Structural and conformational studies of bacterial polysaccharides employing NMR spectroscopy

Sammanfattning: Methods and approaches for performing structural and conformational analysis of bacterial polysaccharides in solution are described. The structure of one exopolysaccharide from Lactobacillus helveticus and two O-antigens from Escherichia coli have been determined using NMR spectroscopy, chemical degradation and mass spectrometry. This thesis discusses different problems encountered in connection with these structural elucidations, namely, overlap of NMR signals, heterogeneity in the polysaccharide sequence and the configurations of a nonulosonic acid.A conformational analysis of the enterobacterial common antigen (ECA) in the form of a cyclic dodecamer is described. Nuclear Overhauser effects (NOEs) and heteronuclear three-bond coupling constants (3JCH) over the glycosidic linkages were determined and used to calculate proton-proton distances and torsion angles. Residual dipolar couplings were measured in a liquid crystalline medium. These couplings provide orientational information distinct from that contained in the local NOE and 3JCH data. The experimental data were compared to the averaged distances, three-bond proton-carbon coupling constants and C-H vector orientations in a model obtained by molecular dynamics simulations. From a fragment that were in agreement with experimental data, a three-dimensional structure was generated for the cyclic ECA in solution