Chapter VIII
).
The best-known yellow pigment which is a _xanthone_ derivative is =euxanthic acid=, known as "Indian yellow," which is a "paired" compound of glucuronic acid (see page 42) and euxanthone. The latter is a 2, 3', dioxyxanthone. The pigment is found in the urine of cattle which have been fed on mango leaves.
The soluble red, blue, and violet pigments are glucosides of various hydroxy-derivatives of the anthocyan nucleus. Their constitution and properties will be discussed in detail in the chapter dealing with the Pigments. These compounds are isomeric with similar flavone and xanthone derivatives, and the transition from one color to the other in plants takes place very easily under the action of oxidizing or reducing enzymes. This accounts for the change of reds and blues to yellows and browns, and vice versa, under changing temperature conditions.
The following red or blue plant pigments, which are anthocyan glucosides, have been isolated and studied (for the structural arrangement of the characteristic groups, see pages 116): from cornflower and roses, _cyanin_, C_{28}H_{31}O_{16}Cl (2 molecules glucose + cyanidin); from cranberries, _idain_, C_{21}H_{21}O_{10}Cl (galactose + cyanidin); from geranium, _pelargonin_, C_{27}H_{30}O_{15}Cl (2 molecules glucose + pelargonidin); from pæony, _pæonin_, C_{28}H_{33}O_{16}Cl (2 molecules glucose + pæonidin, a monomethyl cyanidin); from blue grapes, _[oe]nin_, C_{23}H_{25}O_{12}Cl (glucose + [oe]nidin); from whortle berry, _myrtillin_, C_{22}H_{23}O_{12}Cl (glucose + myrtillidin); from larkspur, _delphinin_, C_{41}H_{39}O_{21}Cl (2 molecules glucose + 2 molecules _p_-oxybenzoic acid + delphinidin); and from mallow, _malvin_, C_{29}H_{35}O_{17}Cl (2 molecules glucose + malvidin).
The blue dye, indigo, is derived from a glucoside of an entirely different type, known as _indican_. Indican is readily extracted from the leaves of various species of indigo plants. When hydrolyzed, it yields glucose and _indoxyl_ (colorless). Indoxyl is easily oxidized to _indigotin_ (the deep blue dye known as "indigo"). The equations illustrating these changes are as follows:
(_a_) C_{14}H_{17}O_{6}N + H_{2}O = C_{6}H_{12}O_{6} + C_{8}H_{7}ON Indican Glucose Indoxyl
(_b_) 2C_{8}H_{7}ON + O_{2} = C_{16}H_{10}O_{2}N + 2H_{2}O Indoxyl Indigotin
The structural relationships of indoxyl and indigotin may be illustrated by the following formulas:
O O /\ /\ ¦ ¦ / \ / \___COH / \___C C____/ \ | | ¦ | | | | | | | | C-H | | C=C | | \ /\ / \ /\ / \ / \ / \/ N \/ N N \/ | | | H H H
Indoxyl Indigotin
Natural indigo dye is prepared by fermentation of indigo leaves, the decay of the cell-walls liberating the enzymes in the tissues, which bring about the chemical changes illustrated in the above equations.
THE CYANOPHORE GLUCOSIDES
Several glucosides which yield hydrocyanic acid as one of the products of their hydrolysis are of common occurrence in plants. These are generally spoken of as the "cyanogenetic" glucosides; but as they do not actually produce cyanogen compounds, but only liberate them when hydrolyzed, the recently suggested term "cyanophore" undoubtedly more correctly indicates their properties.
The best known and most widely distributed of these is =amygdalin=. Amygdalin was first discovered in 1830, and was one of the first substances to be recognized as a glucoside. It is found in large quantities in bitter almonds and in the kernels of apricots, peaches, and plums; also in the seeds of apples, etc., in fact in practically all the seeds of plants of the Rose family. It is the mother substance for "oil of bitter almonds," which is widely used as a flavoring extract.
