Disaccharides

Disaccharides (sugar-like polysaccharides, biose), carbohydrates that split during hydrolysis (inversion) to form from 1 molecule of D. 2 molecules of monosaccharides. D. are soluble in water, giving true solutions; most crystallize well and have a sweet taste. The residues of monosaccharides are connected in D. by an ether bond through the O atom. The bond between monosaccharides is formed either as a result of water elimination from both hydroxyls at the carbonyl groups - diglycosidic bond (I), for example in cane sugar: -CH2OH CH2OH CH2OH CH2OH | | | | CH2OH CH2OH CH2OH CH2OH | | | | O O O O | | | | C=O C=O C=O C=O | | | | H H H H | | | | OH OH OH OH fructose α-1glycosido-β-2fructoside, or by elimination of water from the hydroxyl at the carbonyl group of one monosaccharide and the alcoholic hydroxyl of another - monoglycosidic bond (II); an example is maltose:

CH2OH

I

I

1 O

CH2OH O CH2OH

O I I

I CH2OH

I

TT

I CH8.OH

CH2.OH α-glycosido-4glucose. With monoglycosidic bond (II), the monosaccharide residue that retains the carbonyl group free exhibits mutarotation. D. of type II have reducing capacity and react with phenylhydrazine. D. built on type I do not possess these properties; they are designated as glycosido-glycosides, with numbers before the names of monosaccharides indicating the points of their connection and numbering the C atoms in formulas, counting from the carbonyl group or the nearest C atom to it. Before the name of the monosaccharide participating with its alcoholic hydroxyl in the formation of D. of type II, the number of the C atom to which this hydroxyl belongs is indicated by a number. The numbers of carbon atoms connected by oxygen in the heterocyclic ring are denoted by numbers in < >. The letters α or β indicate in what diastereoisomer form the monosaccharide is located; unstable forms are denoted by γ. In D. of type II, the ending 'ido' marks the monosaccharide that participates in the formation of the bond with its hydroxyl at the carbonyl group. Among the various groups of D. formed by hexoses, the following two are particularly important. 1. The maltose group with the general formula C12H22O9, of type II. Maltose, lactose (see.), cellulose (cellobiose), β-glycosido-4glucose, is formed during the incomplete hydrolysis of cellulose. Gentiobiose, β-glycosido-6glucose, is a D. that enters the molecule of amygdalin, as well as obtained during the incomplete hydrolysis of the trisaccharide gentianose. 2. The trehalose group, general formula C12H22O9, of type I. Trehalose (mycose) α-1glycosido-α-1glycoside, is contained in many fungi. Cane sugar (beet sugar, sucrose).-D. built from a hexose molecule and a pentose molecule are very rare. These include vicianose C9H20O10, isolated from the glycoside Vicia angustifolia.-The synthesis of D. is carried out either chemically (by the action of Ag2CO3 on halogen derivatives of acetylated monosaccharides, condensation of acetylated monosaccharides, action of acids on monosaccharides, etc.) or biologically (from monosaccharides as a result of the reverse reaction under the influence of the same enzymes that cause the splitting of D.; thus, for example, from D-glucose, the D. maltose and reversionose are obtained by the action of yeast maltase, gentiobiose and cellobiose by the action of emulsin; from D-galactose, galactobiose is obtained; isolactose is formed by the action on a mixture of D-glucose and D-galactose of the aqueous extract from kefir grains). On the biological and practical significance of D., on their formation in the body-see Carbohydrates and individual disaccharides. Absorption of D. occurs after their splitting to monosaccharides by corresponding enzymes, mainly through the blood vessels of the intestinal villi. D. are subjected to the most intense splitting in the duodenum. When large amounts of D. are introduced with food, they can be absorbed partly unchanged (lactose, cane sugar); in some animals, lactose is absorbed without inversion; D. are absorbed more slowly than monosaccharides, maltose-more slowly than cane sugar. When feeding large amounts of D., the processes of putrefaction in the intestine decrease, and at the same time, diarrhea may occur. In alimentary glycosuria, D. were found in the urine. Lactose is found in the urine during pregnancy, during lactation, in newborn calves, in infants with severe gastrointestinal disorders. D. that give fermentable monosaccharides upon splitting are converted in the animal body when introduced per os into glycogen. Lactose is a poor glycogen-former. When introduced into the blood, cane sugar and lactose, without being converted to glycogen, are excreted in the urine; maltose, due to the presence of maltase in the blood, does not appear in the urine; after repeated parenteral administration of cane or milk sugar, invertin or lactase sometimes appears in the blood.