Pectic polysaccharide from immature onion stick (Allium cepa): Structural and immunological investigation

The structure of a water-soluble pectic polysaccharide (PS) isolated from immature onion stick (Allium cepa) was investigated using acid hydrolysis, methylation analysis, periodate oxidation study, and NMR studies (1H, 13C, DQF-COSY, TOCSY, NOESY, ROESY, HSQC, and HMBC). The results of the above experiments indicated that the PS contained d-galactose, 6-O-Me-D-galactose, 3-O-acetyl-d-methyl galacturonate and d-methyl galacturonate in a molar ratio of nearly 1:1:1:1 and possesses a backbone of [→4)-α-d-GalpA6Me-(1 → 4)-α-d-GalpA6Me-(1→] in which one methyl galacturonate was substituted at O-3 position by an acetyl group and the neighboring methyl galacturonate being substituted at O-2 with a side chain, α-D-Galp-(1 → 4)-6-O-Me-β-D-Galp-(1→. The probable structure of repeating unit of the pectic polysaccharide was established as: ctic polysaccharide showed in vitro splenocyte, thymocyte as well as macrophage activations.