The structure of the O-antigen from Escherichia coli reference strain O188 (E. coli O188:H10) has been investigated. The lipopolysaccharide shows a typical nonrandom modal chain-length distribution and the sugar and absolute configuration analysis revealed D-Man, D-Glc, D-GlcN and D-GlcA as major components. The structure of the O-specific polysaccharide was determined using one- and two-dimensional H-1 and C-13 NMR spectroscopy experiments, where inter-residue correlations were identified by H-1,C-13-heteronuclear multiple-bond correlation and H-1,H-1-NOESY experiments, which revealed that it consists of pentasaccharide repeating units with the -> 4)-beta-D-GlcpA-(1 -> 2)-beta-D-Manp-(1 -> 4)-beta-D-Manp-(1 -> 3)-beta-D-GlcpNAc-(1 -> following structure: vertical bar alpha-D-Galp-(1 -> 3) Biosynthetic aspects and NMR analysis are consistent with the presented structure as the biological repeating unit. The O-antigen of Shigella boydii type 16 differs only in that it carries O-acetyl groups to similar to 50% at O6 of the branchpoint mannose residues. A molecular model of the E. coli O188 O-antigen containing 20 repeating units extends similar to 100 angstrom, which is similar to the height of the periplasmic portion of polysaccharide co-polymerase Wzz proteins that regulate the O-antigen chain length of lipopolysaccharides in the Wzx/Wzy biosynthetic pathway.

Carbohydrate structure can be elucidated or confirmed by using NMR spectroscopy as the prime technique. Prediction of ¹H and ¹³C NMR chemical shifts by computational approaches makes this assignment process more efficient and the program CASPER can perform this task rapidly. It does so by relying on chemical shift data of mono-, di-, and trisaccharides. In order to improve accuracy and quality of these predictions we have assigned ¹H and ¹³C NMR chemical shifts of 30 monosaccharides, 17 disaccharides, 10 trisaccharides and one tetrasaccharide; in total 58 compounds. Due to different rotamers, ring forms, α- and β-anomeric forms and pD conditions this resulted in 74 ¹H and ¹³C NMR chemical shift data sets, all of which were refined using total line-shape analysis for the ¹H resonances in order to obtain accurate chemical shifts. Subsequent NMR chemical shift predictions for three sialic acid-containing oligosaccharides, viz., GD1a, a disialyl-LNnT hexasaccharide and a polysialic acid-lactose decasaccharide, and NMR-based structural elucidations of two O-antigen polysaccharides from E. coli O174 were performed by the CASPER program (http://www.casper.organ.su.se/casper/) resulting in very good to excellent agreement between experimental and predicted data thereby demonstrating its utility for carbohydrate compounds that have been chemically or enzymatically synthesized, structurally modified or isolated from nature.