Solubility of triacylglycerols or stearylamine in phospholipid vesicles

Abstract

The solubilities of two types of lipids, triglycerides (TG), weakly polar compounds, and stearylamine (SA), a polar molecule, within phospholipid bilayers of vesicles are examined by time-resolved selective solubilisation and compared. Both TG and SA were added to phosphatidylcholine (Lipoid E80, PL) vesicles at different concentrations ranging from 0 to 20% (weight per weight of PL). TG insertion in lipid bilayers is a known phenomenon, which develops up to a limit of solubility beyond which emulsion droplets coexist with liposomes. The characterisation of the various colloids formed is obtained by selective solubilisation of the aggregates resulting from continuous detergent addition (Triton X-100). Turbidity monitoring allowed this identification thanks to the well-known “vesicle-to-micelle transition process”. The low TG concentration only slightly modifies the solubilisation profiles with low turbidity values at the solubilisation completion corresponding to mixed-micelle formation. In contrast, the high TG concentration results in high residual turbidity values interpreted as the coexistence of mixed micelles and emulsion droplets. The amount of detergent used to reach solubilisation completion is proportional to both PL and TG concentrations. The process is dramatically different with SA, although high turbidity values are also observed at solubilisation completion. Indeed, when the SA proportion increases in the vesicles, the amount of detergent necessary to solubilise the vesicles is reduced. So, depending on the type and polarity of molecules added to conventional PL vesicles, the solubilisation can be delayed (TG) or promoted (SA).