Mutarotation, of a- and /3-D-glucose

Keston305-307 showed that the kidney, liver, and lens tissues of many animals contain an enzyme that catalyzes the mutarotation of a- and /3-D-glucose and other sugars it was called mutarotase.3 8,309 This, or a similar, enzyme had earlier been found in preparations of D-glucose oxidase from Penicillium notatum by Bentley and Neu-berger310 and Keilin and Hartree.309 The enzyme was extensively purified by Bentley and Bhate.140 The enzyme is also widely distributed in plant tissues.311... 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

As hemiacetals, a- and )3-D- +)-glucose are readily hydrolyzed by water. In aqueous solution either anomer is converted—via the open-chain form—into an equilibrium mixture containing both cyclic isomers. Thus mutarotation results from the ready opening and closing of the hemiacetal ring (Fig. 34.9). ... 

From this point of view, both the pyranoses and the furanoses may participate in the mutarotation reaction, and both a- and /3-d 1uco-pyranose will have similar mutarotation constants. We may expect a heterocyclic five-membered ring with two adjacent cis hydroxyl groups to have a A of about 1000 units for 0.5 M solutions. In the case of D-glucose it is between 93 and 80 hence, the concentration of a-D-glucofuranose is probably less than 10 per cent. This quantity is sufficient, however, to explain the formation of derivatives of the furanoses. 

Although ring structures were proposed for D-glucose by Colley in 1870, anomeric and ring interconversions were not established as being the principal basis of mutarotations until the isolation of a and /3 isomers of D-glucose by Tanret, in 1895. (Isomers of lactose had... 

Monosaccharides undergo mutarotation, the interconversion of a- and /3-anomers in aqueous solution. One can separately prepare pure a-o-glucose and pure /8-o-glucose as crystalline solids. However, addition of either anomer to water results in the same equilibrium mixture of 64 percent a-D-glucose and 36 percent -o-glucose. 

Certain procedures make it possible to obtain the a and 3 anomers of glucose in pure form. A 1-molar solution of a-D-glucose has a rotation value [a]o of +112°, while a corresponding solution of p-D-glucose has a value of +19°. These values change spontaneously, however, and after a certain time reach the same end point of +52°. The reason for this is that, in solution, mutarotation leads to an equilibrium between the a and p forms in which, independently of the starting conditions, 62% of the molecules are present in the P form and 38% in the a form. 

Define, by reference to glucose, the meaning of each of the following terms (a) a monosaccharide, (b) mutarotation, (c) a pyranose ring system, (d) anomers and (e) a- and (3-glucosides. 

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