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Mutarotation

Carbohydrates are polyhydroxy aldehydes and ketones which can be represented with different configuration. We know that an organic molecule will be optically active if there is no plane of symmetry in the molecule and there is at least one asymmetric carbon atom in the molecule. The asymmetric carbon atom is bonded with 4 different atoms or groups.  The optical rotation plays a very important role in the determination of absolute configurations of sugar molecules because they have more than one asymmetric carbon atoms in the molecule.

According to stereoisomerism, an optically active molecule can exist in two isomeric forms which are known as dextro- and leavo- forms. The dextrorotatory form can rotate the plane polarised light in a clockwise manner whereas laevorotatory isomer can rotate the plane polarised light anticlockwise manner. Both of these isomers show same chemical and physical properties but have different optical properties.

Carbohydrates can also classify on the basis of optical rotation. Sugar molecules also show mutarotation which play an important role in the determination of structure of a carbohydrate. Carbohydrate is an important class of naturally occurring organic compounds such as glucose, fructose, maltose, sucrose, lactose starch, etc. The general formula of sugar molecules is $C_{m}(H_{2}O)_{n}$. Let’s discuss about mutarotation in sugar molecules.

 

D and L Designations - Optical Isomers

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Optical Isomers


An optically active compound is one which can rotate a plane polarized light to right or left. An optically active compound can exist in two isomeric forms which rotate the plane-polarized light in opposite directions. The isomer which rotates the light to right are called dextro rotatory or (+) form and the other one, is leavo or (-) form.

Glyceraldehyde contains a central asymmetric carbon atom. Therefore, it exists in two enantiomers, or mirror image isomers.
Optical Isomers

The enantiomer which rotates the plane of polarized light to right is written as (+) isomer and the other, which rotates plane polarized light to left is (-) isomer. The + and - sign indicates the direction of plane polarized light To left or right.

In 1906, Rosanoff decided arbitrarily that the enantiomer (1) with OH to the right may be designated as D- glyceraldehyde and the other enantiomer (2) (as
L-glyceraldehyde.

This was similarly applied to the carbohydrates. The sugars are thus, divided into the D- family and the L-family. The sugars having same configuration as of D- glyceraldehyde at the asymmetric carbon most distant from the carbonyl group are designated as D- sugars. Those with opposite configuration are called as L- sugars. Thus, natural glucose is D(+) Glucose.

What is Mutarotation?

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The change in the optical rotation of a solution of either form f glucose until a constant value is obtained is called Mutarotation.
Many sugars, like glucose, fructose, etc exist in α and β form and undergo mutarotation. Example: α -D fructose has a specific rotation of -21o , β - D fructose -133 o and constant value 92o .

Mutarotation of Glucose

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Glucose is the most common monosaccharide. It is known as Dextrose because it occurs in nature principally as the optically active dextroroatory isomer. It is a white crystalline solid, with a melting point of 146o .

Glucose is optically active and the ordinary naturally occurring form is (+) glucose.
Two crystalline forms of D-glucose have been isolated.
  1. α - D glucose which crystallizes from a concentrated aqueous solution at 30oC and it has a melting point of 146 degree Celsius and specific rotation of +112o .
  2. β- D glucose which crystallizes from a hot, glacial acetic acid solution. It melts at 148 to 150 degree Celsius and has a specific rotation of +19 degrees.
When either of these forms of D- glucose is dissolved in water and allowed to stand, a gradual change in specific rotation occurs. The specific rotation of the alpha form falls and that of the beta form rises until a constant value of + 53o is obtained.

Thus, two forms of glucose undergoes a phenomenon called as mutarotation. The form with the higher positive rotation is called α-D glucose and that with the lower rotation β-D glucose.
Mutarotation Glucose

Explanation


Glucose forms a stable cyclic hemiacetal between the aldehyde group and the -OH group on the fifth carbon of glucose. In this process the first carbon becomes asymmetric, giving two isomers (I and II) which differ only in the configuration of the asymmetric carbon.



These are called as anomers and the new asymmetric carbon is referred to as Anomeric carbon. The anomer I with OH to the right is designated as the α - D glucose and the other with OH to the left is called as β-D glucose.

The mutarotation occurs because of the slow interconversion of α - D glucose via the open chain form until equilibrium is established giving a constant specific rotation of + 53o. The concentration of the open chain form and the cyclic forms I and II at equilibrium are 0.01% , 36% and 64% respectively.

Confirmation of Mutarotation


D- (+) Glucose forms two isomeric methyl D-glucosides. Aldehydes, react with alcohols in the presence of anhydrous HCl to form acetals. If the alcohol is, methanol, the acetal contains two methyl groups.
Confirmation of Mutarotation

  1. When D- (+) - glucose is treated with methanol and HCl, the product, methyl D- glucoside, contains only one -CH3 group, yet it has properties resembling those of a full acetal. It does not spontaneously revert to aldehyde and alcohol on contact with water, but requires hydrolysis by aqueous acids.
  2. Furthermore, not just one but two of those mono methyl derivatives of D- (+) glucose are known, one with melting point of 165o C and specific rotation +158o and the other with melting point 107oc and specific rotation -33o.
  3. The isomer of higher positive rotation is called methyl α - D glycoside and the other is called methyl β - D glycoside. These glycosides do not undergo mutarotation and do not reduce Tollen's reagent or Fehling's solution, as Glucose.
  4. The two isomers, or anomers, α - D glucose and β- D glucose are diastereomers, differing in configuration about C-1 (first carbon). The mutarotation of these isomers or anomers results from the ready opening and closing of the hemiacetal ring.

Cyclic Structure of Glucose

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  1. The phenomenon of mutarotation was helpful in deciding the cyclic structure of d- glucose.
  2. The french chemist, Tarnet established the existence of two crystalline forms of glucose, α - glucose and β - glucose.
  3. Alpha- glucose had a specific rotation of +112o and β - glucose, +19o .
  4. The optical rotation of each of these forms changed gradually with time till finally, a constant value of +53o was established.
  5. To explain this phenomenon of mutarotation, it was visualized that the α and β forms of glucose were in reality the cyclic hemiacetal forms of glucose which were inter convertible via the open chain form.
  6. The constant value of +53o represented the state of equilibrium between α and β forms of glucose.
  7. This fact established why D-glucose can react both as an aldehyde and a cyclic hemiacetal in which CHO is absent.
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