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 dextrorotatoryd-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.
Arbutin, C12H16O7, 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.
C12H16O7+ H2O = C6H12O6+ C6H4(OH)2.
Hydroquinone has strongly antiseptic properties. Arbutin is both an antiseptic and a diuretic, and is used in medicine.
Phloridzin, C21H24O10, is found in the bark of apple, pear, cherry, plum, and similar trees. Mineral acids (but not emulsin) hydrolyze it to glucose andphloretin(C15H14O5), according to the equation
It is used in medicine as a remedy for malaria, having marked anti-periodic properties.
Glycyphyllin, C21H24O9, found in leaves of Smilax, yields rhamnose and phloretin, when hydrolyzed.
Iridin, C24H26O13(glucose and irigenin), found in rootstocks of Iris, is used in medicine as a cathartic and diuretic.
Baptisin, C26H32O14·9H2O (two rhamnose and baptigenin), found in roots of wild indigo (Baptisia), has strong purgative properties.
Hesperidin, C50H60O27(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:
Salicin, C13H18O7(glucose + saligenin, oro-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, C16H22O8(glucose and coniferyl alcohol), is found in the bark of fir trees. The coniferyl alcohol obtained from coniferin by hydrolysis can be easily oxidized tovanillin, and is, therefore, the source for the artificial flavoring extract used as a substitute for the true extract of the vanilla bean.
Populin, C20H22O8(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;
Salinigrin, C13H16O7(glucose andm-oxy benzaldehyde), is found in the bark of one species of willow (Salix discolor). Its isomer, known ashelicin(glucose ando-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;
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 (seepage 86).
The most common example of this group isgaultherin, C14H18O8, 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, C44H56O16(glucose and jalapinic acid), andconvolvulin, C54H96O27(glucose + rhodeose + convolvulinic acid), are glucosides of very complex organic acids, found in jalap resin, which are used in medicine as cathartics or purgatives.
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, C15H16O8(glucose and skimmetin), is found inSkimmia japonica;æsculin, C15H16O9(glucose and æsculetin), is found in the bark of the horse-chestnut,Æsculus hippocastanum, and its isomer,daphnin(glucose and daphnetin), in several speciesofDaphne; and fraxin, C16H18O10(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.
Scopolin, C22H28O14, found inScopolia japonica, contains two glucose molecules united to a monomethyl ether of æsculin; whilelimettin, found in certain citrus trees, is the dimethyl ether of æsculin.
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:
The red dyes which were formerly obtained from madder, the powdered roots ofRubia 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 isruberythric acid, composed of two molecules of glucose linked with one of alizarin (1,2, dioxyanthraquinone).Xanthopurpurincontains 1,3, dioxyanthraquinone, which is isomeric with alizarin; andrubiadinis a monomethyl (the CH3being in the 4 position), derivative of this compound.Purpurinis 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,C21H20O11, 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, C15H10O7, 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, C21H20O12, is derived from the same flavone, but contains glucose instead of rhamnose, as the sugar constituent of the glucoside.
Apiin, C26H20O9, the yellow glucoside found in the leaves of parsley, celery, etc., contains apiose (a pentose sugar of very unusual structure, represented by the formula,
and apigenin, which is a 1,3,4', trioxyflavone.
Xanthorhamnin, C34H42O20, 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 CH3replacing 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 ofRhamnus, 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 axanthonederivative iseuxanthic acid, known as "Indian yellow," which is a "paired" compound of glucuronic acid (seepage 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, C28H31O16Cl (2 molecules glucose + cyanidin); from cranberries,idain, C21H21O10Cl (galactose + cyanidin); from geranium,pelargonin, C27H30O15Cl (2 molecules glucose + pelargonidin); from pæony,pæonin, C28H33O16Cl (2 molecules glucose + pæonidin, a monomethyl cyanidin); from blue grapes,œnin, C23H25O12Cl (glucose +œnidin); from whortle berry,myrtillin, C22H23O12Cl (glucose + myrtillidin); from larkspur,delphinin, C41H39O21Cl (2 molecules glucose + 2 moleculesp-oxybenzoic acid + delphinidin); and from mallow,malvin, C29H35O17Cl (2 molecules glucose + malvidin).
The blue dye, indigo, is derived from a glucoside of an entirely different type, known asindican. Indican is readily extracted from the leaves of various species of indigo plants. When hydrolyzed, it yields glucose andindoxyl(colorless). Indoxyl is easily oxidized toindigotin(the deep blue dye known as "indigo"). The equations illustrating these changes are as follows:
The structural relationships of indoxyl and indigotin may be illustrated by the following formulas:
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.
