CHAPTER VI

GLUCOSIDES

Strictly speaking, the term _glucoside_ should be applied only to such compounds as contain glucose as the characteristic basic group. But in common usage, it refers to any compound which, when hydrolyzed, yields a sugar as one of the products of the hydrolysis. In all the natural glucosides which occur in plant tissues, the other organic constituent, which is represented by the R in the formula for glucosides (R·C_{6}H_{11}O_{5}, or R·(CHOH)_{5}CHO) is some aromatic group, or closed-ring benzene derivative.[3] The different organic constituents of glucosides are of a great variety of types, such as phenols, alcohols, aldehydes, acids, oxyflavone derivatives, mustard oils, etc. It is noteworthy, however, that no nitrogenous groups of the protein type have been found combined with sugars in glucosides.

Some glucosides contain more than one saccharide group, possibly as di- or trisaccharides. Under proper conditions of hydrolysis, one or more of the saccharide groups can be removed from such compounds, resulting in glucosides of simpler structure.

Most of the common glucosides are derived from _d_-glucose. Some are known, however, which are derivatives of galactose or rhamnose; while in some cases the exact nature of the sugar which is present has not yet been determined.

FOOTNOTES:

[3]

HC // \ HC CH The structural formula for benzene, C_{6}H_{6}, | ¦ is one which it is HC CH \\ / CH

difficult and inconvenient to reproduce in type. On that account, it is

/\ customary to indicate this formula by a plane hexagon, thus | |. \/

It is understood, in all such cases, that the figure represents six carbon atoms arranged in a closed ring, with alternate double and single bonds, and with a hydrogen atom attached to each carbon. The printing of some other group as OH, CH_{3}, adjacent to an angle of the hexagon means that this group replaces the H atom in the compound which is being illustrated.

HYDROLYSIS OF THE NATURAL GLUCOSIDES

All natural glucosides are hydrolyzed into a sugar and another organic residue by boiling with mineral acids; although they vary widely in the ease with which this hydrolysis is brought about.

In most cases, the glucoside is easily hydrolyzed by an enzyme which occurs in the same plant tissue, but in different cells than those which contain the glucoside. Injury to the tissues, germination processes, and perhaps other physiological activities of the cells, result in bringing the enzyme in contact with the glucoside and the hydrolysis of the latter takes place. A large number of such enzymes have been found in plants, many of which hydrolyze only a single glucoside. However, two enzymes, namely, the emulsin of almond kernels, and _myrosin_ of black mustard seeds, each hydrolyze a considerable number of glucosides. In general, _emulsin_ will aid in the hydrolysis of any glucoside which is a derivative of [beta]-glucose, and myrosin will help to split up any sulfur-containing glucoside. Glucosides which are derivatives of rhamnose require a special enzyme, known as _rhamnase_, for their hydrolysis.

The following reactions for the hydrolysis of arbutin and of amygdalin are typical of this action, and will serve to illustrate the general structure of these compounds:

------O------- | | CH_{2}OH·CHOH·CH·CHOH·CHOH·CH·O·C_{6}H_{4}OH + H_{2}O Arbutin = C_{6}H_{12}O_{6} + HOC_{6}H_{4}OH Glucose Hydroquinone

C_{6}H_{5} | (_a_) C_{6}H_{11}O_{5}·O·C_{6}H_{10}O_{4}·O·CH + H_{2}O Amygdalin | CN C_{6}H_{5} | = C_{6}H_{11}O_{5}·O·CH + C_{6}H_{12}O_{6} Mandelo-nitrile | Glucose glucoside CN

C_{6}H_{5} | (_b_) C_{6}H_{11}O_{5}·O·CH+H_{2}O = C_{6}H_{5}·CHOH·CN+C_{6}H_{12}O_{6} Mandelo-nitrile | Mandelo-nitrile Glucose glucoside CN

