The primary acceptor of bacterial photosynthesis: Its operating midpoint potential?

Abstract

1. 1. The equilibrium oxidation-reduction midpoint potentials of the primary acceptors of the purple photosynthetic bacteria Rhodopseudomonas sphaeroides, Chromatium vinosum and Rhodospirillum rubrum have been determined over a wide range of pH values (pH 4.7–11.0). 2. 2. At physiological values of pH the equilibrium reduction of the primary acceptor involves both an electron and a proton, but at higher values of pH, a p K of the reduced form is titrated so that above the p K only an electron is involved in the reduction. 3. 3. The value of the p K in Rps. sphaeroides is approximately pH 10, while in C. vinosum it is close to pH 8, and in R. rubrum it is close to pH 9. 4. 4. During light-induced electron flow, the reduction of the primary acceptor involves only an electron, implying that the kinetically operational midpoint potential is the equilibrium value measured above the p K of the reduced form. In C. vinosum this is −160 mV, in Rps. sphaeroides−180 mV, and in R. rubrum it is −200 mV.