Path Integral Simulation of Charge Transfer Dynamics in Photosynthetic Reaction Centers

Abstract

We present accurate path integral simulations of the primary charge separation in bacterial photosynthesis. The process is modeled in terms of the three coupled electronic states corresponding to the photoexcited special pair (the electron donor), the reduced accessory bacteriochlorophyll (the bridge), and the reduced bacteriopheophytin (the primary electron acceptor) of the L branch which interact with a dissipative medium of protein and solvent degrees of freedom. The electronic state populations are followed over 17 ps via an iterative procedure that employs a propagator functional [Comput. Phys. Commun. 1997, 99, 335]. In a previous article [Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3926] the free energy of the reduced accessory bacteriochlorophyll state and its coupling to the excited special pair were estimated by comparing the simulation results against available experimental observations on wild-type and modified reaction centers. The determined optimal parameters correspond to a simple two-step electron transfer mechanism. Additional simulations presented in this article demonstrate that the obtained parameters and the inferred mechanism are in accord with the temperature dependence of the primary charge separation and other kinetic effects observed in wild-type and modified reaction centers. We point out that the superexchange mechanism implies a large temperature effect in modified reaction centers which should be directly amenable to experimental testing. We also investigate the sensitivity of the calculated dynamics to various assumptions of the model. The results are found to be rather stable with respect to reasonable changes of the medium spectral density and the specifics of the nonequilibrium configuration o the photoexcited donor state, implying that the picture emerging from our simulations is robust and the conclusions are reliable.