Unbinding Transition Induced by Osmotic Pressure in Re- lation to Unilamellar Vesicle Formation

– Small-angle X-ray scattering and phase-contrast microscopy experiments were performed to investigate the effect of the osmotic pressure on vesicle formation in a dioleoylphosphatidylcholine (DOPC)/water/NaI system. Multi-lamellar vesicles were formed when a pure lipid film was hydrated with an aqueous solution of NaI. On the other hand, uni-lamellar vesicles (ULVs) were formed when a lipid film mixed with an enough amount of NaI was hydrated. To confirm the effect of the osmotic pressure due to NaI, a free-energy calculation was performed. This result showed that the osmotic pressure induced an unbinding transition on the hydration process, which resulted in ULV formation. Introduction. – All biomembranes mainly consist of lipid bilayers, and functional proteins float on them. Such bilayers also appear in aqueous solutions of synthesized phospholipids. Therefore, bilayers in such solutions are intensively studied to understand the physical properties of biomembranes. Moreover, these bilayers form vesicles that attract attention for studying model cells. Such vesicles can be effectively obtained by hydrating dry lipid films on substrates or test tubes. (natural swelling method [1]) For example, dioleoylphosphatidylcholine (DOPC) films, one of the typical phospholipids in biomembranes, provide micrometersize vesicles. These DOPC vesicles normally grow with a multi-bilayer shell (multi-lamellar vesicle; MLV), although uni-lamellar vesicles (ULVs) are prefered for using as model cells. Therefore, effective methods to create ULVs have been proposed [2–9]. One of the authors has recently designed a novel method for preparing ULVs by hydrating dry lipid films mixed with sugar and salt [10]. As some previous studies have pointed out, an osmotic pressure would promote ULV formation [6, 8]. The mechanism of ULV formation induced by osmotic pressure, however, has not been yet clarified. (∗) E-mail: [email protected]