Some effects of o-phenanthroline on electron transport in chromatophores from photosynthetic bacteria

The midpoint potentials of the primary electron acceptors in chromatophores from Rhodopseudomonas spheroides and Chromatium have been studied by titrating the laser-induced P605 and cytochrome c oxidations, respectively. Both midpoint potentials are pH dependent (60 mV/pH unit).o-Phenanthroline shifts the midpoint potentials of the primary acceptors, by +40 mV in Rps spheroides and +135 mV in Chromatium. A similar though less extensive change in midpoint potential was observed in the presence of batho-phenanthroline, but not with 8-hydroxyquinoline. The shifted midpoints retain the same dependence on pH.Some of the effects of o-phenanthroline can be explained by assuming that it chelates the reduced form of the primary electron acceptor. This suggests the presence in the primary electron acceptor of a metal chelated by o- and batho-phenanthroline.In Rps spheroides chromatophores o-phenanthroline inhibits the laser- and flash-induced carotenoid shift at all redox potentials, stimulates the laser-induced P605 oxidation at redox potentials between +350 and +420 mV and slows the decay of the laser-induced cytochrome c oxidation below +180 mV. These effects show that o-phenanthroline may have more than one site of action.