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The extent of hydrogen ion gradient across membranes from photosynthetic bacteria has been determined measuring the energy‐dependent quenching of fluorescence of 9‐aminoacridine in different experimental conditions. Examinations of the absorption or emission spectra of 9‐aminoacridine in aqueous solution reveal an ideal behaviour of this amine up to millimolar concentrations. The decrease in fluorescence observed in the presence of energized membranes corresponds to a general quenching in all regions of the absorption or emission spectra and is related to the amount of dye and of vesicles present in the assay according to the model of distribution of an ideal amine. The onset of a pH difference across the membrane is stimulated by the presence in the assay of lipophilic anions or of K + and valinomycin; in these conditions a parallel decrease of membrane potential, as measured by carotenoid band shift, has been observed. The total protonmotive force evaluated by a combination of these two techniques however remains relatively constant during time and in almost all conditions tested.
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Casadio et al. (1974) studied this question.
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