Structural Characterization and in Vitro Hypoglycemic Activity of Polysaccharides from Dendrobium nobile Lindl.

This study explored the structural characteristics and in vitro hypoglycemic activity of polysaccharides from the stems of Dendrobium nobile Lindl.. Two homogeneous polysaccharides, DNP-6 and DNP-7, were obtained by water extraction followed by alcohol precipitation and subsequent successive anion exchange and gel column chromatography. Their structures were analyzed by high performance liquid chromatography (HPLC), ultraviolet-visible (UV-vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The results showed that DNP-6 and DNP-7 were neutral polysaccharides with molecular mass of 2.70 × 10³ and 1.23 × 10³ Da, containing 90.45% and 93.49% total sugar and 2.28% and 1.29% protein, respectively. They were mainly composed of glucose and mannose, with molar ratios of 6.76:1 and 6.63:1, respectively. The glycosidic bonds were mainly of α-configuration, with a small amount of β-pyranose. One-dimensional and two-dimensional NMR results showed that the main chain composition of DNP-6 and DNP-7 was: →4)-α-D-Glcp-(1→4)-α-D-Glcp-(1→4)-α-D-Glcp-(1→4)-α-D-Glcp-(1→4)-α-D-Glcp-(1→4)-β-D-Manp-(1→4)-α-D-Glcp/β-D-Glcp. In addition, the results of in vitro glucose consumption and insulin resistance tests showed that both polysaccharides had good hypoglycemic activity, and DNP-6 possessed stronger activity than DNP-7.