K. Gawrisch et Ll. Holte, NMR INVESTIGATIONS OF NON-LAMELLAR PHASE PROMOTERS IN THE LAMELLAR PHASE STATE, Chemistry and physics of lipids, 81(2), 1996, pp. 105-116
A delicate balance of attractive and repulsive forces between lipid mo
lecules determines the average area per lipid in the liquid crystallin
e 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 hex
agonal (H-II) and cubic phases. If assembled in bilayers, non-lamellar
forming lipids are under a curvature related lateral stress. This ten
sion imbalance influences molecular cross-sectional area in the lamell
ar phase which is reflected in order parameter changes of a perdeutera
ted lipid chain. H-2 NMR order parameters are sensitive to variations
as small as 0.2 Angstrom(2) in area per molecule. Curvature stress cau
sed by a reduction of repulsive forces between headgroups, e.g. replac
ement of PC with PE raises chain order and reduces area per lipid by a
few square angstrom. Stress caused by increased repulsive tension bet
ween lipid hydrocarbon chains, e.g. an increase in chain length, chain
unsaturation or a rise in temperature, lowers chain order and increas
es area per molecule. Addition of alkanes, alcohols, cholesterol and o
ther substances also changes lipid order, but interpretation of these
changes in terms of tension in headgroup and chain regions does not fo
llow simple patterns. In addition to altering tension, these additives
may stabilize non-lamellar phases by lowering interstitial energies.