Structural features of pectic polysaccharides from the skin of Opuntia ficus-indica prickly pear fruits

After removal of the mucilage with water at room temperature, pectic polysaccharides were solubilized from Opuntia ficus-indica fruit skin, by sequential extraction with water at 60 degreesC (WSP) and EDTA solution at 60 degreesC (CSP). Polysaccharides with neutral sugar content of 0.48 and 0.36 mol/mol galacturonic acid residue were obtained, respectively, in the WSP and CSP extracts. These pectic polysaccharides were de-esterified and fractionated by anion-exchange chromatography, yielding for each extract five fractions, which were thereafter purified by size-exclusion chromatography. Two of these purified fractions were characterized by sugar analysis combined with methylation and reduction-methylation analysis. The study was then supported by H-1 and C-13 NMR spectroscopy. The results showed that the water-soluble fraction WSP3 and the EDTA soluble fraction CSP3, consisted of a disaccharide repeating unit -->2)- alpha-L-Rhap-(1 --> 4)-alpha-D-GalpA-(1--> backbone, with side chains attached to O-4 of the rhamnosyl residues. The side chains contained highly branched alpha-(1 --> 5)-linked arabinan and short linear beta-(l - 4)-linked galactan. (C) 2004 Elsevier Ltd. All rights reserved.