Characterization of primary reactants in bacterial photosynthesis. II. Kinetic studies of the light-induced EPR signal (g = 2.0026) and the optical absorbance changes at cryogenic temperatures.

We have shown that the rise and decay kinetics of the light-induced EPR signal are identical to the kinetics of the optical changes at 80 degrees K. This identity provides independent evidence that the EPR signal is due to the oxidized primary electron donor which is bacteriochlorophyll. The EPR and optical changes could be described by a model photochemical reaction scheme that takes into account spin-lattice relaxation. The optical decay rate was found to be temperature independent between 1.5 and 80 degrees K and to obey approximately first order kinetics. These results are consistent with the hypothesis that the charge recombination occurs via tunneling through a potential barrier. The decay constants at these temperatures were found to be the same for different bacterial species and strains. No differences were found between purified reaction centers of R. spheroides R-26 and whole cells. Reaction centers treated with sodium dodecylsulfate or urea were still photochemically active but showed a markedly different kinetic behavior. The decay constant may, therefore, serve as a probe to investigate the molecular environment of the primary reactants.