Structural characterisation of the olive pomace pectic polysaccharide arabiana side chains

摘要

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 HNO_3, at 80 deg C, followed by graded precipitation with ethanol. It was separated from acidic pecitic 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. ~(13)C 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 (~1H and ~(13)C), it was possible to conclude that the T-#beta#-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.