A Novel Core Region, Lacking Heptose and Phosphate, of the Lipopolysaccharide from the Gram‐Negative Bacterium Pseudomonascichorii (Pseudomonadaceae RNA Group 1)

Abstract

AbstractThe 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‐(2→4)‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)