Solutions of, mutarotation

The optical rotations just cited for each isomer are those measured immediately after each one is dissolved m water On standing the rotation of the solution containing the a isomer decreases from +112 2° to +52 5° the rotation of the solution of the p isomer increases from +18 7° to the same value of +52 5° This phenomenon is called mutarotation What is happening is that each solution initially containing only one anomeric form undergoes equilibration to the same mixture of a and p pyranose forms The open chain form is an intermediate m the process... 

Properties. Physical properties of the three crystalline forms of dextrose are Hsted in Table 1. In solution, dextrose exists in both the a- and P-forms. When a-dextrose dissolves in water, its optical rotation, [cc], diminishes gradually as a result of mutarotation until, after a prolonged time, an... 

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

In 1899, Lowry discovered the change in the rotatory power over time of a solution of nitrocamphor in benzene, an effect previously encountered only with aqueous solution of sugars. He named this effect "mutarotation," and its discovery was taken as a prominent achievement for Armstrong s laboratory research group. 50 Lowry ascribed the phenomenon to tautomeric conversion (from a CH-N02 form to a C = NO-OH form), that is, the shift of a hydrogen atom and the shift of a double bond. In 1909, he and Desch concluded that this reversible transformation occurs very quickly because they could not find an ultraviolet absorption spectral band characteristic of either isomer. 51 But what triggered this reversible transformation ... 

Faulkner (1923 1927) reported that mutarotation occurred much more readily in a mixture of pyridine and cresol than in either by itself. Swain and Brown (1952) extended this work and found that a-pyridone, where an acidic and a basic group are incorporated in the same molecule, is an excellent catalyst, 0 05 M concentration giving rise to a reaction 50 times faster in benzene solution than a solution of 0 05 M phenol and 0-05 M pyridine. A 0-001 M solution of a-pyridone was 7000 times more effective than equivalent concentrations of phenol and pyridine. A concerted mechanism was proposed [4]. 

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. 

Due to mutarotation, a solution of pure a or pure p will change to a mixture. 

In our first measurements with solutions of 0.5 M a-D-galactose in 0.5 M boric acid, the constants of the mutarotation and of the change in the conductivity agreed very well. 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

The term mutarotation means the variation of optical rotation with time, ohserved in a solution of sugar on standing. Let us have a look at this phenomenon in a glucose solution. The pure a anomer of glucose has an m.p. of 146 °C and a specific rotation [a]o +112.2°, and the specific rotation on standing is +52.6°, while pure (3 anomer has an m.p. of 148-155 °C and a specific rotation [a]D + 18.7°, and the specific rotation on standing is + 52.6°. When a sample of either pure anomer is dissolved in water, its optical rotation slowly changes and ultimately reaches a constant value of + 52.6°. Both anomers, in solution, reach an equilibrium with fixed amounts of a (35 per cent), (3 (64 per cent) and open chain ( 1 per cent) forms. 

Mutarotation has been shown to be a first-order reaction, the velocity constant being independent of reaction time and concentration of reactants. The rate of mutarotation increases 2.8 times with a 10°C rise in temperature. By applying the law of mass action, equations have been developed to measure the rate of the reversible reaction between the a and (3 forms of lactose. If a dilute lactose solution at constant temperature contain a moles of a and b moles of /3, then the amount of (3 formed (x) per unit of time is... 

The presence of sugars and salts can also affect the rate of mutarotation. Although the effect is small in dilute solutions, a combination of salts equal to that found in solution in milk nearly doubles the rate of mutarotation (Haase and Nickerson 1966). This catalytic effect is attributed primarily to the citrates and phosphates of milk. The presence of high levels of sucrose, on the other hand, has the opposite ef-... 

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. 

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