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

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 Cu(II)-phenoxyl radical systems as model compounds of GO. We present here the solution chemistry of these ligands under various copper and dioxygen statuses: N(3)O ligands first chelate Cu(II), leading, in the presence of base, to [Cu(II)(ligand)(CH(3)CN)](+) complexes (ortho-tert-butylated ligands) or [(Cu(II))(2)(ligand)(2)](2+) complexes (ortho-methoxylated ligands). Excess copper(II) then oxidizes the complex to the corresponding mononuclear Cu(II)-phenoxyl radical species. N(2)O(2) 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 Cu(II)-bis(phenoxyl) diradical species. The donor set of the ligand directs the reaction towards comproportionation for ligands possessing an N(3)O donor set, while disproportionation is observed for ligands possessing an N(2)O(2) 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.

Galactose Oxidase Models: Creation and Modification of Proton Transfer Coupled to Copper(II) Coordination Processes in Pro‐Phenoxyl Ligands

“…This is not the case, showing that a comproportionation reaction consumes the radical once it is formed (as in the case of GO [70] and other N 3 O ligands [50] ) according to Equation 1. copper(II) phenoxyl + copper(I) Ǟ 2 copper(II) -phenolate (1)…”

“…being too high to allow it to be oxidized by copper(II) to produce the corresponding phenoxyl radical. [50] After addition of one molar equiv. of copper(II) to HL H or HL Me deprotonation of the phenol occurs, induced by proton transfer to the benzimidazole or N-methylbenzimidazole substituent, which lowers its redox potential, making oxidation by exogenous copper(II) possible in the absence of exogenous base.…”

“…The protonation constants were determined from pHmetric titrations that were monitored by UV/Vis spectroscopy (Figure 4) for phenol copper(II) complexes of tripodal ligands (6.66-7.31), [50] and for GO itself (7.9). [53] This can be explained by the strong electron-withdrawing effect of the benzimidazolium (or N-methylbenzimidazolium) substituent that very efficiently stabilizes the phenolate form, and therefore lowers its pK a .…”

“…In this context, we have studied the solution chemistry of several tripodal ligands, [49] involving one pro-phenoxyl group (mimicking the amino acids of the GO active site), under various copper(II), base, and dioxygen reagent conditions. [50] We have shown that Cu II phenoxyl radical species could be obtained by adding two equiv. of copper(II) to ligands possessing a N 3 O coordination moiety, such as HLt Bu (Scheme 1), in the presence of one equiv.…”

First Total Synthesis of (±)‐Latifolin and Its Antioxidant Mechanism

“…2,4-Dimethoxyphenol was prepared according to a literature procedure. [21] All calculations were performed with the Gaussian09 suites of programs. [22] The DFT calculations employed the B3LYP functional using the standard 6-31G(d,p) basis set.…”