Medicinal Plants Classification Biosynthesis and ... - Index of

Quality Control of Polysaccharides from Medicinal Plants and Fungi
3.4. The Features of Glycosidic Linkages
The primary structure of a polysaccharide is defined by monosaccharide composition,
configuration of glycosidic linkages, position of glycosidic linkages, sequence of
monosaccharides, as well as the nature, number and location of appended non-carbohydrate
groups. It is known that the position and configuration of glycosidic linkages have close
relationship with their biological activities [198,199]. Unlike protein, there are more potential
linkages in the repeated unit of polysaccharide. The multiplicity of possible linkages between
monomeric units adds a further layer of structural complexity to carbohydrates because they
may be linked between the anomeric hydroxyl group of one monosaccharide and any other
hydroxyl-bearing carbon in another monosaccharide. The wide variety of positional and
anomeric structures makes it possible for saccharides to form as many as 10 12 distinct
structures from as few as six different monosaccharide units [200]. The positions of
glycosidic linkages can be analyzed by enzyme digestion, methylation analysis and NMR
spectroscopy (Table 4). The last two are main methods because exoglycosidic digestion is
limited to a few enzymes of high specificity.
In methylation analysis, polysaccharides should be sequentially methylated, hydrolyzed,
reduced, and acetylated to form partially methylated alditol acetates which separated and
analyzed using GC-MS. Various methylation methods have been developed and the dimsyl
anion or alkali-metal hydroxide (e.g., NaOH) is typically employed to deprotonate free
hydroxyls on the saccharide prior to methylation. Dimethyl sulfoxide (DMSO) is a commonly
used solvent for the methylation reaction due to the good solubility of many saccharide
species in this anhydrous solvent. However, solubility of high-molecular weight (HMW)
polysaccharides is limited in DMSO, and often these polysaccharides are chemically or
enzymatically degraded prior to methylation and linkage analysis. Glycerol could also be
used for improving solubilization of HMW polysaccharides in methylation and linkage
analysis [201]. Resulting chromatographic peaks are identified by a combination of their
retention times and their electron impact-mass spectrometry (EI-MS) fragmentation patterns.
In this way, which residues are terminal and how each monosaccharide is substituted are
indicated, and the occurrence of branching points could also be identified
[33,35,36,76,83,86,89,90,93,98,101, 107,159].
However, methylation analysis does not provide information on the sequence of
constituent residues and the configuration of glycosidic linkages. NMR analysis is a powerful
method for the structural analysis of polysaccharides. It is a fast, reliable, and nondestructive
technique.
289