Chemical Approach to the Cu(II)-Phenoxyl Radical Site in Galactose Oxidase: Dependence of the Radical Stability on N-Donor Properties

Abstract

Abstract Copper(II) complexes of several new N3O-type tripodal ligands, 2,4-di(t-butyl)-6-{[bis(2-pyridyl)methyl]aminomethyl}phenol (HtbuL), 2,4-di(t-butyl)-6-{[(6-methyl-2-pyridyl)methyl](2-pyridylmethyl)aminomethyl}phenol (HtbuL(Mepy)), 2,4-di(t-butyl)-6-{bis[(6-methyl-2-pyridyl)methyl]aminomethyl}phenol (HtbuL(Mepy)2), and 2,4-di(t-butyl)-6-{[(1-methyl-2-imidazolyl)methyl][(6-methyl-2-pyridyl)methyl]aminomethyl}phenol (HtbuL(im)(Mepy)), were prepared. They were structurally characterized by the X-ray crystallographic method to have a square-pyramidal structure with a weakly coordinating group at an apical position. The phenol moiety of [CuCl(tbuL(Mepy))] (2), [CuCl(tbuL(Mepy)2)] (3), and [CuCl(tbuL(im)(Mepy))] (4) was revealed to be coordinated equatorially; it was converted to the phenoxyl radical upon oxidation with Ce(IV), giving a new absorption peak at 405—420 nm. ESR measurements at low temperatures and resonance Raman spectra established that the radical species has a Cu(II)-phenoxyl radical bond. The cyclic voltammograms exhibited a quasi-reversible redox wave at E1/2 = 0.56—0.61 V (vs. Ag/AgCl) corresponding to the formation of the phenoxyl radical, which displayed a first order decay with a half life of 65 and 62 min at -20 °C for 2 and 4, respectively, while that for 3 was only 9.0 min at -40 °C. The radical stability increased with the donor ability of the equatorial N ligands.