Chain-Substituted Lipids in Monolayer Films. A Study of Molecular Packing

Article abstract of DOI:10.1021/ja00228a032

A series of highly purified fatty acids and phospholipids, each possessing a chain substituent of varying size (methyl, n-butyl, or phenyl) at varying locations (carbon 4, 8, 12, or 16 of an 18-carbon chain) were synthesized.Pressure-area isotherms, obtained with the aid of a film balance, revealed how these molecules, either individually or admixed, pack in monomolecular films.Two examples will illustrate here the type of information secured by the method. (a) The presence of a methyl at carbon 16 of a single phospholipid chain has a negligible effect on the molecular packing within a "liquid" film.A methyl on carbon 16 of both chains, on the other hand, is highly expansive.Even a single methyl at position 8 perturbs the film packing appreciably, thus indicating less available space near the center of the chain. (b) Deviation from ideality in mixed fatty acid films can be explained by interdigitation that reduces steric repulsion among the substituents.It is also possible to observe by means of film balance techniques the extrusion of 8-butylstearic acid when it contaminates a film composed largely of stearic acid.Approximate SHADOW calculations could, in many cases, provide film areas in reasonable agreement with experiment.