The facts that in the arrangement (B) the central carbon atom of the glycerol would be asymmetric, and that both lecithin and the glycero-phosphoric acid derived from it by hydrolysis are optically active, prove that formula (B) correctly represents the arrangement of that part of the lecithin molecule; and there is ample theoretical and experimental evidence to prove that the choline linkage is through the alcoholic (OH) group. Hence the formula for lecithin indicating the linkage as shown above is the correct one.
The fatty acids in the lecithin molecule may be different in lecithins from different sources, just as they are different in fats from different sources. Both oleic acid and a solid fatty acid have been found in the hydrolysis products of lecithin from leguminous seeds. In certain lupine seeds, the fatty acids present in the lecithin appear to be palmitic and stearic.
Phosphatides other than lecithin are common in plants. In these, various sugars replace part or all of the glycerol as the alcoholic part of the ester. Percentages of sugar varying from mere traces up to 17 per cent of the weight of material taken, have been found in the products of hydrolysis of phosphatides prepared from vetch seeds, potato tubers, plant pollens, and whole wheat meal.
Furthermore, betaine
betaine
and perhaps other vegetable amines (seeChapter XII) sometimes replace choline as the basic group in the phosphatides.
Bodies similar to the animal cerebrosides seem to occur in many plant tissues, since plant lipoids which yield no phosphorus when hydrolyzed have often been isolated. The sugar which constitutes the alcoholic portion of their structure appears to be galactose in every case which has been reported. Beyond this, little is known of the structure of these plant cerebrosides, as they are very difficult to prepare in pure form and not easily hydrolyzed.
Lipoids are so universally present in plant and animal tissues and so commonly found in those parts of the organism in which vital phenomena are most pronounced (brain, heart, embryo of egg, embryo of seeds, etc.), that it is evident that they must play some important rôle in the activity of living protoplasm. There is, as yet, however, no definite and certain knowledge of what this rôle is. Various theories concerning the matter have been put forward in recent years. For example, Overton, in 1901, presented the idea that every living cell is surrounded by a semi-permeable membrane consisting of lipoid material, which regulates the passage into and out of the cell of substances necessary to its metabolism and growth. Recent investigations by Osterhout and others indicate, however, that Overton's hypothetical lipoid membrane is not essential to a proper explanation of the migration into and out of the cell protoplasm of nutritive materials, etc. Other investigators have cited results which appear to indicate that lipoids play an important, but as yet unknown, part in the process of fat metabolism. Others go even further than this, and argue that since the extraordinary rapidity of the chemical changes which take place in plant protoplasm indicates the necessity of the presence there of exceedingly labile substances, and since both fats and proteins are relatively stable compounds, it is possible that the lipoids, which contain both nitrogenous and fatty acid groups, play an exceedingly important part in the metabolism processes. Bang, in particular, has pointed out (in 1911) that the lipoids are probably the most labile of all the components which constitute the colloidal system known as plant protoplasm. The importance of such considerations will be more apparent after therelation of colloidal phenomena to the activities of plant cell contents has been more fully discussed (seeChapter XVI).
Experimental studies of the physiological uses of lipoids have thus far been devoted almost exclusively to those of animal tissues. They have been seriously hampered by the difficulty of securing properly purified extracts of lecithin and similar lipoids. The same labile character which apparently makes them so important in the chemical changes in the cell makes them equally unstable compounds to work with in attempting to secure pure preparations for the purposes of experimental study. On this account, there is, as yet, no certain knowledge concerning their actual physiological uses. It is evident, however, that they have some really important rôle to play, which opens up a promising field for further study.
Abderhalden, E.—"Biochemisches Handlexikon, Band 3, Fette, Wachse, Phosphatide, Cerebroside, ..." 340 pages, Berlin, 1911.Hopkins, E.—"The Oil-Chemist's Handbook," 72 pages, New York, 1902.Leathes, J. B.—"The Fats," 138 pages,Monographson Biochemistry, London, 1913.Lewkowitsch,J.—"Chemical Technology and Analysis of Oils, Fats, and Waxes," Vol. I, 542 pages, 54 figs.; Vol. II, 816 pages, 20 figs.; and Vol. III, 406 pages, 28 figs., London, 1909.Maclean, H.—"Lecithin and Allied Substances," 206 pages,Monographson Biochemistry, London, 1913.Southcombe, J. E.—"Chemistry of the Oil Industries," 204 pages, 13 figs., London, 1918.
