Kinetically trapped uniform nano-size unilamellar vesicles made of thermodynamically stable multilamellar vesicular phospholipid solutions

We report a study of kinetically trapped unilamellar vesicles (ULVs) of lipid mixtures composed of dimyristoyl phosphatidylcholine (DMPC), dihexanoyl phosphatidylcholine (DHPC) and (dimyristoyl phosphatidylglycerol) DMPG. Two membrane charge densities (namely, [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities ([NaCl] = 0 and 0.2 M) are investigated. Small angle neutron scattering (SANS) is used to identify the morphology of aggregates. As high concentration samples are diluted at 50°C, thermodynamically stable multi-lamellar vesicles (MLVs) are observed for both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to unbind the multilamellar structure. At low temperature (i.e. 10°C) nanodiscs (also known as bicelles) or bilayered ribbons are observed and they slowly evolve into uniform-size nanoscopic unilamellar vesicles (ULVs) when incubated at 10°C over a period of ∼20 hours. The ULVs persist after being heated to 50°C, where thermodynamically stable MLVs are observed. This result clearly indicates that these ULVs are kinetically trapped and that the mechanical properties (e.g. bending rigidity) of the nanodiscs at 10°C favor the formation of nanoscopic ULVs over MLVs. From a practical point-of-view, this method of forming uniform-size ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on lipid-based systems for therapeutic and diagnostic purposes.