Abstract
General equations are formulated to describe the competition between primary photochemistry (trapping) by photosynthetic reaction centers (RCs) and bimolecular singlet-singlet annihilation which occur upon excitation with picosecond laser flashes. These equations are solved under the assumption of fast equilibration and free energy migration between photosynthetic units (lake model) with homogeneous antenna and of irreversible trapping (the initial charge separation does not repopulate an excited state). The yield and the kinetics of fluorescence and of trapping are calculated as a function of the energy of the picosecond flash and of the fraction of RCs closed before excitation. Theoretical curves are adjusted to fluorescence yield measurements in the purple bacterium Rhodospirillum rubrum (Bakker, J.C.G., Van Grondelle, R. and Den Hollander, W.T.F. (1983) Biochim. Biophys. Acta 725, 508–518) and are used to analyse time-resolved photovoltage measured on Rhodobacter sphaeroides R26.1 whole cells. From these analyses, a parameter that relates the competition between trapping and annihilation as well as the ratio of the quenching efficiency for open and closed RCs are determined.
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