the p-system of the arom. ligands and are essentially spin-paired while the unpaired electron is localized mainly in a lower-lying dx2-y2 orbital of the copper. Using ab initio mol. dynamics simulations, we detd. for the first time the structure of the active form complexed with a substrate analog. Our calcns. reveal that upon substrate binding one of the phenolate ligands is pushed away from the copper center into an axial position and the electronic structure rearranges to an unusual antiferromagnetic diradical state. As in the inactive form, the unpaired a-spin d. is located in the copper dx2-y2 orbital. The unpaired b-spin d., instead, is localized on the axial ligand in agreement with the ligand-based radical mechanism that has been proposed for galactose oxidase. [on SciFinder (R)]