Influence of a Spreading Method on the Properties of Amphotericin B-Dipalmitoyl Phosphatidic Acid Mixed Films at the Air/Water Interface

Using two different spreading techniquessmixed spreading and separate spreadingsand film surface pressure measurements, the mixing behavior of the amphotericin B (AmB)/dipalmitoyl phosphatidic acid (DPPA) system was investigated.The π-A isotherms were found to exhibit various "plateaus" suggesting the occurrence of different phase transitions of the monolayer. On the basis of the surface phase rule, the phase diagram of mixed films was elucidated. The AmB molecule takes two orientations in the monolayer state, i.e., horizontal and vertical, depending on the surface pressure. Horizontally oriented AmB and vertically oriented phospholipid form a mixed monolayer that deviates from ideal behavior. On the other hand, vertically oriented AmB molecules were found to be immiscible with DPPA in a monolayer, although both molecules are normal to the surface and parallel to each other. The deviations from ideality can be ascribed to the formation of an AmB/DPPA complex of 2:1 stoichiometry as a result of the existence of hydrogen bonds and ionic attractive forces between polar groups of the two components.