Electron and energy-transfer reactions of [formula omitted] with copper-phenolates

Abstract

Covalently-modified tyrosines are found as cofactors in enzymes such as galactose oxidase and the copper-dependent amine oxidases. In these cases, cofactor formation has been proposed to occur via oxidation of a copper–tyrosine precursor, making electron transfer chemistry in copper-phenolates a key to cofactor biogenesis. Our work was motivated by a desire to understand the ET reactivity of simple metal coordinated phenolate model complexes. Copper and zinc complexes of phenolate ligands were prepared, and their electron and energy-transfer reactivity toward Ru ( R 2 bpy ) 3 2 + ∗ / 3 + studied to test the electron-transfer reactivity of metal-coordinated phenolates. M(salxn) (M = Cu 2+, Zn 2+; salxn = salen, N, N′-disalicylidene-1,2-ethylenediamine; salpen, N, N′-disalicylidene-1,3-propylenediamine; salben, N, N′-disalicylidene-1,4-butylenediamine; salophen, N, N′-disalicylidene- o-phenylenediamine) were synthesized along with the more easily oxidized complexes Cu(bppa) (Hbppa, bis-pyridyl phenolamine) and Cu(icoph) (H 2icoph, bis-iminocatechol o-phenylenediamine). Zn(salophen) and Cu(salophen) were oxidized by Ru (bpy) 3 3 + , indicating that electron transfer was thermodynamically favorable. Cu(salxn) complexes were observed to be efficient energy-transfer quenchers of Ru (bpy) 3 2 + ∗ ; in contrast, Cu(bppa) and Cu(icoph) quenched Ru (bpy) 3 2 + ∗ by electron transfer, with observed reorganization energy λ = 22 kcal/mol. The large self-exchange reorganization energy calculated for the Cu-coordinated phenolates suggest that the CAO and GalOx cofactors may be poor 1e − redox centers, with very slow rates for cofactor biogenesis.