Studies on uronic acid materials : Part XXVII. The structure of the gum from acacia nubica benth.

Abstract

The polysaccharide exuded by Acacia nubica trees as a high, positive specific rotation, has low methoxyl and L-rhamnose contents, and contains D-galactose, L-arabinose, and D-glucuronic acid, which is present in two aldobiouronic acids, 6- O-(β- D-glucopyranosyluronic acid)- D-galactose and 4- O-(α- D-glucopyranosyluronic acid- D-galactose. Autohydrolysis experiments gave 3- O-β- L-arabinofuranosyl- L-arabinose, 3- O-β- L-arabinopyranosyl- L-arabinose, β-(1 → 3)-linked- L-arabinose trisaccharides, and a degraded gum A of molecular weight 5,730, which was studied by linkage and methylation analysis. Partial hydrolysis with acid gave 3- O-β- D-galactopyranosyl- D-galactose and 6- O-β- D-galactopyranosyl- D-galactose. An examination of the O-methyl derivative of degraded gum A gave 2,3,4,6-tetra-, 2,3,4-, 2,3,6-, and 2,4,6-tri-, and 2,4-di- O-methyl- D-galactose; 2,3,4-tri- O-methyl- L-arabinose; and 2,3,4-tri- O-methyl- D-glucuronic acid. Degraded gum A was subjected to a Smith degradation, and the product was examined by linkage and methylation analysis. The O-methyl derivative of the whole gum gave 2,3,4- and 2,3,5-tri-, and 2,5- and 3,5-di- O-methyl- L-arabinose; 2,3,4,6-tetra-, 2,4,6-, 2,3,6-, and 2,3,4-tri-, 2,6- and 2,4-di-, and 2- O-methyl- D-galactose; and 2,3,4-tri- O-methyl- D-glucuronic acid. The whole gum was subjected to five successive Smith-degradations, and the Smith-degraded polysaccharides S1–S5 were each examined by linkage and methylation analysis. The structural evidence suggests that A. nubica gum molecules possess highly branched D-galactan frameworks, to which are attached D-glucuronic acid residues and L-arabinose-containing side-chains, some of which are at least six units long. The gum contains the largest proportion of L-arabinose in any of the Acacia gum exudates studied to date.