Time-resolved chlorophyll fluorescence studies on pigment-protein complexes from photosynthetic membranes

Room-temperature single-photon timing measurements at low excitation energies have been carried out on Photosystem II (PS II) pigment-protein complexes containing 40-70 chlorophylls per reaction centre (RC) as a function of PS II trap closure, Chl fluorescence emission spectra and detergent concentration. The lifetime and amplitude values obtained, using a four exponential model, have been compared with the values exhibited by the isolated light harvesting Chl a b complex of PS II (LHC II) and a Photosystem I (PS I) complex containing 100 Chl/RC. The results suggest that the two rapid decays seen in the PS II preparations arise from the 47 kDa (t{cyrillic} = 54 ps) and 43 kDa (t{cyrillic} = 180 ps) Chl containing polypeptides but that in vivo these two proteins give rise to a single variable decay. The differences are discussed in terms of a perturbation in excitation energy migration by detergent molecules, although the presence of high detergent levels does not alter the kinetic and amplitude characteristics of these decays. The two long-lived decays seen in the PS II-core complexes have the same lifetimes and characteristics with respect to detergent addition as the isolated LHC II, suggesting that they arise from LHC II contamination. Unlike the two fast decays, the two long-lived components are extremely sensitive to detergent concentration. The requirement of two decay components in LHC II suggests the presence of a heterogeneous population. Isolated PS I also gives rise to four decay components with 99% of the excitation energy located in pigment-proteins which exhibit lifetimes of 110 ps and 27 ps. The lifetime and amplitude values have been used to calculate estimations for the single-step transfer time between interacting Chl molecules, the number of visits required before trapping takes place and the quantum yield of photochemistry.