Lipid structure and the behavior of cholesteryl esters in monolayer and bulk phases.

The behavior of cholesteryl esters at the air-buffer interface was studied as a function of molecular area and the presence of noncholesterol-containing lipids (colipids). The data obtained indicate that cholesteryl esters with other than long, saturated acyl groups can be present in surface phases up to packing densities approximately those in natural membranes. Their apparent molecular areas in such phases, which are largely determined by colipid structure, suggest their orientation with the ester function toward the interface. The extent of miscibility in the surface phase is also a strong function of colipid structure. Reversibility of the monolayer to bulk phase transition is determined exclusively by the acyl structure of the cholesteryl ester. Of the esters examined, only those with cis unsaturation collapsed reversibly. Our data predict that cholesteryl esters should be present in small, but finite amounts on the surface of arterial lipid deposits and that a prerequisite for the removal of such deposits is that the bulk lipid phase be in a liquid or liquid crystalline state.