Electrophoretic behavior of stearylamine-containing liposomes

When ζ potentials of liposomes formed by phosphatidylcholine (PC) and stearylamine (STE) at variable concentrations were compared with those corresponding to liposomes formed by phosphatidic acid (PA), a decline and a lack of linearity in their ζ potentials compared to the logarithm of ionic strength were found at concentrations of STE above 10% (molar ratio). Despite the fact that STE is fully protonated at the interval of pH used, ζ potentials were found to be dependent not only on the ionic strength of the medium, as predicted by the classical double-layer theory, but also on the pH. Determination of the STE distribution by spectrofluorometry following the labeling of STE with fluorescamine showed that STE seemed to be preferentially located in the outer monolayer. This apparent contradiction can be explained by the migration of STE molecules from the liposomal surface to the medium, where it is organized in the form of micelles with a diameter of about 2 nm. The presence of micelles in addition to liposomes involves a large adsorption-desorption equilibrium, which, in turn, is influenced by the variation in the electrostatic free energy of the double layer on the membrane. Thus, the surface charge density varies with the change in ionic strength and pH, and consequently, the electrophoretic behavior of STE liposomes differs from that of PA liposomes.