Arabinan-rich polysaccharides isolated and characterized from the endosperm of the seed of Opuntia ficus-indica prickly pear fruits

After removal of starch, the reserve storage polysaccharide of the endosperm seed of Opuntia ficus-indica fruit was studied. Cell Wall Material (CWM) was extracted successively by boiling water (WSF), hot calcium chelating agent solution (CSF) and cold mild alkaline solution (CASF). All polysaccharides extracted were fractionated by ion-exchange chromatography into five fractions. The resulting major fractions were purified by size-exclusion chromatography and analyzed by sugar composition and glycosyl linkage analyses. The investigations were also supported by 1H and 13C NMR spectroscopy analysis. The results showed that the major fraction of WSF consisted of an arabinan. The backbone contained α-(1→5)-linked arabinofuranosyl residues with high percentage of arabinose units substituted at O-2. The predominant fractions from CSF and CASF were related to rhamnogalacturonan type I which consisted of a disaccharide repeating unit →2)-α-l-Rhap-(1→4)-α-d-GalpA-(1→backbone with α-(1→5)-linked arabinan side-chains attached to O-4 of the rhamnosyl residues.