A comparison of the temperature dependence of charge recombination in the ion-radical pair P870+QA- and tryptophan fluorescence in the photosynthetic reaction centers of Rhodobacter sphaeroides

Abstract

The temperature dependences of the charge-recombination rate in the ion-radical pair P870⁺Q_A^- in photosynthetic reaction centers of Rhodobacter sphaeroides were investigated. Recombination kinetics were measured in the individual absorption bands of the donor (600 nm) and an electron acceptor (335 and 420–450 nm) for the reaction center in the water–glycerol and trehalose environment after freezing preparations to–180°С in the dark and on the actinic light and after their subsequent heating. In similar conditions the fluorescence lifetime of tryptophanyls in reaction centers (λ_reg = 325 and 345 nm), which is an internal indicator of the dynamic state of the protein matrix, was measured. A correlation between the temperature dependences of functional and dynamic parameters of reaction centers in different solvents was shown. The differences in the average fluorescence lifetime of tryptophanyls in reaction centers of preparations frozen in the dark or on the actinic light were found. These results are explained due to transitions of reaction centers between different conformational states and processes of proton relaxation in the structure of the hydrogen bonds in the environment of reaction-center cofactors.