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 (R) was treated with endo-beta-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 (R) and was highly O-acetylated. A D2O solution of HDOP (6%, w/w) generated strong signals in H-1, C-13, 2D H-1-H-1 COSY, 2D H-1-H-1 TOCSY, 2D H-1-H-1 NOESY, 2D H-1-C-13 HMQC, and 2D H-1-C-13 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 6-0-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 H-1-C-13 HMBC and 2D H-1-H-1 NOESY spectra. (1)J(C-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 (R) is shown as follows: -(4)M(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)G(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)G(4)(1-)M(4)(1-)M(4)(1-)M(4)(1-)M(1-)(sic) M: beta-D-mannopyranose; G: beta-D-glucopyranose; a: O-acetyl group Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.