Arabinose-containing oligosaccharides from tamarind xyloglucan

Oligosaccharide mixtures were obtained by enzymatic degradation of tamarind xyloglucan from the seeds of Tamarindus indica by commercial Aspergillus niger endo-(1→4)-β-D-glucanase, and their composition analyzed by HPAE-PAD liquid chromatography revealing two major groups of oligosaccharides. The first group contained oligomers of DP 7, 8 and 9, while the second group consisted of a large variety of oligomers of DP 12 to 18. After purification by SEC and HPLC in semi-preparative scale, single oligomers were characterized by glycosyl-residue composition, glycosyl-linkage analysis and plasma desorption mass spectrometry. Beside β-D-glucosyl, β-D-galacto-syl and α-D-xylosyl units, some higher oligomers contained β-D-galactosyl-(1→5)-α-L-arabinosyl units, linked to O-6 in the glucosyl units of the (1→4)-β-D-glucan backbone. Other L-arabinosyl residues were located in terminal positions of α-D-xylosyl side chains. The variety of isolated oligomers reflected a complicated random distribution of side chains in tamarind xyloglucan. Based on the characteristics of the oligomers, four structure forming units for the polysaccharide are proposed.