A study of the behaviour of mixed monolayers of amphotericin B and dipalmitoyl phosphatidic acid based on hysteresis experiments

Abstract

Amphotericin B (AmB) monolayers consist of three distinct regions: a liquid expanded phase at low surface pressures, a liquid condensed phase at high surface pressures and a transitional phase that gives a plateau at 10 mN/m in the π-A curves. The presence of such a plateau is ascribed to a change in the orientation of AmB molecules from a horizontal to a vertical position relative to the air/water interface. Dipalmitoyl phosphatidic acid (DPPA) gives a condensed monolayer, the π-A curves for which exhibit a slope change at 25 mN/m that has been ascribed to an orientational change in the nonpolar chains from a tilted to a vertical position relative to the interface. An equimolar mixture of AmB and DPPA exhibits a surface behaviour in-between those of the pure components, which includes a phase transition at 20 mN/m that suggests the existence of interactions between the components; such interactions result in deviations from the ideal behaviour reflected in plots of the mean molecular area against the mole fractions of the components. On spreading separately on the interface, the two components are initially immiscible, but become miscible gradually with time. Hysteresis experiments reveal that AmB is desorbed into the subphase by effect of its solubility, being decreased by DPPA; this is attributed to interactions between the polar heads of the two components causing AmB molecules to be retained on the surface and giving rise to positive deviations from the ideal behaviour as a result. © Springer-Verlag 1999.