Study on Dendrobium officinale O-acetyl-glucomannan (Dendronan®): Part I. Extraction, purification, and partial structural characterization

It is believed that some polysaccharides with specific structures may play important roles in the medicinal properties of Dendrobium plants. In the current study, a crude polysaccharide preparation, containing 71.4% non-starch polysaccharide and 17.1% starch, was obtained from Dendrobium officinale herbal materials using sequential hot water extractions. A polysaccharide fraction, with a Mw of 312 kDa, was isolated by subjecting the crude polysaccharide to sequential treatments of thermo-stable α-amylase, multiple freeze–thaw processes, dialysis, ethanol precipitation (3 volumes), and freeze-drying. The purified polysaccharide was free of starch and with a purity of 90.4%. The analysis of monosaccharide compositions showed that the purified polysaccharide consisted dominantly of mannose and glucose, in a molar ratio of 6.9:1. Methylation analysis indicated that the glucomannan was a linear polymer without any branches. Both FT-IR and NMR spectroscopic analyses suggested that the glucomannan was O-acetylated and was primarily composed of β-1,4-d-mannopyranosyl and β-1,4-d-glucopyranosyl residues. Acetyl groups were found attached to the O-2 or O-3 positions on some mannosyl residues based on 1D 1H NMR, 2D 1H–1H TOCSY, and 2D 1H–1H COSY spectra. In conclusion, the major polysaccharide from Dendrobium officinale is an O-acetylated glucomannan.