Two-dimensional nuclear magnetic resonance at 500 MHz: the structural elucidation of a Salmonella serogroup N polysaccharide antigen

Abstract

High resolution 1H and 13C nmr spectroscopy at 500 MHz and 125 MHz was used for the structural analysis of the O-antigen of Salmonellalandau, which belongs to the Kauffmann–White serogroup N. This bacterial lipopolysaccharide was extracted from whole cells and hydrolyzed by mild acid to give lipid-free O-polysaccharide. Conventional one-dimensional 1H and 13C nmr data showed the polysaccharide to contain four monosaccharides in each repeating unit and, in addition, to carry an average of one O-acetyl group for every two repeating units. Two-dimensional nmr experiments aided the unambiguous assignment of the 1H and 13C resonances and thereby permitted the structural analysis of this polysaccharide by nmr techniques alone. The structure of the de-O-acetylated repeating unit was established as [Formula: see text][Formula: see text] through the use of methods which included homonuclear shift correlated (COSY and NOESY) experiments. The interpretation of this data was supported and simplified by consideration of firmly established 13C chemical shift assignments obtained from a heteronuclear 1H/13C shift correlated experiment. A three-dimensional model of the O-antigen obtained by semi-empirical calculations is shown to be consistent with interatomic distance constraints imposed by data from 2-D NOESY and one-dimensional nOe difference spectroscopy.