Monomolecular Layers of Synthetic Phosphatides

Abstract In studying the functions of phosphatides in biological membranes, investigations have been made into monomolecular layers of synthetic phosphatides. Force-area curves of monolayers of phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine, all containing identical fatty acids, showed small differences, obviously to be attributed to differences in size and charge of the end groups. The shifts of the force-area curves within one class of phosphatides were more pronounced and are brought about by variations of the apolar moiety. Shortening of the chain length and particularly unsaturation of the fatty acid constituents greatly expanded the films of the l-α-lecithins, thereby increasing the closest stable packing attainable. Force-area curves of structurally isomeric mixed-acid l-α-phosphatides, carrying dissimilar fatty acids in different positions, were identical. Mixed films consisting of phosphatides and cholesterol in molar equivalents—at proportions also found in red cell membranes—revealed a condensing effect of cholesterol on the film of phosphatides containing certain unsaturated fatty acid constituents.