Structural carbohydrates monosaccharide compositions

Structure elucidation of carbohydrate side chains requires knowledge of the (1) monosaccharide composition, (2) sequence and branching of each monomer, (3) stereochemistry, (4) interglycosidic linkage positions, and (5) anomeric configuration of each glycosidic bond. 

With the development of MALDI and ESI, mass spectrometry has assumed a pivotal role in the characterization of oligosaccharides. A complete structure elucidation of carbohydrate side chains inclndes monosaccharide composition. 

The analysis of monosaccharide mixtures as the permethylated derivatives was proposed early in the application of gas-liquid chromatography to carbohydrates, but the method has now been superseded by more convenient procedures.230,231 There are, however, situations in which this method is useful, such as during a structural study of a polysaccharide by the methylation technique. The mixture of partially methylated monosaccharides obtained by methanolysis may then be fully methylated, and the proportions of the various monosaccharides determined. This approach has been used, for example, in studies on a galactomannan392 and on tamarind-kernel polysaccharide.393 Such an analysis also constitutes a useful check to ensure that no significant change in the composition of the polysaccharide occurred during methylation. 

Structural studies of glycosphingolipids involves determination of the structure of the oligosaccharide chain and of the lipid moiety. For the oligosaccharide chain, it is necessary to determine the composition, molar ratio, and sequence of the monosaccharides, their pyranose or furanose nature, and the position of glycosidic bonds and their configuration for the lipid moiety, the composition of the fatty acids and sphingosine bases must be determined. Used for these purposes are the classical, chemical methods, conventionally accepted in the chemistry of carbohydrates and lipids and based on the degradation of compounds, enzymic, and physicochemical methods, primarily mass spectrometry and n.m.r. spectroscopy. 

Because poiysaccharides are natural macromolecules occurring in all living organisms, the structure of some polysaccharides can be much more complex, as they are not made only from simple monosaccharides. In the composition of natural polymeric carbohydrates, a wide variety of sugars are found. Among these, the most common are pentoses and hexoses. The structural formulas of three common pentoses are shown below. They frequently form cyclic structures with five-member rings (furanoses). 

Enzyme-lectin histochemistry has many features in common with EIH. Lectins are of non-immune origin but recognize fine differences in complex saccharide structures. Usually lectin specificity is expressed as reactivity to a given monosaccharide, but it has been demonstrated repeatedly (e.g.. Debray et al., 1981) that this is an oversimplification (Section 3.4). It is becoming increasingly clear that cellular differentiation, maturation and neoplastic transformation are associated with changes of carbohydrate composition of the cell membrane (Ponder, 1983). 

Occurrence. D-Xylose (10) is present in widely abundant polysaccharides of plant tissues. Xylan is the main carbohydrate found in the hemicellulosic fraction, and accounts for one third of all renewable organic carbon available on earth. The structure and composition of xylans are variable, from linear (3-(l->4)-linked xylose chains to highly branched heteropolysaccharides. The branches may involve short oligosaccharides, usually of L-arabinofuranosyl units. Xyloglucans are also important hemicellulose polysaccharides consisting of a backbone of ( I — 4)-(i-D-glucopyranosyl residues heavily substituted with a-D-Xylp. Other monosaccharides may also be present.44... 

Although NMR analysis applied to the general carbohydrate composition in LCCs presents some fundamental interest, from a practical point of view a routine wet chemistry monosaccharide analysis is much simpler and it provides more reliable and, more importantly, quantitative information compared to NMR methods (see, e.g.. Table 1). NMR analysis of carbohydrate units in LCCs should be focused on detailed structural characteristics, a piece of information that routine monosaccharide compositimial analysis methods cannot provide, similarly to the NMR investigatimis conducted on isolated hemicel-luloses (see, for instance, [83]). 

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