CHAPTER IV

CARBOHYDRATES

These substances comprise an exceedingly important group of compounds, the members of which constitute the major proportion of the dry matter of plants. The name "carbohydrate" indicates the fact that these compounds contain only carbon, hydrogen, and oxygen, the last two elements usually being present in the same proportions as in water. As a rule, natural carbohydrates contain six, or some multiple of six, carbon atoms and the same number of oxygen atoms less one for each additional group of six carbons above the first one; e.g., C_{6}H_{12}O_{6}, C_{12}H_{22}O_{11}, C_{18}H_{32}O_{16}, etc.

Carbohydrates are classed as open-chain compounds, that is, they may be regarded as derivatives of the aliphatic hydrocarbons. From the standpoint of the characteristic groups which they contain, they are aldehyde-alcohols. In common with many other polyatomic open-chain alcohols, they generally possess a characteristic sweet, or mildly sweetish, taste. In the case of the more complex and less soluble forms, this sweetish taste is scarcely noticeable and these compounds are commonly called the "starches," as contrasted with the more soluble and sweeter forms, known as "sugars."

The characteristic ending _ose_ is added to the names of the members of this group. As systematic names, the Latin numeral indicating the number of carbon atoms in the molecule is combined with this ending; e.g., C_{5}H_{10}O_{5}, pentose, C_{6}H_{12}O_{6}, hexose, etc.

In recent years, as a matter of scientific interest, many sugarlike substances which contain from two to nine carbon atoms combined with the proper number of hydrogen and oxygen atoms to be equivalent to the same number of molecules of water in each case, have been artificially prepared in the laboratory and designated as dioses, trioses, tetroses, pentoses, hexoses, heptoses, octoses, and nonoses, respectively. Substances corresponding in composition and properties with the artificial tetroses and one or two derivatives of heptoses are occasionally found in plant tissues, and a considerable number of pentoses and their condensation products are common constituents of plant gums, etc.; but the great majority of the natural carbohydrates are hexoses and their derivatives.

GROUPS OF CARBOHYDRATES

Since the simpler carbohydrates are sugars, i.e., they possess the characteristic sweet taste, the name "saccharide" is used as a basis for the classification of the entire group. The simplest natural sugars, the hexoses, C_{6}H_{12}O_{6}, are known as _mono-saccharides_. The group of next greater complexity, those which have the formula C_{12}H_{22}O_{11} and may be regarded as derived from the combination of two molecules of a hexose with the dropping out of one molecule of water at the point of union, are known as _di-saccharides_. Compounds having the formula C_{18}H_{32}O_{16} (i.e., three molecules of C_{6}H_{12}O_{6} minus two molecules of H_{2}O) are _tri-saccharides_; and the still more complex groups, having the general formula (C_{6}H_{10}O_{5})_n_, are called the _poly-saccharides_. The mono-, di-, and tri-saccharides are generally easily soluble in water, have a more or less pronouncedly sweet taste, and are known as the _sugars_; while the polysaccharides are generally insoluble in water and of a neutral taste, and are called _starches_. As will be seen later, there are many natural plant carbohydrates belonging to each of these groups.

In addition to these saccharide groups, there are other types, or groups, of compounds which resemble the true carbohydrates in their chemical composition and properties and are often considered as a part of this general group. These are the pentoses, C_{5}H_{10}O_{5}, and their condensation products, the pentosans (C_{5}H_{8}O_{4})_n_, and their methyl derivatives, C_{6}H_{12}O_{5}; certain polyhydric alcohols having the formula C_{6}H_{8}(OH)_{6}; pectose and its derivatives, pectin and pectic acid; and lignose substances of complex composition. It is doubtful whether these compounds are actual products of photosynthesis in plants, or have the same physiological uses as the carbohydrates and it has seemed wise to consider them in a separate and later chapter.

ISOMERIC FORMS OF MONOSACCHARIDES

Four sugars having the formula C_{6}H_{12}O_{6}, namely, glucose, fructose, mannose, and galactose, occur very commonly and widely distributed in plants. In addition to these, thirteen others having the same percentage composition have been artificially prepared, while seven additional forms are theoretically possible. In other words, twenty-four different compounds, all having the same empirical formula and similar sugar-like properties are theoretically possible. In order to arrive at a conception of this multiplicity of isomeric forms, it is necessary to understand the two types of isomerism which are involved. One of these is _structural_ isomerism, and the other is _space_- or _stereo_-isomerism.

