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A method for the measurement of the internal pH of chloroplasts is described. The method is based on the uptake of a fluorescent amine by chloroplasts, which can be observed by the lowering of the fluorescence emitted from a suspension of chloroplasts. It is shown that this uptake is dependent, as expected, on the dissociation constant of theamine and also on the number of ionisable amines in the molecule. Amines attached to two typesof chromophores, acridine and naphthalene have been investigated. As previously shown for other amines, this uptake was found to be a prerequisite for their acting as uncouplers. The technique provides a simple way to continuously follow changes in ΔpH in chloroplasts.
Schuldiner et al. (1972) studied this question.