The surface and membrane potentials of chromatophores of photosynthetic bacteria as studied by carotenoid absorbance changes

Abstract

The change in surface potential induced by addition of mono- or divalent cations to a chromatophore suspension was monitored by carotenoid absorbance changes (a probe which is intrinsic to the membrane). The change in carotenoid absorbance elicited by an alteration of the surface potential is strongly dependent on the presence of ionophores; the absorbance changes (due to addition of MgCl 2) in the presence of valinomycin or gramicidin are larger than those in the presence of carbonyl cyanide m-chlorophenylhydrazone or cabonyl cyanide p-trifluoromethoxyphenylhydrazone. These differences in carotenoid absorbance change reflect the degree in which the membrane resistance has been shunted. Gramicidin or high concentrations of valinomycin (10 −6 M) appear to be sufficiently effective as shunt in order that the totality of the change in external surface potential is seen as an intramembrane potential difference as sensed by the carotenoids. It is also shown that the decay of the carotenoid changes induced by the addition of salt to the medium is a measure of the intrinsic permeability of the chromatophore membrane for the added cation.