Relationship between conformation of polysaccharides in the dilute regime and their interaction with a phospholipid bilayer

Interactions between polysaccharides and phospholipid bilayers have already been demonstrated in the literature but little is known about the influenc e of macromolecule conformations related to the solvent characteristics (pH , ions, ionic strength). In this study we have investigated the conformatio n of iono- and thermo-sensitive polysaccharides, iota- and kappa-carrageena ns, and their interaction with a dimyristoylphosphatidylcholine (DMPC) mode l bilayer. The study was performed in two different media (NaCl 150 mmol/L, pH 6.5, and NaCl 300 mmol/L, pH 6.5). In the first part. the iota- and kap pa-carrageenan samples have been characterized by size exclusion chromatogr aphy (SEC) coupled with a multi-angle laser light-scattering detector (MALL S). The SEC-MALLS results clearly show polysaccharide chain association at high ionic strength. In the second part, the polysaccharide-membrane intera ction has been studied, using fluorescent probes embedded in the membrane. The thermotropic properties of the membrane were investigated by fluorescen ce depolarization of 1-(4-trimethylammonium-phenyl)-6-phenyl-1.3,5-hexatrie ne (TMA-DPH). The membrane surface accessibility was evaluated by fluoresce nce quenching of 2-(9-anthroyloxy) stearic acid (2-AS). Whatever the ionic strength tested, the polysaccharide presence notably enhances the membrane fluidity below the T-m. This sign of an interaction in the polar level of t he membrane is more marked at low NaCl concentration. In contrast, the lipo somes bilayer accessibility is drastically lowered when increasing the ioni c strength. This is induced by macromolecular chain adsorption on the lipos ome surface, enhanced by the polysaccharide chain association. An ionic str ength enhancement induces a conformational modification of the polysacchari de chains which modifies their ability to interact with the bilayer. Copyri ght (C) 2001 John Wiley & Sons, Ltd.