POLYMERIZATION OF THE INVERTED HEXAGONAL PHASE

The hydration of polar natural and synthetic lipids yields a variety o f lipid phases including various inverted cubic phases and the inverte d hexagonal (H-II) phase. The H-II 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 interfac e, whereas the lipid tails are disordered to fill the volume between t he tubes of water. A particularly interesting characteristic of the H- II phase is the large temperature effect on the basis vector length d of the hexagonal lattice. Previous studies indicate that polymerizatio n of the lipid region of the H-II phase might reduce the sensitivity o f the basis vector to temperature. A phosphoethanolamine (PE) was desi gned and synthesized with dienoyl groups in each lipid tail in an atte mpt to cross-link the lipids around and along the water core of the H- II 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 ac id, followed by deprotection, then phosphorylation with trichloroethox y)carbonyl]-2-amino]ethyl]phosphinic acid to give the Troc-PE, which w as converted to the PE with activated zinc and acetic acid, The hydrat ed PE (1/1 weight lipid/water) formed the H-II phase over an extended temperature range. Polymerization to high conversion was accomplished at 60 degrees C with the aid of redox initiators. Polymerization was f ollowed in-situ using X-ray diffraction over a period of 48 h. The sca ttering, which weakened over the course of the reaction, remained cons istent with a hexagonal phase. Temperature cycling of the polymerized H-II phase showed an unaltered pattern on decreasing temperature while maintaining the same lattice parameter, unlike that of the unpolymeri zed phase where the value increased with decreasing temperature. Thus it is possible to fix the dimensions of the H-II phase by cross-linkin g polymerization of appropriately designed reactive lipids.