Monoses

MONOSES (monosaccharides), sugars, representatives of which are widely distributed in the organism of plants and animals either in a free state or in the form of disaccharides (see), polyoses, glycosides (see), and esters. Monoses are colorless, sweet-tasting, odorless, neutral to litmus substances, easily soluble in water, difficultly soluble in alcohol, insoluble in ether; they are non-volatile. In pure form, most monoses crystallize well. Monoses are polyatomic aldehyde or ketone alcohols, depending on which aldoses (see) and ketoses (see) are distinguished. Natural monoses contain mostly 5 or 6 carbon atoms in the molecule, but monoses with a different number of carbon atoms have been obtained artificially and are also found in nature. Natural monoses are optically active substances. When designating their stereoisomeric forms, one usually proceeds from the configurations of three glucoses: dextrorotatory (d), levorotatory (l), and racemic (dl), and the given letters denote not the direction of rotation, but the genetic connection of various monoses with glucose isomers; for example, levorotatory fructose is designated as l-fructose, as it is close in structure to dextrorotatory d-glucose.

The configuration of monoses has great biological significance. For example, only natural isomers of monoses are fermented; d-mannose and l-arabinose are assimilated by the animal organism more easily than the l-mannose and d-arabinose not found in nature. Trioses of biological significance include glyceraldehyde and dioxyacetone (see). Of the tetroses, digitoxose CH₃(CHOH)₃.CH₂.CH:O is found 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 are not found in nature. Of the hexoses, the following are widely distributed in nature: the aldoses d-glucose (see), d-galactose (see), and d-mannose, and the ketose d-fructose (see). d-mannose (seminose) is found in a free state, but most often in the form of higher carbohydrates—mannans. Under the influence of weak alkalis, d-glucose, d-fructose, and d-mannose convert into one another, so that an equilibrium mixture of these three monoses is established in the solution. A similar transition is observed in the animal organism: the transition of mannose into the corresponding glucoses, glucose into galactose during the formation of milk sugar (see Lactose). The phenylhydrazone of mannose is distinguished by its difficult solubility, which serves for its detection and isolation.

Upon reduction, monoses convert into the corresponding 6-atomic alcohols. Upon oxidation, acids are formed: for example, CH₂(OH).[CH(OH)]₄.COOH, O:HC.[CH(OH)]₄.COOH, HO.OC.[CH(OH)]₄.COOH. When racemic (dl)-mannose is introduced into an animal organism, it partly converts into (d)-glucose; in the urine, besides (dl)-mannose and (d)-glucose, l-mannose and l-glucose appear, because the d-forms are more easily subjected to destruction in the organism. The ketohexose sorbose is formed during the biological oxidation of the 6-atomic alcohol sorbitol, found in rowanberry juice, under the influence of Bacteria xylinum. Of interest is mannonose (containing 9 carbon atoms), which is fermented by yeast just as easily as d-glucose. Close to monoses are aminohexoses (see Glucosamine) and glucuronic acid (see).

The presence of a carbonyl group in monoses determines their reducing ability, their ability to form hydrazones and osazones with phenylhydrazine and its substituted derivatives, oximes with hydroxylamine, and to add HCN. When heated with dilute acids, pentoses yield furfural: (OH)CH-CH(OH)CH-CH:O, and hexoses yield oxymethylfurfural and further levulinic acid.

Reactions for monoses can include Trommer's test (see), Boettger-Nylander's test (see), Molisch's test (see), the reduction of an ammoniacal silver oxide solution, and the formation of osazones. To distinguish aldoses from ketoses, E. Fischer's reaction can be used: 2 cm³ of the test liquid is mixed with 0.2 g of resorcinol and saturated while cooling with gaseous HCl; 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 di- and polysaccharides. A reaction for hexoses is the formation of levulinic acid upon boiling with 20% HCl, which is isolated in the form of a Zn or Ag salt.

Methods for the quantitative determination of monoses are based either on their reducing ability (Fehling's, Bertrand's methods, 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 during incubation with yeast; the amount of aldoses—by the Willstätter and Schudel method: under the action of iodine in an alkaline medium, aldoses quantitatively convert into aldonic acids: CH₂OH.(CHOH)₄.CH:O + J₂ + 3NaOH = CH₂OH.(CHOH)₄.COONa + 2NaJ + 2H₂O. Ketoses do not change under these conditions.

Recent work has confirmed the correctness of Tollens' cyclic formulas for monoses and established the position of oxygen bridges in them. Thus, for example, the formula for d-glucose must be assigned: 1 HOCH 2 HCOH 3 HOCH 4 HCOH CH₂OH stable d-glucose HOCH | I o HC.....----1 CH₂OH unstable gamma-form of d-glucose.

The cyclic structure of monoses is confirmed by the fact that monoses do not give certain aldehyde reactions: they do not add NaHSO₃, do not decolorize fuchsin-sulfurous acid, and do not give the hydroxamic reaction. A direct indication is the existence of each monose in two isomeric forms (alpha and beta), on which the phenomenon of mutarotation (see) depends. Depending on the position of the oxygen bridge, the stability of monoses 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). For d-fructose, the relations are the opposite: the stable form is the one containing a butylene oxide bridge, and the unstable one is that containing an amylene oxide bridge. Unstable forms are designated as (gamma) regardless of the position of the oxygen bridge; gamma-forms are very easily oxidized and possess high reactivity. It is possible that they play a significant role in the processes of carbohydrate transformation in the organism of plants and animals. There is a hypothesis (Winter and Smith), not yet fully proven, that the organism is capable of utilizing only the gamma-form of glucose, formed from alpha- and beta-forms under the influence of hormones. Under pathological conditions (diabetes), the organism loses the ability to convert glucose into the gamma-form. The action of insulin, according to these authors, boils down to the conversion of alpha- and beta-forms of d-glucose into the gamma-form. Radium rays are apparently capable of converting glucose into the gamma-form (Slosse). Upon the action of an ammoniacal solution of Zn(OH)₂ on glucose, methylimidazole is formed (Knoop and Windaus), with methylglyoxal and formaldehyde CH₃-CO-CH being the apparent intermediate products.

This reaction makes a genetic connection between carbohydrates and amino acids (histidine) and purines in the animal organism probable. Monoses are obtained in industry by the hydrolysis of the corresponding polyoses and glycosides. (On the photosynthesis of monoses—see Carbohydrates.) Glucose anhydrides have acquired great theoretical significance, as it seems possible that they are the elementary molecules of polyoses (see Starch). Glucose anhydrides are also beginning to acquire practical significance as a nutrient for diabetics ("salabrose"). See also Fermentation, Hexosephosphoric acids, Glycolysis, Carbohydrates, Metabolism.