The functional unit of electrical events and phosphorylation in chromatophores from Rhodopseudomonas sphaeroides.

1. From electron micrographs of chromatophores from Rhodopseudomonas spaeroides and from the estimated bacteriochlorophyll per chromatophore was estimated. The mean diameter of the chromatophore vesicles was 600 A. 2. The decay of the flash-induced electric potential across the chromatophore membrane measured by the carotenoid band shift was 20% accelerated by about one valinomycin molecule per 4700 bacteriochlorophyll, i.e. by one ionophore molecule per chromatophore. 3. The inhibition of the flash-induced ATP formation by valinomycin followed a similar pattern to the accelerated decay of the electric potential. 4. The single turnover flash yield of ATP synthesis gave a mean value of one ATP per 1470 bacteriochlorophyll or about 3 ATP per vesicle. 5. With regard to the partitioning of the ionophore between the membrane (85%) and aqueous phase (15%) we conclude that one molecule of valinomycin per chromatophore is sufficient to begin to collapse the electrical potential and inhibit ATP synthesis. It is therefore suggested that the membrane potential is an essential component of the energized state which is used for phosphorylation. The results correspond to those obtained for the 100-fold larger vesicles in chloroplasts (thylakoids) where one molecule of ioophore is also sufficient to quench both events.