New structural features of the polysaccharide from gum ghatti (Anogeissus latifola) Original Research Article

Methylation and 13C NMR analyses were carried out on the high-arabinose, acidic heteropolysaccharide of gum ghatti and the products obtained on three successive, controlled Smith degradations. The side chains contained mainly 2-O- and 3-O-substituted Araf units. Of these the second degradation eliminated remaining α-Araf units, and their β anomers became evident. The proportion of Galp units gradually increased in the form of nonreducing end- and Galp units, although 3,6-di-O- and 3,4,6-tri-O-substituted Galp units diminished. After three degradations groups with consecutive 3-O-substituted β-Galp units were formed. The proportion of periodate-resistant 3-O- and 2,3-di-O-substituted Manp units was maintained. As a guide to side-chain structures in the polysaccharide, seven of the 10 free reducing oligosaccharide fractions (PC) present in the gum were isolated and examined (NMR, ESIMS, and sometimes methylation analysis). Characterized are α-Araf-(1→2)-Ara and three Ara-containing oligosaccharide fractions containing 2-O- and 3-O-substituted units. These gave respectively, ESIMS molecular ions arising from Ara2, β-Araf oligosaccharides with four units, β-Araf oligosaccharides with seven units, and Hex2-Ara4. α-Rhap-(1→4)-GlcA, α-Rhap-(1→4)-β-GlcpA-(1→6)-Gal, and α-Rhap-(1→4)-β-GlcpA-(1→6)-β-Galp-(1→6)-Gal represented other side chains.