Vesicles of Mixtures of the Bolaform Amphiphile Sodium Di-n-decyl α,ω-Eicosanyl Bisphosphate and Sodium Di-n-decyl Phosphate

Various properties of vesicles prepared from different ratios of the bolaform (bipolar) amphiphile disodium di- n -decyl α,ω-eicosanyl bisphosphate (EBP) and the nonbolaform (monopolar) sodium di- n -decyl phosphate (DDP) have been studied. The colloidal stability decreased upon the increasing sodium di- n -decyl phosphate content. The main phase transitions, studied by fluorescence depolarization and differential scanning calorimetry, become less cooperative and occur at decreasing temperatures upon increasing sodium di- n -decyl phosphate content. The bolaform amphiphile was found to be membrane spanning as indicated by the lamellar repeat spacing obtained by X-ray powder diffraction. Electron microscopic studies of the small vesicles prepared from pure EBP revealed that addition of the fusogen Ca 2+ caused formation of tubules for the EBP. The tubules did not revert to vesicles upon addition of the Ca 2+ chelator EDTA, whereas mixed vesicles with 20−50 mol % DDP content showed enlarged vesicles after subsequent addition of Ca 2+ and EDTA. A preliminary conclusion is that a substantial amount of nonbolaform amphiphiles must be present in a bolaform membrane to enable fusion to occur.