A calorimetric study of the formation and stability of phospholipid vesicles

In this thesis, the application of calorimetric techniques to the investigation of the formation and stability of vesicles, produced via the hydration of a phospholipid film, is described. The processes of formation of vesicles from natural egg phosphatidylcholine (egg PC) or synthetic dimyristoylphosphatidylcholine (DMPC) films were investigated using solution calorimetry. The formation of phospholipid vesicles in the solution calorimeter comprised hydration of the phospholipid film and subsequent reorganisation of phospholipid molecules into vesicles. The enthalpy of formation of phospholipid vesicles was exothermic for egg PC, and endothermic for DMPC. Solution calorimetry was also used to evaluate the effect of cholesterol, non-ionic surfactants, and beclomethasone dipropionate (BDP) on the enthalpy of formation of phospholipid vesicles. The inclusion of 30 or 50 mol % cholesterol in egg PC films significantly increased the enthalpy of formation of vesicles. The inclusion of increasing amounts of non-ionic surfactants in DMPC films resulted in the solubilisation of the phospholipid bilayer via the lamellar to micelle transition, and consequently, the enthalpies of reaction decreased. The results obtained from films containing cholesterol or non-ionic surfactants indicated that these molecules modified the arrangement of the phospholipids, and had an energetic and molecular effect on the processes of formation of vesicles. Contrarily, no significant interaction was detected between BDP and egg PC or DMPC. The thermodynamic stability of phospholipid dispersions produced by the film hydration method was analysed using isothermal microcalorimetry. Storage of egg PC vesicles at 25 °C in the calorimeter produced an exothermic response that was further investigated. Transmission electron microscopy examination of annealed or vortexed phospholipid dispersions (pure egg PC or mixed) showed the transformation of multilamellar vesicles (MLVs) into bilayered phospholipid discs demonstrating the metastability of MLVs