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The lipid head groups in the inner leaflet of unilamellar bilayer vesicles of the synthetic lipids DHPBNS and DDPBNS can be selectively oligomerised. Earlier studies have established that these vesicles fuse much slower and less extensively upon oligomerisation of the lipid head groups. The morphology and calcium-induced fusion of vesicles of DHPBNS and DDPBNS were investigated using cryo-electron microscopy. DHPBNS vesicles are not spherical but flattened, ellipsoidal structures. Upon addition of CaCl2, DHPBNS vesicles with an oligomerised inner leaflet were occasionally observed in an arrested hemifused state. However, the evidence for hemifusion is not equivocal due to potential artefacts of sample preparation. DDPBNS vesicles show the expected spherical morphology. Upon addition of excess CaCl2, DDPBNS vesicles fuse into dense aggregates that show a regular spacing corresponding to the bilayer width. Upon addition of EDTA, the aggregates readily disperse into large unilamellar vesicles. At low concentration of calcium ion, DDPBNS vesicles with an oligomerised inner leaflet form small multilamellar aggregates, in which a spacing corresponding to the bilayer width appears. Addition of excess EDTA results in slow dispersal of the Ca2+-lipid aggregates into a heterogeneous mixture of bilamellar, spherical vesicles and networks of thread-like vesicles. These lipid bilayer rearrangements are discussed within the context of shape transformations and fusion of lipid membranes.

Various properties of vesicles prepared from different ratios of the bolaform (bipolar) amphiphile disodium di-n-decyl R,ˆ-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 din-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 Ca2+ 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.