Structure of amylovoran, the capsular exopolysaccharide from the fire blight pathogen Erwinia amylovora

The acidic exopolysaccharide (EPS) of Erwinia amylovora, amylovoran, was purified from culture supernatants of bacteria in minimal medium and cleaved chemically either by treatment with trifluoroacetic acid or hydrofluoric acid, and enzymatically by digestion with depolymerase from E. amylovora phage φ-Ea1h. Structural characterization of the resulting oligosaccharides was performed by a combination of mass spectrometric and NMR [one- and two-dimensional (1D and 2D)] spectroscopic techniques. A branched repeating unit with five monosaccharide residues and various substituents was determined: The terminal monosaccharide of the side branch, which bears a 4,6-bound pyruvate residue in the R-configuration, was found to be modified with 2-linked (26%), 3-linked (24%), 2-,3-linked (40%) O-acetyl groups, or these were absent (10%). An additional glucose residue is linked to approximately 10% of the core α-galactose of the repeating unit.