Structural analysis of a rhamnoarabinogalactan from the stems of Dendrobium nobile Lindl

A water-soluble heteropolysaccharide DNP-W3 with a molecular mass of 710 kDa was obtained from the stems of Dendrobium nobile Lindl, a well-known edible and medicinal plant in Southeast Asia. It had [ α ] D 20 +19.6° (c 0.5, H2O) and contained galactose, rhamnose and arabinose in a molar ratio of 3.1:1.1:1.0. Structural features of the purified polysaccharide were elucidated by a combination of chemical and instrumental techniques, including FT-IR, GC–MS, periodate oxidation–Smith degradation, methylation analysis, partial acid hydrolysis, and NMR. The results indicated that the polysaccharide had a backbone of (1 → 3)-linked β-d-galactopyranosyl residues, with occasionally branches at O-4. The branches were mainly composed of (1 → 4)-linked α-l-rhamnopyranosyl residues and terminated with β-l-arabinopyranosyl residues. Bioactive tests in vitro revealed that DNP-3 could stimulate ConA and LPS-induced T and B-lymphocyte proliferation. It was suggested that DNP-3 could be a potential immunostimulant used in the food industry.