The response of fluorescent amines to pH gradients across liposome membranes

Abstract

Phospholipid liposomes were used to test atebrin and 9-aminoacridine as fluorescent probes for measuring pH gradients across membranes. Quenching of 9-aminoacridine fluorescence could be quantitatively related to the magnitude of pH gradients across liposome membranes, and the relation indicated that the distribution of this amine between inner and outer volumes in the liposome system was that of an ideal monoamine. Quenching of atebrin fluorescence could not be related to that predicted from the theoretical equation of a diamine. We conclude that 9-aminoacridine is the preferred fluorescent probe for use in the range of ΔpH = 2–4 pH units. Quenching of 9-aminoacridine fluorescence was used to measure the development of pH gradients in liposomes. In one system, an oxidation-reduction reaction mediated by a dye which accepted H + upon reduction, was established across liposome membranes. It was found that gradients of at least 4 pH units could develop under these conditions. In a second system, nigericin mediated the exchange of K + for H + across the liposome membranes, and it was found that the pH gradient developed depended upon the original K + gradient and could be at least 2.2 pH units. Liposomes offer a model membrane system in which controlled pH gradients may be established.