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The discovery of mutarotation by Augustin Pierre Dubrunfaut dates back to 1846. All of his results were based on sugar. His observations confirmed that sugar dissolves in water and its optical activity changes over time. It fluctuates from 110° to 52°. He termed this phenomenon “birotation.” Nonetheless, Lowry changed it to “mutarotation” in 1899, a title more akin to the concept it represents.Â
Mutarotation is the change in rotation of a solution that occurs when equilibrium between the alpha (α) and beta (β) anomers shift upon dissolution in aqueous solutions. Anomerization is another name for the process. Rotational change caused by the change between isomers causes mutarotation. An optical rotation changes when the corresponding stereocenters of two anomers interconvert, which involves a change in equilibrium between the two anomers. Muta means “change”, and therefore, literally means a shift in rotation. Molecules never change their exact rotation, but the rotation of an entire mixture can change. It happens because some molecules change from one isomer to another.
Examples of Mutarotation
Here are some examples of mutarotation in sugars to explain the concept. Mutarotation of Glucose Because the α and β isomers are diastereomers, their physical properties differ accordingly. In water, following the dissolution of α-d-glucose, the optical rotation slowly decreases from 112.2° to +54 °, indicating an equilibrium. A solution of α-d-glucose in water gradually rotates from an initial value of + 18.7° to the same equilibrium value of + 54° as the initial value. Mutarotation occurs over a gradual transition to an equilibrium point. The cyclic hemiacetals undergo an interconversion in solution due to the cyclic hemiacetals’ open-chain nature. By opening the ring and recycling it, a α or β Anomer can be formed. Glucose in equilibrium is made up of 36% α anomeric form, 64% β anomeric form, and less than 0.01% open-chain form. It is catalyzed by a chemical called mutarotase that turns glucose into a different form. While the mutarotation of glucose leads to primarily anomeric pyranoses, some aldohexoses contain both anomeric pyranoses and anomeric furanoses. The four anomeric furanoses and pyranoses in fructose form another mixture. Mutarotation of Lactose Lactose crystallization is significantly affected by the conversion of α- to β-lactose forms in aqueous solutions (mutarotation). Likewise, changes in lactose solubility affect the metastable zone (MZ), a region between lactose solubility and its supersolubility. There are two glucose molecules and one galactose molecule that are linked together by beta (1-4) glycosidic bonds. In general, milk sugar is known as lactose. Anomeric C1 of glucose residue causes mutarotation in an aqueous solution. Beta-lactose and alpha-lactose are the two structural components of 62.7% and 37.3%, respectively, of the mixture at equilibrium. During the heating of monohydrate crystallized lactose, mutarotation occurs. A first-order reaction of mutarotation, lactose mutarotation is slow due to lactose’s solubility. Furthermore, the amount of sugar and salt in a solution affects the rate of mutarotation. The rate of mutarotation quickly drops to half the normal rate if the sucrose concentration is increased to above 40%.-
Mutarotation of Fructose
