Comparison of the orientational order of lipid chains in the L alpha and HII phases.

The orientational order profile has been determined by using deuterium nuclear magnetic resonance (2H NMR) for POPE in the lamellar liquid-crystalline (L alpha) and the hexagonal (HII) phases and is shown to be sensitive to the symmetry of the lipid phase. In the HII phase, as compared to the L alpha phase, the acyl chains are characterized by a greater motional freedom, and the orientational order is distributed more uniformly along the lipid acyl chain. This is consistent with a change from a cylindrical to a wedge-shaped space available for the lipid chain. 2H NMR studies of POPE dispersions containing tetradecanol or decane, both of which can induce HII phase structure, show very different behavior. Tetradecanol appears to align with the phospholipid chains and experience the L alpha to HII phase transition with a similar change in motional averaging as observed for the phospholipid chains themselves. In contrast, decane is apparently deeply embedded in the lipid structure and exhibits only a small degree of orientation. The L alpha to HII phase transition for systems containing decane leads to a dramatic increase of the motional freedom of decane which is more pronounced than that observed for the lipid chains. This is consistent with a preferential partition of the decane molecules into a disordered environment such as the intercylinder spaces in the HII phase. The presence of decane in the HII phase structure does not modify the order of the lipid chains.(ABSTRACT TRUNCATED AT 250 WORDS)