Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Chemistry of Carbohydrates
Polysaccharides

Polysaccharides are high-molecular-weight polycondensation products of Monosaccharides linked together by glycosidic bonds to form linear or branched chains. D-glucose is the most common monosaccharide unit found in polysaccharides. Other components can include D-mannose, D- and L-galactose, D-xylose, L-arabinose, D-glucuronic, D-galacturonic, and D-mannuronic acids, as well as D-glucosamine, D-galactosamine, sialic acids, and aminouronic acids.

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Fig. 5.2. Structure of two-component Heteropolysaccharides (diagram). a - unbranched; b - branched.

Each monosaccharide incorporated into the polymer molecule can exist in a pyranose or furanose form and may be attached to any of the free hydroxyl groups of the adjacent monosaccharide residue via an $\alpha$- or $\beta$-glycosidic bond. Polysaccharides vary not only in their monosaccharide composition but also in molecular weight and structural features. Thus, some polysaccharides are linear polymers, while others are highly branched.

The Molecular Weight of polysaccharides is relatively high and can only be measured with a certain degree of approximation using existing Methods. This distinguishes polysaccharides from Oligosaccharides, the degree of polymerization of which can be precisely determined by classical chemical methods. In other words, structurally identical molecules of chemically homogeneous polysaccharides most commonly differ in size. Therefore, isolated individual polysaccharides are generally mixtures of polymer homologues.

Based on their General structural principles, polysaccharides can be divided into two groups: Homopolysaccharides, consisting of monosaccharide units of a single type, and heteropolysaccharides, characterized by the presence of two or more types of monomer units (Fig. 5.2). This chapter focuses primarily on homopolysaccharides. Heteropolysaccharides are discussed in greater detail in subsequent chapters (see chapters 21 and 22).