Mechanism and Macromolecular Organization in Photosynthetic Reaction Centers

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AbstractMCB 0416623The 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. Towards understanding electron transfer mechanism, several areas are described: (i) application of newly developed instrumentation for measuring Stark effects in the IR and theory for analyzing intervalence band Stark effects associated with the oxidized special pair; (ii) determination of the parameters that control alternative electron transfer pathways using a newly-developed experimental method, resonance Stark spectroscopy; (iii) application of the resonance Stark method to other pathways created by engineering the RC; (iv) measurement of electric field effects on the populations of charge-separated species in photosystem II RCs, in particular the role of carotenoid cations; and (v) development and application of RC strains in which all native cysteine residues are deleted and new cysteines are inserted as site-specific targets for different types of labels. Towards understanding photosynthetic membrane organization and assembly several areas are described: (i) characterization of the lateral mobility and assembly of antenna protein complexes in supported membranes; and (ii) imaging of components of the photosynthetic membrane in supported lipid bilayers by atomic force microscopy and a novel type of secondary ion mass spectrometry with high lateral resolution. This research uses photosynthetic proteins in supported membranes both as a tool to advance imaging and structural methods and to determine the long-range organization of this important membrane-associated assembly.By integrating this range of approaches, the basic understanding of the mechanism of the initial electron transfer and the related phenomenon of unidirectional electron transfer can be advanced and brought into the larger context of structure-function relationships in complex biological assemblies. Research in the photosynthesis area addresses the general problem of the mechanisms by which the organized environment in proteins modulates the properties of bound prosthetic groups and particular amino acid functional groups with extraordinary functional consequences. An important part of this work involves the development of new experimental and theoretical methods that can have a broad impact in other areas of science and technology. 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. This general area of research has also been a tremendous source of training for undergraduates, graduate students and postdoctoral fellows, often serving as a bridge between the physical sciences and biology. Viewed in broadest terms, concepts and results from photosynthesis research have impacted solar energy research and the emerging area of molecular electronics.