Molecular structure and conformation in phospholipid vesicles sterically stabilized by (tri)-block copolymers investigated by multi-nuclear magnetic resonance techniques

Soybean lecithin was dispersed in water and sonicated to form unilamellar vesicles of 40-45 nm in diameter. (Tri)-block copolymer molecules of the A-B-A type (A is polyethylene oxide (PEO) and B is polypropylene oxide (PPO)) were added to enhance the stability of the vesicles. The hydrophobic (PPO) group of the copolymer was thought to be buried inside the vesicle bilayer after sonication of the phospholipids in the presence of copolymer. C-13- and P-31-NMR were used to investigate the specific localization of the block copolymer and its interaction with the phospholipids. Line broadening of the P-31 signals (phosphatidylcholine) and the C-13 NMR signals of the methyl groups (PPO) was observed for low molar ratios of copolymer:phospholipid. This effect was thought to be due to restriction of molecular mobility of the groups examined, suggesting incorporation of the copolymer inside the bilayer. Other studies investigated the thermotropic behaviour of the phospholipid aggregates formed and the consequences of copolymer presence in the vesicle systems. P-31 NMR offered a way to observe comparatively the effect of vesicle formation in the presence of copolymer molecules. According to this method of vesicle preparation (i.e. with copolymer present) the interaction between the lipid molecules and the copolymers is much more evident. (C) 1998 Elsevier Science B.V.