The electrochemical proton gradient generated by light in membrane vesicles and chromatophores from Rhodopseudomonas sphaeroides.

Abstract

The electrochemical proton gradient, , generated upon illumination of membrane vesicles and chromatophores from Rhodopseudomonas sphaeroides. has been determined. The components, the transmembrane pH gradient, ΔpH, and the membrane potential, ΔΨ, were measured with several methods. The ΔpH, determined from the pH changes in the membrane suspensions, is in good agreement with the ΔpH obtained from the distribution of acetate or methylamine. The ΔpH values obtained from the fluorescence quenching of 9‐aminoacridine in chromatophores are more than two‐fold larger. The ΔΨ was determined from the distribution of the permeant ions triphenylmethylphosphonium and thiocyanate. The ΔΨ values in chromatophores obtained from the absorbance changes of carotenoids were several‐fold larger.Illumination of membrane vesicles results in a ΔpH, inside alkaline and a ΔΨ inside negative. The ΔpH, determined from the distribution of acetate, depends on the composition of the medium and is optimally 0.6 at an external pH 7; the ΔΨ, measured from the distribution of triphenyl‐methylphosphonium, is about 70 mV. The in membrane vesicles is thus at the most 110 mV.Illumination of chromatophores results in a ΔpH, inside acid, and a ΔΨ inside positive. The ΔpH, calculated from the distribution of methylamine, depends on the external pH and increases from 0.98 at pH 6 to 1.36 at pH 8. The ΔΨ is not affected and remains about 50 mV. The in chromatophores is thus at the most 140 mV.The ΔpH and ΔΨ are in both membrane preparations strongly dependent on the composition of the medium. So is for instance the ΔΨ decreased by the presence of chloride ions and the ΔpH increased. Phosphate ions decrease the ΔpH in membrane vesicles. The ΔpH is generated at a lower rate than the ΔΨ; once the has reached a maximum the ΔpH continues to increase at the expense of the ΔΨ.