Kinetic process of formation and reconstruction of small unilamellar vesicles consisting of long- and short-chain lipids.

Whereas the phase separation of normal phospholipids induces formation of microdomains on the surface of spherical vesicles, the separation of a long- and short-chain lipids can induce perforation of small unilamellar vesicles (SUVs) and transformation into bilayered micelles (bicelles) because the edges of the bilayers are stabilized by the short-chain lipid microdomain. In this study, the effect of the phase separation of lipids on the transformation behavior of SUVs consisting of a mixture of long- and short-chain lipids was investigated using small-angle neutron scattering. At the temperature jump from below to above the chain melting temperature of the long-chain lipid, T(c), bicelles fused together and transformed into SUVs when their size reached a critical radius. In contrast, a sequential transformation of small SUVs to small bicelles, small bicelles to large bicelles, and large bicelles to large SUVs occurred when the temperature jumped from a value far above T(c) to one slightly above T(c). To the best of my knowledge, this is the first report of such reconstruction of vesicles. By considering the bending energy of the membrane, the line tension of the rim, and the perforation due to the phase separation, the mechanism of the transformation processes was clarified.