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

The interaction of extruded liposomes (VETs), prepared from dipalmitoylphos phatidylcholine (DPPC) - phosphatidylinositol IPI) and DPPC - cholesterol-d imethyldioctadecylammonium bromide (DDAB) mixtures with suspensions of zinc citrate particles has been investigated by isothermal titration calorimetr y (ITC). Measurements were made at 25 degreesC, as a function of liposomal lipid concentration and charged lipid content (PI and DDAB), in a range fro m 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 w hich association constants and Gibbs energies of adsorption were derived. P revious 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 wer e found to be in ranges from - 24 to -28 kJ mol(-1) for PI-containing lipos omes and -23 to -28 kJ mol(-1) for DDAB-containing liposomes. The entropic contributions to adsorption were positive, this is suggestive of hydrophobi c 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. (C) 2001 Elsevier Science B.V. All rights reserved.