9513229 Parson This proposal describes experimental and theoretical studies of photosynthetic bacterial reaction centers, with a focus on reaction centers that have been modified by site-directed mutagenesis. Amino acid substitutions that are likely to alter the energies of the initial radical-pair states or modify the reaction center's spectroscopic properties have been chosen on the basis of electrostatics calculations. The major targets for mutagenesis are four aspartic acid residues and three arginines, which will be replaced by amino acids with nonionizable or oppositely charged side chains. Effects of the mutations on the spectroscopic properties of the reaction center and on the midpoint potential (Em ) of the primary electron donor (P) will be examined. To study the photosynthetic electron-transfer reactions, mutant and wild-type reaction centers will be excited with flashes of light lasting about 15 fs. The excited state of P and radical-pair states created by charge separation will be probed by measuring the optical absorption and stimulated emission with a second pulse of light at various times after the excitation. These studies will explore how the mutations affect the relaxation dynamics of the excited state, the rate and temperature dependence of the electron-transfer reactions, and the stabilities and free energies of the radical-pair states. Neutron scattering also will be used to study the temperature dependence of nuclear motions in reaction centers. The theoretical studies will include calculations of the free energies of radical-pair states, spectroscopic properties and Em values in mutant and native reaction centers, using a microscopic treatment of dielectric effects. Comparisons of calculated and measured properties will be used to refine and test the theoretical methods, and thus to increase the reliability of calculations of properties that are important for an understanding of the electron-transfer reactions but are not directly accessible experimentally. Tim e-dependent fluctuations of the calculated energies will be examined by molecular-dynamics simulations and will be related to the kinetics of electron transfer. %%% This project concerns pigment-protein complexes termed "reaction centers," which carry out the initial reactions that plants and photosynthetic bacteria use t o capture the energy of sunlight. Dr. Parson's studies will focus on bacterial reaction centers that have been modified genetically to alter the energies of the transient states created by light. The modifications have been chosen from theoretical considertions based on the reaction center's structure. The major targets for mutagenesis are seven amino acids with electrically charged side chains, which will be replaced by amino acids with neutral or oppositely charged side chains. Effects of the mutations on the reaction center's optical absorption spectrum and on the electrochemical properties of the pigments will be examined. To study the photosynthetic electron-transfer reactions, mutant and wild-type reaction centers will be excited with flashes of light lasting about 15 femtoseconds. (A femtosecond is one millionth of a billionth of a second.) The very short-lived states generated by the light pulse will be probed by passing a second pulse of light through the sample at various time after the excitation. These studies will explore how the mutations affect the rate and temperature-dependence of the electron-transfer reactiions. Neutron scattering also will be used to study the temperature dependence of nuclear motions in reaction centers. Parallel theoretical studies will include calculations of the energies of the transient states in mutant and native reaction centers. Comparisons of calclated and measured properties will be used to refine and test the theoretical methods, and thus to increase the reliability of calculations of properties that are important for an understanding of the electron-transfer reactions but are not directly accessible experimentally. ***
