DETRAPPING OF EXCITATION-ENERGY FROM THE REACTION-CENTER IN THE PHOTOSYNTHETIC PURPLE BACTERIUM RHODOSPIRILLUM-RUBRUM

Using picosecond absorption and fluorescence spectroscopy we have measured the yield of energy detrapping from the reaction centre of the photosynthetic purple bacterium Rhodospirillum rubrum at room temperature. By selective excitation of the reaction centre pigments at approx. 800 nm and probing of the antenna bleaching or fluorescence, 25 +/- 5% and 40 +/- 5% of the excitation energy was found to be transferred back to the antenna for photochemically fully active (PBIQ) and prereduced (PBIQ-) reaction centres, respectively. Based on these data and the solution of a simple kinetic model, conclusions regarding the overall trapping time, antenna aggregation state and reaction centre coordination were obtained. Thus, the overall trapping time was concluded to be approx. 35 ps, in very good agreement with previous measurements, and the light-harvesting antenna is suggested to be organized in (alphabeta)2 clusters which surround the reaction centre in a six-fold coordination.