Carbohydrates

Carbohydrates, organic compounds consisting of carbon, hydrogen, and oxygen, with the latter two elements usually in the same ratio as in water. This definition, being based on a purely formal characteristic, is of course insufficient, and it is more correct to characterize C. as aldehyde and ketone derivatives of polyhydric alcohols of varying degrees of complexity. The simplest of them, monoses (see), with 3, 4, 5, 6, or 7 carbon atoms, are such aldehydo-alcohols (see Aldoses) or keto-alcohols (see Ketoses), while the more complex C. can be regarded as simple esters or anhydrides formed by the combination of two or more monose particles with the elimination of water (see Polysaccharides). In 1927, the International Commission for the Reform of Chemical-Biological Nomenclature proposed the following nomenclature for C.: glucides (carbohydrates); oses (monoses); osides: holosides (polysaccharides), heterosides (glycosides). C. are extremely widespread in the plant world, serving here, on one hand, as skeletal substances (cellulose, hemicellulose, pectin), and on the other, participating in metabolism (starch, inulin, sugars). In animal organisms, the quantity of various forms of carbohydrates is significantly smaller, but here too they play an enormous role in metabolism (glycogen, sugars), participating in the construction of skeletal tissues only in tunicates (Tunicata). The animal organism is incapable of synthesizing C. from mineral substances and receives them from the outside either directly with vegetable food (herbivores) or through herbivorous animals (carnivores). In plants, C. are formed during the process of photosynthesis in green plants and chemosynthesis in prototrophic organisms (nitrifying, sulfur bacteria). At present, only photosynthesis has been studied, and even then not in all its aspects, during which carbon dioxide and water combine with the molecule of the green pigment chlorophyll; this compound absorbs solar rays, predominantly of the red-orange part of the spectrum, and at the expense of the energy of the latter, a chemical rearrangement occurs, apparently leading to the formation of a peroxide organic compound. The latter then splits off from chlorophyll with the release of gaseous oxygen. According to the theory first proposed by Baeyer (A. Bayer, 1870), formaldehyde is formed in this process; six molecules of the latter condense according to the aldol type and form a monose. This can be expressed by the following equations: CO₂+H₂O=CH₂O+O₂; 6CH₂O=C₆H₁₂O₆. However, to this day there are no unconditional proofs of the formation of formaldehyde. The nature of the monose formed during photosynthesis has not been established, but there is a series of data allowing one to think that it is fructose. The process of photosynthesis requires the absorption of significant quantities of solar energy (674 large calories for the formation of one gram-molecule) and is the main energy process of the entire organic world, since all life phenomena of plants and animals depend directly or indirectly on it. The fructose formed during photosynthesis easily rearranges into glucose and mannose, which are similar to it in structure, and also through relatively simple changes can give rise to a whole series of other C. (see also Assimilation).

A large number of studies have been devoted to elucidating the structure of C. (E. Fischer, Kiliani, Tollens, Haworth, Irvine, etc.), and at present the view is accepted that monoses can occur in stable and unstable forms. The former (pyranoses) are derivatives of the six-membered heterocyclic nucleus pyran, the latter (furanoses) of the five-membered heterocyclic nucleus furan: o

o /\

HC CH III

J1

HC CH

HC-CH CH/ CH₂ pyran

furan. Derivatives of furan in the natural state exist only in the combined form (fructofuranose in cane sugar), while in the free state they are unstable and quickly pass into pyranoses. All monoses contain an aldehyde or ketone group, and due to this, on one hand, they are easily reduced, for example, by the action of sodium amalgam on aqueous solutions, giving rise to polyhydric alcohols (arabinose--arabitol, xylose--xylitol, glucose and fructose--sorbitol, galactose--dulcitol), which are often found in plants in the free state. On the other hand, they are very easily oxidized, and for aldoses three types of this can be distinguished: 1) upon oxidation of only the aldehyde group, gluconic acids are formed (gluconic, mannonic, galactonic, arabinonic, xylonic); 2) upon oxidation of the primary alcohol group and preservation of the aldehyde group unchanged, uronic acids (glucuronic); 3) upon oxidation of both the aldehyde and primary alcohol groups, sugar acids (saccharic from glucose, mannosaccharic from mannose, mucic from galactose). Upon oxidation of fructose, the carbon chain breaks at the site of the keto group, and with weak oxidation a mixture of glycolic and <$-erythronic acids is obtained, while with more energetic oxidation a mixture of oxalic, tartaric, mesotartaric, and glycolic acids is obtained. The ability of monoses to be easily oxidized is used for their quantitative determination: most often methods based on the reduction of copper oxide to cuprous oxide are used, and the method of Willstatter and Schudel, in which the aldehyde group is oxidized by the action of iodine in an alkaline solution, which allows the determination of aldoses in the presence of ketoses, since the latter are not oxidized by iodine.

