Study on Dendrobium officinale O-acetyl-glucomannan (Dendronan®): Part V. Fractionation and structural heterogeneity of different fractions

In this study, we used gradient ethanol precipitation method to fractionate Dendrobium officinale O-acetyl-glucomannan sample (DOP) and obtained four major fractions. All of them were O-acetyl glucomannans but with different Man/Glc ratio, molecular weight, and intrinsic viscosity. A method based on 1D 1H NMR spectroscopy was developed to calculate the degree of substitution of O-acetyl group and to evaluate the percentage of O-2 and O-3 position occupied by acetyl groups. It was observed that as ethanol concentration increased, the collected fractions tended to contain more O-acetyl groups. There were more acetyl groups attached to O-2 position than to O-3 position, but as ethanol concentration increased, the difference between O-2 and O-3 substitution tended to decrease. Furthermore, we extensively hydrolyzed the four major fractions using mannanase and cellulose into oligosaccahrides. These oligosaccharides were subjected to MALDI-tof-MS analysis. Results showed that glucomannan fragments rich in O-acetyl groups were highly resistant to cellulase and mannanase hydrolysis. The four fractions varied in the distribution pattern of oligosaccharides produced by enzymatic hydrolysis. In conclusion, all the four fractions were different in detailed structural features, although they shared the same major structure as 2,3-O-acetyl-glucomannan.