An active-site analog of the radical copper enzyme galactose oxidase has been prepared from a synthetic tripod chelate ((2-pyridy)methyl)[(2-hydroxy-3,5-dimeethyphenyl)methyl] [(2-hydroxy-5-methyl-3-(methylthio)phenyl)methyl]amine, duncamine (dnc) that binds a single Cu(II) ion through phenolate, thioether-substituted phenolate, and pyridylamine arms. The Cu complex crystallizes as a dinucleated dimer bridged by phenolate oxygens, and the structure has been determined by X-ray crystallography. Addition of pyridine (or other coordinating bases) dissociates the complex into a monomeric derivative that has been characterized spectroscopically (optical absorption and EPR) and electrochemically. The model provides insight into the properties of a mutant form of galactose oxidase which retains the same copper ligand complement as the wild type protein but tacks catalytic activity.