NMR REINVESTIGATION OF 2 N-ACETYLNEURAMINIC ACID-CONTAINING O-SPECIFIC POLYSACCHARIDES (O56 AND O24) OF ESCHERICHIA-COLI

Structures for the N-acetylneuraminic acid (Neu5Ac)-containing O56 and O24 polysaccharides of Escherichia coli have been reported previously . During these studies unusual chemical shifts had been observed for t he NMR signals for H-3eq and C-3 of the Neu5Ac residues of both polysa ccharides. In further pursuing this phenomenon, we have reinvestigated the O56 and O24 polysaccharides as well as derived oligosaccharides b y one- and two-dimensional NMR spectroscopy. The results showed that s tructures of both polysaccharides (PSs) had to be modified and formula ted as [GRAPHICS] 2D ROESY spectra revealed a strong NOE between H-3eq of Neu5Ac and the protons of the side-chain sugar (H-3 and H-5 of alp ha-D-Gal p in the O56 PS and H-3 of alpha-D-Glc p in the O24 PS) and a lso between H-3ax of Neu5Ac and H-3 of beta-D-Glc p in the main chain. This indicated aclose spatial apposition of the seven-linked alpha-Ne u5Ac and the side-chain residues alpha-D-Gal p (O56 PS) and alpha-D-Gl c p (O24 PS), respectively. The strong long-range spatial contacts cau sed the unusual chemical shifts of H-3eq and C-3 of Neu5Ac.