Mutarotation is a chemical phenomenon, commonly used in carbohydrate chemistry. Mutarotation was discovered by Dubrunfaut, a French chemist in 1844. He noticed that there was a change in the specific rotation of sugar, in an aqueous solution, with time. To understand the concept of mutarotation, one must know about isomers.
Isomers are two molecules, which have the same molecular formula but different chemical properties.
Two isomers have the same molecular formula but differ in the arrangement of the functional groups.
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 centres.
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.
Examples for mutarotation:
1. In aqueous solution, D – Glucose exists as a 36% α- D glucose and 64% of β-D glucose
When β-D-glucopyranose is dissolved in water, it rotates a plane-polarized light by +18.7°. Some amount of β-D-glucopyranose undergoes mutarotation, to give α-D-glucopyranose and it turns a plane-polarized light by +112.2°. The equilibrium mixture of the solution contains about 36% of α-D-glucopyranose and 64% of β-D-glucopyranose.
The mutation occurs when the anomeric position (C1) changes its configuration between α and β form in the solution. As a result, carbohydrates undergo ring-opening to form hemiketal (aldehyde form) and reforms into a hemiacetal (closed ring).
Fructose (hemiketal) and Glucose (hemiacetal) undergo mutarotation. But sucrose and cellulose are not performing mutarotation, because of the absence of OH functional group at the anomeric position.
2. Mutarotation of lactose
The lactose is a disaccharide having an ordinary name milk sugar (reducing sugar), which comprises glucose molecule and galactose molecule linked by β(1→4)-glycosidic linkage. Since lactose has beta acetal, it undergoes mutarotation at 20 °C and its equilibrium mixture comprises 37.3 % α-lactose (β-D-galactopyranosyl-(1→4)-α-D-glucopyranose) and 62.7 % β-lactose (β-D-galactopyranosyl-(1→4)-β-D-glucopyranose).
- Mutarotation is a difference in the specific rotation of plane-polarized light, due to the change in the equilibrium between two anomers in the solution.
- Any molecule to show mutarotation must have hemiketal or hemiacetal group.
- Mutarotation property was first observed in sugar.
- The specific rotation of sugar in aqueous solution varies from sugar to sugar.
1. Does sucrose undergo mutarotation?
An ordinary table sugar (Sucrose) is an example of disaccharides, having α-D-glucopyranose and β-D-fructofuranose linked by a glycosidic bond at their anomeric carbons. Sucrose is not a reducing sugar and because of the absence of the hemiacetals, it does not undergo mutarotation.
2. What is the similarity between D- Lactose and D-maltose?
a) both have a potential aldehyde group that can be oxidized.
b) both contain one pyranose and one furanose.
c) both have α and β anomers that can undergo mutarotation.
d) both contain a hemiketal bond in their structures.