The interaction of phospholipid liposomes with zinc citrate particles: a microcalorimetric investigation

The interaction of extruded liposomes (VETs), prepared from dipalmitoylphosphatidylcholine (DPPC) — phosphatidylinositol (PI) and DPPC — cholesterol-dimethyldioctadecylammonium bromide (DDAB) mixtures with suspensions of zinc citrate particles has been investigated by isothermal titration calorimetry (ITC). Measurements were made at 25°C, as a function of liposomal lipid concentration and charged lipid content (PI and DDAB), in a range from 4 to 19 mole%, and pH 7.2 and 5.2 solutions. The interactions were found to be endothermic and the cumulative enthalpies as a function of the amount of lipid bound were interpreted using Langmuir adsorption isotherms from which association constants and Gibbs energies of adsorption were derived. Previous studies of Catuogno and Jones (Colloids Surfaces A, 163 (1999) 165–176) have demonstrated that the liposomes largely adsorb to zinc citrate to form a monolayer of intact liposomes. The Gibbs energies of adsorption were found to be in ranges from −24 to −28 kJ mol−1 for PI-containing liposomes and −23 to −28 kJ mol−1 for DDAB-containing liposomes. The entropic contributions to adsorption were positive, this is suggestive of hydrophobic and/or dehydration effects on liposome binding. There was no evidence to suggest that the initial interaction of the liposomes with the zinc citrate particles gave a thermal contribution attributable to liposome disruption.