Amygdalin has been the object of very extensive studies, and even yet the exact nature of the linkage between its constituent groups is not certainly known. When completely hydrolyzed, it yields two molecules of glucose and one each of benzaldehyde and hydrocyanic acid. Recent studies indicate that the two sugar molecules are separately united to the other constituents, rather than united with each other in the disaccharide relationship. In other words, amygdalin is a true _glucoside_ rather than a _maltoside_. This is indicated by the fact that when submitted to the action of all known hydrolyzing agents which affect it, it has never been found to yield maltose as one of the products of hydrolysis. Furthermore, the rate of hydrolysis of amygdalin is not affected by the presence of maltose; and the segregation of the two glucose molecules is accomplished by enzymes other than maltase, which is the only enzyme which is known to break up a maltose molecule. Since the exact nature of the linkage is not known, it is customary and convenient to indicate the unit groups as linked together in the following order:
C_{6}H_{11}O_{5}-O-C_{6}H_{10}O_{4}-O-C_{6}H_{5}·CH-C[trb]N (1) (2) (3)(4)
A study of the hydrolysis reactions of amygdalin shows that there are three different linkages in the molecule which may be broken by the simple interpolation of a single molecule of water and a fourth which may be split by a different type of hydrolysis, namely, the C[trb]N linkage. These are indicated by the numbers below the corresponding portion of the formula above. Most hydrolyzing agents break the molecule first at (1), yielding one molecule of glucose and one of mandelo nitrile glucoside (see page 77). The next step usually breaks the latter at the point indicated by (2), yielding glucose and benzaldehyde cyanhydrin, or mandelo nitrile. The latter in turn breaks down at (3) into benzaldehyde and HCN. But when amygdalin is boiled with concentrated hydrochloric acid, the first change is the splitting off at (4) of the nitrogen in the form of ammonia and the consequent conversion of the CN group into a COOH group, producing amygdalinic acid. On further hydrolysis, this breaks up in the same order as before. Similarly, it is possible to convert mandelo nitrile into mandelic acid by splitting off the nitrogen to form a COOH group, instead of splitting off the HCN group leaving benzaldehyde.
The mandelo nitrile glucoside contains an asymmetric carbon atom which is wholly outside its glucose group, thus C_{6}H_{10}O_{5}-O-C_{6}H_{5}·CH·CN. Hence, it may exist in dextro, levo, and racemic forms. In the amygdalin molecule, it exists in the dextro form, which has been named "prunasin." The levo form, known as "sambunigrin," has been obtained by hydrolysis of a compound isomeric with amygdalin, whose composition has not been definitely worked out; while the racemic form, known as "prulaurasin," has been prepared from isoamygdalin, by the action of alkalies. Hence, all the possible compounds indicated by the presence of the asymmetric carbon have been found and identified.
The crude enzyme preparation which is obtained from almond seeds, known as "emulsin," contains two enzymes, _amygdalase_, which breaks the amygdalin molecule at linkage (1), and _prunase_, which breaks it at (2). The action of amygdalase must always precede that of prunase. In other words, it is never possible to break off a disaccharide sugar from the molecule, either by the action of prunase alone, or by means of any other hydrolytic agent.
=Dhurrin=, C_{14}H_{17}O_{7}N, is another glucoside of fairly general occurrence in plants, which yields HCN as one of the products of its hydrolysis. It is found in the leaves and stems of several species of millets and sorghums. Frequent cases of poisoning of cattle from eating of these plants as forage have been reported. On hydrolysis, dhurrin first yields glucose and paraoxy-mandelo nitrile; the latter then breaks down into paraoxy-benzaldehyde and HCN.
=Vicianin=, C_{19}H_{25}O_{10}N, is a cyanophoric glucoside, found in the seeds of wild vetch, etc. On hydrolysis, it yields glucose, arabinose, and _d_-mandelo nitrile. It is, therefore, similar to amygdalin, except that one glucose molecule is replaced by arabinose.
