Detection of functional group on the liposome surface by colloid titration

Suspensions of phospholipid liposomes (phosphatidylcholine, phosphatidylglycerol, phosphatidylethanoline and phosphatidylserine) were examined by colloid titration with polydiallyldimethylammonium chloride and potassium polyvinylsulfate as polycationic and polycationic and polyanionic titrants, respectively. In addition, the electrophoretic mobilities of the liposomes were measured with a cytopherometer. The following conclusions are drawn from the results of the titration: Phosphate and ammonium groups of phosphatidylcholine in liposomes form intramolecular or intermolecular salt linkages. Phosphate on the surface of phosphatidylglycerol liposomes shows a constant dissociation at above pH 3. Liposomes of phosphatidylethanolamine have no charge at low pH and a maximum negative charge above pH 10.5. On the surface of phosphatidylserine liposomes, the carboxyl group is completely dissociated at pH 6-7 and the phosphate group is released completely by the conversion of the ammonium ion to an amino group above pH 10.5. Stemming from these results, colloid titration can reveal the dissociation or the situation on the surface of liposomes more precisely than electrophoresis. Thus colloid titration is useful for determining the states of functional groups on the surface of phospholipid liposomes.