Understanding the copper–phenoxyl radical array in galactose oxidase: contributions from synthetic modeling studies

The two-electron oxidation of primary alcohols with dioxygen to yield aldehyde and hydrogen peroxide that is catalyzed by galactose oxidase (GAO) occurs at an intriguing active site comprising of a copper ion ligated by an unusual cysteine-modified tyrosine group. Both the metal ion and the tyrosinate undergo 1-electron redox interconversions during catalysis, the Cu(II)–tyrosyl radical form being a critical species. Due to the novelty of this coupled metal–radical cofactor unit in chemistry and biology and its importance within the more general context of radical–enzyme biochemistry, chemists have attempted to prepare model complexes for this and other redox-related states of GAO. The primary goals of such research are to better understand the enzyme active site spectral properties, structural attributes, and reactivity. In this review article, progress toward these goals is surveyed, beginning with a discussion of the synthesis and structural and spectroscopic characterization of model complexes of the GAO active site and ending with a description of more recent discoveries of catalytic reactivity by Cu(II)–phenoxyl radical species that replicate and provide insights into GAO function.