Rhodopseudomonas viridis photosynthetic reaction charge separation

See, e.g., J. Deisenhofer, H. Michel, The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas-Viridis. Science 1989, 245, 1463-1473 M. E. Michel-Beyerle, M. Plato, J. Deisenhofer, H. Michel, M. Bixton, J. Jortner, Unidirectionality of Charge Separation in Reaction Centers of Photosynthetic Bacteria. Biochim. Biophys. Acta 1988, 932, 52-70. 

J Breton, J-L Martin, A Migus, A Antonetti and A Orszag (1986) Femtosecond spectroscopy of excitation energy transfer and initial charge separation in the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis. Proc Nat Acad Sci, USA 83 5121-5125... 

The photosynthetic reaction centres (RCs) are transmembrane protein-pigment complexes that perform light-induced charge separation during the primary steps of photosynthesis. RCs from purple bacteria consist of three protein subunits, L, M and H, and bind four bacteriochlorophylls, two bacteriopheophytins, two quinones, one non-haem iron and one carotenoid. The elucidation at atomic resolution of the three-dimensional structures of the bacterial RCs from Rhodopseudomonas (Rps.) viridis (1) and Rhodobacter (Rb,) sphaeroides (2-4) has provided impetus for theoretical and experimental work on the mechanism of primary charge separation in the RCs. The structures revealed that the cofactors are bound at the interface between the L and M subunits and are organised around a pseudo C2 symmetry axis. However, the structural symmetry does not result in functional symmetry as the electron transfer proceeds only along the L branch (5). 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

The primary electron transfer event in photosynthesis is the transfer of an electron from the excited state reaction centre chlorophyll [P]to pheophytin [I]. This charge separation is then stabilised by transfer of the electron to a chain of acceptors and the rereduction of the reaction centre chlorophyll by an electron donor. In the purple photosynthetic bacterium Rhodopseudomonas viridis the electron acceptors are quinones and the electron donors are cytochrome haems. The acceptor complex is thought to consist of a primary quinone [Qa], which is a menaquinone, and a secondary quinone [Qb], which is ubiquinone. Qa is tightly bound to the reaction centre and undergoes... 

The rates of the electron transfer processes in reaction centers (RC s) of photosynthetic bacteria are controlled both by the spatial and the electronic structure of the involved donor and acceptor molecules. The spatial structure of bacterial RC s has been determined by X-ray diffraction for Rhodopseudomonas (Rp.) viridis and for Rhodobacter (Rb.) sphaeroides,- The electronic structure of the transient radical species formed in the charge separation process can be elucidated by EPR and ENDOR techniques. The information is contained in the electron-nuclear hyperfine couplings (hfc s) which, after assignment to specific nuclei, yield a detailed picture of the valence electron spin density distribution in the respective molecules. 

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