Kinetics of recombination of photoseparated charges in Rhodobacter sphaeroides reaction centers analyzed by relaxation rate constant distribution

We studied the kinetics of dark recombination of charges photoseparated between the bacteriochlorophyll dimer (P) and the quinone acceptors of the photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides. The time of light activation was varied from 1 to 60 s. The relaxation rate constant distribution was computed from experimental kinetic curves of charge recombination, using our original method whereby relaxation curves are approximated by a set of Gaussian-like peaks in the time domain that correspond to different conformational sub-states of RCs. With increasing photoactivation exposure, some peaks shift toward longer relaxation times. A ‘bifurcation’ of the slower peak is observed at exposures of 20–30 s. The phenomenon is interpreted as evidence of a conformational transition induced by separated charges in the RC structure.