Polymerization of the Inverted Hexagonal Phase

The hydration of polar natural and synthetic lipids yields a variety of lipid phases including various inverted cubic phases and the inverted hexagonal (HII) phase. The HII phase can be considered as aqueous columns encased with a monolayer of lipids and arranged in a hexagonal pattern. The polar head groups are well-ordered at the water interface, whereas the lipid tails are disordered to fill the volume between the tubes of water. A particularly interesting characteristic of the HII phase is the large temperature effect on the basis vector length d of the hexagonal lattice. Previous studies indicate that polymerization of the lipid region of the HII phase might reduce the sensitivity of the basis vector to temperature. A phosphoethanolamine (PE) was designed and synthesized with dienoyl groups in each lipid tail in an attempt to cross-link the lipids around and along the water core of the HII phase. The synthesis of the the PE was accomplished by acylation of 3-(4-methoxybenzyl)-sn-glycerol with 2,4,13-(E,E,Z)-docosatrienoic acid, followed by deprotection, then phosphorylation with dichloro-[[N-[(2,2,2-trichloroethoxy)carbonyl]-2-amino]ethyl]phosphinic acid to give the Troc-PE, which was converted to the PE with activated zinc and acetic acid. The hydrated PE (1/1 weight lipid/water) formed the HII phase over an extended temperature range. Polymerization to high conversion was accomplished at 60 °C with the aid of redox initiators. Polymerization was followed in-situ using X-ray diffraction over a period of 48 h. The scattering, which weakened over the course of the reaction, remained consistent with a hexagonal phase. Temperature cycling of the polymerized HII phase showed an unaltered pattern on decreasing temperature while maintaining the same lattice parameter, unlike that of the unpolymerized phase where the value increased with decreasing temperature. Thus it is possible to fix the dimensions of the HII phase by cross-linking polymerization of appropriately designed reactive lipids.