Primary charge separation, bacterial

Bixon, M., Jortner, J., and Michel Beyerle, M. E., 1991, On the mechanism of the primary charge separation in bacterial photosynthesis. Biochim. Biophys. Acta, 1056 3019316. 

Creighton, S., Hwang, J. K., Warshel, A., Parson, W. W., and Norris, J., 1988, Simulating the dynamics of the primary charge separation process in bacterial photosynthesis. Biochemistry, 27 7749781. 

Fig. 10. (A) and (B) Two models for the electron-transfer sequence in bacterial reaction centers. (C) Population densities of various intermediary states as a function of time calculated according to the model shown in (B). See text for discussion. Figure source (A) and (8) Holzapfel, Finkele, Kaiser, Oesterheldt, Scheer, Stilz and Zinth (1989) Observation of a bacteriochlorophyll anion radical during the primary charge separation in reaction center. Chem Phys Lett 160 5 (C) S Schmidt, T Arit, P Hamm, H Huber, T NSggle, J WachtveitI, M Meyer, H Scheer and W Zinth (1994) Energetics of the primary electron transfer reaction reveaied by ultrafast spectroscopy on modified bacterial reaction centers. Chem Phys Lett 223 118.

MG Rockley, MW Windsor, RJ Cogdell and WW Parson (1975) Picosecond detection of an Intermediate In the photochemical reaction of bacterial photosynthesis. Proc Nat Acad Sci, USA 72 2251-2255 J Fajer, DC Brune, MS Davis, A Forman and LD Spaulding (1975) Primary charge separation In bacterial photosynthesis Oxidized chlorophylls and reduced pheophytin. Proc Nat Acad Sci, USA 72 4956 960 PL Dutton, KJ Kaufmann, B Chance and PM Rentzepis (1975) Picosecond kinetics of the 1250 nm band of the Rps. sphaeroldes reaction center. The nature of the primary photochemical Intermediary state. FEES Lett 60 275-280... 

This paper is an account of our continuing effort in the investigation of the primary charge separation in bacterial RC. In the next sections we will describe in some detail our advances in the modeling and understanding of the RC photosynthetic proteins. 

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 mechanism of the primary charge separation in the bacterial reaction centre (RC) is of central importance for the elucidation of the energy conversion processes in photosynthesis. All the mechanisms proposed for the primary electron transfer (ET) from the singlet excited state of the bacteriochlorophyll dimer (P) along the A branch of the RC, attribute a special role to the accessory monomer bacteriochlorophyll (B), which is structurally located between P and the bacteriopheophytin (H). Two classes of mechanisms were advanced [1] ... 

In photosynthesis, the primary charge separation occurs in the reaction center. To study this electron transfer chain, magnetic resonance measurements are carried out on bacterial reaction centers of Rhodobacter sphaeroides R 26. The investigated triplet states are used as intrinsic probes of the pigment interactions in this protein complex. 

No evidence for an intermediate acceptor, such as Chi ", was obtained at times > 500 fs. The rate of charge separation increases by a factor of two when the temperature is lowered from 277 to 15K, similar to results observed previously with bacterial RC s. Finally, the apparent rapid recovery of the 672 nm band (x - 25 ps) suggests the presence of a second photophysical process in PSII RC s at 15K (but not 277K) that is not coupled to primary charge separation. 

The photochemical activity of PS II reaction centers associated with primary charge separation has been documented by direct measurements of electron transfer by substrate donors (including water) or acceptors as well as by spectroscopic methods involving optical absorption, fluorescence and EPR. On the picosecond time scale it appears that the sequence of events and even the kinetics associated with the earliest steps are very similar between PS II and the purple bacterial reaction centers (15). Nevertheless, other aspects of this similarity remain to be demonstrated whether the primary electron donor of PS II consists of a special pair of chlorophylls, whether the PS II reaction center possesses a structural two-fold symmetry together with a functional asymmetry and whether there is a portion of the PS II complex that corresponds to the H polypeptide. 

Stable chlorophyll 7r-cation radicals may be generated by either chemical or electrolytic one-electron oxidation of the neutral species, and their properties have been extensively studied [46]. These cation radicals are of particular significance, as there is evidence that radicals of Chi or BChl dimers are formed during the primary charge separation processes in green plant and bacterial photosynthesis, respectively [47, 48]. 

In the light-driven primary charge separation processes of photosynthesis, the initial step involves irreversible electron transfer from the primary electron donor to an acceptor within 10 picoseconds [50]. In bacterial photosynthetic systems, it has been established that a BChl special pair acts as the primary electron donor, and that BChl and BPheo monomers act as acceptors [51]. Moreover, electron transfer in green plant photosynthetic systems is in generally known to involve Chi and Pheo moieties [41]. 

Small, G.J. On the validity of the standard model for primary charge separation in the bacterial reaction center. Chem. Phys. 197, 239-257 (1995)... 

The primary charge separation step in bacterial photosynthesis from an excited bacteriochlorophyll (BChl) dimer to a bacteriopheophytin with a possible intermediate BChl monomer is controlled both by the three dimensional arrangement and by the electronic structure of the reacting pigments [l,2]. The three dimensional structure of bacterial photosynthetic reaction centers (EC s) has been determined for R. viridis [3 4] and for Rb. sphaeroides R-26 [5-7] by X-ray diffraction. 

The primary charge separation in photosynthetic bacterial reaction centres (RC) is charactrized by three unique features (i) Ultrafast rate in the psec domain which precludes energy waste due to back transfer to the antenna, (ii) non-Arrhenius temperature dependence of the rate, which manifests optimal coupling to the medium nuclear motion, and (iii) unidirectionality across the A branch of the quasisymmetric RC. 

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