Non-phospholipid liposomes with high sterol content display a very limited permeability

Abstract

We demonstrate that it is possible to form non-phospholipid fluid bilayers in aqueous milieu with a mixture of palmitic acid (PA), cholesterol (Chol), and cholesterol sulfate (Schol) in a molar proportion of 30/28/42. These self-assemblies are shown to be bilayers in the liquid ordered phase. They are stable between pH 5 and 9. Over this pH range, the protonation/deprotonation of PA carboxylic group is observed but this change does not appear to alter the stability of these bilayers, a behavior contrasting with that observed for binary mixtures of PA/Chol, and PA/Schol. The multilamellar dispersions formed spontaneously from the PA/Chol/Schol mixture could be successfully extruded to form Large Unilamellar Vesicles (LUVs). These LUVs show interesting permeability properties, linked with their high sterol content. These non-phospholipid liposomes can sustain a pH gradient (pHinternal 8/pHexternal 6) 100 times longer than LUVs made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol, with a molar ratio of 60/40. Moreover, the non-phospholipid LUVs are shown to protect ascorbic acid from an oxidizing environment (1 mM iron(III)). Once entrapped in liposomes, ascorbic acid displays a degradation rate similar to that obtained in the absence of iron(III). These results show the possibility to form novel nanocontainers from a mixture of a monoalkylated amphiphile and sterols, with a good pH stability and showing interesting permeability properties.