Influence of the polar group on interactions between phospholipids and sterols in monolayers. Rapid decomposition of phosphatidylethanolamines and cholesterol in monolayers at pH 11.9

Abstract Pressure‐area curves were obtained for monolayers of pure lipids or mixtures of lipids in order to investigate the effect of different polar groups upon interactions between molecules of various structures. Cholest‐4‐ene‐3‐one and cholestan‐3‐one were measured alone and in mixtures with di 16∶0‐lecithin, and 16∶0–18∶1‐lecithin, 16∶0–18∶3‐lecithin, and 16∶0–20∶4‐lecithin. All lecithin films were somewhat condensed by the ketones, but much less so than by cholesterol. Cholesterol was measured alone and with either di 16∶0 phosphatidylethanolamine (PE) or rat liver phosphatidylethanolamine at pH 2.0, 5.4 and 11.9. Cholesterol condensed the natural PE at pH 2.0 and 5.4, but not the di 16∶0 PE. At pH 11.9, both cholesterol and PE were chemically unstable in monolayers. The strong interaction of cholesterol with mixed phosphatidylethanolamines in monolayers suggests that cholesterol may also interact strongly with PE in membranes, of which PE are important components.