Characteristics of the quenching of 9-aminoacridine fluorescence by liposomes made from plant lipids

Several laboratories have determined the surface charge density of membrane s utilizing methods based on vesicle-induced quenching of the fluorescence of 9-aminoacridine and its relief by other cations. However, the computatio nal methods by which surface charge density were calculated have not been v erified in a model system. In this study, the quenching of 9-aminoacridine fluorescence by liposomes made from varying amounts of digalactosyldiacylgl yceride and phosphatidic acid and relief of quenching by salts was examined . Quenching of 9-aminoacridine fluorescence increased with increasing amoun ts of phosphatidic acid added, independent of the composition of the added liposomes. In certain instances, the computational methods did not yield th e surface charge density of the liposomes expected from their composition. However, when the effects of background ionic strength on surface potential were considered, there was a positive correlation between expected and cal culated values. Therefore, the data support the contention that changes in the fluorescence of 9-aminoacridine can be used to calculate surface charge density of membranes.