Novel surface phase containing cholesteryl esters. 1. Structural characteristics determined from surface pressure--area measurements.

The behavior of cholesteryl myristoleate in mixtures with dioleoylphosphatidylcholine was investigated at the air--water interface. In addition to the previously described monolayer phase [Smaby, J. M., Baumann, W. J., & Brockman, H. L. (1979) J. Lipid Res. 20, 789--795], a second surface phase has been identified. Analysis of surface pressure and molecular area data as a function of composition shows that the molecules in the second phase can exist in two miscible, double-layer states or packing arrangements, only one of which contains lecithin. The mixed double-layer state is preferentially formed and has stoichiometry ranging between 2.0 and 9.5 molecules of cholesteryl ester for each lecithin molecule. The structure of this state resembles a mixed monolayer of pressure-dependent composition and area which is covered by a second layer of cholesteryl ester at 38.2 A2/molecule. The cholesteryl myristoleate/lecithin ratio of the layer in contact with the aqueous phase ranges from 0 to 2.8 between 39 and 0 mN/m. The second double-layer state is equivalent to a monolayer of cholesteryl ester at the lipid--water interface, covered by a layer of cholesteryl ester molecules at 38.2 A2. Overall, our data show that the presence of lecithin at a lipid--water inferface has a definite ordering effect on cholesteryl ester molecules at least 30--50 A from the interface.