Structural studies of extracellular polysaccharide produced by Rhizobium fredii Tu6, a polysaccharide with nonasaccharide repeating units

Polysaccharide secreted by Rhizobium fredii Tu6 was investigated with composition analysis, linkage analysis, Svensson degradations, ¹H-NMR, and desorption chemical ionization/mass spectrometry. The polysaccharide is composed of a nonasaccharide repeating unit (seven glucose, two galactose) with five sugar residues in the side chain and four sugar residues in the back bone. The terminal residue of the side chain is a galactose residue. The second residue from the terminal end is a 3-linked glucose substituted by a pyruvate group at the 4, 6-positions. Monosaccharide sequence was determined by Svensson degradations followed by linkage analysis. After removal of a pyruvate group from the methylated polysaccharide, the sugar residues in the side chain were removed sequentially by Svensson degradations. The reaction product of each degradation was then analyzed by methylation analysis. The polysaccharide secreted at pH 5 medium has a structure very similar to the polysaccharide secreted in the medium of pH 7 except that about 30% of the terminal galactose residues are also substituted at the 4, 6-positions by a second pyruvate group.