Optical rotation, glucopyranose anomers

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. 

When a-D-glucopyranose is dissolved in water, the optical rotation value decreases with time until it reaches+52.7°. This is because of equilibration between the a- and [3-anomers in a process known as mutarotation. The (3-anomer undergoes reversible ring opening and re-closure, leading to the formation of some of the a-anomer. 

Glucose precipitates from concentrated solutions at room temperature to give crystals that melt at 146°C. Stmctural analysis by X-ray diffraction reveals that these crystals contain only the a-D-(+)-glucopyranose anomer (Figure 24-3). When crystalline a-D-(+)-glucopyranose is dissolved in water and its optical rotation measured immediately, a value = +112... 

Mutarotation The change in optical rotation associated with the equilibration, in solution, of the anomeric and ring forms of a saccharide beginning with a single crystalline form. Thus, dissolving a-D-glucopyranose in water allows formation of the /3-anomer as well as the furanose forms. Since all have different optical rotations, the initial value changes until the equilibrium mixture is achieved. 

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