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 isamygdalin. 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 trueglucosiderather than amaltoside. 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:
A study of the hydrolysis reactions ofamygdalinshows 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≡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 (seepage 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 C6H10O5—O—C6H5·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), andprunase, 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, C14H17O7N, is another glucoside of fairly general occurrence in plants, which yields HCN as one of the products ofits 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, C19H25O10N, is a cyanophoric glucoside, found in the seeds of wild vetch, etc. On hydrolysis, it yields glucose, arabinose, andd-mandelo nitrile. It is, therefore, similar to amygdalin, except that one glucose molecule is replaced by arabinose.
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 enzymemyrosin, 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, C10H16O9NS2K, found in black mustard seeds, when hydrolyzed yields glucose, acid potassium sulfate, and allyl isosulfocyanide (mustard oil), as indicated by the equation.
C10H16O9NS2K + H2O = C6H12O6+ C3H5≡N=C=S+KHSO4.
The acid potassium sulfate group separates first and most readily, leaving a compound known asmerosinigrin, for which the following formula has been suggested:
Merosinigrin
This compound usually breaks down into glucose and mustard oil; but by special treatment it is possible to obtain from it thioglucose, C6H11O5·SH. This indicates that in the original glucoside the glucose is linked with the mustard oil through the sulfur atom.
Sinalbin, C30H42O15N2S2, 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
The sinalbin mustard oil may be represented by the formula. Hydrolysis of the sinapin acid sulfate converts it into sinapinic acid, C6H2OH·(OCH3)2·CH=CH·COOH, choline, N(CH3)4C2H4OH (seepage 152), and H2SO4. It is, therefore, a very complex glucoside.
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, C34H54O11, which is the most active of these glucosides in its physiological effects, when hydrolyzed, yields digitoxigenin, C22H32O4, and a sugar having the formula C6H12O4, which is known as "digitoxose" and is supposed to be a dimethyl tetrose.
Digitalin, C35H56O14, is also strongly active. When hydrolyzed, it yields digitaligenin, C22H10O3, glucose, and digitoxose.
Digitonin, C54H92O28, 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 (seepage 90) in type. On hydrolysis, it yields 2 molecules of glucose, 2 of galactose, and one of digitogenin.
Gitonin, C49H80O23, containing 3 molecules of galactose, one of a pentose sugar, and one of gitogenin; andgitalin, C28H48O10, 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," C7H14O7, which seems to be a monomethyl derivative of digitoxose, and cymarigenin, C23H30O5, a compound which is either identical or isomeric with the organic residue obtained from other members of this group.
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, C10H16O2. Other sapotoxins are obtained from the roots of soapwort and from several species ofGypsophila. Digitonin and digito-saponin are glucosides of this type which are found in the extracts from various species ofDigitalis.
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 insuresnatural 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.
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.
Armstrong, E. F.—"The Simple Carbohydrates and Glucosides," 239 pages,Monographson Biochemistry, London, 1919 (3d ed.).Van Rijn, J. J. L.—"Die Glykoside," 511 pages, Berlin, 1900.
Armstrong, E. F.—"The Simple Carbohydrates and Glucosides," 239 pages,Monographson Biochemistry, London, 1919 (3d ed.).
Van Rijn, J. J. L.—"Die Glykoside," 511 pages, Berlin, 1900.
Using the term in its general application to a group of substances having similar chemical and physical properties, rather than in its limited application to a single definite chemical compound known commercially as "tannin," thetanninsare a special group of plant substances, mostly glucosides, which have the following characteristic properties. First, they are non-crystalline[4]substances, which form colloidal solutions with water, which have an acid reaction and a sharp astringent taste. Second, they form insoluble compounds with gelatine-containing tissues, as shown by the conversion of hide into leather. Third, they form soluble, dark-blue or greenish-black compounds with ferric salts, the common inks. Fourth, they are precipitated from their solutions by many metallic salts, such as lead acetate, stannous chloride, potassium bichromate, etc. Fifth, they precipitate out of solution albumins, alkaloids, and basic organic coloring matters. Finally, most tannins, in alkaline solutions, absorb oxygen from the air and become dark brown or black in color.
FOOTNOTES:[4]The needle-like forms, in which commercial "tannin" comes on the market, are not true crystals, but are broken fragments of the threads into which the colloidal tannin is "spun-out" from the syrupy extracts of nutgalls, etc.
[4]The needle-like forms, in which commercial "tannin" comes on the market, are not true crystals, but are broken fragments of the threads into which the colloidal tannin is "spun-out" from the syrupy extracts of nutgalls, etc.
[4]The needle-like forms, in which commercial "tannin" comes on the market, are not true crystals, but are broken fragments of the threads into which the colloidal tannin is "spun-out" from the syrupy extracts of nutgalls, etc.
