Energy Transfer Patterns in Photosynthetic Antenna System of Purple Bacteria

Sammanfattning: Steady state and time-resolved absorption and fluorescence spectroscopy have been used to investigated the energy transfer in photosynthetic purple bacteria. Picosecond light pulses were used in measurements of Rb. sphaeroides WT and a LH1-deleted mutant, demonstrating that the energy trapping into the reaction center in both species occur on the same time-scale, and that the excitation energy is efficiently transmitted from LH2 to the RC despite the absence of LH1. Femtosecond infrared pulses were used to study the ultrafast energy transfer among the LH2 antenna pigments of Rb. sphaeroides Steady state and time-resolved absorption and fluorescence spectroscopy have been used to investigated the energy transfer in photosynthetic purple bacteria. Picosecond light pulses were used in measurements of Rb. sphaeroides WT and a LH1-deleted mutant, demonstrating that the energy trapping into the reaction center in both species occur on the same time-scale, and that the excitation energy is efficiently transmitted from LH2 to the RC despite the absence of LH1. Femtosecond infrared pulses were used to study the ultrafast energy transfer among the LH2 antenna pigments of Rb. sphaeroides WT, Rps. acidophila and Rps. palustris LL showing that the B800®B850 transfer time is 0.6-0.8 ps for all species. The excitation dynamics within LH2 have also been investigated in various specifically mutated LH2 complexes in which individual amino-acids close to the Bchl binding site have been altered. These mutations produced changes of the Bchl binding environment and leads to changes of the spectral properties. The results demonstrated a correlation of the donor-acceptor spectral overlap with the energy transfer rate which is in agreement with the trend predicted by Förster theory. Ultrafast two-color pump-probe measurements have allowed us to resolve temporally and spectrally the various steps of the energy transfer between and within the antenna complexes of Rb. sphaeroides, enabling us to suggest a model for a possible association between LH2 and LH1. Fast depolarization of the B800 excited states observed in anisotropy absorption measurements in the B800 band suggested a transfer time of 0.2-0.3 ps for transfer within the B800 band.