Study on Dendrobium officinale O-acetyl-glucomannan (Dendronan®): Part II. Fine structures of O-acetylated residues

Main objective of this study was to investigate the detailed structural information about O-acetylated sugar residues in Dendronan®. A water solution (2%, w/w) of Dendronan® was treated with endo-β-mannanase to produce oligosaccharides rich in O-acetylated sugar residues. The oligosaccharides were partly recovered by ethanol precipitation (70%, w/w). The recovered sample (designated Hydrolyzed Dendrobium officinale Polysaccharide, HDOP) had a yield of 24.7% based on the dry weight of Dendronan® and was highly O-acetylated. A D2O solution of HDOP (6%, w/w) generated strong signals in 1H, 13C, 2D 1H–1H COSY, 2D 1H–1H TOCSY, 2D 1H–1H NOESY, 2D 1H–13C HMQC, and 2D 1H–13C HMBC NMR spectra. Results of NMR analyses showed that the majority of O-acetylated mannoses were mono-substituted with acetyl groups at O-2 or O-3 position. There were small amounts of mannose residues with di-O-acetyl substitution at both O-2 and O-3 positions. Minor levels of mannoses with 6-O-acetyl, 2,6-di-O-acetyl, and 3,6-di-O-acetyl substitutions were also identified. Much information about sugar residue sequence was extracted from 2D 1H–13C HMBC and 2D 1H–1H NOESY spectra. 1JC–H coupling constants of major sugar residues were obtained. Evidences for the existence of branches or O-acetylated glucoses in HDOP were not found. The major structure of Dendronan® is shown as follows: