Modelling intramolecular electron transfer reactions in cytochromes and in photosynthetic bacteria reaction centres

Electron transfer rates within protein systems with various donor acceptor distances, reaction-free energies and temperatures, are calculated as the product of an electron tunneling probability and a nuclear distortion activation term. The electronic factor is given by the frequency of electronic motion in the donor, the donor electron energy, the donor-acceptor distance and the protein refractive index. Nuclear distortion is obtained from bond lengths, force constants and bond orders of the co-factor bonds involved in the reaction coordinate. The nuclear factor is calculated according to thermal activation and nuclear tunneling mechanisms. The calculation of distance, free-energy and temperature dependence of photoinduced-intraprotein electron transfer rates in Ru/Zn-modified cytochromes and myoglobins does not rely on fitting unknown parameters to kinetic data and is in good agreement with the experiment. Systems with reduced masses lower than 100 a.m.u. may undergo sizable nuclear tunneling at room temperature. (C) 1998 Elsevier Science S.A. All rights reserved.