Isolation and structure characterization of a low methyl-esterified pectin from the tuber of Dioscorea opposita Thunb.

A low methyl-esterified pectin (33.2% methyl-esterification degree) was isolated from the tuber of Dioscorea opposita Thunb., which was an edible and medicinal material in China. This pectin ( M w of 1.3 × 10 4 g/mol) contained the ~59.1% homogalacturonan (HG) and ~38.1% highly branched rhamnogalacturonan I (RG-I) region with possible side chains embracing arabinogalactan II, arabinan or arabinogalactan I. The fragments including HG backbone consisting of → 4)-α-Gal p A-(1 → and → 4)-α-Gal p A-6- O -methyl-(1 → with molar ratio of ~2:1, and repeating unit of arabinogalactan II side chain composed of α-Ara f -(1 → and → 3,6)-β-Gal p -(1→, were speculated through methylation analysis and NMR spectra. However, the linkage pattern for RG-I backbone and side chains were indiscernible due to limited resolution of NMR spectra. Besides, the pectin adopted a flexible chain conformation in 0.1 M NaNO 3 solution. These results provided a structural basis for study on polysaccharide from D. opposite , which was benefit for development of functional food of yam.