The reaction between primary and secondary electron acceptors in bacterial photosynthesis

Abstract

Following a 20-nsec actinic flash, which causes oxidation of P870 and cytochrome C422, Chromatium chromatophores enter a refractory state. While the chromatophores are in this state, a second flash does not cause further oxidation of P870 or cytochrome C422. The quanta of the second flash are wasted as fluorescence (and heat); apparently they do not energize an alternative photochemical reaction. The refractory state probably reflects the accumulation of the primary electron acceptor in a reduced form. By following the reappearance of the capacity for photochemistry, one can measure the kinetics of electron transfer between the primary electron acceptor and the secondary agent which reoxidizes it. In Chromatium chromatophores, this process requires about 60 μsec to proceed half-way to completion at pH 7, and 80 μsec at pH 8. The rate of the reaction increases with decreasing pH, but not in direct proportion to the proton concentration. It increases with temperature, with an E a of about 8.3 kcal/mole. The kinetics are approximately second order in the concentration of the reduced acceptor.