ELECTRON TRANSFER IN PROTEINS

Electron-transfer (ET) reactions are key steps in a diverse array of biological transformations ranging from photosynthesis to aerobic respiration. A powerful theoretical formalism has been developed that describes ET rates in terms of two parameters: the nuclear reorganization energy (γ) and the electroniccoupling strength (HAB). Studies of ET reactions in ruthenium-modified proteins have probed γ and HAB in several metalloproteins (cytochrome c, myoglobin, azurin). This work has shown that protein reorganization energies are sensitive to the medium surrounding the redox sites and that an aqueous environment, in particular, leads to large reorganization energies. Analyses of electronic-coupling strengths suggest that the efficiency of long-range ET depends on the protein secondary structure: β sheets appear to mediate coupling more efficiently than α-helical structures, and hydrogen bonds play a critical role in both.