Protein dynamics control of electron transfer in reaction centers from Rps. viridis

Abstract

Electron transfer (ET) in the reaction center from Rhodopseudomonas viridis has been studied experimentally by our group in the range of temperatures of 153–295 K. The kinetics of ET reaction from the proximal heme of cytochrome to the special pair was found to be non-exponential. The degree of non-exponentiality strongly depends on temperature, with increasing non-exponentiality at lower temperatures. Here, the experimental kinetic data for ET are analyzed in the frames of a theoretical Sumi–Marcus-type model, which allows establishing a connection between the observed kinetics and local structural dynamics of the protein in a close vicinity of the donor and acceptor sites. The phenomenological model subdivides the multi-time-scale dynamics of the protein into two groups: fast and slow. The division is determined by the rate of ET, which is dynamically controlled by the slow modes of the protein medium. The slow modes are described by a phenomenological collective coordinate , for which a diffusion type of dynamics is assumed. The analysis of the temperature dependence of the kinetic curves for ET allows a complete characterization of the slow time-scale protein dynamics: we find the corresponding activation barrier for the dynamics of , around 0.5 eV and the exponential pre-factor, which is in the range of 10− 16 s. The pre-exponential factor is lower than is expected for a typical activation process described by the transition state theory. We discuss the nature of the collective modes , the nature of its activation barrier (breaking of hydrogen bonds) and possible explanations for a low-pre-exponential factor of its dynamics. The main conclusion of the paper is that the kinetics of ET at low-temperatures can be used as a probe of protein structural dynamics in the microsecond time-scale, the time-scale that is not easily accessible by the computer simulations methods and can be complementary to other experimental techniques.