Phase diagram of lipid A from Salmonella minnesota and Escherichia coli rough mutant lipopolysaccharide

We have reported here on the structural polymorphism of lipid A, the “endotoxic principle” of bacterial lipopolysaccharide. For lipid A of rough mutant lipopolysaccharide from Salmonella minnesota and Escherichia coli, the three-dimensional supramolecular structures were determined with x-ray diffraction utilizing synchrotron radiation. The investigations were performed in the water concentration range 10 to 95% by weight, at lipid A]:Mg²⁺] molar ratios from 1:0 to 0.1:1, and in the temperature range from 20 to 70°C. These data were correlated with measurements of the β→α phase behaviour which was monitored with differential scanning calorimetry and Fourier-transform infrared spectroscopy. We found that the transition temperature of the acyl chains ranges—in the absence of Mg²⁺—from 45°C at high to 56°C at low water content, and—at an equimolar content of Mg²⁺—from 52°C at high to 59°C at low water concentrations. In the gel phase—in which the lipid A acyl chains are more disordered than those from saturated phospholipids—cubic phases are adopted at high water content (>60%) and at high lipid A):Mg²⁺] molar ratios. At low water contents, lamellar states are assumed exclusively. In the liquid crystalline state of lipid A, the hexagonal H_II, state is adopted under all conditions. The structural variability of lipid A is highest at high water concentrations, and structural changes may be induced by only slight changes in temperature, water content, and Mg²⁺ concentration. Under physiological conditions, however, the lipid A assemblies exhibit a strong preference to cubic structures.