What is the Mutarotation?

The rotation of a particular structure of the chiral compound because of the epimerization is called mutarotation. It is the repercussion of the ring chain tautomerism. In terms of glucose, this can be defined as the modification in the equilibrium of the α- and β- glucose anomers upon its dissolution in the solvent water. This process is usually seen in the chemistry of carbohydrates.

Isomers

Isomers are two molecules, which have the same molecular formula but different chemical properties.

Structural isomers

Two isomers having the same molecular chemical formula but differ in the arrangements of the functional groups present.

Stereoisomers

Two isomers have the same molecular formula but vary in the group’s spatial arrangement. Stereoisomers are further classified as:

• Enantiomers are non-superimposable mirror images.
• Diastereomers are neither superimposable nor mirror images. Diastereomers have varied configurations at the stereoisomeric centers.

Mutarotation

A phenomenon in which one of the tautomers is represented in a cyclic form is called tautomerism of the ring chain structure. Mutarotation was discovered by Augustin Dubrunfaut. It is defined as a rotation of the anomer’s optical structure when dissolved in a substance called the solvent. This particular solvent should be an aqueous solution. The solvent’s polarity is also significant.

Mutarotation is promoted by the polar solvents. The mixture that is resulted contains a ratio of β and α sugar anomers. Open chain aldehyde also exists in the mixture but it does not exhibit any features. Mutarotation is specifically led by reducing sugars. The sugar and the salt present in the system are capable of influencing mutarotation.

Mutarotation plays an important role in biochemical reactions. It also plays a significant role in determining the crystallization rate of sugars. The pH as well as the temperature both affects the rate of mutarotation. D-glucose and D-lactose interconvert between anomer forms and tautomeric forms. While interconversion between the rings occurs, it has to pass through a straight-chain form through the ring-opening and ring-closing reactions. These reactions are acid-based catalyzed. The kinetics of crystallization is affected by mutarotation.

Mechanism

The carbohydrates or sugars are known to form a cyclic ring structure as well as an open-chain form. Because of such a tendency of flipping between the cyclic structure to open chain structure and vice versa, they are found to form anomers. The formation of anomers leads to the rotation of the plane-polarized light to a different extent in the solution. The process of converting one anomer to another anomer is termed anomerization and is responsible for the difference in the total optical activity of the carbohydrates. The carbohydrates forming alpha (α) and beta (β) -anomers maintain an equilibrium which results in a certain rotation of the molecule, but if this equilibrium mixture is disturbed due to anomerization, the specific optical activity gets changed, leading to mutarotation.

Ring chain tautomerization

Laar found the word ‘tautomerism’ in the year 1886. This word was used for describing the equilibrium of two similar compounds. In ring-chain tautomerism, one is cyclic. This should contain a minimum of two functional groups. This phenomenon is a reversible isomeric change. A particular functional group out of the 2 functional groups should contain the property of double bonds and other functional groups should be able to perform reaction called additive reactions at multiple bonds.

Dubrunfaut who was a French chemist 1 st discovered the mutarotation phenomenon in a factory where he was involved in the alcohol production from the source of beet sugar. This phenomenon is explained as a modification in the optical rotation of β-and α-anomers due to the process of epimerization. The α-and β- anomers of glucose are capable of rotating optically when dissolved in the solvent water. Mutarotation is considered a general property of chiral cyclic hemiacetals and sugars. Fructose is referred to as the most rapidly muta-rotating sugar.

Glucose Mutarotation

The α- and β- glucose anomers undergo the phenomenon of mutarotation through an open-chain aldehyde form. The glucose’s mutarotation occurs only in anomeric forms. There are certain aldohexoses that are capable of forming 4 specific mixtures of anomeric furanoses and pyranoses. The α-D-glucose has exhibited an optical rotation of +112° in the molecule of water and the β-D-glucose has exhibited an optical rotation of +19°. The rotating value of α- and β- glucose anomers come to +52.5°. When analysis of solutions take place, it was found that the anomers in which rotation happened were not a hundred percent α- and β- anomeric forms of the particular compound, both the anomeric form were in a certain ratio. The mixture usually consists of around 36% α-D-glucose and around 64% β-D-glucose and precisely less than 0.02%of the hydrocarbons of the open chain of an aldehyde form at the equilibrium stage. A ratio of 36:64is the ratio of α- to β- anomers of D-glucose was observed in the water at 77°F. The planar hexagons molecules are drawn using darkened edge which represents the side facing the viewer. The molecules are drawn in the Fisher projection or Haworth projection. The functional group that was written in right in the Fisher projection seems to be below the plane in the projection of Haworth and vice versa. The difference in α and βglucose anomeric forms is often significant in biochemical reactions. The reaction of glucose mutarotation is usually an acid-base catalyzed wherein two protons are transferred.

Lactose Mutarotation

Lactose is a reducing sugar. It is a disaccharide. The monosaccharide galactose and fructose are linked through β- linkage between the first carbon C1 of galactose and the fourth carbon C4 of fructose. The lactose molecule exhibits mutarotation due to the presence of anomeric C1 of the glucose residue. The temperature of the solution, the wavelength of the light source, and the solution’s concentration are significant for the optical rotation of the lactose structure. The mutarotation rate is slow at low temperature but tends to increase as the elevation of temperature occurs. The rate becomes instantaneous at about the temperature of 167°F. Lactose undergoes mutarotation in an aqueous solution. The equilibrium mixture is composed of 62.7% of β-lactose and 37.3% α-lactose, at 68°F. The mutarotation properties result in a different variety of crystalline states of lactose, that includes anhydrous α, and β pure forms, monohydrates, and various α, β defined molecular compounds. Mutarotation is observed in solid-state lactose after heating a monohydrate crystalline sample. The studies showed that lactose mutarotation is a first-order reaction. Lactose mutarotation is slow as it is much more soluble. The mutarotation rate is affected by the existence of salt and sugar in the specific solution. As the concentration of sucrose increases above 40%, mutarotation rapidly decreases to half the normal rate of specific rotation.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, specifically, for Biochemistry and Chemistry.