Abderhalden, E.—"Biochemisches Handlexikon, Band 3, Fette, Wachse, Phosphatide, Cerebroside, ..." 340 pages, Berlin, 1911.
Hopkins, E.—"The Oil-Chemist's Handbook," 72 pages, New York, 1902.
Leathes, J. B.—"The Fats," 138 pages,Monographson Biochemistry, London, 1913.
Lewkowitsch,J.—"Chemical Technology and Analysis of Oils, Fats, and Waxes," Vol. I, 542 pages, 54 figs.; Vol. II, 816 pages, 20 figs.; and Vol. III, 406 pages, 28 figs., London, 1909.
Maclean, H.—"Lecithin and Allied Substances," 206 pages,Monographson Biochemistry, London, 1913.
Southcombe, J. E.—"Chemistry of the Oil Industries," 204 pages, 13 figs., London, 1918.
Included in this group are all those substances to which the characteristic odors of plants are due, along with others similar in structure and possessing characteristic resinous properties. They have no such uniformity in composition as is exhibited by the oils which are included among the fats and waxes; but belong to several widely different chemical groups. Furthermore, there is no sharp dividing line between the essential oils and certain esters of organic acids on the one hand and the fats on the other. For example, if an aromatic fluid essence is a light fluid, non-viscid, and easily volatile, it is usually classed with the organic esters; denser liquid substances, of oily or waxy consistency, and with comparatively slight odor and taste are usually fats, while oils of similar physical properties but possessing strong characteristic odors are classed as essential oils, regardless of their chemical composition.
Included in this general class are compounds having a great variety of chemical structures; e.g., hydrocarbons, alcohols, phenols, organic sulfides and sulfocyanides, etc. Many of these compounds are crystalline solids at ordinary temperatures, but melt to oily fluids at higher temperatures. The characteristic property which assigns any given plant extract to this group is that it has a strikingly characteristic odor or taste, often accompanied by some definite physiological effect, or medicinal property.
These compounds may be either secretions or excretions of plants, sometimes normally present in the healthy tissue, and sometimes produced as the result of injury or disease.
The essential oils and the resins often occur associated together in the plant; or, the resins may develop from the oily juice of the plant after exposure to the air.
These may be divided, according to their chemical composition, into two major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated andsulfurettedoils.
Theterpenesare of three different types, namely: (a) the hemiterpenes, C5H8, unsaturated compounds of the valerylene series, of whichisoprene(found in crude rubber) is the best-known example; (b) the terpenes proper, C10H16, which constitute the major proportion of the whole group; and (c) the polyterpenes (C5H8)n, of whichcolopheneandcaoutchoucare the most common examples.
Eleven different terpenes having the formula C10H16have been isolated from various plant juices, and their molecular arrangement carefully worked out. The following three examples will serve as typical of the general structural arrangement of these hydrocarbons:
A discussion of the evidence which supports these formulas as properly represented the molecular arrangements of the various isomeric forms would be out of place here, as its only particular interest is in connection with the medicinal effects of the different compounds. It is clear, however, that they are six-membered hydrocarbon rings, with additional hydrocarbon groups attached to one or more of the carbon atoms in the ring.
Different modifications, or varieties, of the terpenes constitute the main proportions of the oils of turpentine, bergamot, lemon, fir needles, eucalyptus, fennel, pennyroyal, etc.
Theoxygenated essential oilsmay be either alcohols, aldehydes, ketones, acids, esters, or phenols, derived from either five-membered or six-membered closed-ring hydrocarbons. They are usually present in the plant oil in mixtures with each other or with a terpene. Since most of them have pronounced physiologicalor medicinal properties, their structure has been well worked out, in most cases; but it seems to be hardly worth while to present these matters in detail here, as they are of interest chiefly on account of their medicinal properties rather than their botanical functions.
Borneol, C10H17OH, andmenthol, C10H19OH, are typicalalcohols. The latter is a crystalline substance, which melts at 42°, which is present in peppermint oil, both as the free alcohol and as an ester of acetic acid.
Amyl acetate, CH3·COOC5H11, and linalyl acetate, CH3·COOC10H17, the latter occurring in the oils of lavender and bergamot, are typical esters classed as essential oils.
As examples of thealdehydeoils, benzoic aldehyde, C6H5CHO, "oil of bitter almonds," and cinnamic aldehyde, C6H5CH=CHCHO, found in the oils of cinnamon and cassia, may be cited.
Camphor, C10H16O, is aketone, having the following structural formula:
There are a considerable number of essential oils which arephenols.Thymol, C6H3·(CH3)·(C3H7)·OH, in oil of thyme, and carvacrol, its isomer, in oil of hops, are familiar examples.
Coumarin, the anhydride of cinnamic acid,; is an example of an acid substance which is classed as an essential oil, even though it is a solid at ordinary temperatures. It has an odor and flavor similar to that ofvanillin, the essential flavoring material of the vanilla bean, and is often used as a substitute for the latter in the preparation of artificial flavoring extracts.
Of theessential oils containing sulfur, there are two common examples; oil of mustard, allyl isosulfocyanide, C3H5NCS, and oil of garlic, allyl sulfide (C3H5)2S. The latter is present in onions, garlic, water cress, radishes, etc., the difference in flavor of these vegetables being due to the fact that the allyl sulfide is united with other different groups in the glucoside arrangement,in the different plants. Similarly, mustard oil is not present in mustard seeds as such, but as a glucoside which, when hydrolyzed by the enzymemyrosinwhich is always present in other cells of the same seeds, yields C3H5NCS, KHSO4, and C6H12O6.
The resins were formerly supposed to be the mother substances from which the terpenes are derived. It is now known, however, that they are the oxidation products of the terpenes. Their exact structure is still a matter of some uncertainty, as their peculiar "resinous" character makes them very difficult to study by the usual methods of chemical investigations.
Resins are divided into two classes: (a) the balsams, and (b) the solid or hard resins. Canada balsam and crude turpentine are familiar examples of the first class. They consist of resinous substances, dissolved in or mixed with fluid terpenes. Ordinary resin, orcolophony, consists chiefly of a monobasic acid having the empirical formula C20H30O2, known as sylvinic acid, whose exact structure is not known. Its sodium salt is used as the basis for cheap soaps.
The hard resins are amorphous substances of vitreous character, which consist of very complex aromatic acids, alcohols, or esters, combined with other complicated structures, known asresenes, whose definite chemical nature is not yet known. Among the hard resins are many substances which are extensively used in the manufacture of varnishes, such as copal, amber, dammar, sandarach, etc.
There are also resinous substances, such as asafœtida, myrrh, gamboge, etc., which are mixtures of gums (seeChapter VI) and true resins. Some of these have considerable commercial value for medicinal or technical uses.
No theory has yet been advanced concerning the possibility of the use of essential oils and resins by plants in their normal metabolic processes. The very great diversity in their chemical nature makes it impossible that they should all be considered as havingthe same physiological function, if indeed any of them actually have any such function.
It is evident that those aromatic compounds which occur as normal secretions of plants and which give to the plants their characteristic odors may act either as an attraction to animals which might utilize the plants as food and so serve to distribute the seed forms, or as a repellent to prevent the too rapid destruction of the leaves, stems, or seeds of certain species of plants whose slow-growing habits require the long-continued growth of these portions of the plant for the perpetuation of the species. The presence of these compounds in larger proportions in those species of conifers, etc., which grow in tropical regions, in competition with other rapid-growing vegetation, suggests the latter possibility. It must be admitted, however, that their presence in such cases may be the result of climatic conditions, as indicated by the fact that most spice plants are tropical in habit, rather than the result of their protective influence in the struggle for survival during past ages.
Many of the oils and resins which are secreted as the result of injury by disease or wounds have marked antiseptic properties and undoubtedly serve to prevent the entrance into the injured tissue of destructive organisms.
But apart from these possible protective influences which may have had an important effect upon the preservation and perpetuation of the species of plants which secrete them, there is no known biological necessity for the presence of these aromatic substances in plants.
Abderhalden, E.—"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele,Harze,Kautschuk," 822 pages, Berlin, 1912.Allen'sCommercial Organic Analysis, Vol. IV, "Resins, Rubber, Guttapercha, and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th ed.).Heusler,F.,trans. by Pond, F. J.—"The Chemistry of the Terpenes," 457 pages, Philadelphia, 1902.Parry, E. J.—"The Chemistry of Essential Oils and Perfumes," 401 pages, 20 figs., London, 1899.
Abderhalden, E.—"Biochemisches Handlexikon, Band 7, Gerbstoffe, Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele,Harze,Kautschuk," 822 pages, Berlin, 1912.
Allen'sCommercial Organic Analysis, Vol. IV, "Resins, Rubber, Guttapercha, and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th ed.).
Heusler,F.,trans. by Pond, F. J.—"The Chemistry of the Terpenes," 457 pages, Philadelphia, 1902.
Parry, E. J.—"The Chemistry of Essential Oils and Perfumes," 401 pages, 20 figs., London, 1899.
We come, now, to the consideration of the characteristically nitrogenous compounds of plants. None of the groups of compounds which have been considered thus far have, as a group, contained the element nitrogen. This element is present in the chlorophylls and in certain other pigments, but not as the characteristic constituent of the molecular structure of the group of compounds, nor do these compounds serve as the source of supply of nitrogen for the plant's needs.
The characteristic nitrogen-containing compounds may all be regarded as derived from ammonia, or ammonium hydroxide, by the replacement of one or more hydrogen atoms with organic radicals of varying type and complexity. If the group, or groups, which be considered as having replaced a hydrogen atom in ammonia, in such compounds, is an alkyl group, the compound is strongly basic in character and is known as anamine; whereas if the replacing group is an acid radical, the resulting compound may be neutral (known asacid amides), or weakly acid (known asamino-acids) in type. Compounds of the first type constitute thevegetable bases; while those of the second type are theproteins.
The vegetable bases may be divided into three groups. These are (a) theplant amines, which are simple open-chain amines; (b) thealkaloids, which are comparatively simple closed-ring amines, containing only one nitrogen atom in any single ring; and (c) thepurine bases, which are complex compounds containing a nucleus with four carbon atoms and four nitrogen atoms arranged alternately to form a double-ring group.
The simple amines bear the relation to ammonia, or ammonium hydroxide, represented by the following formulas, in which the R indicates any simple alkyl radical:
The simple amines which occur in animal tissues are known as "ptomaines" and "leucomaines." The ptomaines are all decomposition products resulting from the putrefactive decay of proteins caused by moulds or bacteria. Some of these are highly toxic, producing the so-called "ptomaine-poisoning"; while others are wholly innocuous. They are all simple amines. Putrescine, di-amino butane, NH2·CH2·CH2·CH2·CH2·NH2, and cadaverine, di-amino pentane, HN2·(CH2)5·NH2, are common non-toxic ptomaines, resulting from the decay of meat. Neurine, trimethyl-ethylene ammonium hydroxide, (CH3)3(C2H3)·NOH, is a violently poisonous ptomaine produced in the decay of fish. Amines of similar structure to these are occasionally found in living animal tissues. Such compounds are known asleucomaines, to distinguish them from theptomaines, which are found only in dead material.
Corresponding in structure and properties to these amines of animal origin, there is a series of basic substances, found in many plants, known as theplant amines. The following are common examples:
Trimethyl amine, (CH3)3N, is a very volatile compound, found in the flowers of several species of the Rose family, the leaves of certain weeds, etc. When crushed, these tissues give off a very fetid odor, which is due to this amine.
Choline,muscarine, andbetaineare plant amines which are closely related to each other and to neurine (the toxic ptomaine) in composition and structure, as shown in the following formulas:
Choline and betaine are non-toxic; while muscarine and neurine are violent poisons.
Choline and muscarine occur in certain toadstools. Betaine and choline often occur together in the germs of many plants. Betaine is found in the beet root and the tubers of Jerusalem artichoke. Choline occurs alone in the seeds and fruits of many plants, sometimes as the free amine, but more often as a constituent of lecithin (seepage 141).
Phenyl derivatives of simple amines are sometimes found in plants.Hydroxyphenylethyl amine,
found inergot, andhordein,
found in barley, are examples. The former has marked medicinal properties.
There is no known physiological use for these simple amines in plants. By some investigators, they are regarded as intermediate products in the synthesis or decomposition of proteins; but it would seem that if this were a normal procedure, these amines would occur in varying proportions in all plants, under different conditions of metabolism, instead of in practically constant proportions in only a few species, as they do.
These are a group of strong vegetable bases whose nitrogen atom is a part of a closed-ring arrangement.
As a rule, alkaloids are colorless, crystalline solids, although a few are liquids at ordinary temperatures. They are generally insoluble in water, but easily soluble in organic solvents. Being strong bases, they readily form salts with acids, and these salts are usually readily soluble in water.
Alkaloids are usually odorless; although nicotine, coniine, and a few others, have strong, characteristic odors. Most of them have a bitter taste, and many of them have marked physiological effects upon animal organisms, so that they are extensively used as narcotics, stimulants, or for other medicinal purposes.
Most of the alkaloids contain asymmetric carbon atoms and are, therefore, optically active, usually levorotatory, although a few are dextrorotatory.
The alkaloids are precipitated out of their solutions by various solutions of chemical compounds, known as the "alkaloidal reagents": iodine dissolved in potassium iodide solution gives a chocolate-brown precipitate; tannic acid, phosphotungstic acid, phosphomolybdic acid, and mercuric iodide solutions give colorless, amorphous precipitates; while gold chloride and platinic chloride solutions give crystalline precipitates, many of which have sharp melting points and can be used for the identification of individual alkaloids. There are a great many specific color reactions for individual alkaloids, which are important to toxicologists and pharmacists, but which it would not be desirable to consider in detail here.
The alkaloids are conveniently divided into groups, according to the characteristic closed-ring arrangements which they contain. The several closed-ring arrangements which are found in common alkaloids, and upon which their grouping is based, may be illustrated by the following formulas:
The common alkaloids are distributed in the several groups as follows:
Pyrridine—piperidine group; piperine, coniine, nicotine.Pyrrolidine group; hygrine and stachydrine.Tropane group; atropine, hyoscine, cocaine, lupinine.Quinoline group; quinine, cinchonine, strychnine, brucine.Isoquinoline group; papaverine, hydrastine, morphine, codeine, berberine.
The composition and properties of the individual alkaloids have been extensively studied, because of their medicinal uses. As they have no known metabolic use to the plants which elaborate them, it will not be worth while to consider all of these investigations in detail here. The following facts with reference to certain typical members of each group will serve to illustrate the general constitution and properties of the alkaloids.
Piperine, C17H19O3, is found in black peppers. Its constitution is represented by the following formula, the group which is united to the piperidine ring, in this case, being piperic acid:
Coniine, C8H17N, is found in the umbelliferous plant,Conium maculatum. Structurally, it is a propyl-piperidine, represented by the following formula:
Coniine
Nicotine, C10H14N2, is the alkaloid of tobacco leaves. It is an extremely poisonous, oily liquid, with a strong odor and a burning taste. Its structural formula shows it to contain both a pyrridine ring and a pyrrolidine ring, linked together thus
Nicotine
Hygrine, C7H13NO, from coca leaves, is an acetic acid salt of pyrrolidine, represented by the following formula:
Hygrine
Atropineandhyoscyamine, C17H23NO3, are optical isomers. Atropine is an extremely poisonous, white crystalline compound, which is obtained from deadly nightshade and henbane, and used in medicine, in minute doses, as an agent for reducing temperature in acute cases of fevers. Structurally, it is a tropic acid ester of tropane, represented by the following formula:
Atropine
Cocaine, C17H21NO4, is found in coca leaves. It is a white crystalline solid, which is largely used as a local anæsthetic for minor surgical operations. Its structural formula is
Cocaine
It is, therefore, a di-ester of acetic and benzoic acids with tropane.
Cinchonine, C19H22N2O, andquinine, C20H24N2O2, are alkaloids found in cinchona bark. They are white crystalline solids, which are extensively used in medicine. They have been shown to contain a quinoline group combined with modified piperidine groups, as represented in the following formulas:
Cinchonine
Strychnine, C21H22N2O2,brucine, C21H20(OCH3)N2O2, andcurarineare three alkaloids which are present in the seeds of severalspecies ofStrychnos. They are all highly poisonous. Beyond the fact that when they are hydrolyzed they yield quinoline and indole, their composition is unknown.
