Studies on uronic acid materials. Part XXI. Some structural features of Acacia arabica gum
The polysaccharide exuded by Acacia arabica trees has a high positive specific rotation and contains residues of D-galactose, L-arabinose, L-rhamnose, D-glucuronic acid, and 4-O-methyl-D-glucuronic acid. Linkage analysis affords 3-O-β-L-arabinopyranosyl-L-arabinose and 3-O-β-L-arabinofuranosyl-L-arabinose. An examination of the O-methyl derivative of the gum yields 2,3,5- and 2,3,4-tri, and 2,5-, 3,5-, and 3,4-di-O-methyl-L-arabinose, 2,4-di-O-methyl-D-galactose, and 2,3,4-tri-O-methyl-D-glucuronic acid together with small amounts of 2,3,4-tri-O-methyl-L-rhamnose, 4-O-methyl-L-arabinose, and 2,3,4,6-tetra-, 2,3,6-, 2,4,6-, and 2,3,4-tri-, and 2-O-methyl-D-galactose. The degraded gum obtained after controlled acid hydrolysis is examined by linkage and methylation analysis. Partial acid hydrolysis affords 3-O-β-D-galactopyranosyl-D-galactose and 6-O-β-D-galactopyranosyl-D-galactose. An examination of the O-methyl derivative of the degraded gum yields 2,3,4,6-tetra-, 2,3,6-, 2,4,6-, and 2,3,4-tri-, 2,6- and 2,4-di-, and 2-O-methyl-D-galactose, 2,3,4-tri-O-methyl-D-glucuronic acid, and trace amounts of 2,3,4-tri-O-methyl-L-arabinose. The degraded gum is submitted to a Smith degradation and the whole gum to four successive Smith degradations. The products are examined by linkage and methylation analysis. The structural evidence suggests that A. arabica gum molecules possess highly branched galactan frameworks to which are attached uronic acid residues and arabinose-containing side-chains. This conclusion is supported by the results of viscosity and light-scattering measurements carried out on aqueous solutions of the gum.