(_c_) C_{6}H_{5}·CHOH·CN + H_{2}O = C_{6}H_{5}·CHO + HCN Mandelo-nitrile Benzaldehyde Hydrocyanic acid

GENERAL PROPERTIES OF GLUCOSIDES

As a rule, glucosides are easily soluble in water. They are generally extracted from plant tissues by digestion with water or alcohol. In most cases, the enzyme which is present in other cells of the same tissue must be killed by heating the material, in a moist condition, to the temperature of boiling water, before the extraction is begun, as otherwise the glucoside will be hydrolyzed as rapidly as it is extracted from its parent cell. Maceration or otherwise bruising the tissue, after the enzyme has been destroyed, facilitates the extraction. The glucosides, after extraction and purification by recrystallization, are generally colorless, crystalline solids, having a bitter taste and levorotatory optical activity. This latter property is remarkable, as most of them are compounds of the strongly dextrorotatory _d_-glucose.

Many of the natural glucosides have marked therapeutic properties and are largely used as medicines; others are the mother-substances for brilliant dyes; for example, indican, from which indigo is obtained, and the alizarin glucosides.

Several hundred different glucosides have been isolated from plant tissues, and their properties described, and this number is being added to constantly, as the methods of isolation and study are improved. They may be classified into groups, according to the nature of the organic compound other than sugars which they yield when hydrolyzed. The following descriptions of the occurrence, constitution, products of hydrolysis, and special properties of typical members of each of the several different classes of glucosides will serve to illustrate their general relationship to plant growth.

THE PHENOL GLUCOSIDES

=Arbutin=, C_{12}H_{16}O_{7}, is obtained from the leaves of the bear berry (_Arctostaphylos uva-ursi_), a small evergreen shrub. When hydrolyzed by mineral acids or emulsin, it yields glucose and hydroquinone.

C_{12}H_{16}O_{7}+H_{2}O = C_{6}H_{12}O_{6}+C_{6}H_{4}(OH)_{2}.

Hydroquinone has strongly antiseptic properties. Arbutin is both an antiseptic and a diuretic, and is used in medicine.

=Phloridzin=, C_{21}H_{24}O_{10}, is found in the bark of apple, pear, cherry, plum, and similar trees. Mineral acids (but not emulsin) hydrolyze it to glucose and _phloretin_ (C_{15}H_{14}O_{5}), according to the equation

CH_{3} C_{21}H_{24}O_{10} + H_{2}O = C_{6}H_{12}O_{6} + | (OH)_{3}C_{6}H_{2}·CO·CH·C_{6}H_{4}OH.

It is used in medicine as a remedy for malaria, having marked anti-periodic properties.

=Glycyphyllin=, C_{21}H_{24}O_{9}, found in leaves of Smilax, yields rhamnose and phloretin, when hydrolyzed.

=Iridin=, C_{24}H_{26}O_{13} (glucose and irigenin), found in rootstocks of Iris, is used in medicine as a cathartic and diuretic.

=Baptisin=, C_{26}H_{32}O_{14}·9H_{2}O (two rhamnose and baptigenin), found in roots of wild indigo (_Baptisia_), has strong purgative properties.

=Hesperidin=, C_{50}H_{60}O_{27} (one rhamnose+two glucose+hesperitin), is found in the pulp of lemons and oranges.

The characteristic phenol group which is present in these glucosides has the following structural formula, in each case, the X indicating the H atom which is replaced by the sugar molecule to form the glucoside:

Phloretin

/\ /\ / \----C----CH------/ \ | | ¦ | | |OX HO| |OH O CH_{3} | |OH \ / \ / \/ \/

Irigenin

O / \ -CH_{2}-C-O--/\ /\ \ / \O·CH_{3} / \ O-| | | | | |OX CH_{3}O-| | \ / \ / \/ O | CH_{3}

Hesperitin

/\ /\ / \ / \ HO| |-CH=CH-C-| |OX CH_{3}O| | ¦ | | \ / O \ / \/ \/

THE ALCOHOL GLUCOSIDES

=Salicin=, C_{13}H_{18}O_{7} (glucose+saligenin, or _o_-oxy benzyl alcohol) is found in the bark, leaves, and flowers of most species of willow, the proportion present depending upon the season of the year, and the sex of the tree. It is used as a remedy against fevers and rheumatism, causing less digestive disturbances than the salicylic acid which is the oxidation product of saligenin and which is sometimes used as a remedy for rheumatism.