=Structural Isomerism.=--This refers to an actual difference in the characteristic groups which are present in the molecule. As has been said, all carbohydrates, from the standpoint of the characteristic groups which they contain, are aldehyde-alcohols. The hexoses all contain five alcoholic groups and one primary aldehyde, or one secondary aldehyde (ketone), group. If the aldehyde oxygen is attached to the carbon atom which is at the end of the six-membered chain, the structural arrangement is | that of an aldehyde, C=O and the sugar is of the type known | H as "aldoses"; whereas, if the oxygen is attached to any other | carbon in the chain, the ketone arrangement, C=O results and | the sugar is a "ketose." This difference is illustrated in the Fischer open-chain formulas for glucose (an aldose) and fructose (a ketose) as follows:

Glucose Fructose

CH_{2}OH CH_{2}OH | | CHOH CHOH | | CHOH CHOH | | CHOH CHOH | | CHOH C=O | | CHO CH_{2}OH

=Stereo-isomerism=, or space isomerism, as its name indicates, depends upon the different arrangement of the atoms or groups in the molecule in space, and not upon any difference in the character of the constituent groups. This possibility depends upon the existence in the molecule of the substance in question of one or more _asymmetric carbon atoms_ and manifests itself in differences in the optical activity of the compound.[1] Thus, in the formula for glucose shown above there appear four asymmetric carbon atoms, namely, those of the four secondary alcohol groups (in the terminal, or primary alcohol, group, carbon is united to hydrogen by two bonds, and in the aldehyde group it is united to oxygen by two bonds). Similarly, fructose contains three asymmetric carbon atoms.

As an example of how the presence of these asymmetric carbon atoms results in the possibility of many different space relationships, the following graphic illustrations of the supposed differences between dextro-glucose and levo-glucose, and between dextro- and levo-galactose, may be cited.[2]

_d_-glucose _l_-glucose _d_-galactose _l_-galactose

CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH | | | | H-C-OH H-C-OH H-C-OH HO-C-H | | | | H-C-OH H-C-OH HO-C-H H-C-OH | | | | HO-C-H H-C-OH HO-C-H H-C--OH | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | CHO CHO CHO CHO

Comparisons of the above formulas will show that the difference between the formulas for _d_- and _l_-glucose lies in the arrangement of the H atoms and the OH groups around the two asymmetric carbon atoms next the aldehyde end of the chain; while the _d_- and _l_-galactoses differ in that this arrangement is in the reverse order around all four of the asymmetric carbons. By similar variations in the grouping around the four asymmetric atoms, it is possible to produce the sixteen different space arrangements shown on page 37 for the groups of an aldohexose. Sugars corresponding to fourteen of these different forms have been discovered, three of which are of common occurrence in plants, either as single mono-saccharides or as constituent groups in the more complex carbohydrates; the remaining two forms have only theoretical interest.

Similarly, for a ketohexose, which contains three asymmetric carbon atoms, there are eight possible arrangements. Three sugars of this type are known, only one (fructose) being common in plants; the others are of only theoretical interest.

FOOTNOTES:

[1] It is assumed that the reader, or student, is familiar with the theoretical and experimental evidence in support of the existence of the so-called "asymmetric" carbon atom and its relation to the effect of the compound which contains it, when in solution, in rotating the plane of polarized light. For purposes of review, or of study of this most interesting and important phenomenon, the reader is referred to any standard text-book on Organic Chemistry.

[2] Attention should be called, at this point, to the fact that such formulas as these cannot possibly accurately represent the actual arrangement of the constituent groups of a carbohydrate molecule around an asymmetric carbon atom. The limitations of a plane-surface formula prevent any illustration of the three-dimension relationships in space. Furthermore, there are certain facts in connection with the birotation phenomenon and the relation of the molecular configuration to biochemical properties (which see) that cannot be explained on the basis of the open-chain arrangement represented by the Fischer formulas used here. A closed-ring arrangement, showing the aldehyde oxygen as linked by its two bonds to the first and the fourth carbon atoms of the chain, thus forming a closed-ring of four carbon and one oxygen atoms, instead of being attached by both bonds to a single carbon atom, as in the above formulas, is undoubtedly a more nearly accurate representation of the actual linkage in the molecule than are the open-chain formulas used above.