Tannins occur widely distributed in plants. Practically every group of plants, from the fungi up to the flowering plants, contains many species of plants which show tannin in some of their tissues. Among the higher plants, tannins occur in a great variety of organs. Thus, they are found in the roots of several species of tropical plants; in the sterns, both bark and wood, of oaks, pines, hemlock, etc.; in the leaves of sumac, rhododendron, etc.; in many fruits, especially in the green, or immature, stages; and inthe seeds of several species, either before or after germination. Tannins are also found in certain special structures, such as gland cells, cells of the pulvini, laticiferous tissues, etc. Further, they are especially abundant in the pathological growths known as galls, which often contain from 40 to 75 per cent of tannin and constitute the most important commercial source for these materials.
The principal commercial sources of tannin, which is used in the manufacture of inks, in the tanning of leather, in certain dyeing operations, etc., are oak-galls, the bark and wood of oak, hemlock, acacia, and eucalyptus, the bark of the mangrove, the roots of canaigre, and the leaves of several species of sumac.
Tannins are either free phenol-acids or, more commonly, glucosides of these acids. Common "tannin," when hydrolyzed, yields from 7 to 8 per cent of glucose, which indicates that it is a penta-acid ester of glucose, i.e., each glucose molecule has five acid groups attached to it. The formula for such a tannin is, therefore, as follows,
in which the R represents a complex phenol-acid like tannic acid, or digallic acid. These acids are derivatives of the common phenols, whose constitution will be brought to mind by the following series of formulas:
These phenols themselves do not occur as constituents of tannins, although they are often found in other glucosides, gums, etc. The following mono-carboxyl acid derivatives of these phenols are, however, found both free and in glucoside formation as constituents of many of the common tannins.
Pyrocatechuic acid, derived from pyrocatechol, represented by the formula,
Pyrocatechuic acid
Gallic acid,derived from pyrogallol, and represented by the formula,
Gallic acid
In most of the common tannins, however, the characteristic acids are oxy-derivatives of the so-called "tannon" group, represented by the formula, C6H5·CO·O·C6H5. For example,digallic acid, which is a constituent of many common tannins, is a tetra-oxy, mono-carboxyl derivative of this group, having the structural formula,
tannins
Ellagic acid, which is an hydrolysis product of many of the pyrogallol tannins (see below) and which produces the characteristic "bloom" on leather tanned by this type of tannins, has the following formula,
Ellagic acid
The tannins are divided into two general classes, known respectively as thepyrogallol tanninsand thecatechol tannins. These differ in their characteristic reactions as follows:
Pyrogallol tannins contain approximately 52 per cent of carbon; while the catechol tannins usually contain 59 per cent to 60 per cent, the difference being due to the absence of glucose from the molecule in the latter types.
The two types are distributed in plants as follows: pyrogallol tannins in oak-galls, oak wood, sumac, chestnut, divi-divi, and algaro billa; catechol tannins in the barks of pines, hemlocks, oaks, acacias, mimosas, cassia, and mangrove, in quebracho wood, canaigre roots, cutch and gambier. The so-called "pseudo-tannins" (i.e., compounds which do not tan leather but possess other properties like tannins) are found in hops, tea, wine, fruits, etc.
Ordinary commercial "tannin," or "tannic acid," is a compound of one molecule of glucose with five of digallic acid. It is found in many plants, and is prepared commercially from the Turkish oak-galls and the Chinese sumac-galls. It exhibits all the characteristic properties which have been listed above for tannins in general and responds to all the characteristic reactions of a pyrogallol tannin. It is extensively used for the manufacture of blue-black ink, and in many technical processes.
Catechu tanninandcatechinare compounds of the catechol tannin type. The latter is obtained from acacia wood, mahogany wood, mimosa wood, etc. It is not a true tannin, since it does not convert hide into leather; but when heated to 120° or above, it is easily dehydrated, forming catechu tannin which is identical with that which is obtained directly from gambier and Bombay cutch (products made by evaporating water extracts from the bark ofvarious tropical trees). This latter is a true tannin, which is much used in dyeing and other technical processes.
"Quercitannic acid," obtained from oak bark, etc., is likewise a catechol tannin. It yields no glucose on hydrolysis.
A great many other tannins are known, and their possibilities for technical use in tanning, dyeing, etc., have generally been investigated; but so little has been learned about their composition and relation to the plant's own needs, that it seems unnecessary to discuss them in detail here.
Tannins are probably not direct products of photosynthesis. They are, however, elaborated in the green leaves of plants and translocated from there to the stems, roots, etc. Their close association with the photosynthetic carbohydrates has led many investigators to seek to establish for them some significant function as food materials, or as plastic substances in cell metabolism. Many conflicting views have been advanced, but a careful review of these leads inevitably to the conclusion that tannins probably do not serve in any significant way as food material. The glucose which is generally present in the tannin molecule may, of course, serve as reserve food material, but it seems probable that it functions as a constituent of the tannins only to assist in making them more soluble and hence more easily translocated through the plant tissues.