Characteristic compounds serving for the separation of monoses from each other are hydrazones and osazones (see), i.e., compounds with one or two molecules of phenylhydrazine. Two molecules of monose, combining with each other with the elimination of one particle of water, give rise to disaccharides (see) or biose, the best known of which are sucrose, maltose, and lactose. Disaccharides can be formed by two identical monoses (two molecules of glucose form maltose and trehalose when connected according to the type of α-glycoside, gentiobiose and cellobiose when connected according to the type of β-glycoside, etc.) or the hexoses can belong to different types (sucrose consists of glucopyranose and fructofuranose, melibiose - of galactose and glucose, lactose - also of galactose and glucose). The same is observed for trisaccharides: amylotriose consists of three particles of glucose, and raffinose of glucose, fructose, and galactose. Both monoses and disaccharides and trisaccharides crystallize well, have characteristic optical rotation, occurring in right, left, and inactive modifications. Spatial isomerism is also characteristic of them. Thus, for example, glucose occurs in two isomers - α and β - and gives two series of derivatives: α- and β-glycosides, which are characterized among other things by their relation to enzymes: α-glycosides are split by enzymes of the yeast maltase type (α-glycosidases), β-glycosides - by enzymes of the emulsin type (β-glycosidases). Their structure is evident from the following formulas: CH₂OH I

O

II \У ^α

\ |\ OH

OH

0-glucopyranose If one takes into account all possible cases of optical isomerism, spatial isomerism, and the fact that each monose can occur in both pyranose and furanose forms, it is easy to understand that the number of possible sugars can be enormous and that at present only a part of them are known. Monoses enter into glycosidic bonds not only with other monoses but also with compounds that do not possess the properties of the latter: fatty and aromatic alcohols, aldehydes, ketones, flavones, anthocyanins, purine bases, mustard oils, etc. All these compounds are called glycosides and, with respect to enzymes, behave as indicated above. Molecules of monoses, combining with each other, can give rise not only to di-, tri-, and tetrasaccharides but also to substances of much greater complexity, such as starch (see), inulin (see), glycogen (see), hemicelluloses, cellulose, pectic substances, mucilages, gums, which are grouped under the name of polysaccharides or polyoses. Common characteristics of all these compounds are their amorphous nature and insolubility in water (inulin is an exception in this respect). Starch and glycogen form colloidal solutions with water. In this process, more or less profound changes usually occur in the form of partial hydrolysis, the breakdown of complex aggregates into simpler ones. When heated with acids or under the action of specific enzymes, polysaccharides break down with the formation of monoses, most often glucose (cellulose, starch, glycogen, some hemicelluloses), and in some cases mannose, galactose, fructose (inulin), arabinose, xylose. Sometimes hydrolysis produces a complex mixture of monoses and their derivatives, as for example in the hydrolysis of pectic substances, where arabinose, galacturonic acid, and galactose are formed. The exact structural formulas are unknown for any colloidal polysaccharide. The same must be said regarding their molecular weight. One can only suppose that individual monoses are connected pairwise within the molecules of polysaccharides: this is evidenced by the formation of maltose during diastatic breakdown of starch, cellobiose during the breakdown of cellulose. Some sugars have been synthesized artificially; for the first time, a sugar-like substance was obtained by Butlerov in 1861 by the condensation of formaldehyde with the action of lime water; the similarity of the substance he obtained to sugars was established by E. Fischer, who obtained a similar substance by the action of barium water on the dibromide of acrolein and named it α-acrose. In recent years, many works have appeared devoted to the artificial photochemical synthesis of carbohydrates, however, the results of these works are very contradictory. The nutritional significance of carbohydrates is extremely great, since they are one of the main sources of energy in the body. 1 g of carbohydrates gives upon combustion about 4.1 large calories. The human body obtains the necessary amount of carbohydrates partly from meat food (glycogen), from milk (lactose), but mainly from plant food in the form of starch, sucrose, various monoses, etc. The significance of starch is particularly great, being the main constituent part of many food products (see). Thus, for example, wheat grains contain 65% of it, corn-65%, rye-69%, rice-75%, sorghum and barley-60%, oats-53%, potato tubers-from 14% to 25% (of raw weight), sweet potato tubers-from 10% to 20%. Second place belongs to sucrose, the content of which in sugar cane stems reaches 20% of raw weight, in sweet sorghum stems-up to 15%, and in sugar beet roots-up to 20-22%. In banana fruits, the sugar content reaches 23% of raw weight, in grapes-up to 22%, etc. Besides the carbohydrates themselves, material for their formation in the body is provided by proteins. Carbohydrates in turn provide material for the synthesis of fats in the body. See also Metabolism, Glycolysis, Muscles. Substances of the carbohydrate group also have great technical and industrial importance, for example, cellulose is used for the production of paper, collodion, artificial silk. A derivative of sugars-furfural, obtained as a by-product in paper production, has found wide application in the production of artificial resins, bakelite, etc. Carbohydrates serve as material for obtaining ethyl alcohol, acetone, lactic acid, and other technically important products.