Effects of the Medium Composition on the Components of the Electrochemical Proton Gradient in Rhodopseudomonas sphaeroides

The magnitude and composition of the proton motive force (Δµ~H+) has been measured in chromatophores and whole cells of Rhodopseudomonas sphaeroides as a function of the ionic composition of the buffer in which the energy-transducing membranes are suspended. Measurements with the flow-dialysis technique are compared with spectrophotometric measurements of the absorbance changes of carotenoids and of the quenching of the fluorescence of 9-aminoacridine. It is concluded that the optical techniques give rise to an overestimation of the gradients of pH and electrical potential. However, with the optical techniques the relative permeability of the membrane of R. sphaeroides for various cations and anions can be estimated. The results indicate that in general anions are more permeant than cations; the relative permeability of the anions is: SO4= < HPO4= < Cl- < NO3- < ClO4-.