Isolation and structural characterization of protopectin from the skin of Opuntia ficus-indica prickly pear fruits

Dilute HCl extraction of Opuntia ficus-indica fruits, previously disencrusted by hot water followed by hot chelating treatments, yielded a series of soluble pectic polysaccharides, which were de-esterified and separated into five fractions by anion exchange chromatography. Three of these fractions were thereafter purified by size exclusion chromatography and characterized by sugar analysis combined with methylation and reduction-methylation. Their study was further supported by 1H and 13C NMR spectroscopy. The results showed that one of these fractions (ASP1) was neutral and consisted of a linear β-(1→4)-galactan. One of the acid fractions (ASP2), with a galacturonic content of about 40%, consisted of a disaccharide repeating unit [→2)-α-l-Rhap-(1→4)-α-d-GalpA-(1→] backbone, with short linear β-(1→4)-linked galactan side-chains attached to O-4 of the rhamnosyl residues. Another acid fraction (ASP3), with a high galacturonic content (about 62%), consisted of alternating homogalacturonan blocks (1→4) α-linked and rhamnogalacturonan blocks with approximately the same amount of galactopyranosyluronic acid residues in each block. 13C T1 relaxation times were measured and noticeable differences in molecular motion of the different sugar rings in the repeating units were found.