Active site models for galactose oxidase containing two different phenol groups

Model complexes of the active site of galactose oxidase (GAO) have been developed using a new ligand carrying two different phenol groups, N-[(2-hydroxy-3-methylthio-5-tert-butylphenyl)methyl]-N-[(2-hydroxy-3,5-di-tert-butylphenyl)methyl]-2-(2-pyridyl)ethylamine (L1H2). Deprotonated ligand L12− forms a dimeric Cu(II) complex, [Cu(II)2(L12−)2], in the solid state, the structure of which has been determined by X-ray crystallographic analysis. The dimeric Cu(II)–diphenolate complex can be converted into the monomeric complex, [Cu(II)(L12−)(X)] (X=py, AcO, and PhCH2OH), in solution by adding exogenous ligands such as pyridine (py), acetate (AcO−), or benzyl alcohol (PhCH2OH). The structure and physicochemical properties (UV–Vis, ESR, redox potential) of [Cu(II)(L12−)(X)] have been explored as a model for the resting state of the enzyme. One-electron oxidation of [Cu(II)(L12−)(py)] and [Zn(II)(L12−)(py)] by (NH4)2[CeIV(NO3)6] (CAN) yielded the corresponding phenoxyl radical/phenolate complexes, Cu(II)(L1radical dot−) and Zn(II)(L1radical dot−), respectively, which have also been characterized by UV–Vis, resonance Raman, and ESR. The structure, physicochemical properties and reactivities of the diphenolate and phenoxyl radical/phenolate complexes of L1H2 are compared to those of the corresponding monophenolate and monophenoxyl radical complexes in order to obtain further insight into the role of Tyr 495 in the native enzyme.