Decoration of lipid vesicles by polyelectrolytes: mechanism and structure

This study deals with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles decorated with chitosan and hyaluronan, in dependence with respective membrane and polyelectrolyte net charges (tuned by pH). Studies are performed both on micrometric Giant Unilamellar Vesicles (GUVs) and on their nanometric Large Unilamellar Vesicle (LUV) homologues. Fluorescent microscopy observations reveal that GUVs are homogeneously decorated by both polyelectrolytes, even in the case where global charges of membrane and polyelectrolyte exhibit the same charge sign. ζ-Potential and light scattering experiments performed on LUVs suspensions upon chitosan addition are interpreted in terms of reversible aggregation of vesicles within the frame of a patch-like structure model. A similar aggregation-deaggregation mechanism is highlighted for GUVS in the presence of chitosan. Enthalpic variations measured by microcalorimetry and ζ-potential results show that the interaction between membrane and polyelectrolyte, previously demonstrated to be of electrostatic origin, is stronger when they are of opposite charge sign, as expected. For chitosan, the low saturation coverage degree is found to be nearly independent of molecular weight and interpreted in terms of polymer mainly adsorbed flat on the surface. On the contrary, maximum hyaluronan coverage degree dramatically varies with its molecular weight: hyaluronan is assumed to adsorb on the vesicle forming trains and loops. Finally, chitosan- and hyaluronan-vesicle decorations are demonstrated to be strongly resistant in a very large range of pH (2.0 < pH < 10.0). Similar patch-like structure of DOPC membrane coated by hyaluronan and chitosan is demonstrated for GUVs and LUVs even at extreme pH, but conformations of the adsorbed polyelectrolytes are different.