Morphine, C17H19NO3, is the chief alkaloid of opium, which is the dried juice of young pods of the poppy. Both the alcoholic solution of opium (known as "laudanum") and morphine itself are extensively used in medicine as narcotics to deaden pain. Morphine has an exceedingly complex structure, being a combination of an isoquinoline and a phenanthrene nucleus, which is probably correctly represented by the following formula:
Morphine
Codeine, C17H18(OCH3)NO2, which is also found in opium, is a methyl derivative of morphine.Papaverine,laudanosine,narcotine, andnarceineare four other alkaloids found in opium. They each contain an isoquinoline nucleus, combined by one bond to a benzene ring, with one or more methyl groups and three or more methoxy (OCH3) groups attached at various points around the three characteristic rings. The following formula for laudanosine will illustrate their structure:
Codeine
The above discussions of the composition of typical alkaloids clearly indicate the extreme complexity of their molecular structure. It is generally supposed that they are formed by the decomposition of proteins. But they are developed in only a few particular species of plants and are always present in these plants in fairly constant quantities. Hence, it appears that, in these species, the production of alkaloids is in some way definitely connected with protein metabolism; but it is certain that this is not a common relationship, as it is manifested by such a limited number of species of plants, and there is absolutely no knowledge as to its character and functions. Some authorities prefer to regard the alkaloids as waste-products of protein metabolism; but here, again, it is difficult to understand why such products should result in certain species of plants and not in others.
This is a group of compounds, widely distributed in both plant and animal tissues, all of which are derivatives of the compound known aspurine, C5H4N4. All of the naturally occurring compounds of this group may be regarded as derived from purine, either by the addition of oxygen atoms, or by the replacing of one or more of its hydrogen atoms with a methyl (CH3) group or an amino (NH2) group. The following structural formula represents the arrangement of the purine nucleus, the numbers being used to designate the nitrogen or carbon atoms to which the additional atoms, or groups, are attached in the more complex compounds of the group. In purine itself, the four hydrogen atoms are attached in the 2, 6, 7, and 8 positions.
The double bonds, in each case except those between the 4 and 5 carbon atoms, are easily broken apart and readjusted, so that other atoms or groups can be attached to any atom in the nucleus except the 4 and 5 carbon atoms. In all of the statements with reference to the structure of the purine bases, the term "oxy" is usedto mean an oxygen atom attached by both its bonds to one of the carbons in the nucleus, instead of its customary use to mean the monovalent OH group replacing a hydrogen, as in the case of all other nomenclature of organic compounds. With this understanding, reference to the numbered nucleus formula above will make plain the structure of all of the purine bases which are included in the following list:
Hypoxanthine, C5H4N4O,= 6-monoxypurine.Xanthine, C5H4N4O2, = 2,6-dioxypurine.Uric acid, C5H4N4O3, = 2,6,8-trioxypurine.Adenine, C5H3N4NH2, = 6-aminopurine.Guanine, C5H3N4ONH2, = 2-amino-6-oxypurine.Theobromine, C5H2N4O2(CH3)2= 3,7-dimethyl-2,6-dioxypurine, or dimethyl xanthine.Theophylline, C5H2N4O2(CH3)2= 1,3-dimethyl-2,6-dioxypurine.Caffeine, C5HN4O2(CH3)3= 1,3,7-trimethyl-2,6-dioxypurine, or trimethyl xanthine.
In order to make these structural relationships quite clear, the following formulas for uric acid and for caffeine are presented as typical examples:
Uric acidis found in the excrement of all animals; in the urine of mammals, and in the solid excrement of birds and reptiles. It is not known to occur in plants.
Xanthineandhypoxanthineoccur in animal urine, and also in the tissues of both plants and animals.
Adenineandguanineare constituents of all nucleic acids (see below) and, hence, are found in all plant and animal tissues. Guanine is the chief constituent of the excrement of spiders, and is found also in Peruvian guano. It is also a constituent of the scales of fishes.
Caffeine,theophylline, andtheobromineare not found in animal tissues, but are fairly widely distributed in plants. Caffeine and theobromine are the active constituents of tea leaves and coffeeseeds and are found also in cacao beans and kola nuts. The use of these three compounds in the metabolism of the plants which elaborate them is wholly unknown. They are not so directly related to protein metabolism as are the other purine bases.
