MOLECULAR-INTERACTIONS BETWEEN LIPIDS AND SOME STEROIDS IN A MONOLAYER AND A BILAYER .2.

Molecular interactions between L-alpha-dipalmitoylphosphatidylcholine (DPPC) and some steroids having different hydrophilic groups in a monolayer and in a bilayer are investigated in terms of surface pressure, permeability, and microviscosity. The steroids used in this study are stigmasterol, 4,22-stigmastadien-3-one (stadiene), and stigmasterol acetate(acetate). The collapse pressures for the mixed monolayer of DPPC with various steroids depend upon the molar ratio (X) and types of hydrophilic groups in steroids. A one-stage collapse is observed for X less than or equal to 0.5 of stigmasterol, X less than or equal to 0.3 of stadiene and acetate, whereas a two-stage collapse is seen in other molar ratios of the steroids. The mole fraction within which liposomes are formed is dependent upon the types of hydrophilic group within the steroids. In comparing monolayer and bilayer membranes (liposomes), liposomes are formed at the mole fraction of steroids which have a one-stage collapse in DPPC/steroid mixed monolayer. The permeability of the bilayer membrane decreases with increasing cohesive force between DPPC and steroids, but increases with a decrease in microviscosity near the hydrophilic portions, in the following order: stigmasterol < stadiene < acetate. This study demonstrates that the permeability of liposomes is dependent upon the hydrophilic group of the steroids, and the stability of the liposomes increases with an increase in the cohesive forces between DPPC and the steroids.