Structural characteristics of a highly branched and acetylated pectin from Portulaca oleracea L.
The water-extracted polysaccharide from Portulaca oleracea L. mainly consisted of two pectic polysaccharides designated as POWP-H and POWP-L. The POWP-L was rich in linear homogalacturonan with GalA content of 77.6%, while POWP-H might be a rhamnogalacturonan I (RG-I) pectin composed of Rha (15.7%), Ara (31.7%), Gal (42.5%) and GalA (10.1%). The structure of POWP-H was elucidated by partial degradation, methylation analysis combined with GC-MS method and NMR spectroscopy. It had a weight-average molecular weight of 1.2 × 10 6 g/mol and high degree of branch of 0.57. The presence of abundant arabinogalactan-II and certain galactan side chains in POWP-H was verified by the T-Ara f (17.4%), 1,3,6-Gal p (11.8%), 1,3-Gal p (8.1%), T-Gal p (13.4%) and 1,4-Gal p (14.3%). The evidence for the RG-I backbone of POWP-H was demonstrated by the structure analysis of its degraded product POWP-H-H, which had the backbone of →4)-α-Gal p A-(1 → 2)-α-Rha p -(1 → 4)-α-Gal p A-(1 → 2,4)-α-Rha p -(1→ substituted by short side chain of β-Gal p -(1 → 4)-β-Gal p -(1→ at O -4 position of →2,4)-α-Rha p -(1 → . Moreover, the POWP-H-H was mainly O -acetylated at O -3 position of α-Rha p with 24.8% acetylation degree. In conclusion, POWP-H was mainly the highly branched and acetylated RG-I pectin with relative short RG-I backbone and abundant arabinogalactan II and certain galactan side chains. Besides, it adopted a flexible chain conformation in 0.1 M NaNO 3 solution.
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