A slow change in optical rotation to an equilibrium value is known as mutarotation.
Mutarotation was discovered by Dubrunfaut, a French chemist in 1844.
Isomers
Isomers are two molecules, which have the same molecular formula but different chemical properties.
Structural isomers
Two isomers have the same molecular formula but differ in the arrangement of the functional groups.
Stereoisomers
Two isomers have the same molecular formula but differ in the spatial arrangement of the groups. Stereoisomers are further classified as
- Enantiomers: Enantiomers are non-superimposable mirror images.
- Diastereomers: Diastereomers are neither superimposable nor mirror images. Diastereomers have different configurations at the stereoisomeric centers.
Note: Epimer is a diastereomer, differs in a configuration only at one chiral centre. Anomer is a type of epimer that differs in the configuration at the acetal/hemiacetal carbon.
Mutarotation is a deviation from the specific rotation, due to the change in the equilibrium between α anomeric and β anomeric form, in the aqueous solution.
The specific rotation (optical rotation) of the aqueous solution depends on the optical rotation of both the anomers and their ratio in the aqueous solution. For carbohydrates to show mutarotation, it must be hemiketal or hemiacetal.
Usually α, β anomers of carbohydrates are stable solids, but in the aqueous solution, they undergo an equilibrium process to give a mixture of two forms.
When crystals of pure α-D-glucose are dissolved in water, the specific rotation gradually changes from +112.2 to +52.7.
• When crystals of pure β-D-glucose are dissolved in water, the specific rotation gradually changes from +18.7 to +52.7
• This change in rotation occurs because, in water, the hemiacetal opens to form the aldehyde and, when the aldehyde recyclizes, both α -D-glucose and β-D-glucose can be formed. • Eventually, the three forms of glucose reach equilibrium concentrations.
• The specific rotation of the equilibrium mixture is +52.7
• This is why the same specific rotation results whether the crystals originally dissolved in water are α - D-glucose or β-D-glucose.
When 100% α-D-glucose the specific rotation changes from +112.2 to 52.7
When crystals of pure β-D-glucose are dissolved in water, the specific rotation gradually changes from +18.7 to +52.7
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