Energy trapping and detrapping in the photosynthetic bacterium Rhodopseudomonas viridis: transfer-to-trap-limited dynamics Original Research Article

The efficiency of energy back-transfer from the reaction center to the antenna in chromatophores of the photosynthetic purple bacterium Rhodopseudomonas viridis was measured at room temperature by means of picosecond time-resolved fluorescence spectroscopy. It was found that 20 ± 5% of the excitation energy selectively absorbed by the reaction center pigments in the wavelength region around 830 nm is transferred back to the antenna and gives rise to antenna fluorescence. The measured yield of energy detrapping enabled calculation of the energy detrapping (10–13 ps)−1 and trapping (40–50 ps)−1 rates. These results confirm that in Rps. viridis, like in bacteriochlorophyll a-containing purple bacteria, the energy transfer step from antenna pigments to the reaction center is a rate limiting step in the overall energy trapping by the reaction center. We suggest that this situation is termed ‘transfer-to-trap-limited’ dynamics, to distinguish it from the situation where the charge separation is the rate limiting step (trap limited) or the overall energy diffusion through the antenna is limiting (diffusion limited).