=Coniferin=, C_{16}H_{22}O_{8} (glucose and coniferyl alcohol), is found in the bark of fir trees. The coniferyl alcohol obtained from coniferin by hydrolysis can be easily oxidized to _vanillin_, and is, therefore, the source for the artificial flavoring extract used as a substitute for the true extract of the vanilla bean.

=Populin=, C_{20}H_{22}O_{8} (glucose+saligenin+benzoic acid), found in the bark of poplar trees, is used in medicine as an antipyretic. It can be hydrolyzed, by a special enzyme, into salicin and benzoic acid.

The structure of the two typical closed-ring alcohols which are present in these glucosides is indicated by the following formulas;

Coniferyl alcohol Saligenin CH=CH·CH_{2}OH /\ /\ / \ / \ | |CH_{2}OH | | | |OX | |OCH_{3} \ / \ / \/ \/ OX

THE ALDEHYDE GLUCOSIDES

=Salinigrin=, C_{13}H_{16}O_{7} (glucose and _m_-oxy benzaldehyde), is found in the bark of one species of willow (_Salix discolor_). Its isomer, known as _helicin_ (glucose and _o_-oxy benzaldehyde, or salicylic aldehyde), does not occur naturally in any plant, but is easily produced artificially by the gentle oxidation of salicin. Their relationships are shown on the following formulas;

Salicin Helicin Salinigrin

/\ /\ /\ / \ / \ / \ | |CH_{2}OH | |CHO | |CHO | |OX | |OX | | \ / \ / \ / \/ \/ \/ OX

=Amygdalin=, also contains a benzaldehyde group, but there is linked with it a hydrocyanic acid group; hence, this glucoside is usually classed with the cyanophoric glucosides (see page 86).

THE ACID GLUCOSIDES

The most common example of this group is =gaultherin=, C_{14}H_{18}O_{8}, which is found in the bark of the black birch and is a combination of glucose with methyl salicylate. Both the glucoside itself and the methyl salicylate ("oil of wintergreen") which is derived from it are used as remedies for rheumatism.

=Jalapin=, C_{44}H_{56}O_{16} (glucose and jalapinic acid), and =convolvulin=, C_{54}H_{96}O_{27} (glucose+rhodeose+convolvulinic acid), are glucosides of very complex organic acids, found in jalap resin, which are used in medicine as cathartics or purgatives.

THE OXY-CUMARIN GLUCOSIDES

Cumarin itself is widely distributed in plants. No glucoside containing cumarin as such has yet been isolated; but several glucosides of its oxy-derivatives are known. The following are common ones:

=Skimmin=, C_{15}H_{16}O_{8} (glucose and skimmetin), is found in _Skimmia japonica_; =æsculin=, C_{15}H_{16}O_{9} (glucose and æsculetin), is found in the bark of the horse-chestnut, _Æsculus hippocastanum_, and its isomer, =daphnin= (glucose and daphnetin), in several species of _Daphne_; and fraxin, C_{16}H_{18}O_{10} (glucose and fraxetin), is found in the bark of several species of ash.

The structural arrangement of the oxy-cumarin groups which are found in these glucosides is shown in the following formulas. It is not known to which OH group the sugar is attached, in each case.

