COMPARISON OF PERMEANT ION UPTAKE AND CAROTENOID BAND SHIFT AS METHODS FOR DETERMINING THE MEMBRANE-POTENTIAL IN CHROMATOPHORES FROM RHODOPSEUDOMONAS-SPHAEROIDES GA

A comparison was made of 2 methods for estimating the membrane potential in chromatophores from R. sphaeroides. Illuminated chromatophores generated a potential that is apparently much larger when estimated on the basis of the red-band shift of carotenoids rather than from the extent of uptake of the permeant SCN- ion. When the chromatophores were oxidizing NADH or succinate the uptake of SCN- indicated a larger membrane potential than was estimated from the carotenoid band shift. The extent of SCN- uptake and the carotenoid-band shift respond differently to changes in the ionic composition of the reaction medium. The effects of antimycin on the carotenoid band shift and SCN- uptake are reported. The carotenoid band shift and the uptake of SCN- are responding to different aspects of the energized state.