Structural studies of the O-specific polysaccharide from the lipopolysaccharide of Mesorhizobium huakuii strain S-52, the symbiotic partner of Astragalus sinicus

The O-specific polysaccharide (OPS) obtained by mild-acid degradation of the lipopolysaccharide isolated from Mesorhizobium huakuii strain S-52 was studied by sugar and ethylation analyses along with H-1 and C-13 NMR spectroscopy. It was concluded that the OPS was composed of trisaccharide repeating units containing two residues of 6-deoxy-L-talose (6dTal) and one L-rhamnose (Rha), whose sequence in the OPS was determined by NOESY and HMBC experiments. The minor 3-O-acetylation (about 10%) of 6-deoxytalose glycosidically substituted at position-2 was judged by relative signal intensities of corresponding O-acetylated and non-acetylated 6dTal residues. Moreover, it was found that the non-reducing end of the OPS repeating unit was occupied by 3-O-methyl-D-fucose, which terminated the O-chain as a cap-residue. These data defined the structure of the OPS as: alpha-3-OMe-D-Fucp-(1 ->[2)-alpha-L-6dTalp-(1 -> 3)-alpha-L-6dTalp-(1 -> 2)-alpha-L-Rhap-(1 ->](n) (C) 2011 Elsevier Ltd. All rights reserved.