Structural characterisation of the olive pomace pectic polysaccharide arabinan side chains Original Research Article

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 80 °C, 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. 13C NMR spectroscopy confirmed this linkage composition, and allowed to assign the α anomeric configuration for the arabinofuranosyl residues, except for some terminally linked ones, that were seen to occur as T-β-Araf. By 2D NMR spectroscopy (1H and 13C), it was possible to conclude that the T-β-Araf 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.