A study of the kinetic properties of the stable semiquinone of the reaction-center secondary acceptor in chromatophores of non-sulfur purple bacteria

Abstract

Flash-induced absorption changes at 450 nm were investigated in isolated chromatophores of Rhodopseudomonas sphaeroides and Rhodospirillum rubrum non-sulfur purple bacteria to follow the redox changes of the semiquinone species of the secondary quinone acceptor of the photosynthetic reaction center. Excitation of a dark-adapted chromatophore suspension by a series of successive flashes in the presence of electron donors capable of rapidly reducing the photooxidized reaction-center pigment causes the formation of a stable semiquinone species (Q ⨪ B) with a lifetime which is shown to be proportional to the amount of the oxidized redox mediator in the incubation medium. It is shown that the disappearance of the flash-induced absorption changes at 450 nm on lowering the ambient redox potential ( E h) to 200–300 mV is the result of increasing the lifetime of Q ⨪ B, as the amount of the oxidized mediator diminishes; consequently, in these circumstances, the 2–5 min dark interval between the flash cycles appears insufficient for Q ⨪ B recovery. After the addition of redox mediators with a low midpoint potential, acting as an oxidant for Q ⨪ B, the flash-induced redox changes of Q ⨪ B were observed at low E h values unless E h reached a value at which Q B underwent reduction at equilibrium to form Q BH 2. The data provide evidence that reaction centers with a fully oxidized secondary acceptor can donate electrons to the cyclic electron-transport chain only after two turnovers, leading to the formation of the doubly reduced ubiquinone species (Q BH 2) of the secondary acceptor.