Mechanism and Macromolecular Organization in Photosynthetic Reaction Centers

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The objective of this research is to understand the mechanisms of charge separation and recombination in photosynthetic reaction centers (RCs) and the higher-order organization of photosynthetic membranes. This research project focuses on three areas: (i) the mechanism of electron transfer in RCs where the primary electron acceptor has been removed; (ii) the use of vibrational spectator probes to quantify electrostatics in the RC; and (iii) the development of advanced imaging techniques using well-defined photosynthetic complexes and assemblies as a model system. By working with RCs in which normal electron transfer is blocked, new pathways can be revealed in a clean background and important intermediates can be trapped. In particular, the kinetics and energetics of electron transfer along the ordinarily non-functional side of the RC can be studied in depth. This project will investigate the orientation of a particular amino acid that is believed to modulate reaction energetics. A suite of methods will introduce spectator vibrational probes into the RC to measure differences in electrostatic fields in symmetry-related positions, as well as probe changes in fields as electron transfer occurs. Areas (i) and (ii) focus on specific, well-defined questions that arise in bacterial photosynthesis; the results and methods will apply to more complex photosynthetic organisms and the broad area of biological electron transfer. Area (iii) uses photosynthetic systems, but will impact the development of new approaches for characterizing membranes and membrane proteins in general. Imaging mass spectrometry will be pushed to its limit using the highly organized photosynthetic membrane, in particular to see whether integral membrane protein organization can be visualized. A "membrane interferometer" has been developed that combines advanced fabrication, membrane assembly and optics in a device that is highly sensitive to membrane curvature and osmotic pressure. Photosynthetic proteins will be used to test the precision of this interferometer, to test hypotheses concerning the impact of photosynthetic membrane proteins on membrane curvature and budding, and as reporters of membrane curvature upon incorporation of channels that are sensitive to membrane tension.Sustained support in this area has led to the development of many new approaches for studying electron transfer reactions, one of the most important classes of reactions in physical and biological systems. Nearly every spectroscopic, structural, and theoretical method has been applied to some aspect of RC and photosynthetic membrane function. In many cases this was the first example of new physical approaches applied to a biological problem, often leading to significant improvements in the physical and theoretical methods, and these concepts and methods are now used in many other fields. Assembly of RCs on surfaces opened an entirely new area of technology, membrane patterning, which is now used in many laboratories and is the basis of efforts to use membrane and membrane-protein arrays for screening in several biotechnology companies. This general area of research has been a tremendous source of training for undergraduates, graduate students and postdoctoral fellows, often serving as a bridge between the physical sciences and biology. The PI has given more than 100 invited talks at universities, national labs, companies and scientific conferences worldwide over the past 5 years. Viewed in broadest terms, concepts and results from photosynthesis research have impacted solar energy research and the emerging area of molecular electronics.