Polysaccharides form a diverse and complex family of biological macromolecules. They are polymeric carbohydrate molecules composed of long chains of monosaccharide units joined together by glycosidic linkages and on hydrolysis give the constituent monosaccharides or oligosaccharides. Polysaccharides occurring in nature often have high molecular weights. A large number of polysaccharides contain between 100 and several thousand monosaccharides. They are usually polydispersed in nature and may contain a wide range of closely related structures.
Polysaccharides can be categorized according to their molecular characteristics (type, number, bonding and sequence of monosaccharides), physicochemical characteristics (water solubility, viscosity, surface activity) and nutritional function (digestible or non-digestible). Polysaccharides containing only one type of monosaccharide repeating in the chain are termed homopolysaccharides, such as starch, cellulose and glycogen which are formed from only glucose, whereas those which contain two or more constituent monosaccharides are known as heteropolysaccharides, such as pectin, hemicellulose and gums. Polysaccharides may be linear, branched or even cyclic in nature and this has important consequences for their physicochemical nature.
Figure 1. Polysaccharides of glucose
Polysaccharides are abundant in nature and found in nearly every living organism. They can function as storage polysaccharides, such as starch in plants and glycogen in animals, structural elements in the cell walls of bacteria and plants and skeletal elements in arthropods. They can also be found linked to proteins and lipids, and have been implicated in cell–cell recognition and molecular targeting.
In order to discover their structure, it is necessary to determine not only monosaccharides composition and their linkage positions and sequence, but also the anomeric configuration of linkages, the ring size (furanose or pyranose), the absolute configuration (D or L) and identify any other substituents present. There is no one method that can be used to determine their fine structure. Instead, we must use a combination of analytical methods to gain as much information as we can. For polysaccharide analysis, separation and extraction techniques, component analysis, methylation analysis, glycosidic hydrolysis, mass spectrometry (MS) methods, and nuclear magnetic resonance (NMR) spectroscopy are required to determine the fine structure and size exclusion chromatography (SEC) coupled with light scattering (LS) detectors to determine the molecular size distribution.
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