SPECTRA OF FLUORESCENCE LIFETIME AND INTENSITY OF Rhodopseudomonas sphaeroides AT ROOM AND LOW TEMPERATURE. COMPARISON BETWEEN THE WILD TYPE, THE C 71 REACTION CENTER-LESS MUTANT AND THE B800–850 PIGMENT-PROTEIN COMPLEX

Abstract— Spectra of the fluorescence lifetime and intensity of chromatophores from the wild type Rhodopseudomonas sphaeroides , from the C 71 reaction center-less mutant and of the B800–850 light harvesting pigment-protein complex have been studied by phase fluorimetry techniques at different light modulation frequencies at room and low temperature.
As already known, closed reaction centers (saturating light) are still quenchers of antenna fluorescence although with a lower efficiency than when they are opened. The fluorescence yields and lifetimes of both the C 71 mutant strain and the B800–850complex are found to increase by about 30% between room and low temperature.
The fluorescence lifetimes obtained for the C 71 strain (0.65 ns at 20C; 0.85 ns at 77 K) and for the B850 complex (1 ns at 20C; 1.3 ns at 77 K) indicate that the non-radiative deactivation pathways, in the antenna, remain important in the absence of the reaction centers even at low temperature. We suggest that these data arise from the presence of special antenna molecules which act as intrinsic quenchers of the B875 antenna fluorescence. Between room and low temperature, the fluorescence yield and lifetime of the wild type are found roughly constant. This result suggests that the energy trapping by the reaction centers is independent of the temperature. The mechanism governing the energy transfer from the antenna to the reaction centers may differ from the mechanism leading to the energy transfer within the antenna. We suggest that a partially irreversible trapping of the excitation energy, on its way to the reaction center, takes place in the vicinity of the reaction center.