Monosaccharides

MONOSACCHARIDES (monosaccharides), sugars which are widely distributed in the plant and animal organism either in free form or in the form of disaccharides (see), polysaccharides, glycosides (see), and esters. Monosaccharides are colorless, sweet-tasting, odorless, neutral to litmus substances, easily soluble in water, sparingly soluble in alcohol, insoluble in ether; they are not volatile. In pure form, most monosaccharides crystallize well. Monosaccharides are polyhydric aldehyde- or ketones, depending on which they are classified as aldoses (see) and ketoses (see). Natural monosaccharides contain 5 or 6 carbon atoms in the molecule, but artificially obtained monosaccharides and those found in nature also have a different number of carbon atoms. Natural monosaccharides are optically active substances. In designating their stereoisomeric forms, one usually starts from the configurations of three glucose forms: dextrorotatory (d), levorotatory (l), and racemic (dl), and these letters denote not the direction of rotation, but the genetic relationship of various monosaccharides with glucose isomers; for example, levorotatory fructose is designated as l-fructose, since it is similar in structure to dextrorotatory d-glucose. CH₂OH CHO CH₂OH d-mannose CH₂OH CHO CH₂OH d-fructose CH₂OH CH₂OH CHO CH₂OH l-mannose CH₂OH CH₂OH CHO CH₂OH l-fructose CH₂OH CHO CH₂OH d-glucose CH₂OH CHO CH₂OH l-glucose The configuration of monosaccharides has great biological significance. For example, only natural isomers of monosaccharides are fermented; d-mannose and l-arabinose are assimilated by the animal organism more easily than the naturally occurring l-mannose and d-arabinose. Trioses with biological significance include glyceraldehyde and dihydroxyacetone (see). Among tetroses, digitoxylose CH₃(CHOH)₃CH₂CHO occurs in nature in the form of the glycoside digitoxin in the leaves of Digitalis. Aldopentoses are widely distributed in the plant and animal kingdoms. Ketopentoses do not occur in nature. Among hexoses, aldoses are widely distributed in nature: d-glucose (see), d-galactose (see), and d-mannose, and the ketose d-fructose (see). d-mannose (seminose) occurs both in free form, but most often in the form of higher carbohydrates-mannans. Under the influence of weak alkalis, d-glucose, d-fructose, and d-mannose pass into each other, so that in solution an equilibrium mixture of these three monosaccharides is established. A similar transition is also observed in the animal organism: mannose passes into the corresponding glucose, glucose into galactose in the formation of milk sugar (see Lactose). Phenylhydrazone of mannose differs in difficult solubility, which serves for its discovery and isolation. Upon reduction, monosaccharides pass into the corresponding 6-atom alcohols. Upon oxidation, acids are formed: for example, CH₂(OH)[CHOH]₄COOH, OHC[CHOH]₄COOH, HOOC[CHOH]₄COOH. When introduced into the animal organism, racemic (dl)-mannose partly passes into (d)-glucose; in the urine, in addition to (dl)-mannose and (d)-glucose, l-mannose and l-glucose appear, because d-forms are more easily destroyed in the organism. The keto-hexose sorbose is formed during the biological oxidation of the 6-atom alcohol sorbit, found in the juice of rowan, under the influence of Bacteria xylinum. Mannose (containing 9 carbon atoms), which ferments as easily as d-glucose, is of interest. Close to monosaccharides are amino-hexoses (see Glucosamine), glucuronic acid (see). The presence of a carbonyl group in monosaccharides determines their reducing ability, ability to form phenylhydrazones and its substituted hydrazones and osazones, with hydroxylamine-oximes, to add HCN. When heated with dilute acids, pentoses give furfural: (OH)CH-CH(OH) HC- CH(OH) CH₂OH hexoses give oxymethylfurfural and then levulinic acid. Reactions for monosaccharides can serve as Trommer's test (see), Bethe-Nilander's test (see), Molisch's test (see), reduction of ammoniacal silver oxide solution, formation of osazones. To distinguish aldoses from ketoses, E. Fischer's reaction can be used: 2 cm³ of the test liquid are mixed with 0.2 g of resorcinol and saturated with gaseous HCl while cooling; after 12 hours, the liquid is diluted with water, alkalized with NaOH, mixed with a few drops of Fehling's solution and heated; in the case of aldoses, a red-violet coloration appears. This reaction is also given by some polysaccharides. A reaction for hexoses is the formation of levulinic acid when boiled with 20% HCl, which is isolated as Zn- or Ag-salt. Methods for quantitative determination of monosaccharides are based either on their reducing ability (methods of Fehling, Bertrand, etc.) or on their optical activity (see Polarimetry). The amount of hexoses capable of alcoholic fermentation can be determined by the amount of CO₂ formed when standing with yeast, the amount of aldoses-by the method of Willstätter and Schudel: under the action of iodine in an alkaline medium, aldoses pass quantitatively into aldonic acids: CH₂OH(CHOH)₄CHO + I₂ + 3NaOH = CH₂OH(CHOH)₄COONa + 2NaI + 2H₂O. Ketoses do not change in this case.

HC-

\/

OH

O

The works of recent years have confirmed the correctness of the cyclic formulas for monosaccharides proposed by Tollens (Tolien) and established the position of the oxygen bridges in them. Thus, for example, d-glucose should be assigned the formula: 1 HOCH 2 HCOH 3 HOCH 4 HCOH CH₂OH stable form of d-glucose HOCH | O HC-----| CH₂OH unstable γ-form of d-glucose The cyclic structure of monosaccharides is confirmed by the fact that they do not give some aldehyde reactions: they do not add NaHSO₃, do not decolorize fuchsin-sulfurous acid, do not give the hydroxamic reaction. A direct indication can be the existence of each monosaccharide in two isomeric forms (α and β), which determines the phenomenon of mutarotation (see). Depending on the position of the oxygen bridge, the stability of monosaccharides changes. Thus, for aldohexoses, the stable forms are those containing an amylene oxide bridge (1,5). Unstable forms of aldoses contain a butylene oxide bridge (1,4). In the case of d-fructose, the relationship is reversed: the stable form is the one containing a butylene oxide bridge, and the unstable one containing an amylene oxide. Unstable forms are denoted (γ) regardless of the position of the oxygen bridge; γ-forms are very easily oxidized and have high reactivity. It is possible that they play a significant role in the processes of carbohydrate transformation in the plant and animal organism. There is a hypothesis (Winter and Smith), not yet fully proven, that the organism can utilize only the γ-form of glucose, which is formed from α- and β-forms under the influence of hormones. Under pathological conditions (diabetes), the organism loses the ability to convert glucose into the γ-form. According to these authors, the action of insulin consists in the conversion of α- and β-forms of d-glucose into the γ-form. Radium lupins apparently can convert glucose into the γ-form (Slosse). When glucose is acted upon by an ammoniacal solution of Zn(OH)₂, methylimidazole is formed (Knoop and Windaus), with methylglyoxal and formaldehyde CH₃CO-CH₂OH apparently being intermediate products. This reaction makes it probable the genetic relationship between carbohydrates and amino acids (histidine) and purines in the animal organism.

Monosaccharides are obtained technically by hydrolysis of the corresponding polysaccharides and glycosides. (On photosynthesis of monosaccharides-see Carbohydrates.) The anhydrides of glucose have acquired great theoretical significance, as it seems possible that they are the elementary molecules of polysaccharides (see Starch). Anhydrides of glucose are beginning to acquire practical significance as a nutrient for diabetics ('salabrose'). See also Fermentation, Hexosephosphoric acids, Glycolysis, Carbohydrates, Metabolism.