Trioses(C3H6O3)Tetroses(C4H8O4)Aldose—Glyceric aldehyde, or glyceroseAldoses—d- andl-Erythrose,l-ThreoseKetose—DioxyacetonePentoses(C5H10O5)Methyl Pentoses(C6H12O5)Aldoses—d- andl-ArabinoseAldoses—Rhamnosed- andl-XyloseFucosel-RiboseRhodeosel-LyxoseChinovoseHexoses(C6H12O6)Mannitol seriesDulcitol seriesAldoses—d- andl-Glucosed- andl-Galactosed- andl-Mannosed- andl-Talosed- andl-Gulosed- andl-Idosed-Altrosed-AlloseKetoses—d-Fructosed-Tagatosed-Sorbose
Heptoses(C7H14O7)Octoses(C8H16O8)Nonoses(C9H18O9)GlucoheptoseGluco-octoseGlucononoseMannoheptoseManno-octoseMannononoseGalactoheptoseGalacto-octosePersueloseSedoheptose
The hexoses are by far the most important group of monosaccharides. They are undoubtedly the first products of photosynthesis, and all the other carbohydrates may be considered to be derived from them by condensation. Because of their biochemical significance and their immense importance as the fundamental substances for all plant and animal energy-producing materials, the following detailed studies of their chemical composition and molecular configuration are fully warranted.
That all the hexoses contain five alcoholic groups is proved by the experimental evidence that each one forms a penta-ester, by uniting with five acid radicals, when treated with mineral or organic acids under proper conditions. Thus, glucose penta-acetate, penta-nitrate, penta-benzoate, etc., have all been prepared. The presence of the aldehyde group is proved by the fact that all aldohexoses have been converted, by gentle oxidation, into pentaoxy-monobasic acids, and the ketohexoses broken down into shorter chain compounds by similar gentle oxidations; these reactions being characteristic of compounds containing an aldehyde and a ketone group respectively. This experimental evidence establishes the nature of the characteristic groups in the molecule, in each case.
The molecular configurations illustrated in the following table are those suggested by Emil Fischer, as a result of his exhaustivestudies of the chemical constitution of the various carbohydrates. There is, of course, no thought that the printed formulas here presented accurately represent the actual relationships in space of the different groups; but there is fairly conclusive evidence that the variations in special groupings in the different sugars are properly referable to the particular asymmetric carbon atoms as indicated in the several formulas as presented.
Reference will be made in subsequent paragraphs to the probable chemical constitution of the monosaccharides other than hexoses; but the above discussion of the structure of the hexoses will serve as a sufficient introduction to the study of the composition of the common carbohydrates.
Specific Rotatory Power.—All soluble carbohydrates, since they contain asymmetric carbon atoms, with the consequent larger groups on one side of the molecule than the other, rotate the plane of polarized light when it passes through a solution of the carbohydrate in question. The amount of the rotation depends upon the nature of the carbohydrate, the concentration of the solution, and the length of the column of solution through which the ray of polarized light passes. But the same definite amountof the same sugar, dissolved in the same volume of water, and placed in a tube of the same length, will always cause the same angular deviation, or rotation, of the plane in which the polarized light which passes through it is vibrated. In other words, the same number of molecules of the optically active substance in solution will always produce the same rotatory effect. This is called the specific rotatory power of the substance in question. It is expressed as the number of degrees of angular deviation of the plane of polarized light caused by a column of the solution exactly 200 mm. in length, the concentration of the solution being 100 grams of substance in 100 cc. at a temperature of 20° C. Actual determinations of specific rotatory power are usually made with solutions more dilute than this standard, and the observed deviation multiplied by the proper factor to determine the effect which would be produced by the solution of standard concentration. If the direction of the deviation is to the right (i.e., in the direction in which the hands of the clock move) it is spoken of as "dextro" rotation and is indicated by the sign +, or the letterd; while if in the opposite direction, it is called "levo" rotation and indicated by the sign -, or the letterl. For example, the specific rotation of ordinary glucose is +52.7°; of fructose, -92°; of sucrose, +66.5°.
