Primary charge separation processes in reaction centers of an antenna-free mutant of Rhodobacter capsulatus

The primary charge separation processes of membrane-bound reaction centers from an antenna-free mutant of the purple bacterium Rhodobacter capsulatus have been investigated by picosecond time-resolved fluorescence and femtosecond stimulated emission measurements. Target analysis has been applied to decompose the time-resolved fluorescence emission spectrum into its components for open and closed (quinone-reduced) reaction centers. The reaction center environment, i.e. membrane vs. detergent, exerts a remarkable influence on the primary charge separation kinetics. For fully open reaction centers the main component of the primary electron transfer process from the excited special pair to bacteriopheophytin — measured both in fluorescence and in femtosecond stimulated emission — takes 5–6 ps, which is a factor of nearly two longer than in the corresponding detergent-isolated reaction centers (2.7 ps). For quinone-reduced RCs a very long charge separation time of approx. 10 ps is observed, concomitant with a more than five-fold increase in fluorescence yield. All of these parameters differ substantially from those of detergent-isolated RCs. Possible origins for the observed kinetics are discussed.