A first model for the oxidized active form of the active site in galactose oxidase: A free-radical copper complex

Copper(II) complexes derived from the tripodal ligand bis(3'-t-butyl-2'-hydroxybenzyl) (2-pyridylmethyl)amine (LH(2)) have been studied in order to mimic the redox active site of the free radical-containing copper metalloenzyme galactose oxidase. In noncoordinating solvents such as dichloromethane, only an EPR-silent dimeric complex was obtained (L(2)Cu(2)). The crystal structure of L(2)Cu(2) revealed a ''butterfly'' design of the [Cu(mu OR)(2)Cu] unit, which is not flattened and leads to a short Cu-Cu distance, the t-butyl groups being localized on the same side of the [Cu(mu OR)(2)Cu] unit. The dimeric structure was broken down by acetonitrile or by alcohols, leading quantitatively to a brown mononuclear copper(II) complex. UV-visible and EPR data indicated the coordination of the solvent in these mononuclear complexes. Electrochemical as well as chemical (silver acetate) one-electron oxidation of acetonitrile solutions of the monomeric complex led to a yellow-green solution. Based on EPR, UV-visible and resonance Raman spectroscopy, the one-electron oxidation product was identified as a cupric phenoxyl radical system. It slowly decomposes into a product where the ligand has been substituted (dimerization) in the para position of the hydroxyl group, for one of the phenolic groups. The data for the one-electron oxidized species provides strong evidence for a free-radical copper (II) complex.