Structures for the N-acetylneuraminic acid (Neu5Ac)-containing 056 and 024 polysaccharides of Escherichia coli have been reported previously. During these studies unusual chemical shifts had been observed for the NMR signals for H-3eq and C-3 of the Neu5Ac residues of both polysaccharides. In further pursuing this phenomenon, we have reinvestigated the 056 and 024 polysaccharides as well as derived oligosaccharides by one- and two-dimensional NMR spectroscopy. The results showed that structures of both polysaccharides (PSs) had to be modified and formulated as056 PS: ~ 3)-fl-D-Glcp-(1 ~ 3)-fl-D-GlcpNAc-(1 ~ 7)-a-Neu5Ac-(2 2 t 1 a-D-Galp 024 PS: ~ 3)-fl-D-Glcp-(1 ~ 3)-fl-o-GalpNAc-(1 ~ 7)-ot-Neu5Ac-(2 2 t 1 a-o-Glcp2D ROESY spectra revealed a strong NOE between H-3eq of NeuSAc and the protons of the side-chain sugar (H-3 and H-5 of ~-o-Galp in the 056 PS and H-3 of Ot-D-Glcp in the 024 PS) and also between H-3ax of Neu5Ac and H-3 of fl-D-Glcp in the main chain. This indicated a close spatial apposition of the seven-linked a-Neu5Ac and the side-chain residues a-D-Galp (056 PS) and a-D-Glcp (024 PS), respectively. The strong long-range spatial contacts caused the unusual chemical shifts of H-3eq and C-3 of Neu5Ac.