Isolation and structural characterization of a neutral polysaccharide from the stems of Dendrobium densiflorum

A novel neutral heteropolysaccharide (DDP-1-D) was purified from hot water extracts of dried stem of Dendrobium densiflorum by DEAE-52 and Sephacryl S-200 High-Resolution Chromatography. The heteropolysaccharide had an average molecular weight about 9440 Da. It was composed mainly of glucose and mannose in the ratio of 3.01:1. Structural features of DDP-1-D were elucidated by a combination of chemical and instrumental techniques, including FT-IR, GC–MS, periodate oxidation–Smith degradation, 1H and 13C NMR spectroscopies (including COSY, TOCSY, HSQC, and HMBC spectra). The results indicated that DDP-1-D is a mannoglucan and has a backbone consisting of (1→4)-linked α-d-Glcp, (1→6)-linked α-d-Glcp, (1→2)-linked α-d-Manp and (1→4)-linked β-Manp. This is the first study to provide clear evidence for the structure of the polysaccharide in D. densiflorum.