| The work presented in this thesis is a comparative study of singlet-triplet exciton annihilation in in vitro chlorophyll, in chlorophyll bound to chloroplast membranes, and to proteins isolated from chloroplast membranes.;Fluorescence quenching curves, i.e., plots of the relative fluorescence yield as a function of excitation intensity, of chlorophyll a in solution, free pigments, the light-harvesting chlorophyll-protein complex (LHCP), and the aggregates of LHCP were determined.;In photosynthetic systems, chlorophyll fluorescence is observed after absorbed energy migrates among the photosynthetic pigments. The number of molecules participating in this energy-migration sequence defines the domain size of the photosynthetic system. Samples were chosen to establish and develop the concept that singlet-triplet exciton annihilation occurring in domains of different sizes gives rise to fluorescence quenching curves that differ in shape and their position along the logarithmic excitation intensity axis. The effect of the triplet quantum yield on the characteristics of the fluorescence quenching curve is discussed.;These features were used to determine the possibility of energy transfer among chlorophyll molecules bound to unconnected proteins, and to obtain the domain size of aggregates. The singlet-triplet annihilation constant of in vivo chlorophyll was determined. The triplet quantum yield in chloroplasts and in LHCP was calculated from a master-equation theory of singlet-triplet annihilation in photosynthetic systems.;Utilizing laser pulses of microsecond duration and varying intensities, singlet-triplet exciton annihilation is manifested as a decrease in the flourescence yield as the excitation intensity is progressively increased above a critical intensity. Ground state depletion, excited state absorption, and lasing, may produce effects analogous to fluorescence quenching, and therefore these processes were also investigated.;Fluorescence quenching experiments were conducted at cryogenic temperatures in order to investigate the role of preferential exciton annihilation in different pigment beds.;Our results establish that fluorescence quenching techniques may be used to probe pathways of energy migration, and to determine the physical and functional sizes of various photosynthetic systems. |
