Structural characterisation of the olive pomace pectic polysaccharide arabinan side chains

An arabinan (97% of Ara and 3% of hexuronic acid) was isolated from the alcohol-insoluble residue (AIR) of olive pomace by treatment with 0.02 M HNO3, at 80degreesC, followed by graded precipitation with ethanol. It was separated from acidic pectic polysaccharides by anion-exchange chromatography, and by size-exclusion chromatography its molecular weight was estimated as 8.4 kDa. By methylation analysis, the linkage composition was established as 5:4:3:1 for (1 --> 5)-Araf, T-Araf, (1 --> 3,5)-Araf and (1 --> 3)-Araf, respectively. C-13 NMR spectroscopy confirmed this linkage composition, and allowed to assign the alpha anomeric, configuration for the arabinofuranosyl residues, except for some terminally linked ones, that were seen to occur as T-beta-Araf. By 2D NMR spectroscopy (H-1 and C-13), it was possible to conclude that the T-beta-Alaf was (1 --> 5)-linked to a (1 --> 5)-Araf residue. Also, in the arabinan (1 --> 5)-Araf backbone, the branched (1 --> 3,5)-Araf residues were always adjacent to linear (1 --> 5)-Araf residues. A tentative structure is proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.