Charge Recombination at Low Temperature in Photosynthetic Bacteria Reaction Centers
The first photochemical event in purple bacterial photosynthesis occurs in an integral membrane protein-pigment complex, the so-called reaction center. The reaction center is composed of three polypeptide subunits and a number of cofactors: four bacteriochlorophylls, two bacteriopheophytins, one (or two) ubiquinones, and one non-heme iron atom (Fe2+) (see ref. 1 for a review). The three-dimensional structure of the reaction center from the photosynthetic bacterium, Rhodopseudomonas viridis, has been recently determined by X-ray diffraction at a resolution of 2.9 Å (2,3). Upon light absorption, a bacteriochlorophyll dimer (P) is raised to an excited state (P*). In a few picoseconds, an electron leaves the primary donor P* and a transient radical pair state (P+Bpheo−) is formed (4, 5, 6). The electron then migrates from the reduced Bpheo molecule to one of the quinone molecule, QA, in about 200 ps. (7, 8, 9). The possibility of structural changes linked to this light-induced charges separation has been discussed by several authors. NOKS et al (10) found that electron transfer kinetics are affected when chromatophores are incubated with the cross-linker glutaraldehyde, but only if the incubation is performed under illumination. ARATA and PARSON (11, 12) have presented evidences for a decrease in the volume of the reaction center-solvant system from calorimetric studies. WOODBURY and PARSON (13) suggest that there is a discrete distribution of states involved on the time scale of the early electron-transfer steps. VERMEGLIO and PAILLOTIN (14) have proposed from photodichroism studies at low temperature, that the voyeur bacteriochlorophyll molecule framework moves in the reaction center complex after charge separation. Similar results have been obtained by VASMEL et al (15) in their linear dichroism study on oriented Chloroflexus aurantiacus reaction centers in squeezed polyacrylamide gel. KIRMAIER et al (16, 17) have observed different kinetics components as a function of probe wavelength for the formation of state P+QA − in both Rhodobacter sphaeroides and Chloroflexus aurantiacus reaction centers. They postulated that this behaviour is due to readjustments of the pigments and/or the protein following the charges separation process. KLEINFELD et al (18) have modeled the non-exponential behaviour for the charges recombination kinetics observed at low temperature in terms of a distribution of structural configurations. These authors also observed that, when reaction centers are cooled under continuous illumination, the recombination time of a subsequent light-induced charge separation at low temperature was lengthened by a factor of 5. They proposed (18) that the light-pretreatment induces structural changes which lead to an increase of the donor-acceptor electron-transfer distance.
KeywordsReaction Center Charge Recombination Rhodobacter Sphaeroides Primary Electron Donor Reaction Center Complex
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