COUPLING OF PROTEIN MOTION TO ELECTRON-TRANSFER - MOLECULAR-DYNAMICS AND STOCHASTIC QUANTUM-MECHANICS STUDY OF PHOTOSYNTHETIC REACTION CENTERS

We investigate how electron transfer is controlled by protein motion in photosynthetic reaction centers. Our study is based on molecular dynamics (MD) simulations of two electron transfer steps in the reaction center of Rps. viridis at physiological and at lower temperatures. The classical simulations of protein nuclear motions are complemented by a quantum mechanical description for the electron transfer, incorporating in a two-state model a coupling to the classical protein motion through a fluctuating diagonal contribution which is determined as the energy difference DELTA-E(t) between reactant and product states at each instance of time. The properties of DELTA-E(t), the distribution p(DELTA-E) and correlation function <DELTA-E(t + tau)DELTA-E(tau)>, are investigated and a stochastic quantum mechanical model for electron transfer is introduced that incorporates three characteristics of DELTA-E(t), namely its mean value, its rms-deviations from the mean, and the mean relaxation time of its correlation function. The calculations which go beyond second-order perturbation theory predict a bell-shaped dependence of the electron transfer rate on redox energies with a so-called inverted region and with a width of about 20 kcal/mol (about 10 kcal/mol for the stochastic model). Rapid (0.05 ps) dielectric relaxation after electron transfer induces a shift of the mean <DELTA-E> which causes reactant and product states to become sufficiently out of resonance and which, thereby, prevents electron back-transfer. It is shown that all components of photosynthetic reaction centers contribute rather evenly to the coupling between electron transfer and medium.