An arabinogalactan from the skin of Opuntia ficus-indica prickly pear fruits

The cold-water extract from the skin of Opuntia ficus-indica fruits was fractionated by anion-exchange chromatography. The major fraction, which was purified by size exclusion chromatography, consisted of a polysaccharide composed of galactose and arabinose residues in the ratio 6.3:3.3, with traces of rhamnose, xylose and glucose, but no uronic acid. The results of methylation analysis, supported by 13C NMR spectroscopy, indicated that this polysaccharide corresponded to an arabinogalactan having a backbone of (1 → 4)-linked β-d-galactopyranosyl residues with 39.5% of these units branched at O-3. The side-groups consisted either of single l-arabinofuranosyl units or l-arabinofuranosyl α-(1 → 5)-linked disaccharides. This polysaccharide is thus an arabinogalactan that can be classified in the type I of the arabinogalactan family.