MOLECULAR-INTERACTIONS BETWEEN LIPID AND SOME STEROIDS IN A MONOLAYER AND A BILAYER

The molecular interactions between L-alpha-dipalmitoylphosphatidylcholine (DPPC) and some steroids in a monolayer and in a bilayer have been investigated in terms of surface pressure, permeability, polarity, and fluidity. The compressibilities obtained from the surface pressure-area isotherms of the mixed monolayer of DPPC with steroids having a hydrophilic group (OH group) are almost the same, but the DPPC-4,22-stigmastadien-3-one mixed monolayer shows different behavior. The collapse pressures for the mixed monolayer of DPPC with steroids having a short side chain are higher than that of DPPC with steroids having a long side chain. At similar collapse pressures, the limiting area of DPPC with steroids which have a few double bonds is smaller than that with steroids having more double bonds. The interaction between hydrophobic groups in the mixed monolayer increases with a decrease in both the number of side chains and double bonds in steroid molecules. Permeability of the mixed bilayer is dependent on the steroids involved, and the order of the magnitude is cholesterol < beta-cholesterol < beta-sitosterol < stigmasterol. It is found that the interaction between hydrophobic groups of DPPC and steroids in both the monolayer and the bilayer membranes increases with decreases in the number of side chains and double bonds in the steroid molecules involved.