Effect of lipid phase transition on the binding of anions to dimyristoylphosphatidylcholine liposomes.

Temperature dependence of the electrophoretic mobility of multilamellar liposomes prepared from dimyristoylphosphatidylcholine was measured in the presence of salts with different anions in aqueous solutions. It was established that specific binding of anions to liposome surface induced a pronounced zeta potential (electrostatic potential at the hydrodynamic plane of shear). A combination of Langmuir, Gouy-Chapman, and Boltzmann equations was used to describe the dependence of the zeta potential on the concentration of anions. The values of binding constants (K) and maximum numbers of binding sites per unit area (sigma max) were determined by this method. The sequence for anion affinities to liposome surface was found to be as follows: trinitrophenol greater than ClO-4 greater than I- greater than SCN- greater than Br- greater than NO-3 greater than Cl- approximately or equal to SO2-4. A sharp increase in the negative zeta potential was detected at the temperature of phase transition of the lipid from the gel to liquid-crystalline state. It was found that the parameter K did not change at lipid phase transition and the shifts in zeta potential might be due to alterations of sigma max. The binding sites were considered as defects in the package of lipid molecules in membranes.