Weak acid-induced release of liposome-encapsulated carboxyfluorescein

Leakage of the entrapped anionic fluorophore carboxyfluorescein was used as a measure of the permeability of liposomes to several different acids. Carboxyfluorescein leakage increased with increasing buffer concentration at a given pH and depended on its chemical nature: apolar weak acids such as acetic or pyruvic acids induced fast leakage at relatively high pH (4 to 5), while glycine, aspartic, citric and hydrochloric acids induced leakage only at lower pH. Fluorescence leakage measurements reflected the acidification of the liposomes' aqueous spaces, which was primarily caused by the diffusion of undissociated acid molecules across the lipid bilayer. A simple mathematical model in accord with this hypothesis and assuming that carboxyfluorescein leakage was directly related to the proportion of its neutral lactone form, described satisfactorily the carboxyfluorescein leakage kinetics and allowed rough estimation of permeability coefficients for carboxyfluorescein (neutral lactone form; 9 · 10^?9 cm · s^?1), acetic acid ($\text{>1 · 10}{}^{\mathrm{?7}}\text{cm}\text{·}\text{s}{}^{\mathrm{?1}}$) and glycine (cation: 6 · 10^?9 cm · s^?1). These results are consistent with low effective proton permeability of liposomes ($\text{<5 · 10}{}^{\mathrm{?12}}\text{cm}\text{·}\text{s}{}^{\mathrm{?1}}$) and with the permeability coefficient of HCl (3 · 10^?3 cm · s^?1) reported by Nozaki and Tanford ((1981) Proc. Natl. Acad. Sci. U.S.A. 78, 4324–4328). Diffusion of weak acid molecules across lipid membranes has implications for drug encapsulation and delivery, and may be of biological significance.