ENERGY-TRANSFER AND CHARGE SEPARATION KINETICS IN PHOTOSYSTEM-I .2. PICOSECOND FLUORESCENCE STUDY OF VARIOUS PS I PARTICLES AND LIGHT-HARVESTING COMPLEX ISOLATED FROM HIGHER-PLANTS

Two different Triton X-100 isolated photosystem (PS) I particle preparations as well as isolated light-harvesting complex (LHC) I from spinach have been studied by steady state and time-resolved fluorescence with a time resolution of about 5 ps. The dependence of the fluorescence kinetics and spectra on two parameters was investigated in detail: firstly, the solubilizing detergent used in the measurements was varied between Triton X-100, octylglucoside and SB12 (zwitterionic detergent). Secondly, the PS I particle size and type were varied, studying what is believed on the basis of literature data to be PS I-native and PS I core, respectively. All of these changes had pronounced effects on the kinetics. By global analysis procedures, we generally find four and in some cases five lifetime components in all data sets. Our results indicate that all PS I preparations studied, including the PS I-core complex lacking LHC I, are heterogeneous, displaying at least a bimodal distribution in antenna size. We attribute this size distribution to the action of Triton X-100 used in the isolation procedure. The fastest lifetime component in the range of 6-14 ps, depending on antenna size, appears as a rise term at wavelengths above 710 nm in all particles and is attributed to an energy transfer in the antenna system from the main pool of short wavelength absorbing pigments (F690) to pool(s) of long wavelength absorbing and fluorescing pigments (F720 and/or F735). The observation of such an antenna equilibration component at room temperature is crucial for the understanding of the excitation kinetics. Furthermore, for the sample that is believed to be ''native'' PS I, we found two lifetime components of 31-49 ps (depending on the detergent) and 130 ps. We attribute both of them to an overall charge separation, but originating from different particle types in the sample. We thus propose a fundamental antenna size heterogeneity. The shorter time most probably corresponds to intact PS I cores, whereas only the longer one originates from ''native'' PS I with antenna size of about 200 Chl. The data suggest that the excitation kinetics in all of these PS I particles is close to the trap-limit.