Emergence of superstructures from a homogeneous lipid sphere

The spontaneous generation of a periodic hexagonal superstructure on a giant phospholipid sphere (GPS) with a diameter of 20-200 microm was studied. The GPS was composed of ternary phospholipids consisting of dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), and dioleoylphosphatidylinositol-bisphosphate (DOPIP(2)). GPSs were prepared by natural swelling of a lipid film formed on a glass substrate. A GPS with a homogeneous lipid mixture tends to form a two-layered structure between the surface and inner parts; the surface layer is attributed to a DOPIP(2) rich region (we call this layer SL), and the interior is rich in DOPE and DOPC (we call this layer IL). A hexagonal superstructure develops in the SL, and the topology then changes to form multiple-doughnut structures. Finally, myelin-like tubes are generated through symmetry breaking of the doughnutlike structures. The time-dependent change in the surface-area expansion of a GPS is shown to obey the logistic growth model, and this is attributed to the kinetic process of phase segregation between the surface and bulk phase of the GPS.