Electron and energy-transfer reactions of Ru(bpy)32+∗/3+ with copper-phenolates

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(R2bpy)32+∗/3+ studied to test the electron-transfer reactivity of metal-coordinated phenolates. M(salxn) (M = Cu2+, Zn2+; 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) (H2icoph, bis-iminocatechol o  -phenylenediamine). Zn(salophen) and Cu(salophen) were oxidized by Ru(bpy)33+, indicating that electron transfer was thermodynamically favorable. Cu(salxn) complexes were observed to be efficient energy-transfer quenchers of Ru(bpy)32+∗; in contrast, Cu(bppa) and Cu(icoph) quenched Ru(bpy)32+∗ 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.

Copper-phenolate models of tyrosyl cofactors found in proteins undergo electron transfer or energy transfer with Ru(bpy)32+∗/3+. The self-exchange of Cu(phenolate)/Cu(phenoxyl) has a high reorganization energy, λ = 22 kcal/mol, suggesting that metal-associated tyrosyl cofactors may undergo slow electron transfer.Figure optionsDownload as PowerPoint slide