Short-chain lecithins: How to form unilamellar vesicles from micelles and phospholipid multibilayers

Abstract

Short-chain lecithins (6–8 carbon fatty acyl chains) form micelles in aqueous solutions. NMR and SANS studies show that a short-chain lecithin molecule in these micelles has the same conformation as a long-chain lecithin in bilayer aggregates. When small amounts of these short-chain lecithins (typically 20 mol% of total phospholipid) are added to aqueous dispersions of long-chain phospholipids, unilamellar vesicles form spontaneously. Induction of unilamellar vesicles from multibilayers does not occur with comparable concentrations of single-chain detergents. Short-chain lecithin/long-chain phospholipid unilamellar vesicles (SLUVs) are extremely stable and form with a wide variety of long-chain phospholipids (with different head groups, T m, unsaturation, etc.). We have characterized them in terms of (1) size distribution and coexistence of multilamellar or mixed micellar structures, (2) phase behavior of the long-chain phospholipid, (3) distribution of the short-chain lecithin across the bilayer, (4) dynamics and interactions of the two phospholipids, and (5) susceptibility to water-soluble phospholipases. The results suggest a mechanism for vesicle formation and are critical in understanding the action of phospholipase-A 2 and phospholipase-C on bilayer surfces.