TRAPPING KINETICS, ANNIHILATION, AND QUANTUM YIELD IN THE PHOTOSYNTHETIC PURPLE BACTERIUM RPS-VIRIDIS AS REVEALED BY ELECTRIC MEASUREMENT OF THE PRIMARY CHARGE SEPARATION

The kinetics and the yield of the primary charge separation in whole cells of Rps. viridis was studied with the light-gradient technique using 30 ps and 12 ns flashes at 532 nm and 1064 nm. When the menaquinone acceptor, QA, of the reaction centers (RCs) is oxidized the primary charge separation occurs with two electrogenic phases. The faster phase with a time constant of 45 ± 20 ps contributed with 40% and the slower phase with a time constant of 140 ± 15 ps contributed with 60% to the total electrogenicity. We interpret the fast phase as the trapping time, monitored by the charge separation between the primary donor P-965 and the intermediary pheophytin acceptor, H. The second phase is ascribed to the electron transfer from H to QA. The reduction of QA, either chemically or photochemically, prior to the flash leaves only the fast rising phase. This signal decays with a time constant of 2.3-2.8 ns. At the excitation wavelength of 1064 nm the quantum yield of primary photochemistry is 0.97 ± 0.07. The reduction of QA does not change the quantum yield. The recombination of the state P+H- when QA is reduced can repopulate the excited state (ant.·P)* as demonstrated by a fluorescence phase with the same time constant. The fast phase of fluorescence was not affected by the reduction of QA, indicating an unaffected primary photochemistry. Fluorescence measurements with double flashes delayed by nanoseconds showed that the quenching power of P+ is approx. 1.4-times smaller than that of P. A comparison of the photovoltage amplitude evoked by 12 ns flashes and 30 ps flashes at 1064 nm revealed marked competition between annihilation and trapping. At 532 nm the competition was less pronounced. The influence of the fraction of closed RCs (as defined by the state P+Q-A on the trapping efficiency was studied by excitation with trains of 30 ps flashes. These data and the data resulting from the nanosecond versus picosecond measurements are analyzed with a theory for the light-gradient photovoltage (Leibl and Trissl (1990) Biochim. Biophys. Acta 1015, 304-312) and a theory of exciton dynamics in the lake model (Deprez et al. (1990) Biochim. Biophys. Acta 1015, 295-303). © 1990.