Reducing Action.—All of the hexose sugars are active reducing agents. This is because of the aldehyde group which they contain. Many of the common heavy metals, when in alkaline solutions, are strongly reduced when boiled with solutions of the hexose sugars. Alkaline copper solutions yield a precipitate of red cuprous oxide; ammoniacal silver solutions give silver mirrors; alkaline solutions of mercury salts are reduced to metallic mercury, etc. Any sugar which contains a potentially active aldehyde group will exhibit this reducing effect and is known as a "reducing sugar." In some of the di- and tri-saccharides, the linkage of the hexose components together is through the aldehyde group, in such a way that it loses its reducing effect; such sugars are known as "non-reducing." Advantage is taken of this property for both the detection and quantitative determination of the "reducing sugars." A standard alkaline copper solution of definite strength, known as "Fehling's solution," is added to the solution of the sugar to be tested and the mixture boiled, when the characteristic brick-red precipitate appears. If certain standardconditions of volume of solutions used, length of time of boiling, etc., are observed, the quantity of cuprous oxide precipitated bears a definite ratio to the amount of sugar which is present, so that if the precipitate be filtered off and weighed under proper conditions, the weight of sugar present in the original solution can be calculated. The proper conditions for carrying on such a determination and tables showing the amounts of the various "reducing sugars" which correspond to the weight of cuprous oxide found, are given in all standard text-books dealing with the analysis of organic compounds.
Fermentability.—The common hexoses are all easily fermented by yeast, forming alcohol and carbon dioxide, according to the equation
C6H12O6= 2C2H5OH + 2CO2.
The importance and biochemical significance of this reaction will be considered in detail in connection with the discussions of the relation of molecular configuration to biochemical properties (seepage 56) and the nature of enzyme action (seepage 194).
Formation of Hydrazones and Osazones.—Another property of the hexoses which is due to the presence of an aldehyde group in the molecule, is that of forming addition products with phenyl hydrazine, known as "hydrazones" and "osazones." For example, glucose reacts with phenyl hydrazine in acetic acid solution, in two stages. The first, which takes place even in a cold solution may be represented by the equation
C6H12O6+C6H5·NH·NH2=C6H12O5:N·NH·C6H5+H2O.GlucosePhenyl-hydrazineGlucose-hydrazone
The structural relationships involved may be represented as follows:
The hydrazones of the common sugars, with the exception of the one from mannose, are colorless compounds, easily soluble inwater. Hence, they do not serve for the separation or identification of the individual sugars. But if the solution in which they are formed contains an excess of phenyl hydrazine and is heated to the temperature of boiling water for some time, the alcoholic group next to the aldehyde group (the terminal alcohol group in ketoses) is first oxidized to an aldehyde and then a second molecule of phenyl hydrazine is added on, as illustrated above, forming a di-addition-product, known as an "osazone." The osazones are generally more or less soluble in hot water, but on cooling they crystallize out in yellow crystalline masses each with definite melting point and crystalline form. All sugars which have active aldehyde groups in the molecule form osazones. These afford excellent means of identification of unknown sugars, or of distinguishing between sugars of different origin and type.
Glucose, mannose, and fructose all form identical osazones. This is because the structure of these three sugars is identical except for the arrangement within the two groups at the aldehyde end of the molecule (see formulas onpage 44). Since it is to these two groups that the phenyl hydrazine residue attaches itself, it follows that the resulting osazones must be identical in structure and properties. All other reducing sugars yield osazones of different physical properties.
When an osazone is decomposed by boiling with strong acids, the phenyl hydrazine groups break off, leaving a compound containing both an aldehyde and a ketone group. Such compounds are known as "osones." The osones from glucose, mannose, and fructose are identical. By carefully controlled reduction, either one of the C=O groups of the osone may be changed to an alcoholic group, producing thereby one of the original sugars again. Hence, it is possible to start with one of these sugars, convert it into the osone and then reduce this to another sugar, thereby accomplishing the transformation of one sugar into another isomeric sugar.