THE MUSTARD OIL GLUCOSIDES
The seeds of several species of plants of the Cruciferæ or mustard family contain glucosides in which the other characteristic group is a sulfur-containing compound. These glucosides yield "mustard oils" when they are hydrolyzed by the enzyme _myrosin_, which accompanies them in the plant. The following glucosides, found in the seeds of white and black mustard, are the best-known representatives of this class.
=Sinigrin=, C_{10}H_{16}O_{9}NS_{2}K, found in black mustard seeds, when hydrolyzed yields glucose, acid potassium sulfate, and allyl isosulfocyanide (mustard oil), as indicated by the equation.
C_{10}H_{16}O_{9}NS_{2}K+H_{2}O = C_{6}H_{12}O_{6} + C_{3}H_{5}N[trb]C=S+KHSO_{4}.
The acid potassium sulfate group separates first and most readily, leaving a compound known as _merosinigrin_, for which the following formula has been suggested:
-----O------ | | CH_{2}OH·CHOH·CH·CHOH·CH·CH | | O S | / |/ C=N[trb]C_{3}H_{5}
This compound usually breaks down into glucose and mustard oil; but by special treatment it is possible to obtain from it thioglucose, C_{6}H_{11}O_{5}·SH. This indicates that in the original glucoside the glucose is linked with the mustard oil through the sulfur atom.
=Sinalbin=, C_{30}H_{42}O_{15}N_{2}S_{2}, from white mustard seeds, when hydrolyzed by myrosin, yields glucose, sinalbin mustard oil (a paraoxybenzyl derivative of allyl isosulfocyanide) and sinapin acid sulfate; according to the equation
C_{30}H_{42}O_{15}N_{2}S_{2}+H_{2}O = C_{6}H_{12}O_{6}+C_{7}H_{7}O·NCS Sinalbin Glucose Sinalbin mustard oil
+ C_{16}H_{24}O_{5}N·HSO_{4}. Sinapin acid sulfate
The sinalbin mustard oil may be represented by the formula ____ / \ HO-CH CH-CH_{2}NCS. Hydrolysis of the sinapin acid sulfate converts \____/ it into sinapinic acid, C_{6}H_{2}OH·(OCH_{3})_{2}·CH=CH·COOH, choline, N(CH_{3})_{4}C_{2}H_{4}OH (see page 152), and H_{2}SO_{4}. It is, therefore, a very complex glucoside.
TEE DIGITALIS GLUCOSIDES
The five, or more, glucosides which are present in the leaves and seeds of the foxglove (_Digitalis purpurea_) have been extensively studied, as they are the active principles in the various digitalis extracts which are used in medicine as a heart stimulant.
=Digitoxin=, C_{34}H_{54}O_{11}, which is the most active of these glucosides in its physiological effects, when hydrolyzed, yields digitoxigenin, C_{22}H_{32}O_{4}, and a sugar having the formula C_{6}H_{12}O_{4}, which is known as "digitoxose" and is supposed to be a dimethyl tetrose.
=Digitalin=, C_{35}H_{56}O_{14}, is also strongly active. When hydrolyzed, it yields digitaligenin, C_{22}H_{10}O_{3}, glucose, and digitoxose.
=Digitonin=, C_{54}H_{92}O_{28}, constitutes about one-half of the total glucosides in the extract which is obtained from most species of the digitalis plants. It is much less active than the others. It is a saponin (see page 90) in type. On hydrolysis, it yields 2 molecules of glucose, 2 of galactose, and one of digitogenin.
=Gitonin=, C_{49}H_{80}O_{23}, containing 3 molecules of galactose, one of a pentose sugar, and one of gitogenin; and =gitalin=, C_{28}H_{48}O_{10}, containing digitoxose and gitaligenin, have also been isolated from digitalis extracts.
The structural arrangement of the characteristic groups in these glucosides has not yet been definitely worked out.
