Delayed fluorescence from Rhodopseudomonas sphaeroides following single flashes

Mode of inhibition of electron transport by orthophenanthroline in chromatophores and reaction centers of Rhodopseudomonas sphaeroides

In Rhodopseudomonas sphaeroides, light causes the transfer of electrons from bacteriochlorophyll to ubiquinone in the photochemical reaction centers. Electrons from this "primary" ubiquinone move on to a secondary ubiquinone. We have studied the manner in which o-phenanthroline inhibits the transfer of electrons from primary to secondary ubiquinone in chromatophores (intracytoplasmic membrane fragments) and isolated reaction centers of Rp. sphaeroides. The formation of anionic semiquinones, both primary and secondary, is signaled by an absorption band at 450 nm and by band shifts of bacteriopheophytin and bacteriochlorophyll in the near infrared. The pattern of band shifts is different for primary and secondary semiquinone, allowing us to distinguish which quinone has become semireduced. This point was established for isolated reaction centers [Vermeglio, A. & Clayton, R. K. (1977) Biochim. Biophys. Acta 461, 159-165] and is confirmed here for chromatophores. By adding o-phenanthroline at various times during a sequence of actinic light flashes and monitoring the resulting optical absorbance changes, we have found that o-phenanthroline inhibits electron transfer from primary semiquinone to secondary quinone if the latter is in its oxidized form, but not if the latter is semireduced. Our findings can explain the decay kinetics of delayed fluorescence from bacteriochlorophyll in Rp. sphaeroides as measured by R. P. Carithers and W. W. Parson [(1976) Biochim. Biophys. Acta 440, 215-232].

Short-lived delayed luminescence of photosynthesizing organisms. II. The ratio between delayed and prompt fluorescence as studied by the modulation method

The ratio between the intensities of delayed and prompt fluorescence was studied for different photosynthetic objects under different conditions by modulation method. The method is based on excitation of luminescing objects by light, modulated harmonically, and on a combined study of phase shifts and demodulation coefficients of the luminescence as related to excitation light. The presence of intense delayed emissions was revealed in purple bacteria, Ectothiorhodospira shaposhinokovii, Rhodospirillum rubrum and Rhodopseudomonas sphaeroides, in the micro- and nanosecond range. Under conditions of saturating light, their proportion was several percent of the total emission. The most striking phenomenon was observed under reducing conditions (addition of 1 . 10(-2) M Na2S2O4 to whole-cell suspensions of purple bacteria) where the intensity of the delayed emissions grew dramitically and became comparable to that of prompt fluorescence. The data obtained indicate that, at room temperature, reversal of some early stages of charge separation in bacterial reaction centres may proceed largely via the channel that includes generation of the reaction-centre bacteriochorophyll in the excited singlet state, followed by excitation-energy migration to antenna bacteriochlorophyll. The relation of these phenomena to the efficiency of solar energy utilization in photosynthetic apparatus is discussed.