Kinetics of reduction of bacteriochlorophyll dimer in reaction centers of photosynthetic bacteria

Kinetic curves of reduction of the oxidized bacteriochlorophyll dimer by the proximal heme of cytochrome in seven mutated reaction centers from Rps. viridis was measured in the temperature range of 295-10 K under an ambient potential when two high-potential hemes were reduced. The data are analyzed in frames of a model which accounts for slow medium dynamics (the diffusion-reaction equation) and the quantum effect in the modes responsible for reorganization of the cofactors and the medium. The model reproduces the observed nonexponential kinetics down to the lowest temperature where the fast kinetic component still survives. The modeling results in determination of an average characteristic time for re-orientation of dipoles of the protein matrix as a function of temperature. At temperatures above the glass-transition temperature, this function shows Arrhenius behavior with the activation energy varying between the values of 0.5 eV for wild type Rps. viridis and 0 eV for Rps. sulfoviridis found previously. Possible reasons for disappearance of the fast kinetic component in some reaction centers under study at low temperatures are discussed.