omega-6 and omega-3 fatty acids: monolayer packing and effects on bilayer permeability and cholesterol exchange.

It has been suggested that the polyunsaturated omega-3 fatty acid, docosahexaenoic acid (DHA), can adopt unique closely packed arrays in lipid bilayers (Glomset and Applegate. (1986) J. Lipid Res. 27, 658-680). These conformations are predicted on the basis of molecular dynamics calculations and are in contrast to the expanded conformations characteristic of omega-6 unsaturated fatty acids. It has also been suggested that close packing of omega-3 acyl chains could have a substantial affect on the physical properties of lipid bilayers (e.g. permeability). We report here some experimental tests of these predictions. Surface pressure-area experiments have been carried out on DHA and its mixtures with stearic and oleic acids. At low surface pressures DHA is more expanded than oleic acid. Extrapolation to the high surface pressures characteristic of lipid bilayers indicates that the area per molecule of DHA is only marginally less than that for oleic acid. Thus there is no compelling evidence to suggest that the average area per molecule of the omega-3 fatty acid is substantially different from the omega-6 fatty acid at high surface pressures. Experiments also show that the permeability of bilayers to glucose and the rates of dissociation of pyrenyl cholesterol from bilayers were similar for bilayers containing DHA compared to bilayers containing oleic acid or linoleic acid.