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

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 Q_a 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.