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.

The 1-palmitoyl-2-oleoyl-phosphatidylethanolamine: 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE:POPC) system has been investigated by measuring, in the inverted hexagonal (HII) phase, the intercylinder spacings (using x-ray diffraction) and orientational order of the acyl chains (using 2H nuclear magnetic resonance). The presence of 20 wt% dodecane leads to the formation of a HII phase for the composition range from 0 to 39 mol% of POPC in POPE, as ascertained by x-ray diffraction and 2H nuclear magnetic resonance. The addition of the alkane induces a small decrease in chain order, consistent with less stretched chains. An increase in temperature or in POPE proportion leads to a reduction in the intercylinder spacing, primarily due to a decrease in the water core radius. A temperature increase also leads to a reduction in the orientational order of the lipid acyl chains, whereas the POPE proportion has little effect on chain order. A correlation is proposed to relate the radius of curvature of the cylinders in the inverted hexagonal phase to the chain order of the lipids adopting the HII phase. A simple geometrical model is proposed, taking into account the area occupied by the polar headgroup at the interface and the orientational order of the acyl chains reflecting the contribution of the apolar core. From these parameters, intercylinder spacings are calculated that agree well with the values determined experimentally by x-ray diffraction, for the variations of both temperature and POPE:POPC proportion. This model suggests that temperature increases the curvature of lipid layers, mainly by increasing the area subtended by the hydrophobic core through chain conformation disorder, whereas POPC content affects primarily the headgroup interface contribution. The frustration of lipid layer curvature is also shown to be reflected in the acyl chain order measured in the L alpha phase, in the absence of dodecane; for a given temperature, increased order is observed when the curling tendencies of the lipid plane are more pronounced.

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