Formation of Glucosides.—By treatment with a considerable variety of different types of compounds, under proper conditions, it is possible to replace one of the hydrogen atoms of the terminal alcoholic group of the hexose sugars with the characteristic group of the other substance, forming compounds known, respectively, as glucosides, fructosides, galactosides, etc. The structural relation of methyl glucoside to glucose, for example, may be illustrated as follows:
A general formula for glucosides is R·(CHOH)5·CHO; and the R may represent a great variety of different organic radicals (see the chapters dealing with Glucosides and with Tannins). When the glucosides are hydrolyzed, they yield glucose and the hydroxyl compound of the radical with which it is united. All the statements which have been made with reference to glucosides, apply equally well with reference to fructosides, galactosides, mannosides, etc.
It is possible, by various laboratory processes, to replace additional hydrogen atoms in the glucose molecule with the same or other organic radicals, thus producing glucosides containing two or more R groups; but most of the natural glucosides contain only one other characteristic group.
Oxidations.—When the hexoses are oxidized they give rise to three different types of acids, depending upon the conditions of the oxidation and the kind of oxidizing agent used. With glucose, for example, the relationships involved may be illustrated as follows:
An important property of the acids of thegluconictype is that when heated with pyridine or quinoline to 130°-150° they undergo a molecular rearrangement whereby the acid corresponding to an isomeric sugar is produced. For example, gluconic acid, under these conditions, becomes mannonic acid, which can be reduced to mannose. The process is reversible; mannose can be converted to mannonic acid, thence to gluconic acid, thence to glucose. Similarly, galactonic acid can be converted into talonic acid, and this to talose, and this process is reversible. These facts afford another means of conversion of one sugar into another.
From the standpoint of physiological processes,glucuronic acidis the most interesting and important oxidation product of glucose. It is often found in the urine of animals, as the result of the partial oxidation of glucose in the animal tissues. Normally, glucose is oxidized in the body to its final oxidation products, carbon dioxide and water. But when many difficultly oxidizable substances, such as chloral, camphor, turpentine oil, aniline, etc., are introduced into the body, the organism has the power of combining these with glucose to form glucosides. These so-called "paired" compounds are then oxidized to the corresponding glucuronic acid derivatives and eliminated from the body in the urine. No phenomenon similar to this occurs in plants, however, and glucuronic acid has never been found in plant tissues.
Synthesis and Degradation of Hexoses.—Monosaccharides of any desired number of carbon atoms can be produced from aldoses having one less carbon atoms, by way of the familiar "nitrile" reaction. Aldoses, like all other aldehydes, combine directly with hydrocyanic acid, forming compounds known as nitriles, which contain one more carbon atom than was present in the original aldehyde; the cyanogen group can easily be converted into a COOH group; and this, in turn, reduced to an aldehyde, thus producing an aldose with one more carbon atom than was present in the initial sugar. These changes may be illustrated by the following equations:
It is possible, by this process, to advance step by step from formaldehyde to higher sugars, Emil Fischer and his studentshaving carried the process as far as the production of glucodecose (C10H20O10). It usually happens, however, that two stereo-isomers result from the "step-up" by way of the nitrile reaction; thus, arabinose yields a mixture of glucose and mannose, glucoseyieldsglucoheptose and mannoheptose, etc.
The reverse process, or the so-called "degradation" of a sugar into another containing fewer carbon atoms, may be readily accomplished in either one or two ways. In Wohl's process, the aldehyde group of the sugar is first converted into anoxime, by treatment with hydroxylamine; the oxime, on being heated with concentrated sodium hydroxide solution, splits off water and becomes the correspondingnitrile; this, on further heating, splits off HCN and yields an aldose having one less carbon atom than the original sugar. This process is the exact reverse of the nitrile synthesis, described above. The second method of degradation, suggested by Ruff, makes use of Fenton's method of oxidizing aldehyde sugars to the corresponding monobasic acid, using hydrogen peroxide and ferrous sulfate as the oxidizing mixture; thealdonic acidthus formed is then converted into its calcium salt, which, when further oxidized, splits off its carboxyl group and one of the hydrogens of the adjacent alcoholic group, leaving an aldose having one less carbon atom than the original aldose sugar.
