Fluorinated Vesicles Made from Combinations of Phospholipids and Semifluorinated Alkanes. Direct Experimental Evidence of the Location of the Semifluorinated Alkane within the Bilayer

Previous work has demonstrated that codispersion of semifluorinated alkanes C n F 2 n +1 C m H 2 m +1 (F n H m diblocks) with phospholipids strongly modifies the properties of the resulting vesicles. The hypothetical segregation and ordered self-organization of F n H m diblocks as a continuous film within the bilayer were suggested to account for the observed changes of bilayer behavior; however, no proof of the diblock location and organization was provided. We now report direct experimental evidence of the diblock's presence and organization within the bilayer using cryogenic transmission electron microscopy (cryo-TEM), high-sensitivity differential scanning calorimetry (micro-DSC), and small-angle X-ray scattering (SAXS). Cryo-TEM demonstrated that a sonicated codispersion of dioleoylphosphatidylcholine (DOPC) and C 6 F 13 C 10 H 21 (F6H10) (1:1) led to an homogeneous population of small unilamellar vesicles (SUVs) with no F6H10 emulsion droplets present. The bilayer of these SUVs appeared as a single, thick and dark ring, characteristic of the bilayer of vesicles made from fluorinated phospholipids. This aspect, very different from that of the bilayer of DOPC vesicles, which consists of two concentric rings, demonstrated unambiguously the presence of a fluorinated core within the bilayer. The micro-DSC study on dimyristoylphosphatidylcholine (DMPC)/F6H10 SUVs showed a shift of the DMPC main transition phase temperature to lower values, indicating interdigitation of the H10 hydrogenated segments of the diblock with the fatty acid chains of the DMPC. In addition, F6H10 was observed to undergo a gel-to-fluid transition at ∼25 °C when incorporated in the DMPC bilayer, giving further evidence for an organized fluorinated film within the bilayer (F6H10 is liquid in the bulk). Finally, SAXS experiments were in agreement with a hollow spherelike particle model constituted by a ∼3 nm thick fluorinated shell, indicating that the fluorinated segments are not interdigitated.