The purine bases, other than the three mentioned in the preceding paragraph, are undoubtedly intermediate products in protein metabolism. In animals, they constitute a large proportion of the waste-products from the use of proteins in the body. It is not clear that there are similar waste-products in plant metabolism, however. In both plants and animals, the purine bases which are a part of the nucleic acids undoubtedly play an important and essential part in growth, since they form the major proportion of the nucleus, from which all cell-division proceeds.
These compounds do not occur free in plants; but since they are constituent groups in the plant nucleic acids (see below), a brief explanation of their composition is desirable. They are nitrogenous bases, similar to, but somewhat simpler than, the purine bases. Their general composition and structural relationships are illustrated by the following typical formulas:
The nuclei of cells are composed almost wholly of complex organic salts, in whichproteinsconstitute the basic part andnucleic acidsthe acid part. These salts, or esters, are known under the general name "nucleoproteins." The composition of the proteins is discussed in detail in the following chapter, and it seems desirable to present a brief discussion of the constitution of the nucleic acids here; although they are essentially acids rather than vegetable bases.
The nucleic acids are complex compounds consisting of a carbohydrate, phosphoric acid, two purine bases, and two pyrimidine bases. So far as is known, all animal nucleic acids are identical and all plant nucleic acids are identical; but those of plant origin differ from those found in animal cells in the character of the carbohydrate and that of one of the pyrimidine bases which are present in the molecule, as shown in the following tabulation of their composition:
Animal nucleic acidPlant nucleic acidPhosphoric acidPhosphoric acidHexose (levulose)Pentose (d-ribose)GuanineGuanineAdenineAdenineCytosineCytosineThymineUracil
The structure of the plant nucleic acid may be represented by the following formula:
That this is probably a correct representation of the general arrangement in this compound, is indicated by the fact that by different methods of hydrolysis it is possible to split off either the purine and pyrimidine bases, leaving a carbohydrate ester of phosphoric acid; or the phosphoric acid, leaving carbohydrate combinations with the nitrogenous bases.
Nucleic acid, prepared from animal glands which contain large proportions of it, is a white powder, which is insoluble in water, but when moistened forms a slimy mass. It is almost insoluble in alcohol, but dissolves readily in alkaline solutions, forming a colloidal solution which readily gelatinizes (seeChapteron Colloids). Solutions of nucleic acids are optically active, probably because of the carbohydrate constituents.
From their structure and properties, it is apparent that nucleic acids are on the border line between carbohydrates, plant amines, and proteins. They undoubtedly play an important part, both in cell-growth and in the synthesis of proteins from carbohydrates and ammonium compounds.
Barger, Geo.—"The Simpler Natural Bases," 215 pages,Monographson Biochemistry, London, 1914.Fischer, E.—"Untersuchungen in der Puringruppe, 1882-1906," 608 pages, Berlin, 1907.Henry, T. A.—"The Plant Alkaloids," 466 pages, Philadelphia, 1913.Jones, W.—"The Nucleic Acids," 118 pages,Monographson Biochemistry, London, 1914.Pictet, A.—"La Constitution Chimique des Alcaloides Vegetaux," 421 pages, Paris, 1897 (2d ed.).Vaughan, V. C.andNovy, F. G.—"Ptomaines, Leucomaines, Toxins and Antitoxins," 604 pages, Philadelphia, 1896, (3d ed.).Winterstein, E.andTrier, G.—"Die Alkaloide," 340 pages, Berlin, 1910.
Barger, Geo.—"The Simpler Natural Bases," 215 pages,Monographson Biochemistry, London, 1914.
Fischer, E.—"Untersuchungen in der Puringruppe, 1882-1906," 608 pages, Berlin, 1907.
Henry, T. A.—"The Plant Alkaloids," 466 pages, Philadelphia, 1913.
Jones, W.—"The Nucleic Acids," 118 pages,Monographson Biochemistry, London, 1914.
Pictet, A.—"La Constitution Chimique des Alcaloides Vegetaux," 421 pages, Paris, 1897 (2d ed.).
Vaughan, V. C.andNovy, F. G.—"Ptomaines, Leucomaines, Toxins and Antitoxins," 604 pages, Philadelphia, 1896, (3d ed.).
