Chapter 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 to 82° 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:

------O------- | | CH_{2}OH·CHOH·CH·CHOH·CHOH·CH | O | CH_{2}OH·CHOH·CHOH·CH·C·CH_{2}OH \ / O

=Trehalose= seems 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:

------O------- | | CH_{2}OH·CHOH·CH·CHOH·CHOH·CH | O | CH_{2}OH·CHOH·CH·CHOH·CHOH·CH | | ------O-------

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.

=Maltose= rarely 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,

-------O------ | | CH_{2}OH·CHOH·CH·CHOH·CHOH·CH | O | CHOH·CHOH·CHOH·CH·CHOH·CH_{2} | | ----------O-----

=Isomaltose= is 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-[beta]-glucoside, while normal maltose is a glucose-[alpha]-glucoside.

=Gentiobiose= is a disaccharide which results from the partial hydrolysis of the trisaccharide _gentianose_ (see page 53). It is very similar in its general properties to isomaltose. =Cellobiose= is 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 disaccharide =lactose= is 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 trisaccharide _raffinose_ (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

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 C_{18}H_{32}O_{16}. 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, namely _rhamnose_, 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, C_{6}H_{12}O_{5}). It is of interest only in connection with the properties of the glucoside in which it is present (see page 84).

Three trisaccharides which are non-reducing sugars are found in plants; namely, raffinose, gentianose, and melizitose.

=Raffinose= occurs 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, C_{18}H_{32}O_{16}, 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:

C_{6}H_{11}O_{5}-C_{6}H_{10}O_{4}-C_{6}H_{11}O_{5} Fructose 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, C_{18}H_{32}O_{16}·2H_{2}O, occurs in the sap of _Larix europea_ and 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.

TETRASACCHARIDES

A complex saccharide, known as _stachyose_, which is found in the tubers of _Stachys tuberifera_, is said by some investigators to be a tetrasaccharide and by others to have the formula C_{36}H_{62}O_{31}·7H_{2}O (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.

THE RELATION OF THE MOLECULAR CONFIGURATION OF SUGARS TO THEIR BIOCHEMICAL PROPERTIES

As will be pointed out later (see