The differences in conception embodied by these two types of formulas may be shown by the following formulas for glucose:

CH_{2}OH CH_{2}OH | | CHOH CHOH | | CHOH CH-CHOH ------O------- | | | | | CHOH O | or CH_{2}OH·CHOH·CH·CHOH·CHOH·CHOH | | | CHOH CH-CHOH | | CHO OH

Fischer's Closed-ring formulas formula

It will be observed that in the closed-ring formula there are five asymmetric carbon atoms, and the asymmetry of the terminal one forms the basis for the explanation of the existence of the so-called [alpha] and [beta] modification of _d_-glucose (see page 46). However, the ordinary aldehyde reactions of the sugars are more clearly indicated by the open-chain formula. Some investigators are inclined to be that sugars actually exist in the open-chain arrangement when in aqueous solution, and in the closed-ring arrangement when in alcoholic solution. The closed-ring formulas will be used in this text in the discussions of the birotation phenomena and of biochemical properties, but for the explanations of the stereo-isomeric forms and similar phenomena, the open-chain formulas are just as useful in conveying an idea of the possibilities of different space relationships, and are so much simpler in appearance and in mechanical preparation, that it seems desirable to use these rather than the more accurate closed-ring formulas.

CHEMICAL CONSTITUTION OF MONOSACCHARIDES

The term "monosaccharides," as commonly used, refers to hexoses. It applies equally well, however, to any other sugar-like substance which either occurs naturally or results from the decomposition of more complex carbohydrates, and which cannot be further broken down without destroying its characteristic aldehyde-alcohol groups and sugar-like properties.

All such monosaccharides, being alcohol-aldehydes, can easily be reduced to the corresponding polyatomic alcohols, containing the same number of carbon atoms as the original monosaccharides, each with one OH group attached to it. All aldose monosaccharides are converted, by gentle oxidation, into the corresponding monobasic acid, having a COOH group in the place of the original CHO group. Further oxidation either changes the alcoholic groups into COOH groups, producing polybasic acids, or breaks up the chain. When ketose monosaccharides are submitted to similar oxidation processes, they are broken down into shorter chain compounds.

The various monosaccharides which have thus far been found as constituents of plant tissues, or as parts of other more complex compounds which occur in plants, are shown in the following table:

_Trioses_ (C_{3}H_{6}O_{3}) _Tetroses_ (C_{4}H_{8}O_{4})

Aldose--Glyceric aldehyde, Aldoses--_d_- and _l_-Erythrose, or glycerose _l_-Threose Ketose--Dioxyacetone

_Pentoses_ (C_{5}H_{10}O_{5}) _Methyl Pentoses_ (C_{6}H_{12}O_{5})

Aldoses--_d_- and _l_-Arabinose Aldoses--Rhamnose _d_- and _l_-Xylose Fucose _l_-Ribose Rhodeose _l_-Lyxose Chinovose

_Hexoses_ (C_{6}H_{12}O_{6})

Mannitol series Dulcitol series Aldoses--_d_- and _l_-Glucose _d_- and _l_-Galactose _d_- and _l_-Mannose _d_- and _l_-Talose _d_- and _l_-Gulose _d_- and _l_-Idose _d_-Altrose _d_-Allose Ketoses-- _d_-Fructose _d_-Tagatose _d_-Sorbose

_Heptoses_ _Octoses_ _Nonoses_ (C_{7}H_{14}O_{7}) (C_{8}H_{16}O_{8}) (C_{9}H_{18}O_{9})

Glucoheptose Gluco-octose Glucononose Mannoheptose Manno-octose Mannononose Galactoheptose Galacto-octose Persuelose Sedoheptose

The hexoses are by far the most important group of monosaccharides. They are undoubtedly the first products of photosynthesis, and all the other carbohydrates may be considered to be derived from them by condensation. Because of their biochemical significance and their immense importance as the fundamental substances for all plant and animal energy-producing materials, the following detailed studies of their chemical composition and molecular configuration are fully warranted.

That all the hexoses contain five alcoholic groups is proved by the experimental evidence that each one forms a penta-ester, by uniting with five acid radicals, when treated with mineral or organic acids under proper conditions. Thus, glucose penta-acetate, penta-nitrate, penta-benzoate, etc., have all been prepared. The presence of the aldehyde group is proved by the fact that all aldohexoses have been converted, by gentle oxidation, into pentaoxy-monobasic acids, and the ketohexoses broken down into shorter chain compounds by similar gentle oxidations; these reactions being characteristic of compounds containing an aldehyde and a ketone group respectively. This experimental evidence establishes the nature of the characteristic groups in the molecule, in each case.

The molecular configurations illustrated in the following table are those suggested by Emil Fischer, as a result of his exhaustive studies of the chemical constitution of the various carbohydrates. There is, of course, no thought that the printed formulas here presented accurately represent the actual relationships in space of the different groups; but there is fairly conclusive evidence that the variations in special groupings in the different sugars are properly referable to the particular asymmetric carbon atoms as indicated in the several formulas as presented.

1. Aldohexoses of the mannitol series:

_d_-Glucose _l_-Glucose _d_-Mannose _l_-Mannose CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | HO-C-H H-C-OH HO-C-H H-C-OH | | | | H-C-OH HO-C-H HO-C-H H-C-OH | | | | CHO CHO CHO CHO

_d_-Gulose _l_-Gulose _d_-Idose _l_-Idose CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH | | | | HO-C-H H-C-OH HO-C-H H-C-OH | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | HO-C-H H-C-OH HO-C-H H-C-OH | | | | HO-C-H H-C-OH H-C-OH HO-C-H | | | | CHO CHO CHO CHO

_d_-Altrose _l_-Altrose _d_-Allose _l_-Allose (unknown) (unknown) CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | HO-C-H H-C-OH H-C-OH HO-C-H | | | | HO-C-H H-C-OH H-C-OH HO-C-H | | | | HO-C-H H-C-OH H-C-OH HO-C-H | | | | CHO CHO CHO CHO

2. Aldohexoses of the dulcitol series:

_d_-Galactose _l_-Galactose _d_-Talose _l_-Talose CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | HO-C-H H-C-OH HO-C-H H-C-OH | | | | HO-C-H H-C-OH HO-C-H H-C-OH | | | | H-C-OH HO-C-H HO-C-H H-C-OH | | | | CHO CHO CHO CHO

3. Ketohexoses:

_d_-Fructose _d_-Sorbose _d_-Tagatose CH_{2}OH CH_{2}OH CH_{2}OH | | | H-C-OH HO-C-H H-C-OH | | | H-C-OH H-C-OH HO-C-H | | | HO-C-H HO-C-H HO-C-H | | | C=O C=O C=O | | | CH_{2}OH CH_{2}OH CH_{2}OH

Reference will be made in subsequent paragraphs to the probable chemical constitution of the monosaccharides other than hexoses; but the above discussion of the structure of the hexoses will serve as a sufficient introduction to the study of the composition of the common carbohydrates.

CHARACTERISTIC REACTIONS OF HEXOSES

=Specific Rotatory Power.=--All soluble carbohydrates, since they contain asymmetric carbon atoms, with the consequent larger groups on one side of the molecule than the other, rotate the plane of polarized light when it passes through a solution of the carbohydrate in question. The amount of the rotation depends upon the nature of the carbohydrate, the concentration of the solution, and the length of the column of solution through which the ray of polarized light passes. But the same definite amount of the same sugar, dissolved in the same volume of water, and placed in a tube of the same length, will always cause the same angular deviation, or rotation, of the plane in which the polarized light which passes through it is vibrated. In other words, the same number of molecules of the optically active substance in solution will always produce the same rotatory effect. This is called the specific rotatory power of the substance in question. It is expressed as the number of degrees of angular deviation of the plane of polarized light caused by a column of the solution exactly 200 mm. in length, the concentration of the solution being 100 grams of substance in 100 cc. at a temperature of 20° C. Actual determinations of specific rotatory power are usually made with solutions more dilute than this standard, and the observed deviation multiplied by the proper factor to determine the effect which would be produced by the solution of standard concentration. If the direction of the deviation is to the right (i.e., in the direction in which the hands of the clock move) it is spoken of as "dextro" rotation and is indicated by the sign +, or the letter _d_; while if in the opposite direction, it is called "levo" rotation and indicated by the sign -, or the letter _l_. For example, the specific rotation of ordinary glucose is +52.7°; of fructose, -92°; of sucrose, +66.5°.

=Reducing Action.=--All of the hexose sugars are active reducing agents. This is because of the aldehyde group which they contain. Many of the common heavy metals, when in alkaline solutions, are strongly reduced when boiled with solutions of the hexose sugars. Alkaline copper solutions yield a precipitate of red cuprous oxide; ammoniacal silver solutions give silver mirrors; alkaline solutions of mercury salts are reduced to metallic mercury, etc. Any sugar which contains a potentially active aldehyde group will exhibit this reducing effect and is known as a "reducing sugar." In some of the di- and tri-saccharides, the linkage of the hexose components together is through the aldehyde group, in such a way that it loses its reducing effect; such sugars are known as "non-reducing." Advantage is taken of this property for both the detection and quantitative determination of the "reducing sugars." A standard alkaline copper solution of definite strength, known as "Fehling's solution," is added to the solution of the sugar to be tested and the mixture boiled, when the characteristic brick-red precipitate appears. If certain standard conditions of volume of solutions used, length of time of boiling, etc., are observed, the quantity of cuprous oxide precipitated bears a definite ratio to the amount of sugar which is present, so that if the precipitate be filtered off and weighed under proper conditions, the weight of sugar present in the original solution can be calculated. The proper conditions for carrying on such a determination and tables showing the amounts of the various "reducing sugars" which correspond to the weight of cuprous oxide found, are given in all standard text-books dealing with the analysis of organic compounds.

=Fermentability.=--The common hexoses are all easily fermented by yeast, forming alcohol and carbon dioxide, according to the equation

C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.

The importance and biochemical significance of this reaction will be considered in detail in connection with the discussions of the relation of molecular configuration to biochemical properties (see page 56) and the nature of enzyme action (see page 194).

=Formation of Hydrazones and Osazones.=--Another property of the hexoses which is due to the presence of an aldehyde group in the molecule, is that of forming addition products with phenyl hydrazine, known as "hydrazones" and "osazones." For example, glucose reacts with phenyl hydrazine in acetic acid solution, in two stages. The first, which takes place even in a cold solution may be represented by the equation

C_{6}H_{12}O_{6}+C_{6}H_{5}·NH·NH_{2} = C_{6}H_{12}O_{5}:N·NH·C_{6}H_{5} Glucose Phenyl-hydrazine Glucose-hydrazone + H_{2}O.

The structural relationships involved may be represented as follows:

CHO H_{2}N·NH CH=N-------NH | /\ | /\ (CHOH)_{4} + | | = (CHOH)_{4} | | | | | | | | CH_{2}OH \/ CH_{2}OH \/

The hydrazones of the common sugars, with the exception of the one from mannose, are colorless compounds, easily soluble in water. Hence, they do not serve for the separation or identification of the individual sugars. But if the solution in which they are formed contains an excess of phenyl hydrazine and is heated to the temperature of boiling water for some time, the alcoholic group next to the aldehyde group (the terminal alcohol group in ketoses) is first oxidized to an aldehyde and then a second molecule of phenyl hydrazine is added on, as illustrated above, forming a di-addition-product, known as an "osazone." The osazones are generally more or less soluble in hot water, but on cooling they crystallize out in yellow crystalline masses each with definite melting point and crystalline form. All sugars which have active aldehyde groups in the molecule form osazones. These afford excellent means of identification of unknown sugars, or of distinguishing between sugars of different origin and type.

Glucose, mannose, and fructose all form identical osazones. This is because the structure of these three sugars is identical except for the arrangement within the two groups at the aldehyde end of the molecule (see formulas on page 44). Since it is to these two groups that the phenyl hydrazine residue attaches itself, it follows that the resulting osazones must be identical in structure and properties. All other reducing sugars yield osazones of different physical properties.

When an osazone is decomposed by boiling with strong acids, the phenyl hydrazine groups break off, leaving a compound containing both an aldehyde and a ketone group. Such compounds are known as "osones." The osones from glucose, mannose, and fructose are identical. By carefully controlled reduction, either one of the C==O groups of the osone may be changed to an alcoholic group, producing thereby one of the original sugars again. Hence, it is possible to start with one of these sugars, convert it into the osone and then reduce this to another sugar, thereby accomplishing the transformation of one sugar into another isomeric sugar.

=Formation of Glucosides.=--By treatment with a considerable variety of different types of compounds, under proper conditions, it is possible to replace one of the hydrogen atoms of the terminal alcoholic group of the hexose sugars with the characteristic group of the other substance, forming compounds known, respectively, as glucosides, fructosides, galactosides, etc. The structural relation of methyl glucoside to glucose, for example, may be illustrated as follows:

Glucose (C_{6}H_{12}O_{6}) Methyl Glucoside (C_{7}H_{14}O_{6}) CHO CHO | | (CHOH)_{4} (CHOH)_{4} | | CH_{2}OH CHOH | CH_{3}

A general formula for glucosides is R·(CHOH)_{5}·CHO; and the R may represent a great variety of different organic radicals (see the chapters dealing with Glucosides and with Tannins). When the glucosides are hydrolyzed, they yield glucose and the hydroxyl compound of the radical with which it is united. All the statements which have been made with reference to glucosides, apply equally well with reference to fructosides, galactosides, mannosides, etc.

It is possible, by various laboratory processes, to replace additional hydrogen atoms in the glucose molecule with the same or other organic radicals, thus producing glucosides containing two or more R groups; but most of the natural glucosides contain only one other characteristic group.

=Oxidations.=--When the hexoses are oxidized they give rise to three different types of acids, depending upon the conditions of the oxidation and the kind of oxidizing agent used. With glucose, for example, the relationships involved may be illustrated as follows:

CHO COOH CHO COOH | | | | (CHOH)_{4} (CHOH)_{4} (CHOH)_{4} (CHOH)_{4} | | | | CH_{2}OH CH_{2}OH COOH COOH Glucose Gluconic acid Glucuronic acid Saccharic acid

An important property of the acids of the _gluconic_ type is that when heated with pyridine or quinoline to 130°-150° they undergo a molecular rearrangement whereby the acid corresponding to an isomeric sugar is produced. For example, gluconic acid, under these conditions, becomes mannonic acid, which can be reduced to mannose. The process is reversible; mannose can be converted to mannonic acid, thence to gluconic acid, thence to glucose. Similarly, galactonic acid can be converted into talonic acid, and this to talose, and this process is reversible. These facts afford another means of conversion of one sugar into another.

From the standpoint of physiological processes, _glucuronic acid_ is the most interesting and important oxidation product of glucose. It is often found in the urine of animals, as the result of the partial oxidation of glucose in the animal tissues. Normally, glucose is oxidized in the body to its final oxidation products, carbon dioxide and water. But when many difficultly oxidizable substances, such as chloral, camphor, turpentine oil, aniline, etc., are introduced into the body, the organism has the power of combining these with glucose to form glucosides. These so-called "paired" compounds are then oxidized to the corresponding glucuronic acid derivatives and eliminated from the body in the urine. No phenomenon similar to this occurs in plants, however, and glucuronic acid has never been found in plant tissues.

=Synthesis and Degradation of Hexoses.=--Monosaccharides of any desired number of carbon atoms can be produced from aldoses having one less carbon atoms, by way of the familiar "nitrile" reaction. Aldoses, like all other aldehydes, combine directly with hydrocyanic acid, forming compounds known as nitriles, which contain one more carbon atom than was present in the original aldehyde; the cyanogen group can easily be converted into a COOH group; and this, in turn, reduced to an aldehyde, thus producing an aldose with one more carbon atom than was present in the initial sugar. These changes may be illustrated by the following equations:

(1) CHO + HCN CHOH·CN CN | | | (CHOH)_{3} = (CHOH)_{3} or (CHOH)_{4} | | | CH{2}OH CH_{2}OH CH_{2}OH Aldopentose Nitrile

(2) CN + H_{2}O COOH + NH_{3} | | (CHOH)_{4} = (CHOH)_{4} | | CH_{2}OH CH_{2}OH Nitrile Acid

(3) COOH - O CHO | | (CHOH)_{4} = (CHOH)_{4} | | CH_{2}OH CH_{2}OH Acid Aldohexose

It is possible, by this process, to advance step by step from formaldehyde to higher sugars, Emil Fischer and his students having carried the process as far as the production of glucodecose (C_{10}H_{20}O_{10}). It usually happens, however, that two stereo-isomers result from the "step-up" by way of the nitrile reaction; thus, arabinose yields a mixture of glucose and mannose, glucose yields glucoheptose and mannoheptose, etc.

The reverse process, or the so-called "degradation" of a sugar into another containing fewer carbon atoms, may be readily accomplished in either one or two ways. In Wohl's process, the aldehyde group of the sugar is first converted into an _oxime_, by treatment with hydroxylamine; the oxime, on being heated with concentrated sodium hydroxide solution, splits off water and becomes the corresponding _nitrile_; this, on further heating, splits off HCN and yields an aldose having one less carbon atom than the original sugar. This process is the exact reverse of the nitrile synthesis, described above. The second method of degradation, suggested by Ruff, makes use of Fenton's method of oxidizing aldehyde sugars to the corresponding monobasic acid, using hydrogen peroxide and ferrous sulfate as the oxidizing mixture; the _aldonic acid_ thus formed is then converted into its calcium salt, which, when further oxidized, splits off its carboxyl group and one of the hydrogens of the adjacent alcoholic group, leaving an aldose having one less carbon atom than the original aldose sugar.

