Supramolecular structure of lipopolysaccharide and free lipid A under physiological conditions as determined by synchrotron small-angle X-ray diffraction.

Lipopolysaccharides, the major amphiphilic components of the outer leaflet of the outer membrane of Gram-negative bacteria, may assume various three-dimensional supramolecular structures depending on molecular properties (e.g. chemical structure) and on ambient conditions (e.g. temperature, concentration of divalent cations). We applied synchrotron small-angle X-ray diffraction to investigate the supramolecular structures of natural and synthetic Escherichia-coli-type lipid A, of lipid A from Salmonella minnesota, and of rough mutant lipopolysaccharides of E. coli and S. minnesota under physiological water content (greater than 90%) at different temperatures (20, 37, and 55 degrees C) and at different lipid/divalent cation molar ratios (20:1 to 1:1). We found that in the absence of divalent cations rough mutant lipopolysaccharide and free lipid A form unilamellar structures with the main reflections centered around 4.50 nm for free lipid A, 4.80 nm for Re lipopolysaccharide, and 5.90 nm for Rd1 lipopolysaccharide at 20 degrees C, i.e. below the beta----alpha acyl-chain-melting transition temperature. Above this temperature, the reflections are shifted to 4.30 nm for free lipid A (at 55 degrees C), 4.60 nm for Re lipopolysaccharide (at 37 degrees C), and to 5.50 nm for Rd1 lipopolysaccharide (at 37 degrees C). The addition of divalent cations leads (at lower concentrations, i.e. lipid/cation molar ratios 20:1 to 5:1) to sharper reflections expressing a higher state of order and to a shift of the center of the main reflections lying now at 5.10 nm for free lipid A, 6.40 nm for Re and 7.20 nm for Rd1 lipopolysaccharide at 20 degrees C. At higher concentrations of divalent cations (e.g. lipid/cation molar ratio 1:1), an increasing tendency to form nonlamellar, inverted cubic structures is observed which is indicated by the occurrence of another main periodicity and/or of reflections with spacing ratios 1: square root of 2, 1: square root of 3 of the main periodicity. The tendency to assume inverted cubic structures is only weakly pronounced for rough mutant lipopolysaccharides but dominant for free lipid A even at physiological temperature and divalent cation concentration.