On the Phase Behaviour of Lipid X, a Precursor of Bacterial Lipopolysaccharides

Abstract

Lipid X forms small micelles of average diameter of 13 ± 2 nm in excess H2O pH 7 to 8, and under physiological conditions. The critical micellar concentration is 4 × 10–5 M, and the critical micellar temperature coincides with the ice melting temperature. Freezing of micellar dispersions of lipid X induces the conversion of the micellar phase to a lamellar gel phase with hexagonal hydrocarbon chain packing. Conversely, melting of ice in frozen lipid X dispersions induces the transition from the lamellar gel phase to a liquid crystalline phase. At room temperature different liquid crystalline phases are observed depending on the water content of lipid X dispersions. In the order of increasing water contents cubic, hexagonal and micellar phases are formed. At room temperature lipid X has therefore a preference for phases with highly curved lipid surfaces. The preference for non-lamellar phases in the fully hydrated state of lipid X at room temperature can be rationalized in terms of hydration. About 22 mol H2O/ mol lipid X are bound which is about twice as much water as is bound to phosphatidylcholine. The large amount of water which is associated with the α-D-glucosamine-l-phosphate group of lipid X gives rise to a wedge-shaped molecule. Molecules of this shape have been shown to have a preference for non-lamellar phases. The hydrocarbon chains of lipid X in the smectic (lamellar) phase pack in a typical, hexagonal lattice. Interdigitation of the hydrocarbon chains in the lamellar phase is not observed.