Relaxation processes accompanying electron stabilization in the quinone acceptor part of Rb. sphaeroides reaction centers

The temperature dependence of the dark recombination rate in photooxidized bacteriochlorophyll (P) and photoreduced quinone acceptors (ubiquinones) Q_A and Q_B of photosynthetic reaction centers of purple bacteria Rhodobacter sphaeroides (Rb. sphaeroides) was studied. Photoinduced changes in the absorption were detected in the Q_x absorption band of photooxidized bacteriochlorophyll at 600 nm and in the bands corresponding to the redox changes of ubiquinones at 335 and 420-450 nm. Kinetic analysis was used to evaluate the activation energy and the characteristic time of the transient process of relaxation accompanying electron stabilization at the final quinone acceptor. A comparative study of the kinetics of oxidation-reduction reactions of photoactive bacteriochlorophyll RC purple bacteria and quinone acceptors in their individual absorption bands is an informative approach to studying the mechanisms of this stabilization. The analysis of the revealed kinetic differences makes it possible to estimate the activation energy and the characteristic times of the transition relaxation processes associated with the stabilization of the electron in the quinone acceptor part of RC. Purple bacterial reaction centers have fundamental similarities with PSII reaction centers. Such a similarity represents evolutional closeness between the two types of RC. So it is possible that the photoinduced charge separation in PSII RC, as well as in purple bacteria RC, is also accompanied by definite conformational changes. The possible role of hydrogen bonds of surrounding protein in the relaxation processes accompanying the electron transfer to quinone acceptors is discussed.