The galactosamine residue in mycobacterial arabinogalactan is α-linked

Previous studies have demonstrated that cell wall arabinogalactan from mycobacteria possesses a single galactosamine (GalN) residue. This moiety, which is one of the rare natural occurrences of galactosamine lacking an acetyl group on the nitrogen, has been identified as a pendant substituent attached to a highly branched arabinofuranose residue in the arabinan core. However, the stereochemistry by which the GalN residue is linked to the polysaccharide remains unknown. We report here the synthesis of two tetrasaccharides, 1 and 2, consisting of GalN attached through either an α- or β-linkage to a trisaccharide fragment of mycobacterial arabinan. These molecules represent the first synthetic GalN-containing oligosaccharides, and the preparation of both targets was achieved from a single donor species by modulation of the reaction solvent. Comparison of the NMR spectra of 1 and 2 with those obtained from a sample derived from the natural glycan revealed that the GalN residue in the polysaccharide is attached via an α-linkage.