=Enolic Forms.=--A final avenue for the interconversion of glucose, mannose, and fructose into one another, is through the spontaneous transformations which these undergo when dissolved in water containing sodium hydroxide or potassium hydroxide. This change is due to the conversion of the sugar, in the alkaline solution, into an _enol_, which is identical for all three sugars, and which may subsequently be reconverted into any one of the three isomeric hexoses. The relationships involved are illustrated in the following formulas:

CHO CHO CH_{2}OH CHOH | | | ¦ H-C-OH HO-C-H C=O C-OH | | | | HO-C-H HO-C-H HO-C-H HO-C-H | | | | H-C-OH H-C-OH H-C-OH H-C-OH | | | | H-C-OH H-C-OH H-C-OH H-C-OH | | | | CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH Glucose Mannose Fructose Enolic Form

The preceding technical discussion of the chemical constitution and reactions of the hexoses has been presented, not because it has any direct connection with the occurrence or functions of these compounds in plant tissues, but for the purpose of giving to the student a graphic conception of the structure and properties of these simple carbohydrates, as a basis for the understanding of the nature, properties, possible chemical reactions, syntheses, etc., of the more complex types of carbohydrates, which, along with these simple monosaccharides, constitute the most important single group of organic components of plants.

THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES

Only two monosaccharides occur as such in plants. These are glucose and fructose. All the other hexoses, whose structure is shown on pages 37 and 38, occur in plants only as constituents of the more complex saccharides, in glucoside-formations, or as the corresponding polyatomic alcohols.

The aldo-hexoses which occur most commonly in plants, either free or in combination, are _d_-glucose, _d_-mannose, and _d_-galactose; while _d_-fructose and _d_-sorbose are the common keto-hexoses.

=Glucose= (often called also dextrose, fruit sugar, or grape sugar) occurs widely distributed in plants, most commonly in the juices of ripening fruits, where it is usually associated with fructose and sucrose, the two hexoses being easily derived from sucrose by hydrolysis. Glucose is also produced by the hydrolysis of many of the more complex carbohydrates, by the action either of enzymes or of dilute acids; lactose, maltose, raffinose, starch, and cellulose, as well as many glucosides all yielding glucose as one of the products of their hydrolysis. In all such cases, it is _d_-glucose which is obtained.

Glucose is a crystalline solid (although it does not form such sharply defined crystals as does sucrose, or "granulated sugar"), which is easily soluble in water. It usually appears on the market in the form of thick syrups, which are produced commercially by the hydrolysis of starch with dilute sulfuric acid, removal of the acid after the hydrolysis is complete, and evaporation of the resulting solution to the desired syrupy consistency. (Since corn starch is commonly used as the raw material for this process, these syrups are often spoken of as "corn syrup.") The sweetness of glucose is about three-fifths that of ordinary cane sugar.

Glucose exhibits all the properties of hexoses which have been described in general terms above. It is a reducing-sugar, and is easily fermented. The specific rotatory power of _d_-glucose is +52.7°. But when glucose is dissolved in water, it exhibits in a marked degree the phenomenon known as "mutarotation"; that is, freshly made solutions exhibit a certain definite rotatory power, but this changes rapidly until it finally reaches another definite specific rotation. In other words, glucose is "birotatory," or possesses two distinct specific rotatory powers, and the changing rotation effect in aqueous solutions is due to the change from one form to the other. When dissolved in alcohol, it does not exhibit this change in rotatory power. In order to explain this phenomenon, it is necessary to assume that there are two modifications of _d_-glucose, which have been designated respectively as the [alpha] and [beta] forms. The possibility of the existence of these two forms is explained by the assumption of the closed-ring arrangement of the glucose molecule, as indicated in the following formulas which represent the two possible isomeric arrangements:

HO-C-H H-C-OH / \ / \ / \ / \ H-C-OH \ H-C-OH \ | O | O HO-C-H / HO-C-H / \ / \ / \ / \ / C-H C-H | | H-C-OH H-C-OH | | CH_{2}OH CH_{2}OH [alpha]-Glucose [beta]-Glucose

It is assumed that the [alpha] modification (with its specific rotatory power of +105°) is the normal form for crystalline glucose, but that when dissolved in water it is changed into an _aldehydrol_, i.e., a compound containing two additional OH groups, which later breaks down again, into the [beta] modification (with its specific rotatory power of +22°). When dissolved in alcohol, this change does not take place because of the absence of the excess of water necessary to produce the intermediate aldehydrol form.

There are other examples of the existence of the [alpha] and [beta] modification of glucose. For example, [alpha]-methyl-glucoside and [beta]-methyl-glucoside (specific rotatory powers, +157° and -33°, respectively) are both known, as well as several other similar glucoside arrangements.

