Effect of calcium ions on the surface charge and aggregation of phosphatidylcholine liposomes

The aggregation behaviour of multilamellar liposomes composed of egg phosphatidylcholine (EPC) and dipalmitoylphosphatidylcholine (DPPC) in the presence of varying concentrations of calcium ions has been studied by measuring changes in the particle size of liposome suspensions. Reversible aggregation occurred with both liposome preparations, showing a maximum rate at approximately 0.9 to 1.8 mM calcium chloride. At higher or lower concentrations aggregation was inhibited in the case of EPC suspensions or, for DPPC systems, occurred only slowly. Furthermore, the EPC systems showed more marked increases in size when aggregation occurred. The zeta potentials of both systems were negative in water, becoming positive on increasing the concentration of calcium ions. A reasonable correlation was found between calcium concentration corresponding to the minimum in initial zeta potential and that producing the greatest aggregation rate.