The interaction of cationic and anionic vesicles with zinc citrate dispersions

The interaction of cationic and anionic extruded liposomes, prepared from dipalmitoylphosphatidylcholine (DPPC)–phosphatidylinositol (PI) and DPPC–cholesterol-dimethyldioctadecylammoniumnium bromide (DDAB) mixtures, respectively with zinc citrate particles has been investigated as a function of liposomal lipid concentration, liposome composition and pH. At pH 7 the anionic liposome adsorption increases with PI content. The cationic liposomes bind optimally when the composition contains 9 mol% DDAB. Anionic liposome adsorption increases with increasing solution acidity and cationic liposomes adsorption increases with solution alkalinity, both these observations are consistent with ionic interactions between liposomes and a negatively charged zinc citrate surface which becomes less negatively charged in acid solution and more negatively charged in alkaline solution. The adsorption data can be fitted to Langmuir isotherms from which association constants and Gibbs energies in a range from −46 to −56 kJ mol−1 are obtained. Using anionic and cationic liposomes encapsulating radiolabelled d-glucose it is found that anionic liposome adsorption results in the disruption of some of the liposomes (≈10%) which form a bilayer on the solid surface. In contrast, the liposome disruption of cationic liposomes on adsorption was within the experimental area of detection (±2%). For both types of liposomes the majority of the adsorption to the zinc citrate particle surface was in the form of intact liposomes.