SOME STRUCTURAL STUDIES OF BREA GUM (AN EXUDATE FROM CERCIDIUM AUSTRALE JOHNST)

The gum exudate (brea gum) from the leguminous tree Cercidium australe contains residues of l-arabinose, d-xylose, d-glucuronic acid, and 4-O-methyl-d-glucuronic acid, in the approximate molar ratios of 1,7:6,3:1,9:0,9. Autohydrolysis of the gum afforded (chromatographic identification) xylobiose (and homologues), 2-O- and 4-O-(α-d-glucopyranosyluronic acid)-d-xylose, and 2-O-(4-O-methyl-glucopyranosyluronic acid)-d-xylose. The principal, neutral components of the methylated gum were 3-O- and 2,3,4-tri-O-methyl-d-xylose; by chromatographic methods, 2,3-di-O-methyl-d-xylose and -l-arabinose, and a tri-O-methylarabinose were also detected. After reduction, the acidic products from the methylated gum gave, inter alia, 3,4-di-O-methyl-d-glucose and 2,3,4-tri-O-methyl-d-glucose (chromatographic identification). The major structural features of brea gum appear to be a β-(1 → 4)-linked d--xylan backbone [possibly containing some (1 → 2)-linkages] that is heavily 2-substituted by short branch-chains containing residues of d-xylose (and l-arabinose) and d-glucuronic acid, in which both types of residue may be terminal. Approximately one-third of the uronic acid is present as the 4-methyl ether, and, in the purified polysaccharide, a similar proportion of the acid residues is lactonised. Supporting evidence for these features was obtained by periodate-oxidation studies of the carboxyl-reduced polysaccharide. © 1969.