Structural characterization and inhibition on α-glucosidase activity of acidic polysaccharide from Annona squamosa

The crude polysaccharide (TASP3) was extracted from the fruit pulp of Annona squamosa and then isolated and purified by the combination of grading-alcoholic precipitation and Sephadex G-200. The structure of purified polysaccharide (GASP3-3-I) was determined based on the physicochemical and instrumental analyses. The results indicated that GASP3-3-I was an acidic heteropolysaccharide and its average molecular weight was 2.28 × 106 Da. The monosaccharide composition was analysed by GC–MS and ion chromatography, respectively. It was revealed that GASP3-3-I was consisted of rhamnose, arabinose, xylose, mannose, glucose, galactose, glucuronic acid and galacturonic acid with a molar ratio of 5.06:45.5:5.26:0.63:6.09:31.76:0.49:5.19. Moreover, periodate oxidation reaction, Smith degrading reaction, methylation, FT-IR and NMR were used to conduct the structural characterization of GASP3-3-I. The results indicated that glycosyl residues of GASP3-3-I were mainly composed of (1→) l-arabinose, (1 → 6), (1 → 3) and (1,3 → 6) d-galactose, (1→) d-xylose, (3→) and (3 → 6) d-glucose, (1 → 2) l-rhamnose. The α-glucosidase inhibitory activity assay showed that GASP3-3-I had a certain inhibition on α-glucosidase activity.