Photoinduced electron transfer in ruthenium- modified cytochrome c

ABSTRACT: Distant heme-Ru electronic couplings have been extracted from intramolecular electron-transfer rates in Ru(histidine-X) (X=33,39,62) derivatives of cytochrome c. The rates (and the couplings) correlate with the lengths of u-tunneling pathways comprised of covalent bonds, hydrogen bonds, and through-space jumps from the histidines to the heme group. The electron-transfer (ET) reactions that occur within and between proteins typically involve prosthetic groups separated by distances that are often greater than 10 A. An understanding of how the intervening medium, driving force, and nuclear reorganization energetics and dynamics modulate protein ET reactions has been a central goal of our research program. In recent years, we have been examining the rates of electron transfer between surface-bound ruthenium complexes and metalloprotein active sites. ' This work has provided a considerable capacity for predicting protein ET rates. We have been guided by semiclassical ET theory, which describes the rate constant for nonadiabatic reaction between a donor and acceptor held at fixed distance and orientatiox2 k, = (?r/h2A knT) "(HAn)2exp {- (AG O + A)2/4A kBT} (1) The tunneling matrix element HAD is a measure of the electronic coupling between the reactants and the products at the transition state. The magnitude of HAD depends upon donor-acceptor separation