Purple bacteria primary charge separation

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 photosystems of green plants and photosynthetic bacteria appear to function with basically the same sort of mechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, but the molecular constituents of the reaction center are quite diflFerent. Photosystem I contains iron-sulphur proteins as electron acceptors so can be called iron-sulphur (FeS) type reaction center, while photosystem II contains pheophytin as the primary electron acceptor and quinone as the secondary acceptors so it can be called pheophytin-quinone (4>-Q) type . The reaction center of purple bacteria, green nonsulphur bacteria, and PSII are (4>-Q) type. Green sulphur bacteria, heliobacteria, and PSI have (FeS) type reaction centers. ... 

Photoreactions that produce chemical energy by excitation of BChl or Chi molecules take place in RCs. The process is referred to as the primary charge separation. Purple bacteria use a type of photosynthesis that, to some extent, resembles green plant photosynthesis in PSll. In the 1980s, two purple bacteria, Rhodopseudomonas viridis and Rhodobacter sphoeroides, reached a prominence that few had expected from species living at the bottom of ponds and similar places. Two German chemists, Johann Deisenhofer and Hartmut Michel, managed to dissolve the protein from the membrane, crystallize it, and determine its structure. 

The primary photochemical charge-separation process, i.e., P870-t-A -> P870 +A in purple photosynthetic bacteria requiresthat there is a reaction partner to accept the electron released by the primary donor. Again, using D-[P-A] to represent the core composition of the bacterial reaction center, we can write the following sequence of events ... 

When electron transfer to the secondary acceptor is disrupted, the separated charges recombine in a few nanoseconds, via the radical pair mechanism, to form the spin-polarized triplet state of the primary donor, P. As shown in Fig. 11, the decay time of P865 in the green filamentous bacterium Cf. aurantiacus is 6 //s at ambient temperature. At 1.2 K it is 75 /js. Reaction centers of Cf. aurantiacus contain two menaquinone molecules, MQa and MQg, which behave the same way as a pair of analogous quinones in purple bacteria and photosystem II. Under non-physiological conditions, MQa recombines with P865 in 60 ms and MQb in 1 s. 

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). 

Photosystem II reaction centers (RC) consist of D1 and D2 polypeptides and a bound cytochrome b 559. Isolated RC, named D1/D2 particles, contain 4-6 Chi a. 1 j3-carotene and 1 or 2 cytochrome b 559 per two pheophytin a molecules (1). Polypeptides D1 and D2 exhibit marked homologies with the L and M subunits of the RC of purple bacteria, respectively (2). PS II shares a number of functional similarities with bacterial RCs. In these particles, absorption of a photon results in a rapid charge separation between the primary electron donor. Peso molecule because of the lack of a secondary electron acceptor, relaxation of the D1/D2 reaction centers occurs through the formation of a 900 /is-lived triplet state involving Chi a molecule(s), named P (3). 

In the best-known purple bacteria, understanding of the primary electron transfer events is stiH rudimentary why is a dimer needed as primary electron donor, what is the function of the monomeric bacteriochlo-rophyll located on the active branch, what makes possible a high yield of charge separation even at low temperature, why are there two branches in the RC structure and why is only one of them active ... 

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