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 influence of macromolecule conformations related to the solvent characteristics (pH, ions, ionic strength). In this study we have investigated the conformation of iono- and thermo-sensitive polysaccharides, iota- and kappa-carrageenans, and their interaction with a dimyristoylphosphatidylcholine (DMPC) model 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 kappa-carrageenan samples have been characterized by size exclusion chromatography (SEC) coupled with a multi-angle laser light-scattering detector (MALLS). The SEC-MALLS results clearly show polysaccharide chain association at high ionic strength. In the second part, the polysaccharide-membrane interaction has been studied, using fluorescent probes embedded in the membrane. The thermotropic properties of the membrane were investigated by fluorescence depolarization of 1-(4-trimethylammonium-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). The membrane surface accessibility was evaluated by fluorescence 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 the membrane is more marked at low NaCl concentration. In contrast, the liposomes bilayer accessibility is drastically lowered when increasing the ionic strength. This is induced by macromolecular chain adsorption on the liposome surface, enhanced by the polysaccharide chain association. An ionic strength enhancement induces a conformational modification of the polysaccharide chains which modifies their ability to interact with the bilayer.