Structural elucidation of the backbone oligosaccharide from the lipopolysaccharide of Moraxella catarrhalis serotype A

The cell-surface lipopolysaccharide produced by Moraxella catarrhalis serotype A is composed of a hydrophobic lipid A moiety and an oligosaccharide, but lacks high-molecular-weight O-polysaccharide chains. The oligosaccharide component is composed of D-glucose, D-galactose, D-glucosamine, and 3-deoxy-D-manno-octulosonic acid. The carbohydrate backbone was obtained from the lipopolysaccharide by employing a reaction sequence involving deacylation, dephosphorylation, and reduction of the reducing D-glucosamine terminus of the lipid A moiety. Structural analysis of the backbone oligosaccharide employed a combination of microanalytical methods and nuclear magnetic resonance spectroscopy. Homo- and hetero-nuclear chemical shift correlation techniques and nuclear Overhauser enhancement (NOE) experiments led to the unambiguous assignment of the H-1 and C-13 resonances associated with each of the component glycosyl residues and established their sequence within the backbone oligosaccharide as shown, [GRAPHICS] This lipopolysaccharide was found to consist of a highly branched, D-glucose-containing inner-core region which possesses a unique and unusual solution conformation. This was established by measurement of transglycosidic NOEs and three-bond H-1-C-13 coupling constants, in conjunction with molecular modeling. A D-galactose-containing disaccharide identified as a terminal group of the lipopolysaccharide was structurally identical to antigenic epitopes expressed by certain mammalian epithelial cells and may be related to the virulence potential of this human pathogen.