Structure of the arabinogalactan from gum tragacanth (Astralagus gummifer)

The polysaccharide obtained by ethanol precipitation from an aqueous solution of gum tragacanth contained arabinogalactan and tragacanthic acid, as well as starch ( approximately 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, beta-Araf, and alpha-Galp units, as well as known terminal, and 2-O- and 3-O-substituted alpha-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 alpha-Araf (1-->2)-alpha-Araf-(1-->4)-Ara and alpha-Araf-(1-->2)-alpha-Araf-(1-->5)-Ara, which correspond to the side chains of the arabinogalactan, beta-Galp-(1-->4)-beta-Galp-(1-->4)-beta-Galp-(1-->4)-Gal; and a mixture of beta-Galp-(1-->4)-beta-Galp-(1-->4)-Gal and beta-Glcp-(1-->4)-beta-Galp-(1-->4)-beta-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 beta-Galp nonreducing end-units.