Structural characterization of the O-antigenic polysaccharide of the lipopolysaccharide from Rhizobium etli strain CE3 -: A unique O-acetylated glycan of discrete size, containing 3-O-methyl-6-deoxy-L-talose and 2,3,4-tri-O-methyl-L-fucose

The O-antigenic polysaccharide of the Rhizobium etli CE3 lipopolysaccharide (LPS) was structurally characterized using chemical degradations (Smith degradation and beta-elimination of uronosyl residues) in combination with alkylation analysis, electrospray, and matrix-assisted laser desorption ionization-time of flight mass spectrometry, tandem mass spectrometry, and H-1 COSY and TOCSY nuclear magnetic resonance spectroscopy analyses of the native polysaccharide and the derived oligosaccharides. The polysaccharide was found to be a unique, relatively low molecular weight glycan having a fairly discrete size, with surprisingly little variation in the number of repeating units (degree of polymerization = 5). The polysaccharide is O-acetylated and contains a variety of O-methylated glycosyl residues, rendering the native glycan somewhat hydrophobic. The molecular mass of the major de-O-acetylated species, including the reducing end 3-deoxy-D-manno-2-octulosonic acid (Kdo) residue, is 3330 Da. The polysaccharide is comprised of a trisaccharide repeating unit having the structure -->4)-alpha-D-GlcpA-(1-->4)-[alpha-3-O-Me-6-deoxy-Talp-(1-->3)]-alpha-L-Fucp-(1-->. The nonreducing end of the glycan is terminated with the capping sequence alpha-2,3,4-tri-O-Me-Fucp-(1-->4)-alpha-D-GlcpA-(1-->, and the reducing end of the molecule consists of the non-repeating sequence -->3)-alpha-L-Fucp-(1-->3)-beta-D-Manp-(1-->3)-beta-QuiNAcp-(1-->4)-alpha-Kdop-(2-->, where QuiNAc is N-acetylquinovosamine (2-N-acetamido-2,6-dideoxyglucose). The reducing end Kdo residue links the O-chain polysaccharide to the core region oligosaccharide, resulting in a unique location for a Kdo residue in LPS, removed four residues distally from the lipid A moiety, Structural heterogeneity in the O-chain arises mainly from the O-acetyl and O-methyl substitution. Methylation analysis using trideuteriomethyl iodide indicates that a portion of the 2,3,4-tri-O-methylfucosyl capping residues, typically 15%, are replaced with 2-O-methyl- and/or 2,3-di-O-methylfucosyl residues. In addition, approximately 25% of the 3,4-linked branching fucosyl residues and 10% of the 3-linked fucosyl residues are 2-O-methylated, A majority of the glucuronosyl residues are methyl-esterified at C-6, These unique structural features may be significant in the infection process.