Secondary electron transfer in reaction centers of Rhodopseudomonas sphaeroides. Out-of-phase periodicity of two for the formation of ubisemiquinone and fully reduced ubiquinone.

Electron transfer between purified reaction centers from Rhodopseudomonas sphaeroides and exogenous ubiquinone has been studied in the presence of electron donors by measurements of light-induced absorbance changes following a sequence of short actinic light flashes. Each odd flash promotes the formation of a molecule of ubisemiquinone; after each even flash the semiquinone disappears and a molecule of the fully reduced quinone appears. We interpret these results by means of a model where a specialized molecule of ubiquinone is reduced by the primary electron acceptor in a one-electron transfer reaction after each flash, and is reoxidized by a molecule of the ubiquinone pool in a two-electron transfer reaction every two flashes.