A NOVEL CORE STRUCTURE, LACKING HEPTOSE AND PHOSPHATE, OF THE LIPOPOLYSACCHARIDE FROM THE GRAM-NEGATIVE BACTERIUM PSEUDOMONS CICHORII (PSEUDOMONADACEAE RNA GROUP 1)

The extracted lipopolysaccharides (LPSs) fraction of the plant-pathogenic bacterium Pseudomonas cichorii contained two different molecules, the first of them an LPS possessing an O-specific polysaccharide made up of four 2-acetamido-2,6-dideoxy sugars linked linearly in a tetrasaccharide repeating unit. The structure of the second LPS component is reported here: it was of the rough form and contained a novel type of core region. The rough form LPS of Ps. cichorii was de-O-acylated by mild hydrazinolysis and then de-N-acylated with hot 4 M KOH. The oligosaccharide representing the complete carbohydrate backbone of the LPS was isolated by high-performance anion-exchange chromatography. Its structural characterization employed compositional and methylation analyses, matrix-assisted laser desorption/ionisation mass spectrometry and 1H, 13C and 31P NMR spectroscopy, with application of various 1D and 2D experiments. The carbohydrate backbone of this core region consisted of a hexasaccharide that contained no heptoses, being made up of L-Rhap, D-GlcpN and D-GalpN, besides the characteristic α-(24)-linked 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) disaccharide. The first Kdo residue linking the core region to the lipid A was substituted by a GalpN residue. This core region thus represents one of the rather few cases in which this position is not substituted by a manno-configured sugar. The structure of the LPS carbohydrate backbone is shown in Scheme 1. If not stated otherwise, sugars were D-configured pyranoses. Kdo is 3-deoxy-D-manno-oct-2-ulosonic acid. The lipid A backbone is N- and O-acylated in the LPSs. − Molecular modelling studies of the LPS carbohydrate backbone and of some of its parts were performed, yielding a preferred conformation that was surprisingly inflexible between the two Kdo residues.