Effects of electron donors and acceptors on the kinetics of the photoelectric responses in Rhodospirillum rubrum and Rhodopseudomonas sphaeroides chromatophores

Abstract

Chromatophores of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides were adhered to one side of a collodion film impregnated with a phospholipid solution in decane and 20 ns laser flashes were delivered to produce an electrical potential difference generated across the collodion film in less than 0.2 μs (resolution time of the apparatus). The kinetics of Δψ decay in the dark was studied. In the absence of additions there occurs a ‘rapid’ decay of photoelectric potential ( τ ≈ 70 ms) corresponding to charge recombination within the primary dipole P-870 +-Q - A. The rapid decay of Δψ is prevented by ascorbate in the presence of permeable redox dyes which can reduce the photooxidized P-870 + rapidly. Under these conditions, Δψ dissipates with τ > 0.5 s typical of a passive discharge of the chromatophore membrane. Prevention of the rapid decay of Δψ by 70–75% can be observed upon addition of excess ubiquinone-10 to the solution of phospholipids used to impregnate the collodion film, and to a lesser extent by addition of some other quinones. The effect of quinones is inhibited by o-phenanthroline. The data obtained show that upon association of chromatophores with the collodion film, the secondary quinone acceptor is extracted from its binding site into a hydrophobic volume of the macroscopic membrane, and this effect can be reversed by exogenous ubiquinone. About 4-times less Q-10 is required to reconstitute Q B function in chromatophores from Rps. sphaeroides than in those from R. rubrum, which points to a tighter binding of the secondary acceptor in the former. No evidence for electrogenic nature of Q − A → Q B electron transfer could be obtained in experiments with Q B-replenished chromatophores.