Mutarotation

MUTAROTATION (multirotation, birotation), a phenomenon consisting in the fact that a freshly prepared solution of a substance shows a different specific rotation than a solution that has stood for some time. Mutarotation is observed in sugars possessing a free carbonyl group, i.e., in all monoses and in disaccharides constructed on the type of a monoglycosidic bond. Mutarotation is caused by the existence of two isomeric forms (a and B) in such sugars in connection with the presence in their cyclic form of an asymmetric carbon atom.

CH₂OH

a-d-glucose, [a]_D = + 19.8°. These two forms are not optical antipodes, but differ in properties and in specific rotation. Both modifications can be isolated in pure form using suitable solvents and temperatures, e.g., a-d-glucose ([a]_D = + 106°) crystallizes out on cooling from aqueous and alcoholic solutions, b-d-glucose ([a]_D = + 19.8°) from aqueous solutions at temperatures above 98° or from hot saturated solutions in pyridine. But both modifications are stable only in the solid state. In solution, the transition of one form into the other occurs, and equilibrium is gradually established for both forms at a certain ratio of both forms, [a]_D = + 52.5°, and is the same for both forms (under identical conditions). This transition causes the phenomenon of mutarotation. The position of equilibrium in a given solvent changes little with external conditions (temperature, concentration), but the rate of mutarotation strongly depends on temperature and the presence of catalysts. Thus, for example, in aqueous solutions of d-glucose, equilibrium is established at room temperature in about a day, upon boiling in a few minutes, and in the presence of OH- ions (traces of ammonia, soda, caustic alkalis) instantaneously. Disaccharides in which both glycosidic hydroxyls participate in the formation of a bond between monose residues (cane sugar) do not show mutarotation. Monoses in which the hydrogen of the glycosidic hydroxyl is substituted possess a more stable configuration than free sugars and may not show the phenomenon of mutarotation. Upon hydrolysis of a disaccharide or glycoside, the liberation of glycosidic hydroxyls occurs, and mutarotation is observed, which makes it possible to establish the nature of the bond and the configuration of the monoses if the course of hydrolysis is controlled simultaneously with the observation of mutarotation by some method independent of optical properties (e.g., by determining the reducing capacity through titration).

L. Brovds