Electron transfer in ruthenium-modified proteins

Photochemical techniques have been used to measure the kinetics of intramolecular electron transfer in Ru(bpy)₂(im)(His)²⁺-modified (bpy = 2,2′-bipyridine; im = imidazole) cytochrome c and azurin. A driving-force study with the His33 derivatives of cytochrome c indicates that the reorganization energy (γ) for Fe²⁺→Ru³⁺ ET reactions is 0.8 eV. Reductions of the ferriheme by either an excited complex,^*Ru²⁺, or a reduced complex, Ru⁺, are anomalously fast and may involve formation of an electronically excited ferroheme. The distance dependence of Fe²⁺→Ru³⁺ and Cu⁺→Ru³⁺ electron transfer in 12 different Ru-modified cytochromes and azurins has been analyzed using a tunneling-pathway model. The ET rates in 10 of the 12 systems exhibit an exponential dependence on metal-metal separation (decay constant of 1.06 å^-1) that is consistent with predictions of the pathway model.