=Mannose.=--This sugar does not occur as such in plants; but complex compounds which yield _d_-mannose when hydrolyzed, known as "mannosans," are found in a number of tropical plant forms. The mannose which is obtained from these by hydrolysis is very similar to glucose in its properties, forms the same osazones as do glucose and fructose, exhibits mutarotation, etc. Mannose may also be obtained by oxidizing mannitol, a hexatomic alcohol, known as "mannite," which occurs in many plants, especially in the manna-ash (_Fraxinus ornus_), the dried sap from which is known as "manna."

=Galactose= occurs in the animal kingdom as one of the constituents of lactose, or milk-sugar. It is also one of the constituents of raffinose, a trisaccharide sugar found in plants, and occurs as "galactans" in many gums and sea-weeds. The _d_-galactose, obtained by the hydrolysis of any of these compounds, is a faintly sweet substance which resembles glucose in many of its properties; having one characteristic difference, however, in that it forms mucic acid instead of saccharic acid when oxidized by concentrated nitric acid. These oxidation products are very different in their physical properties and this difference serves to distinguish between the two sugars from which they are derived.

=Fructose= (levulose, honey sugar, or "diabetic" sugar) occurs along with glucose in the juices of many fruits, etc. It is a constituent of sucrose, of raffinose, and of the polysaccharide inulin, from which it may be obtained by hydrolysis. It is a ketose sugar, reduces Fehling's solution, forms the same osazone as glucose, and is easily fermentable by yeast. Its sweetness is slightly greater than that of ordinary cane sugar. _d_-fructose (the ordinary form) is easily soluble in water, and is strongly levorotatory, its specific rotatory power at 20° C. being -92.5°; it is unique in the very large effect which is produced in its rotatory power by increasing the temperature of the solution; at 87° its specific rotatory power is reduced to -52.7°, exactly equal to but in the opposite direction of the effect of glucose; hence, _invert sugar_, which is a mixture of an equal number of molecules of glucose and fructose, and which has a specific rotatory power of -19.4° at 20° C., becomes optically inactive at 82° C.

=Sorbose= is the only other ketohexose which has any importance in plant chemistry. It does not occur free in plants, but is the first oxidation product from the hexatomic alcohol, sorbitol, which is present in the juice of the berries of the mountain-ash. Sorbose is a crystalline solid, which is not fermentable by yeast, but which otherwise closely resembles fructose.

DISACCHARIDES

The disaccharides, having the formula C_{12}H_{22}O_{11}, may be regarded as derived from the monosaccharides by the linking together of two hexose groups with the dropping out of a molecule of water, in the same way that many other organic compounds form such linkages. That this is a perfectly correct conception, is shown by the fact that, when hydrolyzed, the disaccharides break down into two hexose sugars, thus

C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}.

With all known disaccharides, at least one of the hexoses obtained by hydrolysis is glucose; hence all disaccharides may be regarded as glucosides (C_{6}H_{12}O_{5}·R) in which the R is another hexose group.

Since hexoses have both alcoholic and aldehyde groups, and since either of these types of groups may function in the linkage of the two hexoses to form a disaccharide, it is possible for two hexoses, both of which are reducing sugars to be linked together in three different ways: (1) through an alcoholic group of each hexose, (2) through an alcoholic group of one and the aldehyde group of the other, and (3) through the aldehyde group of each hexose. Disaccharides linked in either of the first two ways will be reducing sugars, since they still contain a potentially active aldehyde group; but those of the third type will not be reducing sugars, since the linkage through the aldehyde groups destroys their power of acting as reducing agents. Examples of each of these three types of linkage are found among the common disaccharides, as will be pointed out below.

The following table shows the general characteristics of the common disaccharides.

_Type 1.--Aldehyde group potentially active, reducing sugars:_

Sugar Components Maltose Glucose and glucose Gentiobiose Glucose and glucose Lactose Glucose and galactose Melibiose Glucose and galactose Turanose Glucose and fructose

_Type 2.--Non-reducing sugars:_

Sucrose Glucose and fructose Trehalose Glucose and glucose

The disaccharides of Type 1 reduce Fehling's solution and form hydrazones and osazones, although somewhat less readily than do the hexoses. They all show mutarotation and exist in two modifications, indicating that the component groups have the closed-ring arrangement.

The disaccharides of Type 2, since they contain no potentially active aldehyde group, do not reduce Fehling's solution, nor form osazones; neither do they exhibit mutarotation. The only disaccharides which occur as such in plants are of this type. Disaccharides of Type 1 may be obtained by the hydrolysis of other, more complex, carbohydrates.

All disaccharides are easily hydrolyzed into mixtures of their component hexoses, by boiling with dilute mineral acids, or by treatment with certain specific enzymes which are adapted to the particular disaccharide in each case (see page 55, also