Skimmetin Æsculetin

CH=CH·CO CH=CH·CO /\ | /\ | / \_O_| / \_O_| | | | | | | HO| | \ / \ / \/ \/ OH OH

Daphnetin Fraxetin

CH=CH·CO CH=CH·CO /\ | /\ | / \_O_| / \_O_| | | HO| | | |OH HO| | \ / \ / \/ \/ OH OCH_{3}

=Scopolin=, C_{22}H_{28}O_{14}, found in _Scopolia japonica_, contains two glucose molecules united to a monomethyl ether of æsculin; while =limettin=, found in certain citrus trees, is the dimethyl ether of æsculin.

THE PIGMENT GLUCOSIDES

Many, if not all, of the red, yellow, violet, and blue pigments of plants either exist as, or are derived from, glucosides. These are of three types: the madder, or alizarin, reds are derivatives of various oxy-anthraquinones; most of the soluble yellow pigments are glucosides derived from flavones or xanthones; and the soluble red, blue, and violet pigments of the cell-sap of plants are mostly anthocyan derivatives. The four basic groups, or nuclei, which are present in these different types of compounds are complex groups consisting essentially of two benzene rings linked together through a third ring in which there are either two oxygen atoms in the ring, or one oxygen in the ring and a second attached to the opposite carbon in the (C=O) arrangement, as shown by the following diagrammatic formulas:

Xanthone Anthraquinone

O O ¦ 1 1 ¦ 1' /\ C /\ /\ C /\ / \ / \ / \ / \ / \ / \ | | | |2 2| | | |2' | | | |3 3| | | |3' \ / \ / \ / \ / \ / \ / \/ O \/ \/ C \/ 4 4 ¦ 4' O

Flavone Anthocyan

1 1 /\ O 5'____4' /\ O 5'____4' / \ / \____/ \ / \ / \____/ \ 2| | | \____/3' 2| | | \____/3' 3| | |5 1' 2' 3| | |6 1' 2' \ / \ / \ / \ / \/ C \/ 5 4 ¦ 4 O

The red dyes which were formerly obtained from madder, the powdered roots of _Rubia tinctoria_, but are now almost wholly artificially synthetized, consist of at least four different glucosides, the organic group of which, in each case, is an hydroxy-derivative of anthraquinone. The most important of these is _ruberythric acid_, composed of two molecules of glucose linked with one of alizarin (1,2, dioxyanthraquinone). _Xanthopurpurin_ contains 1,3, dioxyanthraquinone, which is isomeric with alizarin; and _rubiadin_ is a monomethyl (the CH_{3} being in the 4 position), derivative of this compound. _Purpurin_ is a glucoside of 1,2,4, trioxyanthraquinone.

The soluble yellow pigments are generally glucosides of hydroxy-derivatives of xanthone or flavone, known as oxyxanthones or oxyflavones. The sugars which are united to these nuclei vary greatly, so that there are a great variety of yellow, white, or colorless flavone or xanthone pigment compounds. These compounds are almost universally present in plants. For example, one typical set of examinations of the wood, bark, leaves, and flowers of over 240 different species of tropical plants showed that flavone derivatives were present in every sample which was tested, the pigments being usually located in the powdery coating of the epidermis of the tissues.

The following typical examples will serve to illustrate the composition and properties of the glucosides of this type.

=Quercitrin=, C_{21}H_{20}O_{11}, is found in oak bark, in the leaves of horse-chestnut, and in many other plants, often associated with other pigments. It is a brilliant yellow crystalline powder. Industrially, it ranks next to indigo and alizarin in importance as a natural dye stuff. It is a glucoside of rhamnose with 1,3,3',4', tetraoxyflavonol (i.e., the flavone nucleus with five OH groups replacing the hydrogens in the 1, 3, 5, 3', and 4' positions). =Quercetin=, C_{15}H_{10}O_{7}, which is the tetraoxyflavonol itself, without any sugar in combination with it, is found in the leaves of several species of tropical plants and in the bark of others. =Isoquercitrin=, C_{21}H_{20}O_{12}, is derived from the same flavone, but contains glucose instead of rhamnose, as the sugar constituent of the glucoside.

