Calculation of Connectivity of Photosynthetic Units in Intact Cells of Rhodobacter Sphaeroides

Abstract

The kinetics of the bacteriochlorophyll fluorescence of intact cells of photosynthetic bacterium Rhodobacter sphaeroides was measured under rectangular shape of intense excitation in the microsecond time range. The interest was focused to the initial (sigmoidal) rise of the fluorescence to characterize the connectivity of the photosynthetic units. As the rate of the primary photochemistry (charge separation) was set to be much larger than that of the re-reduction of the oxidized dimer of the reaction center, there was reciprocity between light intensity and photochemical rise time, and therefore a simple model of single fluorescence (photochemical) quencher could be applied. By linearization of the fluorescence induction kinetics, the connectivity parameter p could be directly obtained (p = 0.47 ± 0.01) and was independent on the intensity of the light excitation. The mean value of the number of steps (visits) in the antenna was calculated before an exciton is either trapped by an open reaction center (it is utilized by photochemistry) or dissipated in form of fluorescence emission. According to these calculations, the mean number of steps is less than 1 if p < 0.5, even if all of the reaction centers are closed. The observed small p value includes highly restricted mobility of excitions among the photosynthetic units.