Winterstein, E.andTrier, G.—"Die Alkaloide," 340 pages, Berlin, 1910.
The proteins are the most important group of organic components of plants. They constitute the active material of protoplasm, in which all of the chemical changes which go to make up the vital phenomena take place. Combined with the nucleic acids, they comprise the nucleus of the cell, which is the seat of the power of cell-division and, hence, of the growth of the organism. Germ-cells are composed almost exclusively of protein material. Hence, it is not an over-statement to say that proteins furnish the material in which the vital powers of growth and repair and of reproduction are located. A recognition of their importance is reflected in the use of the name "protein," which comes from a Greek word meaning "pre-eminence," or "of first importance."
In addition to the proteins which constitute the active protoplasm, plants also contain large amounts of reserve, or stored, proteins, especially in the seeds. In the early stages of growth, the proteins are present in largest proportions in the vegetative portions of the plant; but as maturity approaches, a considerable proportion of the protein material is transferred to the seeds.
The plant proteins are fairly uniform in their percentage composition. The analyses of some sixteen different plant proteins show the following maximum limits of percentages of the different chemical elements which they contain: Carbon, 50.72-54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03; oxygen, 20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more widely, both in percentage composition and in properties, than do those of plant origin.
Protein molecules are very large and, in the case of the so-called "conjugated proteins" in particular, their structure is very complex. The molecular weight of some of the proteins has been determined directly, in the case of those particular ones which can be prepared in proper form for the usual determination of molecular weight by the osmotic pressure method; and has been computed for various others, from the percentage of sulfur found on analysis, or (in the case of the hæmoglobin of the blood) from the proportion by weight of oxygen absorbed. From these determinations and computations, the following formulas for certain typical proteins have been calculated: for zein (from Indian corn), C736H1161N184O208S3; for gliadin (from wheat), C685H1068N196O211S5; for casein (from milk), C708H1130N180O224S4P4; for egg-albumin, C696H1125N175O220S8. These few examples will serve to illustrate the enormous size and complexity of the protein molecule. The conjugated proteins are still more complex than the simple proteins whose formulas are here presented.
Fortunately for the purposes of the study of the chemistry of the proteins, however, it has been found that most of the common plant proteins, known as the "simple proteins," can easily be hydrolyzed into their constituent unit groups, which are the comparatively simple amino-acids, whose composition and properties are well understood. A study of the results of the hydrolysis of some twenty common plant proteins has shown that it is rarely possible to recover the amino-acids in sufficient quantities to account for a full 100 per cent of the material used, the actual percentage of amino-acids recovered usually totaling from 60 to 80 per cent. The remaining material is supposed to be also composed of amino-acids which are linked together in some arrangement which is not broken apart by any method of hydrolysis which has yet been devised. This view is borne out by the fact that substances which exhibit all the characteristic properties of proteins have been artificially synthetized, by using only amino-acid compounds. Animal proteins often show a much larger proportion of unhydrolyzable material than do plant proteins.
The products of hydrolysis of the common simple proteins are all amino-acids. These are ordinary organic acids with one (or more) of the hydrogen atoms of the alkyl group replaced by a —NH2(or sometimes by a —NH—) group. They may be regarded as ammonia, NH3, with one of its hydrogen atoms replaced by an acid radical; or as the acid with one of its hydrogens replaced by the NH2group. For example, an amino-acid derived from acetic acid, CH3·COOH, is glycine, or amino-acetic acid, CH2NH2·COOH; from propionic acid, CH3·CH2·COOH, there may be obtained either α-amino-propionic acid,CH3·CHNH2·COOH, or β-amino-propionic acid, CH2NH2·CH2·COOH, etc.
All of the amino-acids which result from the hydrolysis of proteins are α-amino-acids, that is to say, the NH2group is attached to the α-carbon atom, i.e., the one nearest to the COOH group. Hence, the general formula for all the amino-acids which are found in plants is R·CHNH2·COOH.
These amino-acids contain both the basic NH2group and the acid COOH group. For this reason, they very easily unite together, in the same way that all acids and bases unite, to form larger molecules, the linkage taking place between the basic NH2group of one molecule and the acid COOH group of the other, as indicated by the following equation: