NMR investigations of non-lamellar phase promoters in the lamellar phase state

A delicate balance of attractive and repulsive forces between lipid molecules determines the average area per lipid in the liquid crystalline lamellar phase. Formation of inverted non-lamellar phases is linked to an imbalance in lateral tension between lipid headgroups and chains favoring formation of curved lipid monolayers, such as in Inverse hexagonal (H-II) and cubic phases. If assembled in bilayers, non-lamellar forming lipids are under a curvature related lateral stress. This tension imbalance influences molecular cross-sectional area in the lamellar phase which is reflected in order parameter changes of a perdeuterated lipid chain. H-2 NMR order parameters are sensitive to variations as small as 0.2 Angstrom(2) in area per molecule. Curvature stress caused by a reduction of repulsive forces between headgroups, e.g. replacement of PC with PE raises chain order and reduces area per lipid by a few square angstrom. Stress caused by increased repulsive tension between lipid hydrocarbon chains, e.g. an increase in chain length, chain unsaturation or a rise in temperature, lowers chain order and increases area per molecule. Addition of alkanes, alcohols, cholesterol and other substances also changes lipid order, but interpretation of these changes in terms of tension in headgroup and chain regions does not follow simple patterns. In addition to altering tension, these additives may stabilize non-lamellar phases by lowering interstitial energies.