REACTION CENTER DYNAMICS

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We are using optical and infrared spectroscopy to study the mechanism of energy transduction by living organisms. As a model system, we are focusing on light-induced electron transport reactions in the bacterial reaction center (RC). This integral membrane protein initiates the light-induced electron transport reaction of bacterial photosynthesis. We are interested in detecting novel reaction intermediates in the electron transport sequence and interpreting these events with physical chemical models. The molecular nature of the forces that determine the spectroscopic, kinetic and thermodynamic characteristics of this protein will be determined. Kinetic fluorescence spectroscopy will be used to monitor relaxations in delta G degrees of reaction intermediates in the electron transfer sequence. Specific amino acids and protein subunits may play a role in these events; this hypothesis will be tested by examining function of RC's modified by site directed mutations. By altering amino acids at or near L branch glutamic acid-104, we hope to understand how electrostatic hydrogen-bonding interactions determine the unusual ground state electronic and redox properties of BPhL. In the reduced state, a protein (solvent) rearrangement or tautomerization could be important for the redox equilibrium activity of BPhL. This hypothesis will be tested. Similar considerations apply to the first quinone electron acceptor, QA. Conservative (nonpolar) mutations at sites adjacent to the prosthetic group may perturb the existing protein-prosthetic group interactions sufficiently to cause measurable changes in the free energy of the state P+QA-. A role for protein may also be postulated in the dynamics of electron transport. Protein atoms may be vibronically coupled to electron transfer. Solvent-protein dipolar fluctuations may play a rate-limiting role under some conditions. It is also possible that protein-catalyzed tautomerization of the photosynthetic pigments could be linked to electron transport at a rate-limiting step. By measuring how mutations alter the dynamics of electron transport through time-resolve absorbance and fluorescence changes, we hope to build a picture of how nuclear motions are coupled to electron transfer in the reaction center protein. Site directed mutations will also be useful for information-rich spectroscopic assignments (NMR, IR, ENDOR, Raman) in the RC protein.