Structural Elucidation of the Biological Repeating Unit of O-specific Polysaccharide from Citrobacter Serotype O41

Sugar analysis of the O-specific polysaccharide produced by Citrobacter serotype O41 revealed the presence of a hexasaccharide repeating unit, which includes the unusual 3-amino-N -(d-3′hydroxybutyryl)-3,6-dideoxy-d-galactosyl residue (Fuc3NAcyl). The structure of the repeating unit was determined by extensive use of homo- and heteronuclear two-dimensional NMR spectroscopy, including the application of long-range 1H-13C correlation experiments and NOE studies to establish the sequence of sugar units. Identification of the Glcβ1→2Fuc3NAcylβ→6GlcNAcα1→ sequence at the non-reducing terminus establishes the biological repeating unit of this O-specific polysaccharide as: →2Glcβ-2Fuc3NAcyl-β1→6GlcNAcα1→(Glcα1-→2)4Galβ1→3GalNAcβ1→. This structure is similar to that found for the O-specific polysaccharide isolated from Hafnia alvei strain 1211 [Katzenellenbogen, E., Romanowska, E., Dabrowski, U. & Dabrowski, J. (1991) Eur. J. Biochem. 200, 401–407], which differs in having an acetyl substituent at O4 of the Fuc3NAcyl residue and a branch point of GalNAcα substituted by Glcβ at O3; these differences are responsible for the only weak serological cross-reactivity of the two strains.