Abstract

Electron-transfer (ET) reactions are key steps in a diverse array of biological transformation 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 (λ) and the electronic coupling strength (H_AB). Methods have been developed for coordinating photoactive ruthenium-bipyridine complexes to surface histidine residues of native and mutant proteins. ET reactions are initiated by pulsed laser excitation of the Ru groups, and kinetics are monitored by time-resolved spectroscopy. Studies of ET reactions in ruthenium-modified proteins have probed the reorganization energies and electronic coupling strengths in several metalloproteins (cytochrome c, myoglobin, azurin). This work has shown that protein reorganization energies ate 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 a-helical structures, and hydrogen bonds play a critical role in both.

© 1997 Optical Society of America