THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES
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, are d-glucose, d-mannose, and d-galactose; while d-fructose and d-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 is d-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 of d-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 of d-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 an aldehydrol, 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 yield d-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."