Galactose Oxidase Models: Solution Chemistry, and Phenoxyl Radical Generation Mediated by the Copper Status

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Abstract
Galactose oxidase (GO) is an enzyme that catalyzes two‐electron oxidations. Its active site contains a copper atom coordinated to a tyrosyl radical, the biogenesis of which requires copper and dioxygen. We have recently studied the properties of electrochemically generated mononuclear CuII‐phenoxyl radical systems as model compounds of GO. We present here the solution chemistry of these ligands under various copper and dioxygen statuses: N3O ligands first chelate CuII, leading, in the presence of base, to [CuII(ligand)(CH3CN)]+ complexes (ortho‐tert‐butylated ligands) or [(CuII)2(ligand)2]2+ complexes (ortho‐methoxylated ligands). Excess copper(II) then oxidizes the complex to the corresponding mononuclear CuII‐phenoxyl radical species. N2O2 tripodal ligands, in the presence of copper(II), afford directly a copper(II)‐phenoxyl radical species. Addition of more than two molar equivalents of copper(II) affords a CuII–bis(phenoxyl) diradical species. The donor set of the ligand directs the reaction towards comproportionation for ligands possessing an N3O donor set, while disproportionation is observed for ligands possessing an N2O2 donor set. These results are discussed in the light of recent results concerning the self‐processing of GO. A path involving copper(II) disproportionation is proposed for oxidation of the cross‐linked tyrosinate of GO, supporting the fact that both copper(I) and copper(II) activate the enzyme.