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 x 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-Araf (17.4%), 1,3,6-Galp (11.8%), 1,3-Galp (8.1%), T-Galp (13.4%) and 1,4-Galp (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)-alpha-GalpA-(1 -> 2)-alpha-Rhap(1 -> 4)-alpha-GalpA-(1 -> 2,4)-alpha-Rhap-(1 -> substituted by short side chain of beta-Galp-(1 -> 4)-beta-Galp-(1 -> at O-4 position of -> 2,4)-alpha-Rhap-(1 -> Moreover, the POWP-H-H was mainly O-acetylated at O-3 position of a-Rhap 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 NaNO3 solution.