=Cymarin=, the active principle of Indian hemp (_Apocynum cannabinum_), is similar in type to the digitalis glucosides. When hydrolyzed, it yields a sugar known as "cymarose," C_{7}H_{14}O_{7}, which seems to be a monomethyl derivative of digitoxose, and cymarigenin, C_{23}H_{30}O_{5}, a compound which is either identical or isomeric with the organic residue obtained from other members of this group.
THE SAPONINS
The saponins constitute a group of glucosides which are widely distributed in plants, whose properties have been known since early Grecian times. They have been found in over four hundred different species of plants, belonging to more than forty different orders.
The most characteristic property of saponins is that they form colloidal solutions in water which produce a soapy foam when agitated, and are peculiarly toxic, especially to frogs and fishes. In dry form, they have a very bitter, acrid taste, and their dust is very irritating to the mucous membranes of the eye, nose, and throat.
On hydrolysis, the saponins yield a variety of sugars,--glucose, galactose, arabinose, and sometimes fructose, and even other pentoses--and a group of physiologically active substances, known as "sapogenins."
The more toxic forms of these glucosides are known as "sapotoxins."
The chemical composition of the saponins varies so widely that it is scarcely possible to cite typical individuals. Sarsaparilla, the dried root of smilax plants, contains a mixture of non-poisonous saponins, from which at least four individual glucosides have been isolated and studied. Corn cockle contains a highly poisonous sapotoxin which, on hydrolysis, yields four molecules of a sugar and one of sapogenin, C_{10}H_{16}O_{2}. Other sapotoxins are obtained from the roots of soapwort and from several species of _Gypsophila_. Digitonin and digito-saponin are glucosides of this type which are found in the extracts from various species of _Digitalis_.
THE PHYSIOLOGICAL USES OF GLUCOSIDES
It is scarcely conceivable that substances which vary so widely in composition as do the different types of glucosides can possibly all have similar physiological uses in plants. The cyanophoric glucosides, the pigment glucosides, the mustard oil glucosides, and the saponins, for example, can hardly be assumed to have the same definite relationships to the metabolism and growth of the plant. To be sure, they are alike in that they all contain one or more sugar molecules, and it is probable that the carbohydrates which are held in this form may serve as reserve food material, especially when the glucoside is stored in the seeds; but it is obvious that the simpler and more normal form of such stored food is that of the polysaccharides which contain no other groups than those of the carbohydrates. It seems much more probable that the physiological uses of glucosides depend upon their ability to form temporarily inactive "pairs" with a great variety of different types of organic compounds which are elaborated by plants for a variety of purposes.
It has been noted that in most, if not all, instances, the glucosides are accompanied in the same plant tissue (although in separate cells) by the appropriate enzyme to bring about their hydrolysis and so set free both the sugar and the other characteristic component whenever the conditions are such as to permit the enzyme to come in contact with the glucoside. This occurs whenever the tissue is injured by wound or disease, and also during the germination process.
Injury to the plant tissue seems to be a necessary preliminary to the functioning of the active components of the glucoside, except in the case of the seeds. This leads naturally to the supposition that at least some of these glucosides are protective or curative agents in the plant tissues. This conception is further supported by the facts that many of the non-sugar components of glucosides are bactericidal in character and that the glucosides commonly occur in parts of the plant organism which are otherwise best suited to serve as media for the growth of bacteria. Thus, it is known that in the almond, as soon as the tissue is punctured, amygdalin is hydrolyzed and all bacterial action is inhibited. Similarly, the almost universal presence of glucosides containing bactericidal constituents in the bark of trees insures natural antiseptic conditions for all wounds of the outer surfaces of the stem of the plant. In fact, it is easily conceivable that at least one of the reasons for the failure of the processes of decay of plant tissues to set in until after the death of the cells, is that during living, respiratory activity these antiseptic glucosides are so generally present in the tissues.
Further, it has been fairly well established that the "chromogens," or mother-substances of the pigments, which, under the influence of oxidase enzymes, serve to regulate the respiratory activities of the plant are essentially glucosidic in character. This, and other, functions of the pigments, most of which are glucosides, will be discussed at some length in the chapter dealing with the Pigments (