Energy transfer in the photosynthetic antenna system of the purple non-sulfur bacterium Rhodopseudomonas cryptolactis

Abstract

Energy transfer in the antenna of the purple non-sulfur bacterium Rhodopseudomonas cryptolactis was studied by means of fluorescence and transient absorption measurements. Different growth conditions resulted in three cultures which differed in the composition of the peripheral antenna (B800-820, B800-850 or a mixture of both). Oxidation of the primary electron donor caused an increase of the fluorescence yield of the core complex, B880, and of B800-850, but not of B800-820, indicating energy transfer from B880 to B800-850, but not to B800-820. Singlet-singlet annihilation measurements showed that the number of bacteriochlorophylls of B880 between which energy transfer can occur corresponds to one or two photosynthetic units only (40 to 70 BChls), for all the three cultures studied. These results are explained by a model for Rps. cryptolactis in which each 13880 unit is associated with its own complement of B800-850. The B800-850 complexes do not appear to form an interconnecting network. Time-resolved transient absorption measurements showed trapping rates by open and closed reaction centres of (50 ps) −1 and (200 ps) −1, respectively. Time constants of energy transfer from B800-850 to B880 or from B800-820 to other complexes were 12–15 ps.