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: