Controlling the Pathway of Electron Transfer in Bacterial Reaction Centers

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Light-initiated electron transfer within the photosynthetic reaction center is the primary event in solar energy conversion by photosynthetic organisms. The core of the photosynthetic reaction center from purple nonsulfur bacteria is a quasi-symmetric heterodimer, providing two potential pathways for transmembrane electron transfer. Past measurements have demonstrated that only one of the two pathways (the A-side) is used to any significant extent upon excitation with red or near infrared light. Recently, Dr. Woodbury has found that excitation with blue light into the Soret band of the reaction center gives rise to photochemistry along the alternate or B-side pathway apparently within a 200 fs laser pulse. The spectral signature of the states formed, and their long (nanosecond) lifetime at low temperature, are characteristic of charge separated states involving the anion of the B-side bacteriopheophytin. This opens the door to a whole new array of measurements attempting to understand both the mechanism and the function of B-side electron transfer in the reaction center. This research project centers around the interplay between the free energy of the charge separated states, which can be varied over a large range using mutants that alter the P/P+ potential, and the energy of the photon used in excitation. Preliminary data suggests that the more favorable the energetics of B-side electron transfer, the lower the energy of the photon that is required to initiate B-side transfer. The physiological function of B-side electron transfer is not yet known. One possibility is that it could rapidly quench the excited states formed from blue or UV light absorption by reaction center cofactors and surrounding tryptophan residues. Several experiments involving mutants that disrupt B-side transfer are planned to see if reaction centers are more susceptible to UV and blue light damage without functional B-side photochemistry. Finally, in order to generate a series of new mutants with spectrally defined functional properties (loss or A-side electron transfer, increased B-side transfer, etc.), Dr. Woodbury will develop a directed evolution methodology that utilizes entirely optical means of both screening and selecting cells from a large library of reaction center mutants.