=Apiin=, C_{26}H_{20}O_{9}, the yellow glucoside found in the leaves of parsley, celery, etc., contains apiose (a pentose sugar of very unusual structure, represented by the formula,

CH_{2}OH \ COH·CHOH·CHO), and apigenin, which is a 1,3,4',trioxyflavone. / CH_{2}OH

=Xanthorhamnin=, C_{34}H_{42}O_{20}, is a very complex glucoside containing two rhamnose and one galactose groups, united with rhamnetin, which is quercitin with the H of the OH in either the 1, or 3, position replaced by a methyl group. There are several similar pigments which differ from xanthorhamnin only in the number or position of the methoxy groups (i.e., the OH groups with a CH_{3} replacing the H), or in the nature of the sugar which is present in the compound. Rhamnetin itself is found in the fruits of certain species of _Rhamnus_, and is used in dyeing cotton.

The structural arrangement of the characteristic groups of these flavone pigments will be dealt with more in detail in the chapter dealing with Pigments (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 (Chapter VIII).

Many gaseous anæsthetics are known to have a marked effect in stimulating plant growth. In a number of cases, it has been shown that the contact of plant tissues with these anæsthetics brings about an interaction of the enzyme and glucoside which are present in the tissue, with the consequent hydrolysis of the latter, setting free its characteristic components. This observation has led to the supposition that many of the organic constituents of glucosides are definite plant stimulants, to which the name "hormones" has been applied. There is considerable experimental evidence to support this conception that glucosides may be the source of stimulating hormone substances, which will be discussed more in detail in the chapter dealing with these plant stimulants (Chapter XVII).

Glucosides may also serve as the mechanism for putting out of action of harmful products which may appear in the tissues as the result of abnormal conditions. These harmful substances may be rendered soluble by combination with sugars and so transposed by osmosis to some other part of the plant. The abnormally large percentages of glucosides which are present in certain species of plants during unfavorable climatic conditions lends some support to this view.

Finally, it may be assumed that easily oxidizable substances, such as aldehydes and acids, are possibly protected against too rapid, or premature, oxidation by being transformed into glucosides.

In general, it may be said that the glucosides seem to serve as the regulatory, protective, and sanatory agencies of the plant mechanism.

BIOLOGICAL SIGNIFICANCE OF GLUCOSIDES

The bitter taste of glucosides and their almost universal presence in the bark of plants undoubtedly helps to prevent the destructive gnawing of the bark by animals.

Glucosides having either a strong bitter taste, or pronouncedly poisonous properties, likewise undoubtedly serve to protect such important organs of plants as the seeds and fruits from being prematurely eaten by birds and animals. The common disappearance of these bitter substances as the seed or fruit ripens adds to the attractiveness of the material for food for animals at the proper stage of ripeness to provide for wider distribution of the seeds for further propagation. Further, the very general occurrence of these protective glucosides in many of the vegetative parts of plants during the early stages of growth, followed by their disappearance after the seeds of the plant have been formed, certainly serves to protect these plants from consumption as forage by animals before they have been able to develop their reproductive bodies. The lack of palatability, and even the production of digestive disorders resulting from the eating of unripe fruit may be due, in part at least, to the presence of protective glucosides in unripe fruits and vegetables.

On the other hand, the almost universal presence of the brilliant pigment glucosides in the external parts of flowers undoubtedly serves to attract the insects which are biologically adapted to provide for the transportation of pollen from one blossom to another and so to insure the cross-fertilization which is so important in maintaining the vigor of many species of plants.

It is apparent that this important group of compounds, with its exceedingly varied and complex constituent groups, may play a variety of significant rôles in plant growth.

REFERENCES.

ARMSTRONG, E. F.--"The Simple Carbohydrates and Glucosides," 239 pages, _Monographs_ on Biochemistry, London, 1919 (3d ed.).

VAN RIJN, J. J. L.--"Die Glykoside," 511 pages, Berlin, 1900.