Via conformational flexibility, triglycerides stabilize organic-water interfaces of changing area.

Abstract

The role of triglycerides (TGs) in both synthetic and biological membranes has long been the subject of study, involving metabolism, disease, and colloidal synthesis. TGs have been found to be critical components for successful liposomal encapsulation via double emulsion (Water/Oil/Water), which this work attempts to explain. TGs can occupy multiple positions in a biological membranes. The glycerol backbone can sit at the membrane's interface with water, adjacent to phospholipid headgroups (“surface” or “M” conformation), typically with relatively low (∼3%) solubility. TGs can also occupy “oil” phases, where the glycerol backbone is isolated from water, in either mid-membrane positions or lipoprotein-style phospholipid-coated TG droplets. Using 13C-Nuclear magnetic resonance spectroscopy (NMR) to determine the degree of hydration of the TG backbone, it was revealed that TGs transition from surface to oil phases as the organic solvent is removed via evaporation. A new, transitional TG backbone position has been identified, with a level of hydration between surface and oil. These results suggest that TGs are able to temporarily coat and stabilize the large water/organic interfaces present just after emulsification. As the chloroform evaporates and interfaces shrink, the TGs recede into mid-membrane spaces or to bud off into separate coated oil droplets, which are confirmed via transmission electron microscopy. These TG-rich droplets can be removed via high-speed centrifugation. Encapsulation efficiency is found to be inversely related to both chain saturation and length, indicating that membrane fluidization is a key property arising from the presence of TGs. Beyond clarification of a mechanism for high-efficiency liposomal encapsulation, these results implicate TGs as components that are able to stabilize biological membrane transitions involving changing interfacial area. This role for TGs may be of use in designing drug delivery systems and investigation of membrane transitions in life sciences.