Mutarotation point

Both anomers of o-glucopyranose can be crystallized and purified. Pure a-n-glucopyranose has a melting point of 146 °C and a specific rotation, lo-Jn, of +112.2 pure /3-D-glucopyranose has a melting point of 148 to 155 °C and a specific rotation of +18.7. When a sample of either pure anomer is dissolved in water, however, the optical rotation slowly changes and ultimately reaches a constant value of +52.6. That is, the specific rotation of the a-anomer solution decreases from +112.2 to +52.6, and the specific rotation of the /3-anomer solution increases from +18.7 to +52.6. Called mutarotation, this change in optical rotation is due to the slow conversion of the pure anomers into a 37 63 equilibrium mixture. 

Turanose crystallizes in well-formed prisms (see Fig. 5) it is anhydrous, shows the composition CuHhOu by combustion analysis, its melting point is 157°, and it exhibits a large and rapid mutarotation in water from an initial [a]o22 of about +22° to the final value +75.3° in the course of about thirty minutes. Isbell and Pigman26 found for the mutarotation of turanose at 20.7°, [a]n +27.3° (initial) —> +75.8°... 

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. 

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. 

Examination of Figure 9.11 demonstrates an excellent separation of a-glucose from (3-glucose which points out the power of GLC for the separation of these materials as well as a significant complication in the study of carbohydrates. It is well known that solutions of some carbohydrates undergo anomerization and that an initially pure form of a sugar may result in an equilibrium mixture (mutarotation) as in Figure 9.12. 

Mutarotation of 0.3% solutions of the freshly dissolved sugars in 12 ml of 5 mM EDTA, pH 7.4 was followed. Significant differences in mutarotation rates (AK) in the presence and absence of 100 units of bovine kidney enzyme were expressed as the ratio AK/Ksp. Differences of less than 5% in these rate constants were not considered significant. Of the 18 sugars listed, nine have been tested previously as substrates for other mammalian mutarotases with essentially the same pattern as described here. The pattern of specificity indicates that a 3-point attachment of enzyme and substrate is necessary for catalysis of mutarotation. b Data from 72). 

Pointed platelets or prisms from 95% ethanol mp, 115°C (decomposition).1 A mixture of a and (3 stereoisomers aqueous solutions rapidly undergo mutarotation to an equilibrium value of [a]D25 + 3 90.1... 

It is of interest at this point to mention that the optical rotation of L-sorbose has been studied extensively. Lobry de Bruyn and Van Ekenstein21 reported that no mutarotation was observable however, Pigman and Isbell56 later discovered that L-sorbose does possess a small complex mutarotation. For example, at 20°C., L-sorbose (c, 11.3 l, 4) gave an initial observed rotation (°S) of —57.124, which changed to — 57.498 in 2.69 minutes and attained the final value of —57.768 after two hours. Similar results were observed at 0°C. but at a much slower rate. These authors were of the opinion that the smallness of this mutarotation was due to the fact that the equilibrium solution of L-sorbose is... 

The situation was clarified by the isolation of a, )8, and y isomers of D-glucose by Tanret in 1895. He showed that the a and y isomers mutarotate in opposite directions and, at equilibrium, each have the same rotation as the y3 form. The isomers were also found to have the same molecular weight. Tanret s three isomers of D-glucose were considered to be ring and free aldehyde forms by Lobry de Bruyn and Alberda van Ekenstein in 1895, von Lipp-mann in 1896, and Simon in 1901. Fischer in 1893, and von Lipp-mann pointed out that ring formation would produce a new asymmetric carbon atom, and thus the existence of isomeric sugars, glycosides, and acetates was clarified. 

The mutarotation constant, ki + ko is the sum of the constants for the two opposing reactions, and k lki is the equilibrium constant. Lowry and Hudson pointed out and showed that the same value is obtained for ki + kj from the mutarotation of the a and /8 anomers, a situation which has been experimentally confirmed by many others. Hudson found the reaction constant to be independent of the concentration of sugar over a wide range, and dependent on catalysis by both acids and bases, as had also been shown less precisely by Urech and by Levy. The effect of acids, bases, and salts will be considered in more detail in Part II of this review. 

The solubilities are determined experimentally by separation of the solution from the solid phase at various intervals of time and determination of the equilibrium rotation of the solution. Lowry pointed out that a number of earlier workers had noted that the solubility of D-glucose or lactose increases during mutarotation. The rate of change depends on the velocity constant of the forward action. The ratio of the initial and final solubilities for the a anomer is ... 

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