What is the Mechanism of Mutarotation? 

The alteration that occurs in the optical rotation of an aqueous solution which arises due to the existence of change in the equilibrium between beta anomer and alpha anomer, is referred to as mutarotation (also known as anomerization). 

The mechanism of mutarotation was discovered during the analysis of glucose molecules in the year 1846 by Augustin-Pierre Dubrunfaut. This outstanding property of carbohydrate molecules is due to their existence in both cyclic and linear conformation. Thus, alteration in the functional group arrangement causes a change in the optical rotation resulting in mutarotation. 

Properties of Carbohydrates for Mutarotation 

• When a carbon atom is found to be bonded with four different functional groups or atoms, it is referred to as chiral carbon and it shows asymmetry. This carbon plays a key role in the mutarotation phenomenon.  
• Isomers are those compounds that have the same molecular formula but exhibits variants in their chemical properties. 
• Stereoisomers exist when there is an alteration in the spatial arrangement of the functional group while both molecules have the same molecular formula. This is further classified into enantiomer and diastereomer. 
  • Enantiomers: The two isomers exist as non-superimposable mirror images. 
  • Diastereomers: The two isomers do not have a mirror as well as superimposable images.  

Anomer 

The cyclic structures of carbohydrates are found to exist in two forms depending on the location of the substituent positioned at the anomeric center. The pair of forms is usually called "anomers" since they are isomers located at the anomeric center. To designate the cyclic structure of carbohydrate as α- or β anomer, the relative locations of the group and organization at the anomeric center is seen. In the alpha form, the exocyclic O group found at the anomeric center is on the opposing face to the group, and in the beta form, the exocyclic O group at the anomeric center is on the identical face as the group. If a mixture of the alpha and beta anomers is proximal, then this is often described by using a "wavy" line to describe the bond; the wavy bond to the group designates a mixture of the structures.  In general, the two models are found as stable solids, but in solution, they are found to undergo rapid equilibration. 

Anomers are heterogeneous in structure, and thus have distinctive stabilizing and destabilizing influences from each other. The major grantors to the durability of a certain anomer are:  

• The anomeric effect, which preserves the anomer that possesses an electron releasing group (typically an oxygen or nitrogen atom) in axial orientation on the ring. This consequence is eliminated in polar solvents such as water.  
• 1,3-diaxial interactions, which normally destabilize the anomer that has the anomeric group positioned in an axial orientation on the ring. This effect is particularly notable in pyranoses and other six-membered ring compounds. This is found to be a major factor in water. Hydrogen bonds positioned among the anomeric group and other organizations on the ring, causing the stabilization of the anomer. Dipolar repulsion taking place among the anomeric group and other groups on the ring, pointing to destabilization of the anomer. 

Reducing and Non-Reducing Sugars 

The presence of free ketone or aldehyde (functional group of the anomeric carbon) in the molecule is the reducing sugar. Lack of free ketone or aldehyde group forms the non-reducing sugar. 

The availability of free anomeric carbon is essential for the mutarotation and thus non-reducing sugar does not exhibit mutarotation. 

Mutarotation Mechanism 

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. 

• Glucose (monosaccharide) is the main source of energy for the body as its metabolism generates ATP. Glucose exists in two different diastereomeric forms namely alpha and beta anomer, which possess different physical properties. When the hydroxyl group on C-1 is on the opposite side of the ring as the chain on C-5, then it is considered as alpha (α) anomer. When the hydroxyl group on C-1 is on the same side of the ring as the chain on C-5, then it is considered as a (beta) β form. In water, the alpha form of D-glucose has a specific rotation of +112 degrees, and that beta form has a specific rotation of +19 degrees.  

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. 

• Lactose undergoes mutarotation in an aqueous solution due to the availability of anomeric C1 of glucose. At equilibrium, 37.3% exist as alpha-lactose and the remaining 62.7% as beta-lactose. 

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. 

• Fructose is a ketone carbonyl group and exists in two forms β-fructopyranose, and β-fructofuranose. Both of these molecules possess structural differences, that is, one molecule is a 6-ringed structure while the other form is a 5-ringed structure. Due to this structural difference, there occurs a variation in the optical activity and thus possesses mutarotation. 

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. 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.