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. 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.
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. 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. 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. 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. The phenomenon of mutarotation was discovered in 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. After this, several other scientists got involved in understanding the phenomenon of mutarotation. 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? Given below is a series of events that occurred after the discovery of mutarotation, for an in-depth understanding of this phenomenon.
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. 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. 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. 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. Figure: A structural diagram of sucrose showing its two anomeric carbons involved in the O-glycosidic linkage.
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. 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.
But sucrose and cellulose cannot- they are not hemiacetals or hemiketals. They do not have an OH at the anomeric position. You have JavaScript disabled!
MendelSet works best with JavaScript enabled. Please enable JavaScript. What types of sugars can undergo mutarotation? Why does a hemi-acetal open under neutral pH but a full acetal requires acid to open? I assume it must be a hydrogen bond between water lone pairs and the hydrogen on the hydroxyl of the hemi-acetal.
This would weaken the OH bond and facilitate ring opening. Though I am not sure if this applies when it is the solvent doing the catalysis. What is the reason behind this? Does impurity affect the mutarotation of glucose, are the results for a pure and impure glucose be different after mutarotation. Yes, impurities could dramatically affect the mutarotation of glucose. The rotation values given are for pure compounds, for which an accurate calculation of concentration can be made.
If the sample is impure and especially if it is contaminated with an optically active impurity the measured number will be way off. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. This site uses Akismet to reduce spam. Learn how your comment data is processed.
Next Reducing Sugars. Each of these two forms can be synthesized and isolated as pure compounds. Yes — if it is indeed the same molecule!
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