Introduction

The phenomenon of mutarotation was discovered in 1846 while a scientist was learning about the properties of glucose, a carbohydrate. Carbohydrates are the most abundant biological molecule on Earth. They are a vital part of people’s healthy diet and perform functions essential to life. Glucose and fructose are the main sources of energy supplied to the body. Ribose and deoxyribose are essential entities in the structural formulation of the genetic material of the living organism. While polysaccharides are involved in building the cell wall structure of plants and bacteria.

Despite the in-depth understanding of the functions of carbohydrates and intensive studies of their properties, there are still many uncovered territories in carbohydrates studies. One such property also includes mutarotation. Though it’s been more than 150 years since the discovery of this phenomenon, it’s still an ongoing research area in many labs.

This article covers the journey of discoveries of different aspects of mutarotation, its definition, and some examples of molecules exhibiting mutarotations. But, before moving to the concept of mutarotation, one must understand the physical properties of carbohydrates including isomers, as the phenomenon of mutarotation is completely related to the structures of molecules.

Properties of Carbohydrates

Carbohydrates exist in both linear and cyclic structural forms. The cyclic structure is produced as a result of an intermolecular reaction between the carbonyl group and the hydroxyl group. The different arrangements of functional groups of carbohydrate molecules cause a shift in its many properties including mutarotation. So, it’s become essential to learn about these different arrangements and assigned terms to avoid any confusion.

  • Chiral carbon: A carbon atom containing four different atoms or groups attached to it (showing asymmetry) is known as chiral carbon. This carbon is essential to observe the phenomenon of mutarotation in compounds.
  • Isomers: Two molecules having the same molecular formula but different chemical properties. The concept of mutarotation is related to the arrangement of functional groups around carbons. Therefore, one must understand different terms related to isomers.
  1. Structural (constitutional) isomers: Two isomers having the same molecular formula but different arrangements of functional groups.
  2. Stereoisomers: Two isomers having the same molecular formula but different spatial arrangements of the groups are called stereoisomers and the phenomenon is known as stereoisomerism. These are classified into two groups:
  • Enantiomers: Two isomers that are non-superimposable mirror images of each other.
  • Diastereomers: Two isomers that are neither superimposable nor mirror images of each other.

Figure: A simple classification of isomers.

Mutarotation

Mutarotation is a change in the optical rotation of a solution due to a change in the equilibrium between alpha (ɑ) and beta (β) anomers, upon dissolution in the aqueous solution.The process is also known as anomerization.

The concept of mutarotation is related to the optical rotation and activity of the compounds dissolved in the solution. What are these terms? To better understand the concept of mutarotation, some of the frequently used terms are explained below.

What are anomers?

Anomers are epimers (diastereomers differing at one carbon) that differ from each other in the configuration of C1 if they are aldoses and in the configuration of C2 if they are ketoses.

What is optical rotation?

Optical rotation is the angle through which the plane of polarized light moves or rotates when a linearly polarized light travels through a layer of liquid or certain other materials.

What’s polarized light?

When light is transmitted through certain media, the vibrations of the light waves occur in a single plane. This is called the polarization of light.

What does equilibrium between anomers mean?

It’s a state of balance between two forms of a compound (alpha and beta) in a solution.

What is optical activity?

It’s the ability of a compound to rotate the plane of polarized light. It occurs in the compounds possessing chirality (asymmetry or compounds which lack mirror symmetry). The optical activity of chiral compounds is generated when the electromagnetic radiation of polarized light interacts with the asymmetric field of electrons of these compounds.

The compounds are known to be optically active when they rotate the linearly polarized light. And it must be noted that all optically active compounds have their own specific rotation.

History and Origin of The Concept of Mutarotation

The phenomenon of mutarotation was discovered in 1846 by Augustin Pierre Dubrunfaut. His whole study was based on sugars. He noticed that when sugar is dissolved in water, its optical activity changes in time. From the value of 110°, it decreases to 52°. He named this phenomenon “birotation”. Later, in 1899, Lowry gave it a more appropriate name “mutarotation”, which signifies the concept behind it.

After this, several other scientists got involved in understanding the phenomenon of mutarotation. The series of discoveries answered many questions like “how does it occur? Why does it occur? Does it occur in other mediums as well? Is this phenomenon observed in other compounds as well? Is it observed in all kinds of sugar? If no, why not?” The answer to these questions is explained as a different subtopic in this article.

