Photoelectron transfer processes with ruthenium(II) polypyridyl complexes and Cu/Zn superoxide dismutase.

The processes that are photoinduced by Ru(bpz)(3) (bpz = 2,2'-bipyrazyl) in the presence of Cu/Zn superoxide dismutase (Cu/Zn SOD) are investigated by laser flash photolysis and electron paramagnetic resonance (EPR) spectroscopy; they are compared to those of the system [Ru(bpy)(3)(2+)-Cu/Zn SOD]. Although the mechanism is complicated, primary and secondary reactions can be evidenced. First, the excited Ru(bpz)(3) complex is quenched reductively by Cu/Zn SOD with the production of a reduced complex and an oxidized enzyme. The oxidation site of Cu/Zn SOD is proposed to correspond to amino acids located on the surface of the protein. Afterward and only when this reductive electron transfer to the excited complex has produced enough oxidized protein, another electron-transfer process can be evidenced. In this case, however, the charge-transfer process takes place in the other direction, i.e., from the excited complex to the Cu(II) center of the SOD with the formation of Ru(III) and Cu(I) species. This proposed mechanism is supported by the fact that Ru(bpy)(3), which is less photo-oxidizing than Ru(bpz)(3), exhibits no photoreaction with Cu/Zn SOD. Because Ru(III) species are generated as intermediates with Ru(bpz)(3), they are proposed to be responsible for the enhancement of poly(dG-dC) and poly(dA-dT) oxidation observed when Cu/Zn SOD is added to the Ru(bpz)(3)-DNA system.