This proposal describes a continuation of experimental and theoretical studies of photosynthetic bacterial reaction centers. Reaction centers are pigment-protein complexes that carry out the photochemical electron-transfer reactions plants and photosynthetic bacteria use to capture the energy of sunlight. In the experimental work, reaction centers from several species of purple photosynthetic bacteria will be excited with flashes of light. Transient states created by the excitation will be probed by measuring the optical absorption spectrum and stimulated emission with a second pulse of light at various times after the excitation. These studies will focus on the rate and temperature dependence of the initial electron- transfer steps in reaction centers that have been modified by directed mutagenesis. Amino acid substitutions that are likely to alter the energies of various radical-pair states will be chosen on the basis of theoretical calculations. The transient states formed by excitation of reaction centers in which the initial electron acceptor is already reduced or the donor already oxidized will also be examined. The theoretical studies will include calculations of the free energies of radical-pair states in native and mutant reaction centers, starting with the crystal structures of the native proteins and using a microscopic treatment of dielectric effects. Time-dependent fluctuations of these energies will be examined by molecular-dynamics simulations, and will be reacted to the kinetics of the electron-transfer reactions. The goal of this work is to understand how the structure of the protein determines the mechanism, kinetics, and temperature dependence of the first two electron-transfer steps in the photosynthetic process.
