Structure of the arabinogalactan from gum tragacanth (Astralagus gummifer) Original Research Article

The polysaccharide obtained by ethanol precipitation from an aqueous solution of gum tragacanth contained arabinogalactan and tragacanthic acid, as well as starch (∼0.6%). GC–MS, NMR, and ESI–MS analyses showed the structure of the arabinogalactan to be even more complex than previously determined, with core structures containing Arap, β-Araf, and α-Galp units, as well as known terminal, and 2-O- and 3-O-substituted α-Araf units. Analysis was aided by examination of free, reducing oligosaccharides present in the gum. In addition to maltose, maltotriose, maltotetraose, and maltopentaose, the following were characterized: mixed α-Araf (1→2)-α-Araf-(1→4)-Ara and α-Araf-(1→2)-α-Araf-(1→5)-Ara, which correspond to the side chains of the arabinogalactan, β-Galp-(1→4)-β-Galp-(1→4)-β-Galp-(1→4)-Gal; and a mixture of β-Galp-(1→4)-β-Galp-(1→4)-Gal and β-Glcp-(1→4)-β-Galp-(1→4)-β-Galp-(1→4)-Gal, which did not resemble side-chain structures of the arabinogalactan. The latter are suggested to be related to tragacanthic acid, which has been previously found to contain β-Galp nonreducing end-units.