Given below is a series of events that occurred after the discovery of mutarotation, for an in-depth understanding of this phenomenon.

S. no. Scientist’s name Year Observations
1. E. O. Erdmann 1855 ●     Lactose occurs in two crystalline modifications, one having higher rotation and the other, a lower.

●     Each form of lactose shows mutarotation towards the same final rotation, which is 52°.

●     Measured the value to which each form of lactose mutarotate towards the stable value of a stable solution.

2. Anthon 1859 ●     The formation of a saturated solution of glucose in cold water is very slow even after vigorous mixing.
3. Mills and Hogarth 1879 ●     The property of a compound to slowly form a saturated solution in the cold water is due to a slowly balanced chemical reaction, that involves the carbonyl group.

●     The carbonyl group is involved in the process of mutarotation.

●     The slow process of the solution is the property of all the aldehyde and ketone sugars.

●     First attempt to find the physical laws that govern mutarotation.

4. Urech 1882- 1855 ●     Mutarotation follows the law of unimolecular reactions.
5. Brown and Morris and Arrhenius 1888 ●     The reaction which causes the mutarotation is not a polymerization or dissociation of the sugar.
6. Charles Tarnet 1895 ●     A new form of crystalline glucose having a specific rotation of less than 52°.

●     The final rotation of the glucose solution is the same, independent of its initially rotating form.

●     Isolated new crystalline forms of rhamnose, galactose, and arabinose

7. Lowry 1899 ●     The mutarotation of glucose is caused by a balanced reaction between the highest and lowest rotating forms.

Mechanism of Mutarotation

The discovery of mutarotation, which began with glucose, had opened a wide area of research for scientists to study this phenomenon in other crystalline sugars. Later, it was found in lactose, galactose, arabinose, maltose, xylose, fructose, fucose, rhamnose, mannose, rhodeose, gentiobiose, melibiose, perseulose, and several rare synthetic sugars. In conclusion, all reducing sugars undergo mutarotation in an aqueous solution and non-reducing sugars don’t possess this property.

What’s reducing sugar?

Any sugar that has a free aldehyde or ketone group is considered as reducing sugar. They are also called hemiacetal compounds. These sugars are in equilibrium with the open-ring form of the molecule. They contain an aldehyde group which acts as a reducing agent towards certain metal salts. For example, they reduce copper ion (Cu2+) in Benedict’s test and Fehling solution and silver ion (Ag+) in Tollen’s test.

What is a non-reducing sugar and why don’t they show mutarotation?

Non-reducing sugars don’t have any free aldehyde or ketone groups. This is also the reason why they are not oxidized by a weak oxidizing agent or do not possess reducing power. Examples include sucrose and trehalose.

To understand the concept of mutarotation in non-reducing sugars, let’s take the example of sucrose. Sucrose is formed by a condensation reaction between a glucose molecule and a fructose molecule. The condensation reaction involves the anomeric carbons of glucose and fructose that lead to the formation of an O-glycosidic bond between the two molecules. For mutarotation to occur, a compound must have a free-anomeric carbon. But in sucrose, both the anomeric carbons are involved in the formation of glycosidic linkage, because of that, they don’t exhibit the phenomenon of mutarotation.Non-reducing sugars don’t have any free aldehyde or ketone groups. This is also the reason why they are not oxidized by a weak oxidizing agent or do not possess reducing power. Examples include sucrose and trehalose.Figure: A structural diagram of sucrose showing its two anomeric carbons involved in the O-glycosidic linkage.

The Occurrence and Measurement of Mutarotation

Mutarotation involves the mechanism of ring-chain tautomerism. The two different cyclic hemiacetal forms of sugars establish a state of equilibrium with the linear form. It means that even if a compound is 100% pure (containing only one form when it’s dissolved in water), it undergoes the equilibrium state with its linear pattern.  For example, when a 100% alpha-glucose form is added to water, it unmasks itself into a straight chain (or linear pattern). And, when it reforms, it can either change into an alpha form or beta form. Further, after some time, an equilibrium state is achieved between both forms that show that the reaction follows the zeroth law of thermodynamics.

The alpha and beta anomers of the sugars have different specific rotations. A liquid solution of the pure alpha compound will rotate with a different angle and in the opposite direction to that of the solution of the pure beta compound. The individual value of the optical rotation of each anomer and their ratio in the solution determines the optical ratio of a solution.

The optical rotation of the sample is weighed by taking the sum of the optical rotation of each monomer. A polarimeter is used to measure the rotation of a sample. This can also be used to calculate the ratio of two forms of a compound (anomers) present in the solution.

