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Mutarotation is the term given to the change in the specific rotation of a cyclic monosaccharide as it reaches an equilibrium between its α and β anomeric forms.
Though the cyclic forms are usually heavily favoured, liquid monosaccharides (or monosaccharaides in aqueous solution) are always in equilibrium with their straight-chain forms. This equilibrium is established as the hemiacetal bond between C1 (the only carbon bound to two oxygens) and C5 is cleaved (forming the straight-chain compound) and reformed (forming the cyclic compound). When the hemiacetal bond is reformed, the OH group on C5 may attack either of the two stereochemically distinct sides of the aldehyde group that contains C1. Which side it actually does attack on decides whether the α or β anomer is formed.
These α and β anomers usually have different specific rotations. I.e. a solution of pure α anomer will rotate plane polarised light in a different direction or a different number of degrees than a solution of pure β anomer will when such light is passed through them. (These pure anomers can be obtained through techniques such as recrystallisation). However, as time passes, some of the pure α anomer in solution will be converted into the β anomer by the process described above until a stable equilibrium is established between the two. The formation of this equilibrium mixture of α and β anomers results in mutarotation, the observable change in the specific rotation of the original solution.
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