POLYMERIZATION OF NONLAMELLAR LIPID ASSEMBLIES

The formation of nonlamellar lipid structures in model lipid membranes has been extensively studied in recent years. These hydrated lipid phases include the inverted hexagonal phase and various bicontinuous cubic phases, which occur at selected lipid concentrations, temperatures, and pressures. Cubic phases that are bicontinuous with respect to the polar and nonpolar regions are especially interesting as organic analogs of zeolites. The recently developed methods used to polymerize and stabilize lamellar assemblies offer certain strategies that are applicable to nonlamellar phases. Here we report the successful stabilization of a nonlamellar phase via the polymerization of reactive amphiphiles. A 3:1 molar mixture of polymerizable mono-dienoyl-substituted phosphoethanolamine and bis-dienoyl-substituted phosphocholine were hydrated to yield bilayers. X-ray diffraction of the unpolymerized mixture at 60 degrees C showed the formation of an inverted hexagonal phase which on prolonged incubation changed to a bicontinuous cubic phase of Pn ($) over bar 3m symmetry. Polymerization of the hexagonal phase produced a stabilized hexagonal structure over the range of 20 to 60 degrees C. The same lipids at lower concentration were characterized by P-31-NMR and transmission electron microscopy (TEM) before and after polymerization. The NMR shows the formation of a sample with isotropic symmetry as expected for a cubic phase. The polymerized sample retained a nonlamellar structure after cooling and extended storage at room temperature or near 0 degrees C. The TEMs show a polydomain square lattice with 6 +/- 1 nm diameter aqueous channels. This stabilized nonlamellar phase is the first representative of a new family of materials with interpenetrating water channels with high surface area and potentially bicompatible lipid-water interfaces.