Phospholipid Giant Unilamellar Vesicles (GUVS) Melt Like Large (LUVS), Not Mulitilamellar (MLVS), Vesicles

Abstract

The disparity between the excess heat capacity curves for the melting of large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs) has given rise to two hypotheses. The first is that the size of the vesicles results in a difference for how the two types of vesicles undergo their phase transitions. The second is that there is communication between the bilayers in an MLV when it transitions from its ordered gel phase to its liquid disordered phase, resulting in increased cooperativity. To test these hypotheses, differential scanning calorimetry (DSC) was performed on giant unilamellar vesicles (GUVs) of pure DPPC.The GUVs were prepared using electroformation, and visualized with confocal fluorescence microscopy. The average size of the GUVs determined from the images was about 7 micrometers. For comparison, the size of LUVs is about 100 nm. DSC was then performed of the GUVs, and their excess heat capacity curve was recorded. The excess heat capacity curves for the GUVs closely resemble the curves for LUVs. Both GUV and LUV curves are much broader (halfwidth ∼ 1oC) than those of MLV curves (halfwidth ∼ 0.1oC). The similarity of the GUV and LUV excess heat capacity curves indicate that the size of the vesicles does not impact how they undergo their phase transitions. In addition, the results provide evidence that there is cooperativity between the bilayers of an MLV when it undergoes its melting phase transition.