Polysaccharides

POLYSACCHARIDES (polyoses), carbohydrates that break down during hydrolysis to form 2 or more molecules of monoses (see). Two groups of polysaccharides are distinguished: polysaccharides of the 1st order (sugar-like polyoses) and polysaccharides of the 2nd order (higher, colloidal, non-sugar-like polyoses). Polysaccharides of the 1st order approach cane sugar in their properties: they crystallize well, yield true aqueous solutions, possess a definite and relatively small molecular weight, and most also have a sweet taste. According to the number of monose residues included in their molecule, disaccharides, trisaccharides, etc., are distinguished. Among the trisaccharides, raffinose (melitose, gossypose) is significant. It is contained in sugar beets and in significant quantities in molasses; [α]_D²⁰ = +104.5°, therefore, in the polarimetric determination of cane sugar containing an admixture of raffinose, an excessively strong rotation is obtained (plus-sugar). Emulsin splits raffinose into galactose and cane sugar; invertin splits it into fructose and melibiose. Many of the polysaccharides of the 2nd order are insoluble in water; those that are soluble yield only colloidal solutions. Their molecular weight is unknown. Upon heating, they decompose without melting, do not exhibit reducing ability, and do not possess a sweet taste. These include many reserve nutrients of animals and plants, for example, starch, glycogen, inulin (see); cellulose (see) and hemicellulose [lichenin (see) and other hexosans and pentosans] and pectin substances (see) participate in the construction of plant cell walls; tunicin (animal cellulose) is found in crustaceans and snails; chitin, a polysaccharide containing nitrogen (yielding glucosamine upon hydrolysis), constitutes the main part of the integuments of crustaceans, certain insects, fungi, and apparently bacteria (tuberculosis, hay bacillus); sinistrin, 2C₁₂H₂₀O₁₀ + H₂O, which is part of the nucleoprotein of Helix pomatia, and others. Regarding the structure of polysaccharides, see Starch. Large fluctuations in the determinations of the molecular weight of colloidal polysaccharides are the result of changes in the association of molecules under the influence of external conditions (temperature, pressure, solvent, etc.). Polysaccharides have enormous industrial significance, being the main constituent of cotton and linen fabrics and paper, and serving as starting materials for obtaining artificial silk, celluloid, pyroxylin, etc. Literature: see literature for the article Starch.