Mutarotation in Glucose

The first kinetic study of glucose was proposed by Bronsted and Guggenheim, where they proposed that the mutarotation of glucose may follow either acid or base catalysis. Glucose has two diastereomeric forms, alpha and beta forms. Both forms are different in physical properties. For example, the addition of methanol causes the crystallization of D-glucose into alpha-D-Glucose, which melts at 46 °C.  Whereas, the acetic acid addition forms the beta form of the sugar that melts at 150 °C. The specific rotation of the alpha-beta form is +112.2 and +18.7.Figure: The figure shows the mutarotation of glucose anomers. .

So, when a cyclic glucose molecule is added to water, it undergoes reversible epimerization to another form of glucose via the linear open-chain form. Here, the specific rotation of the solution gradually changes to +52.7°, which shows the equilibrium state. The mixture is composed of 36% alpha-D-glucose, 64% beta-D-glucose, and 0.02% open-chain glucose. The three steps involved in the interconversion of the pyranose form of glucose to the aldehyde form are:

  1. The protonation of O5
  2. The breaking of the O1-H bond (intramolecular transfer)
  3. The breaking of the O5-C1 bond

Figure: The schematic diagram showing the three steps of chemical reactions involved in the interconversion of glucose molecules from one form to the other.

Mutarotation in Other Carbohydrates

Mutarotation in Lactose

Lactose is a reducing sugar. It’s a disaccharide molecule composed of one glucose molecule and one galactose molecule linked by beta (1-4) glycosidic linkage. It is commonly known as milk sugar. It undergoes mutarotation in an aqueous solution due to the presence of anomeric C1 of glucose residue. At equilibrium, the mixture is composed of 62.7% beta-lactose and 37.3% alpha-lactose.  In solid-state lactose, mutarotation is observed after heating monohydrate crystalline samples.Figure: The two structural forms of lactose achieved after mutarotation in an aqueous solution.The mutarotation of lactose is a first-order reaction and because of the higher solubility of lactose, the rate of mutarotation is slow. The rate of mutarotation is also affected by the presence or absence of sugar and salt in the solution. For example, if the concentration of sucrose is added above 40% of the solution, the mutarotation rapidly decreases to half the normal rate of specific rotation.

Some other factors that influence the mutarotation of lactose include:

  1. The temperature of the solution (rate of mutarotation is slow at low temperature)
  2. The wavelength of the light source
  3. The concentration of the solution

Mutarotation in Fructose

Fructose is a monosaccharide that is present in both combined and free-form. Unlike glucose, fructose contains a ketone carbonyl group. The change from one form of fructose to another form occurs in the presence of acidic protons (change in pH) or thermal excitations (temperature change). The open-chain form of fructose molecule forms two different structures, β-fructopyranose, and β-fructofuranose. Both these molecules have different optical activity due to their structural differences, one is a 6-ringed structure molecule and the other is a 5-ringed structure molecule.Figure: A schematic presentation of the mutarotation of the fructose molecule via its linear-chain form.

Techniques to Study the Mutarotation of Compounds

The two techniques used to study the phenomenon are polarimeter and dielectric spectroscopy.

  1. Polarimeter: It’s an instrument used to measure the angle of rotation or optical activity of compounds. It occurs after a polarized light is passed through an optically active substance. It can also measure the ratio, purity, and concentration of two enantiomers. It was discovered in 1808 by Etienne-Louis Malus.
  2. Dielectric Spectroscopy: It is a technique that is used to measure the dielectric properties of a sample when subjected to an applied electric field of a fixed or changing frequency. Today, it’s used in labs to explore various scientific fields including fuel cell testing, biomolecular interaction, and microstructural characterization.

Conclusion

Carbohydrates have been intensively studied for a long time because of several mysterious phenomena and properties. One such phenomenon is mutarotation. It is the tautomerization of sugar molecules when added to an aqueous solution. It’s also known as anomerization. It is only observed in hemiacetal compounds due to intermolecular interaction. The sugar, added to the aqueous solution, equilibrate between its two forms via an open-chain structure.

The phenomenon is extensively studied in the liquid medium but its mechanism is still not well understood. It’s an ongoing research area where researchers are also trying to understand the mechanism of this behavior in supercooled liquid states and glass mediums. So, the study of mutarotation and its kinetic reactions builds a solid scope for young researchers interested in the field.

References

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