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Photosynthesis
We report the results of Molecular Dynamics (MD) simulations and formal modeling of the free
energy surfaces and reaction rates of primary charge separation in the reaction center of Rhodobacter
sphaeroides. Two simulation protocols were used to produce MD trajectories. Standard force
field potentials were employed in the first protocol. In the second protocol, the special pair was
made polarizable to reproduce a high polarizability of its photoexcited state observed by Stark
spectroscopy. The charge distribution between covalent and charge-transfer states of the special
pair was dynamically adjusted during the simulation run. We found from both protocols that the
breadth of electrostatic fluctuations of the protein/water environment far exceeds previous estimates
resulting in about 1.6 eV reorganization energy of electron transfer in the first protocol and 2.5 eV
in the second protocol. Most of these electrostatic fluctuations become dynamically frozen on
the time-scale of primary charge separation resulting in much smaller solvation contributions to
the activation barrier. While water dominates solvation thermodynamics on long observation times,
protein emerges as the major thermal bath coupled to electron transfer on the picosecond time of the
reaction. Marcus parabolas were obtained for the free energy surfaces of electron transfer by using
the first protocol while a highly asymmetric surface was obtained in the second protocol. A non-
ergodic formulation of the diffusion-reaction electron transfer kinetics has allowed us to reproduce
the experimental results for both the temperature dependence of the rate and the non-exponential
decay of the population of the photoexcited special pair.
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