Phase behaviour of lipid X.
The phase behaviour of aqueous dispersions of lipid X, a precursor of bacterial lipopolysaccharides has been investigated by a variety of physico-chemical techniques. The results are consistent with the presence of disk-shaped micelles with an average diameter of 13 +/- 1.8 nm. The critical micellar concentration in water and physiological saline is 4 x 10(-5) M. Consistent with the formation of micelles in water and physiological saline is the finding that lipid X is in the liquid-crystalline state at temperatures higher than 0 degrees C. The packing and the dynamics of lipid X are characteristic of micelles. Close to the polar group the hydrocarbon chains are significantly more mobile and disordered than in the corresponding region of lipid bilayers. From monolayer studies an estimate of the molecular area of lipid X is derived; under physiological conditions the area/molecule is about 0.50 nm2 at 30 mN/m indicating that lipid X has a wedge-like shape. The two pK values of the primary phosphate group of lipid X are pK1 approximately 1.3 and pK2 = 8.2. At pH values less than 7, the area/molecule decreases, i.e. the packing of the lipid X molecules becomes tighter, and there is also a decrease in the solubility of lipid X. As is characteristic of charged lipids, the state of aggregation (phase behaviour) of lipid X depends on pH, the ionic strength and the nature of the counterion.
Lipka
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G
Laboratorium fuer Biochemie, Eidgenoessische Technische Hochschule, Zuerich, Switzerland.
Demel
R A
RA
Hauser
H
H
D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002621:Chemistry; D002850:Chromatography, Gel; D003720:Densitometry; D006017:Glycolipids / Q000032:analysis*; D008050:Lipid A / Q000032:analysis*; D008823:Micelles; D008854:Microscopy, Electron; D010316:Particle Size; D012995:Solubility; D013050:Spectrometry, Fluorescence
D006017:Glycolipids; D008050:Lipid A; D008823:Micelles; C044283:lipid X