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
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 follows:
- Enantiomers: They are non-superimposable mirror images.
- Diastereomers: They are neither superimposable nor mirror images. Diastereomers have different configurations at the stereoisomeric centres.
Note: Epimer is a diastereomer, which 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, they 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 of Mutarotation
1. In an aqueous solution, D-Glucose exists as 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 re-form 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 an OH functional group at the anomeric position.
2. Mutarotation of lactose
Lactose is a disaccharide having the ordinary name, milk sugar (reducing sugar), which comprises glucose molecules and galactose molecules 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).
The mutarotation process is based on the mechanism of ring-chain tautomerism. The two different cyclic hemiacetal conformations of sugars establish a state of equilibrium with the linear configuration.
Thus, when one anomer is dissolved in water, it undergoes mutarotation which results in reaching the equilibrium state forming a linear chain. After some time, an equilibrium state is attained between both alpha and beta forms of the reducing sugar.
The specific rotation (optical activity) is different for different forms of sugar.
When the D-glucose is dissolved in water, the specific rotation of the glucose gets changes and reaches equilibrium (that is, alpha-D-glucose get decreased to +52.5° and beta-D-glucose get increased to +52.5°).
Thus, the addition of cyclic glucose in water results in a reversible epimerization the process to another form through the linear open-chain form. Finally, the solution contains 36% alpha-D-glucose, 0.02% open-chain glucose, and 64% beta-D-glucose.
The main steps involved in the interconversion of the glucose pyranose form to the aldehyde form include:
1. The protonation of oxygen at position 5.
2. Cleavage of the O1-H bond.
3. The breaking of the O5-C1 bond.
As the solubility of lactose Is high, the mutarotation process is slow. Other factors that affect mutarotation include the temperature, the wavelength and the absoption of the solution.
Techniques to Study the Mutarotation
The two techniques used to study the mutarotation are polarimeter and dielectric spectroscopy.
- Polarimeter: This instrument measures the angle of rotation or optical activity of compounds. Once the polarized light is passed through the substance the change in the optical rotation is measured. It also measures the concentration of enantiomers in the solution.
- Dielectric Spectroscopy: The change in the dielectric property after an application of an electric field is measured by dielectric spectroscopy.