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 ph ases include the inverted hexagonal phase and various bicontinuous cub ic 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 anal ogs of zeolites. The recently developed methods used to polymerize and stabilize lamellar assemblies offer certain strategies that are appli cable to nonlamellar phases. Here we report the successful stabilizati on of a nonlamellar phase via the polymerization of reactive amphiphil es. A 3:1 molar mixture of polymerizable mono-dienoyl-substituted phos phoethanolamine and bis-dienoyl-substituted phosphocholine were hydrat ed to yield bilayers. X-ray diffraction of the unpolymerized mixture a t 60 degrees C showed the formation of an inverted hexagonal phase whi ch on prolonged incubation changed to a bicontinuous cubic phase of Pn ($) over bar 3m symmetry. Polymerization of the hexagonal phase produ ced a stabilized hexagonal structure over the range of 20 to 60 degree s C. The same lipids at lower concentration were characterized by P-31 -NMR and transmission electron microscopy (TEM) before and after polym erization. The NMR shows the formation of a sample with isotropic symm etry as expected for a cubic phase. The polymerized sample retained a nonlamellar structure after cooling and extended storage at room tempe rature 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 i nterpenetrating water channels with high surface area and potentially bicompatible lipid-water interfaces.