Ethanol-induced aggregation and fusion of small phosphatidylcholine liposome: participation of interdigitated membrane formation in their processes.

The mechanism for the ethanol-induced aggregation/fusion of uniform-sized small liposomes comprised of dipalmitoyl (DPPC) or egg yolk (eggPC) phosphatidylcholines was studied by measuring the average size using a photon correlation spectroscopy, by observing directly the states in the liposomal solutions using freeze-fracture electron microscopy and by attempting resonance energy transfer using flurophore-labeled phospholipids. Abrupt increases in the apparent size of DPPC liposomes were observed in the presence of above 44 mg/ml ethanol, where microscopically plateau membranes form interdigitated structure, in which the acyl chains fully interpenetrate the hydrocarbon chains of the apposing monolayer. On the contrary, in the eggPC liposome, where the membranes cannot form interdigitated structures even in the presence of high concentration of ethanol, such intense aggregation and fusion were not observed, suggesting their intimate relation to the interdigitated structure formation. The formation of interdigitated structures in the adhering region leads to an increase in the interfacial area and an exposure of hydrophobic acyl chain terminal on the surface area, and enhances hydrophobic interactions between two interdigitated bilayers. Thus, the resultant interdigitated structure makes the aggregated state stable and partially initiates the bilayer mixing between the two apposed membranes, leading to fusion.