Total synthesis and structural studies of zwitterionic Bacteroides fragilis polysaccharide A1 fragments

Abstract

Zwitterionic polysaccharides (ZPSs) are exceptional carbohydrates,carrying both positively charged amine groups and negatively chargedcarboxylates, that can be loaded onto MHC-II molecules to activateT cells. It remains enigmatic, however, how these polysaccharidesbind to these receptors, and to understand the structural featuresresponsible for this "peptide-like" behavior, well-definedZPS fragments are required in sufficient quantity and quality. Wehere present the first total synthesis of Bacteroidesfragilis PS A1 fragments encompassing up to 12 monosaccharides,representing three repeating units. Key to our successful syntheseshas been the incorporation of a C-3,C-6-silylidene-bridged "ring-inverted"galactosamine building block that was designed to act as an apt nucleophileas well as a stereoselective glycosyl donor. Our stereoselective synthesisroute is further characterized by a unique protecting group strategy,built on base-labile protecting groups, which has allowed the incorporationof an orthogonal alkyne functionalization handle. Detailed structuralstudies have revealed that the assembled oligosaccharides take upa bent structure, which translates into a left-handed helix for largerPS A1 polysaccharides, presenting the key positively charged aminogroups to the outside of the helix. The availability of the fragmentsand the insight into their secondary structure will enable detailedinteraction studies with binding proteins to unravel the mode of actionof these unique oligosaccharides at the atomic level.