Some fungi, and perhaps other plants as well, can actually utilize tannins as food material under suitable conditions and in the absence of a proper supply of carbohydrates. But this does not prove that tannins can normally replace carbohydrates as food material for these species of plants.
There seems to be ample evidence that tannins are elaborated where intense metabolism is in progress, such as occurs in green leaves during the early growing season; in the rapid tissue formation which takes place after the stings of certain insects, producing galls, etc.; during germination, and as a result of any other unusual stimulation of metabolism. It may be, therefore, that tannins serve as safety accumulations of excessive condensations of formaldehyde, or other photosynthetic products, under such conditions. It seems certain that in all such cases tannins are the result of,and not (as some investigators have supposed) the causative agents for, the abnormally rapid metabolism.
It seems to be fairly well demonstrated that tannins are intermediate products for the formation of cork tissue. This may account for their common occurrence in the wood and bark of trees. Indeed, it has been shown that gallic and tannic acids are present in considerable proportions in those parts of the plant where cork is being formed. Further, that they bear direct relation to cork-formation has been demonstrated in two different ways. First, cork-like substances have been artificially produced by passing a stream of carbon dioxide through mixtures of formaldehyde with various tannic acids. Second, by various treatments of cork, decomposition compounds showing tannin-like properties may be obtained.
Some investigators have held that not only cork tissue but also other lignose, or cell-wall material, may be developed from tannins. Certain observations withSpirogyraseem to indicate that tannin may play an important part in the formation of new cell walls during conjugation, as cells which are ready to conjugate are rich in tannin, which gradually diminishes in quantity until it is practically absent at the time of spore-formation. There seems to be no evidence that tannins perform any such function as this in higher plants, however.
Again, tannins may play a very important part in pigment-formation. They are very similar in structure to the anthocyanin pigments, both being made up of practically identical decomposition units, the phenolic bodies. The disappearance of tannins during the process of ripening of fruits may be connected, in part at least, with the development of the brilliant red, blue, and yellow pigments which give such rich colors to the thoroughly ripe fruits.
Finally, certain of the tannins undoubtedly serve as protective agents to prevent the growth of parasitic fungi in fruits, etc. Recent investigations show that at least some of the varieties of fruits which are resistant to the attacks of certain parasitic diseases utilize tannins for this purpose. This protective effect may be accomplished in two different ways. Either the tannin actually serves as an antiseptic to prevent the growth of the parasitic fungus within the tissues of the host plant, or it assists in the development of a corky layer which "walls-off" the infected area and so prevents further spread of the disease through the tissue.Examples of both types of protective action have recently been reported.
It is obvious that the different forms of tannins may play different rôles in plant life, and the same tannin substance may possibly serve different purposes under different conditions.
The presence of tannins in fruits and the changes which they undergo during the ripening process cannot fail to attract attention to their biological significance in serving to protect the fruit from premature consumption as food by animals.
Tannins are of frequent occurrence in green fruits, imparting to them their characteristic astringent taste. They nearly always disappear as the fruit ripens. The fact that during the ripening process both sugars and fruit esters, as well as attractive surface pigments, are developed has led certain investigators to the conclusion that tannins serve as mother-substances for these materials in the green fruits and are converted into these attractive agencies during ripening. There is nothing in the chemical composition of tannins which indicates, however, that they are precursors of sugars or fruit esters, although (as has been pointed out) they may give rise to anthocyan pigments.
Further, recent researches concerning the tannin of persimmons (the best-known and most striking example of the phenomena under discussion) clearly show that the tannin is not actually used up during the ripening process; that instead it remains in the ripe fruit in practically undiminished quantity; but that when the fruit is ripe, the tannin is enclosed in certain special large cells or sacs, which are surrounded by an insoluble membrane, so that when the fruit is eaten by animals the astringent tannin, enveloped in these insoluble sacs, passes by the organs of taste of the animal without causing any disagreeable effects. This walling-off of the astringent tannins can be stimulated in partially ripe fruits by treating them with several different chemical agents, the simplest method being that of placing the unripe fruit in an atmosphere of carbon dioxide gas for a short period. The artificial "processing" of persimmons to render them edible for a longer period before they become naturally fully ripe and subject to decay is now a commercial enterprise. This process is of interest because ofits possible connection with the conversion of tannins into cork, under the influence of carbon dioxide gas, as mentioned in a preceding paragraph.
From these facts, it is apparent that in persimmons, and probably in other tannin-containing fruits, the process of natural selection has developed a mechanism for the secretion of tannin in green fruits, followed by a process for walling it off in harmless condition when the fruit is ripe, which serves most admirably to protect the fruit from consumption by animals before the enclosed seeds have fully developed their reproductive powers.