Enolic Forms.—A final avenue for the interconversion of glucose, mannose, and fructose into one another, is through the spontaneous transformations which these undergo when dissolved in water containing sodium hydroxide or potassium hydroxide. This change is due to the conversion of the sugar, in the alkaline solution, into anenol, which is identical for all three sugars, and which may subsequently be reconverted into any one of the three isomeric hexoses. The relationships involved are illustrated in the following formulas:
The preceding technical discussion of the chemical constitution and reactions of the hexoses has been presented, not because it has any direct connection with the occurrence or functions of these compounds in plant tissues, but for the purpose of giving to the student a graphic conception of the structure and properties of these simple carbohydrates, as a basis for the understanding of the nature, properties, possible chemical reactions, syntheses, etc., of the more complex types of carbohydrates, which, along with these simple monosaccharides, constitute the most important single group of organic components of plants.
Only two monosaccharides occur as such in plants. These are glucose and fructose. All the other hexoses, whose structure is shown on pages 37 and 38, occur in plants only as constituents of the more complex saccharides, in glucoside-formations, or as the corresponding polyatomic alcohols.
The aldo-hexoses which occur most commonly in plants, either free or in combination, ared-glucose,d-mannose, andd-galactose; whiled-fructose andd-sorbose are the common keto-hexoses.
Glucose(often called also dextrose, fruit sugar, or grape sugar) occurs widely distributed in plants, most commonly in the juices of ripening fruits, where it is usually associated with fructose and sucrose, the two hexoses being easily derived from sucrose by hydrolysis. Glucose is also produced by the hydrolysis of many of the more complex carbohydrates, by the action either of enzymes or of dilute acids; lactose, maltose, raffinose, starch, and cellulose, as well as many glucosides all yielding glucose as one of the products of their hydrolysis. In all such cases, it isd-glucose which is obtained.
Glucose is a crystalline solid (although it does not form such sharply defined crystals as does sucrose, or "granulated sugar"), which is easily soluble in water. It usually appears on the market in the form of thick syrups, which are produced commercially by the hydrolysis of starch with dilute sulfuric acid, removal of the acid after the hydrolysis is complete, and evaporation of the resulting solution to the desired syrupy consistency. (Since corn starch is commonly used as the raw material for this process, these syrups are often spoken of as "corn syrup.") The sweetness of glucose is about three-fifths that of ordinary cane sugar.
Glucose exhibits all the properties of hexoses which have been described in general terms above. It is a reducing-sugar, and is easily fermented. The specific rotatory power ofd-glucose is +52.7°. But when glucose is dissolved in water, it exhibits in a marked degree the phenomenon known as "mutarotation"; that is, freshly made solutions exhibit a certain definite rotatory power, but this changes rapidly until it finally reaches another definite specific rotation. In other words, glucose is "birotatory," or possesses two distinct specific rotatory powers, and the changing rotation effect in aqueous solutions is due to the change from one form to the other. When dissolved in alcohol, it does not exhibit this change in rotatory power. In order to explain this phenomenon, it is necessary to assume that there are two modifications ofd-glucose, which have been designated respectively as the α and β forms. The possibility of the existence of these two forms is explained by the assumption of the closed-ring arrangement of the glucose molecule, as indicated in the following formulas which represent the two possible isomeric arrangements:
It is assumed that the α modification (with its specific rotatory power of +105°) is the normal form for crystalline glucose, but that when dissolved in water it is changed into analdehydrol, i.e., a compound containing two additional OH groups, which later breaks down again, into the β modification (with its specific rotatory power of +22°). When dissolved in alcohol, this change does not take place because of the absence of the excess of water necessary to produce the intermediate aldehydrol form.
There are other examples of the existence of the α and β modification of glucose. For example, α-methyl-glucoside and β-methyl-glucoside (specific rotatory powers, +157° and -33°, respectively) are both known, as well as several other similar glucoside arrangements.
Mannose.—This sugar does not occur as such in plants; but complex compounds which yieldd-mannose when hydrolyzed, known as "mannosans," are found in a number of tropical plant forms. The mannose which is obtained from these by hydrolysis is very similar to glucose in its properties, forms the same osazones as do glucose and fructose, exhibits mutarotation, etc. Mannose may also be obtained by oxidizing mannitol, a hexatomic alcohol, known as "mannite," which occurs in many plants, especially in the manna-ash (Fraxinus ornus), the dried sap from which is known as "manna."
Galactoseoccurs in the animal kingdom as one of the constituents of lactose, or milk-sugar. It is also one of the constituents of raffinose, a trisaccharide sugar found in plants, and occurs as "galactans" in many gums and sea-weeds. Thed-galactose, obtained by the hydrolysis of any of these compounds, is a faintly sweet substance which resembles glucose in many of its properties; having one characteristic difference, however, in that it forms mucic acid instead of saccharic acid when oxidized by concentrated nitric acid. These oxidation products are very different in their physical properties and this difference serves to distinguish between the two sugars from which they are derived.
Fructose(levulose, honey sugar, or "diabetic" sugar) occurs along with glucose in the juices of many fruits, etc. It is a constituent of sucrose, of raffinose, and of the polysaccharide inulin, from which it may be obtained by hydrolysis. It is a ketose sugar, reduces Fehling's solution, forms the same osazone as glucose, and is easily fermentable by yeast. Its sweetness is slightly greater than that of ordinary cane sugar.d-fructose (the ordinary form) is easily soluble in water, and is strongly levorotatory, its specific rotatory power at 20° C. being -92.5°; it is unique in the very large effect which is produced in its rotatory power by increasing the temperature of the solution; at 87° its specific rotatory power is reduced to -52.7°, exactly equal to but in the opposite direction of the effect of glucose; hence,invert sugar, which is a mixture of an equal number of molecules of glucose and fructose, and whichhas a specific rotatory power of -19.4° at 20° C., becomes optically inactive at 82° C.
Sorboseis the only other ketohexose which has any importance in plant chemistry. It does not occur free in plants, but is the first oxidation product from the hexatomic alcohol, sorbitol, which is present in the juice of the berries of the mountain-ash. Sorbose is a crystalline solid, which is not fermentable by yeast, but which otherwise closely resembles fructose.
The disaccharides, having the formula C12H22O11, may be regarded as derived from the monosaccharides by the linking together of two hexose groups with the dropping out of a molecule of water, in the same way that many other organic compounds form such linkages. That this is a perfectly correct conception, is shown by the fact that, when hydrolyzed, the disaccharides break down into two hexose sugars, thus
C12H22O11+ H2O = C6H12O6+ C6H12O6.
With all known disaccharides, at least one of the hexoses obtained by hydrolysis is glucose; hence all disaccharides may be regarded as glucosides (C6H12O5·R) in which the R is another hexose group.
Since hexoses have both alcoholic and aldehyde groups, and since either of these types of groups may function in the linkage of the two hexoses to form a disaccharide, it is possible for two hexoses, both of which are reducing sugars to be linked together in three different ways: (1) through an alcoholic group of each hexose, (2) through an alcoholic group of one and the aldehyde group of the other, and (3) through the aldehyde group of each hexose. Disaccharides linked in either of the first two ways will be reducing sugars, since they still contain a potentially active aldehyde group; but those of the third type will not be reducing sugars, since the linkage through the aldehyde groups destroys their power of acting as reducing agents. Examples of each of these three types of linkage are found among the common disaccharides, as will be pointed out below.
The following table shows the general characteristics of the common disaccharides.
Type 1.—Aldehyde group potentially active, reducing sugars:SugarComponentsMaltoseGlucose and glucoseGentiobioseGlucose and glucoseLactoseGlucose and galactoseMelibioseGlucose and galactoseTuranoseGlucose and fructoseType 2.—Non-reducing sugars:SucroseGlucose and fructoseTrehaloseGlucose and glucose
The disaccharides of Type 1 reduce Fehling's solution and form hydrazones and osazones, although somewhat less readily than do the hexoses. They all show mutarotation and exist in two modifications, indicating that the component groups have the closed-ring arrangement.
The disaccharides of Type 2, since they contain no potentially active aldehyde group, do not reduce Fehling's solution, nor form osazones; neither do they exhibit mutarotation. The only disaccharides which occur as such in plants are of this type. Disaccharides of Type 1 may be obtained by the hydrolysis of other, more complex, carbohydrates.
All disaccharides are easily hydrolyzed into mixtures of their component hexoses, by boiling with dilute mineral acids, or by treatment with certain specific enzymes which are adapted to the particular disaccharide in each case (seepage55, alsoChapter XIV).
Sucrose(cane sugar, beet sugar, maple sugar) is the ordinary "granulated sugar" of commerce. It occurs widely distributed in plants, where it serves as reserve food material. It is found in largest proportions in the stalks of sugar cane, in the roots of certain varieties of beets, and in the spring sap of maple trees, all of which serve as industrial sources for the sugar. In the sugar cane, and beet-roots, it constitutes from 12 to 20 per cent of the green weight of the tissue and from 75 to 90 per cent of the soluble solids in the juice which can be expressed from it. Its universal use as a sweetening agent is due to the combined facts that it crystallizes readily out of concentrated solutions and, hence, can be easily manufactured in solid form, and that it is sweeter than any other of the common sugars except fructose.
Sucrose is a non-reducing sugar, forms no osazone, and is not directly fermentable by yeast, although most species of yeasts contain an enzyme which will hydrolyze sucrose into its component hexoses, which then readily ferment.
When hydrolyzed by acids, or by the enzyme "invertase," it yields a mixture of equal quantities of glucose and fructose. Sucrose is dextrorotatory, but since fructose has a greater specific rotatory action to the left than glucose has to the right, the mixture resulting from the hydrolysis of sucrose is levorotatory. Since the hydrolysis of sucrose changes the rotatory effect of the solution from the right to the left, the process is usually called the "inversion" of sucrose, and the resultant mixture of equal parts of glucose and fructose is called "invert sugar." As has been pointed out, solutions of invert sugar become optically inactive when heated to82 °C., because of the reduction in the rotatory power of fructose due to the higher temperature.
The probable linkage of the two hexoses to form sucrose, in such a way as to produce a non-reducing sugar, is illustrated in the following formula:
Trehaloseseems to serve as the reserve food for fungi in much the same way that sucrose does for higher plants. It is composed of two molecules of glucose linked together through the aldehyde group of each, as trehalose is a non-reducing sugar. This linkage is illustrated in the following formula:
Trehalose may be hydrolyzed into glucose by dilute acids and by the enzyme "trehalase," which is contained in many yeasts and in several species of fungi. It is strongly dextrorotatory (specific rotatory power, +199°). It is not fermentable by yeast.
Trehalose appears to replace sucrose in those plants which contain no chlorophyll and do not elaborate starch. The quantity of trehalose in such plants reaches a maximum just before spore formation begins. Since it is manufactured in the absence of chlorophyll, its formation must be accomplished by some other means than photosynthesis, yet it is composed wholly of glucose—a natural photosynthetic product.
Maltoserarely occurs as such in plants, although its presence in the cell-sap of leaves has sometimes been reported. It is produced in large quantities by the hydrolysis of starch during the germination of barley and other grains. This hydrolysis is brought about by the enzyme "diastase," which is present in the sprouting grain.
Maltose is easily soluble in water, and crystallizes in masses of slender needles. It is a reducing sugar; readily forms a characteristic osazone; is strongly dextrorotatory (specific rotatory power +137°); and is readily fermented by ordinary brewer's yeast, which contains both "maltase" (the enzyme which hydrolyzes maltose to glucose) and "zymase" (the alcohol-producing enzyme). When hydrolyzed, either by dilute acids or by maltase, one molecule of maltose yields two molecules of glucose. Its component hexoses are, therefore, the same as those of trehalose, a non-reducing sugar, this difference in properties being due to the difference in the point of linkage between the two glucose molecules, that for maltose being such as to leave one of the aldehyde groups potentially active, as shown in the following formula,
Isomaltoseis a synthetic sugar, obtained by Fischer, by condensing two molecules of glucose. Its properties are quite similar to those of maltose, but it yields a slightly different osazone and is not fermentable by yeast. These differences are explained by the assumption that this sugar is a glucose-β-glucoside, while normal maltose is a glucose-α-glucoside.
Gentiobioseis a disaccharide which results from the partial hydrolysis of the trisaccharidegentianose(seepage 53). It is very similar in its general properties to isomaltose.Cellobioseis a disaccharide which results from the hydrolysis of cellulose. It is a reducing sugar, forms an osazone, and resembles maltose.
Maltose, isomaltose, gentiobiose, and cellobiose, are all glucose-glucosides, the difference between them being undoubtedly due to linkage being between different alcoholic groups in the glucose molecules.
The disaccharidelactoseis a glucose-galactoside. It is the sugar which is present in the milk of all mammals. It has never been found in plants.Melibiose, which is the corresponding vegetable glucose-galactoside, may be obtained by the partial hydrolysis of the trisaccharideraffinose(see below). It is a reducing sugar; forms a characteristic osazone; and exhibits mutarotation. It is not fermented by ordinary top-yeasts, but is first hydrolyzed and then fermented by the enzymes present in bottom-yeasts.
Trisaccharides, as the name indicates, consist of three hexoses (or monosaccharides) linked together by the dropping out of two molecules of water. Their formula is C18H32O16. When completely hydrolyzed, they yield three molecules of monosaccharides; when partially hydrolyzed, one each of a disaccharide and a monosaccharide.
One trisaccharide of the reducing sugar type, namelyrhamnose, exists in plants as a constituent of the glucoside xanthorhamnin. It is composed of one molecule of glucose united to two molecules of rhamnose (methyl pentose, C6H12O5). It is of interest only in connection with the properties of the glucoside in which it is present (seepage 84).
Three trisaccharides which are non-reducing sugars are found in plants; namely, raffinose, gentianose, and melizitose.
Raffinoseoccurs normally in cotton seeds, in barley grains, and in manna; also, in small quantities in the beet root, associated with sucrose. It is more soluble in water than is sucrose and hence remains in solution in the molasses from beet-sugar manufacture, which constitutes the commercial source for this sugar. Raffinose crystallizes out of concentrated solutions, with five molecules of water of crystallization, in clusters of glistening prisms. It is strongly dextrorotatory, the anhydrous sugar having a specific rotatory power of +185°, and the crystalline form, C18H32O16, showing a specific rotation of +104.5°. It does not reduce Fehling's solution, nor form an osazone, and in its other properties it closely resembles sucrose.
The hydrolysis of raffinose presents several interesting possibilities. If its structure is represented as follows:
C6H11O5——C6H10O4——C6H11O5Fructose Glucose Galactose\_____ _____/ \_____ _____/\/ \/Sucrose Melibiose
it is apparent that it may break down by hydrolysis in three different ways: (1) into sucrose and galactose, (2) into fructose and melibiose, and (3) into fructose, glucose, and galactose. As a matter of fact, it does actually break down in these three different ways, under the influence of different catalysts; invertase or dilute acids break it down into fructose and melibiose, emulsin hydrolyzes it to sucrose and galactose, while strong acids or the enzymes of bottom-yeasts break it down into the three hexoses.
Gentianose, a trisaccharide found in the roots of yellow gentian (Gentiana lutea), is a non-reducing sugar, which when hydrolyzed yields either fructose and gentiobiose, or fructose and two molecules of glucose.
Melizitose, a trisaccharide which, in crystallized form, has the formula, C18H32O16·2H2O, occurs in the sap ofLarix europeaand in Persian manna, and has recently been found in considerable quantities in the manna which collects on the twigs of Douglas fir and other conifers. When hydrolyzed, it yields one molecule of fructose and one of turanose, a disaccharide containing fructose and glucose linked together in a slightly different way than they are in sucrose. Turanose itself is a reducing sugar, but when linked with fructose to form melizitose its reducing properties are destroyed. Melizitose is a very sweet sugar.
A complex saccharide, known asstachyose, which is found in the tubers ofStachys tuberifera, is said by some investigators to be a tetrasaccharide and by others to have the formula C36H62O31·7H2O (i.e., a hexasaccharide). It is a crystalline solid, with a faintly sweetish taste, and a specific rotatory power of+148°.When hydrolyzed it yields glucose, fructose, and two (or more) molecules of galactose.
As will be pointed out later (seeChapter XIV), all chemical reactions which are involved in vital phenomena, including those of plant growth and metabolism, are controlled by enzymes. The biochemical reactions which the soluble carbohydrates undergo afford such excellent illustrations of the relation of the molecular configuration of an organic compound to the possibility of the action of an enzyme upon it, that it seems desirable to discuss this relationship at this point, rather than to postpone it until after the nature of enzyme action has been considered. Undoubtedly, the student, after he has studied the nature of enzymes and their mode of action, as presented inChapter XIV, will find it profitable to return to this section and review the facts here presented, as illustrating the principles and mechanism of enzyme action. But a consideration, at this time, of the relation of the molecular configuration of the sugars to their biochemical reactions cannot fail to add interest to the study of these matters from the chemical and biological standpoints.
It has been known for a long time that the dextro- and levo-isomers of a compound which contains one or more asymmetric carbon atoms are affected differently by biological agents, such as yeasts, moulds, bacteria, etc. Pasteur, as early as 1850, showed that the green mould,Penicillium glaucum, when growing in solutions of racemic acid (a mixture of equal molecules ofd- andl-tartaric acids) uses up only thed-acid, leaving thel-form absolutely untouched. Later, it was found that the same green mould attacksl-mandelic acid in preference to thed- form; whereas theyeast,Saccharomyces ellipsoideus, exhibits the opposite preference for these acids.
These observations upon some of the earlier known forms of optically active organic acids led the way to a general study of this phenomenon as exhibited by the optically active soluble carbohydrates. The results of these studies may be considered in connection with the several different types of reactions which these sugars undergo, as follows:
Glucoside Hydrolysis.—As was pointed out in connection with the discussion of the mutarotation of glucose, this sugar may exist in either the α or the β modification. Glucosides of both α and β glucose are of common occurrence. The difference in molecular configuration, in such cases, may be represented by the following formulas:
The radical represented by the R may be either a common alkyl radical (as CH3, C2H5, etc.), another saccharide group (as in the case of the disaccharides, trisaccharides, etc.), or some other complex organic group (as in the case of the natural glucosides described inChapter VI). But, in every case, the glucoside is easily hydrolyzed by the enzymemaltase(or α-glucase) if the molecular arrangement is that represented by the α-attachment, or by the enzymeemulsin(or β-glucase) if the glucoside is of the β type; but emulsin is absolutely without effect upon α-glucosides, and maltase does not produce the slightest change in β-glucosides. These statements hold true regardless of the nature of the group which is represented by the R in the formulas above. Hence, thebiochemical properties of the glucosides, so far as their hydrolysis by the enzymes which are present in many biological agents is concerned, depends wholly upon the molecular configuration of the glucose itself. Furthermore, neither the mannosides, which differ from glucosides only in the arrangement of the H and OH groups attached to one of the asymmetric carbon atoms in the hexose, nor galactosides in which two such arrangements are different (see configuration formulas onpage 57), are attacked by either maltase or emulsin. But other enzymes specifically attack otherdisacharides, or polysaccharides, or glucoside-like complexes. For example,lactaseacts energetically upon ordinary lactose and all other β-galactosides; but not upon any glucoside, mannoside, etc.
Again, neither α- nor β-xylosides, which correspond with the above-described glucosides in every particular except that the HCOH group next the terminal CH2OH group is missing, are hydrolyzed by either emulsin or maltase.
These instances, selected from among many similar observations, clearly prove that not only the number and kind of groups in the molecule, but also the arrangement of the constituent groups in space, must be identical in order that the compound may be acted upon by any given enzyme acting as a biological hydrolytic agent.
Fermentability.—The enzymezymase, present in all yeasts, promotes the fermentation of the naturald- forms of the three hexoses, glucose, mannose, and fructose, but is without effect upon the artificiall- forms of the same sugars. The uniform action of zymase upon these hexoses is easily explained upon the basis of the same assumption which was used to account for the formation of identical osazones from these sugars and their easy transformation into each other; namely, their easy transformation into anenolicform which is identical for all three.
Further, galactose is fermented by some yeasts (although not by all), but much less readily than are the other sugars, and the temperature reaction is quite different with galactose than with the others. Talose and tagatose are entirely unfermentable. A study of the configuration formulas for these several sugars shows the explanation for these observed facts. These formulas are as follows: