Optical Rotation, Mutarotation

Specific rotation constants, designated as [a] for sodium D-line light at 20-25°C, are listed in Table 4.8 for some important mono- and oligosaccharides. The specific rotation constant [a] at a selected wavelength and temperature is calculated from the angle of rotation, a, by the equation  

A simple mutarotation exists in this example, unlike complex mutarotations of other sugars, e. g., idose which, in addition to pyranose, is also largely in the furanose form. Hence, the order of its mutarotation kinetics is more complex. 

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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... 

The specific optical rotations of pure a and p o mannopyranose are +29 3° and -17 0° respectively When either form is dissolved in water mutarotation occurs and the observed rotation of the solution changes until a final rotation of +14 2° is observed Assuming that only a and p pyranose forms are present calculate the percent of each isomer at equilibrium... 

A particular carbohydrate can mterconvert between furanose and pyra nose forms and between the a and (3 configuration of each form The change from one form to an equilibrium mixture of all the possible hemi acetals causes a change m optical rotation called mutarotation... 

Mutarotation (Section 25 8) The change in optical rotation that occurs when a single form of a carbohydrate is allowed to equilibrate to a mixture of isomeric hemiacetals... 

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... 

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. 

Mutarotation (Section 25.5) The change in optical rotation observed when a pure anomer of a sugar is dissolved in water. Mutarotation is caused by the reversible opening and closing of the acetal linkage, which yields an equilibrium mixture of anomcrs. 

D-ribitol (21), and the structure formulated as 2,4-0-benzylidene-D-er-ythrose (22), would be 2,3-O-benzylidene-D-erythrose (23). These reassignments are supported by comparison of the properties of the product described as 22 with data from the literature. Thus, an authentic sample of 22, obtained by a different route (32), had an optical rotation value of — 20 °, which greatly differs from that found for the product formulated (31) as 22 (—65.2° — — 62.6°). The fact that mutarotation is observed, as well as the correspondence with the [a]D value (—62°) for 23, would indicate that the latter is the correct structure for the product described as 22. In any event, hydrolysis of the acetal function of both (22 and 23), leads to D-erythrose. 

The isomerism of a- and jS-glucose is to be attributed to the spatially different arrangement of the H and OH-groups attached to the asymmetric carbon atom 1. This atom is asymmetric in the cyclic lactol formula (Tollens). The mutarotation of the sugars, i.e. the gradual change to the final stationary value of the optical rotation, is to be explained by an equilibrium occurring in solution between the various... 

For a number of optically active ions of the type cis-M(AA)2XY, where M = Co and Cr, there is an initial optical rotation change (mutarotation) that is similar in rate to that of acid hydrolysis, for example,... 

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. 

When either isomer is dissolved in water, there is a gradual change from one form to the other until equilibrium is established, i.e. mutarotation. These changes may be followed by measuring the change in optical rotation with time until, at equilibrium, the specific rotation is +55.4°. 

The mutarotation coefficient ( i + k2) can be determined by the change in optical rotation with time ... 

In 1946, Berger and Lee102 heated D-ribose with aniline in ethanol, and obtained a crystalline product to which they ascribed an a-n-furanoid structure. This iV-phenyl-a-D-ribofuranosylamine showed mutarotation in water, and was hydrolyzed by water, by aqueous acid, and by alkali. It was distinguished from the pyranoid isomer by differences in optical rotation and mutarotation. The furanoid structure was allegedly established... 

For arabinose the half-times for all components are essentially the same. As long as this constraint is obeyed, the calculated optical rotation curves would be expected to show no rapid initial drop, and this is what was found. Therefore, we must postulate a fifth, as yet undetected, fast component in mutarotating arabinose solutions. Here the only reasonable... 

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