Role of relaxation processes in stabilization of photoinduced charges in a biological membrane

Abstract

Picosecond absorption spectroscopy was used in a study of stabilization of a primary ion-radical pair P +I − in reaction centers of purple bacteria Rb. sphaeroides. Measurements were made on samples with hydrogen bonds modified by isotopic substitution of H2O with D2O and by introduction of cryoprotectors into the structure of the reaction centers. The state P +I −, which appeared as a result of separation of charges in a P *I complex, was stabilized by the interaction of P +I − with charged light groups of the water–protein environment in modified reaction centers after an interval of ~ 1 ns (stabilization in control samples took ~ 100 ps). Moreover, the transport of an electron to the next acceptor Qa was slowed down (by a factor of ~ 5) in modified reaction centers and the probability of recombination of charges increased (by a factor of ~ 3) in the unrelaxed state. Relaxation processes lowered the energy level of P +I − by 0.03–0.05 eV and reduced the energy of the interaction in an ion-radical pair by a factor of 2.