Polysaccharides and Their Synthesis

Cellulose consists of several thousand D-glucose units linked by 1 4-j8-gly-coside bonds like those in cellobiose. Different cellulose molecules then interact to form a large aggregate structure held together by hydrogen bonds. 

Nature uses cellulose primarily as a structural material to impart strength and rigidity to plants. Leaves, grasses, and cotton, for instance, are primarily cellulose. Cellulose also serves as raw material for the manufacture of cellulose acetate, known commercially as acetate rayon, and cellulose nitrate, known as guncotton. Guncotton is the major ingredient in smokeless powder, the explosive propellant used in artillery shells and ammunition for firearms. 

Amylopectin accounts for the remaining 80% of starch and is more complex in structure than amylose. Unlike cellulose and amylose, which are linear polymers, amylopectin contains 1 6-a-glycoside branches approximately every 25 glucose units. 

Starch is digested in the mouth and stomach by a-glycosidases, which catalyze the hydrolysis of glycoside bonds and release individual molecules 

FIGURE 21.10 A representation of the structure of glycogen. The hexagons represent glucose units linked by i— 4 and 1— 6 glycoside bonds. 

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This subject has been of continuing interest for several reasons. First, the present concepts of the chemical constitution of such important biopolymers as cellulose, amylose, and chitin can be confirmed by their adequate chemical synthesis. Second, synthetic polysaccharides of defined structure can be used to study the action pattern of enzymes, the induction and reaction of antibodies, and the effect of structure on biological activity in the interaction of proteins, nucleic acids, and lipides with polyhydroxylic macromolecules. Third, it is anticipated that synthetic polysaccharides of known structure and molecular size will provide ideal systems for the correlation of chemical and physical properties with chemical constitution and macromolecular conformation. Finally, synthetic polysaccharides and their derivatives should furnish a large variety of potentially useful materials whose properties can be widely varied these substances may find new applications in biology, medicine, and industry. 

N. K. Kochetkov, Synthesis of fragments of bacterial polysaccharides and their application for preparation of synthetic antigens, Pure Appl. Chem., 56 (1984) 923-938. 

A. Y. Chernyak, G. V. Sharma, L. O. Kononov, P. Radha Krishna, A. V. Rama Rao, and N. K. Kochetkov, Synthesis of glycuronamides of amino acids, constituents of microbial polysaccharides, and their conversion into neoglycoconjugates of copolymer type, Glycoconj. J., 8 (1991) 82-89. 

N. K. Kochetkov Synthesis of fragments of bacterial polysaccharides and their... 

Nowadays, anhydro sugars constitute very versatile starting materials not only in carbohydrate chemistry but also for the synthesis of noncarbohydrate and nonnatural compounds. In the past two decades, interest in anhydro sugars has increased because they have been shown to be suitable monomers for preparing stereoregular polysaccharides and their specifically substituted derivatives. 

Polymers may be used in medicine either as biomaterials to support or replace body parts, or as pharmaceuticals, for use as drugs or in drug formulations. The biomaterial field is beyond the scope of this discussion and we will restrict our discussion only to the propa-ties of polysaccharides of biomedical interest and chemical methods available for their synthesis. At the present time only natural polysaccharides and their derivatives have been used medicinally but for some applications in physiology and internal medicine, synthetic polysacdiarides might be valuable. At last a few methods for their synthesis are available or in process of development. Some possibilities for the use of other synthetic polymers in medicine may also be suggested by the following survey. 

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