Epimers: The Epimeric Diastereomers

Epimers are stereoisomers, and to be exact, they are a pair of diastereomers with more than one chiral center but differ only at one of those chiral centers. Diastereomers are non-superimposable and non-mirror, images of the same organic compound. They are the carbohydrates with a difference in orientation of one of the hydroxyl (-OH) groups.

A suitable example of diastereoisomer is D-glucose and D-galactose. These monosaccharides are dextrose (D) sugars and with just a difference of the orientation of one hydroxyl group (on carbon no 4), their identification changes. Such diastereoisomers are known as epimers.

Epimers as D glucose and D galactose

For example, D-glucose and D-galactose are C-4 epimers because they have an epimeric carbon (point of difference carbon) at position 4.

Usually, the chiral center having a difference among epimers is written with carbon number to mark the identity of absolute difference point.

Examples of epimers are;

  • D-ribose and D-arabinose (C-2 epimers)
  • D-glucose and D-mannose (C-2 epimers)
  • D-idose and D-talose (C-3 epimers)
  • D-glucose and D-galactose (C-4 epimers)
  • D-glucose and L-idose (C-5 epimers), etc

A structural explanation of above listed epimers is given below:

Epimers on position 2, C-2

D-ribose and D-arabinose are C-2 epimers. it means that they have an absolute difference on carbon number 2. The rest of all structures are necessarily the same. Ribose is one of the sugars of human purine bases and the idea of this small difference to create a diastereomer, an epimer, has been used to synthesize cytosine arabinoside, an artificially designed sugar base similar to human cytosine deoxyribose.

Epimers on position 3, C-3

D-idose and D-talose are C-3 isomers, meaning an absolute difference on third carbon. Idose is a six-membered monosaccharide (hexose). Similarly, talose is also an aldohexose, but an unnatural sugar. It is soluble in water and is slightly soluble in methanol.

Epimers on position 5, C-5

D-glucose and L-idose are epimers with the C-5 epimerization.

The epimers in which the stereogenic center is also the carbonyl carbon in open chained form, those epimers are known as anomers. Such a carbon atom is known to be an anomeric carbon. So all anomers are in fact, epimers first.

Related topics

Epimers of Glucose

The epimers of glucose are;

  1. D-glucose and D-mannose (C-2 epimers)
  2. D- glucose and D-Allose (C-3 epimers)
  3. D-glucose and D-galactose (C-4 epimers)
  4. D-glucose and L-idose (C-5 epimers)

Although glucose is an aldose, several ketoses like fructose also have epimers i.e. D-Fructose and D-Psicose (ribo-2-hexulose).

Epimeric pair (Summarizing Epimers)

To summarize;

  • Epimers are a special class of diastereoisomers.
  • Two diastereoisomers that differ in configuration around only one of the stereogenic centers are termed epimers.

Epimerization

Epimerization is a stereochemical process in which a change in the configuration of orientation of functional groups takes place over a parent structure. This process creates diastereoisomers, and for the creation of one specific type of diastereomers, that change is brought over just a single carbon atom. This becomes the identity of those epimers like C-2, C-3, C-4, etc.

Epimerization is generally a slow process but spontaneous, however, some epimerizations can be catalyzed by enzymes.

Examples of Epimerization

  • Carbohydrate epimerization is a very important technique used to cope with the widespread demand for some types of sugars.
  • N-acetylglucosamine and N-acetylmannoseamine are epimers. This epimerization is catalyzed by renin-building protein.

Epimerization in medicines

  • Tetracycline undergoes epimerization at position 4, to make rather inactive epitetracycline.
  • Doxorubicin-Epirubicin epimerization.
  • Cephalexin epimers.
  • Chenodeoxycholate-Ursodeoxycholic acid (UDCA) epimerizations, etc.

Mutarotation

Due to an equilibrium change, the change in optical properties of a chemical compound i.e. change from one form of anomer to another is termed mutarotation in stereochemistry. cyclic sugars show mutarotation when subjected to a disturbance in equilibrium rate. In actuality, mutarotation is the change in orientation of branches present on the stereogenic center.

As mutarotation involves equilibrium conditions, it is evident that this phenomenon is concentration dependent.

For example, α-D-glucopyranose and β-D-glucopyranose show mutarotation over time. This can be observed by optical properties measurement techniques. Although both of these compounds are dextrorotatory (+), but still they have a large difference among their optical values. α-D-glucopyranose shows +52.7° rotation in clockwise direction and β-D-glucopyranose shows a +18.7° rotation in the same direction.

Key Takeaway(s)

Epimers are the particular diastereomers, the non-mirror, and non-superimposable mirror images but this change is necessary to be put on one stereogenic center only.

If this stereogenic center is also the carbonyl carbon in open chained form, these epimers are then called anomers.

Concepts Berg

What are epimers in biochemistry?

Several sugar structures in biochemistry are diastereomers. Out of those diastereomer structures, some are only different from each other at absolute single point. These structures are known to be epimers.

What is the difference between epimers and enantiomers?

Epimers are diastereomer, so they are non-superimposable, non-mirror images. Enantiomers, on the other hand, are non-superimposable mirror images.

What is Mutarotation? Explain with example.

Mutarotation is the conversion of an anomer to its other form. It involves the interconversion of stereocenters.

What are the differences between epimers and isomers?

Isomers are same chemical compounds with same molecular formulae and different structural formulae. They are differentiated into many types and one of those types is stereochemistry. Now in that stereochemistry, epimers are limited to diastereomers. So in fact epimers are a type of isomers.

What are Epimers-Anomers?

As epimers are diastereomers with the point of different at a single position. Now if that specific positioned carbon, the stereogenic center is also a carbonyl carbon in open chain form, it is called as epimer-anomer.

Which two sugars are epimers?

Sugars having epimeric structures are as:

  • Glucose and Mannose (C-2)
  • Ribose and Xylose (C-3)
  • Glucose and Galactose (C-4), etc.

Can epimers be called isomers?

Epimers can also called isomers but that is a very broad term with respect to epimers. Epimers are sophisticated, less numbered, and very absolute, while isomers have many many types.

Are glucose and fructose epimers?

Glucose and fructose are not epimers. They both have different numbered rings, and different diastereomers.

References

  • Organic Chemistry (Fourth edition), Francis A. Carey, University of Virginia
  • Organic Chemistry (Third Edition), Janice Gorzynski Smith, University of Hawai’i at Manoa
  • Organic Chemistry (Fourth edition), Paula Yurkanis Bruice, University of California, Santa Barbara
  • Epimerization (Science direct)

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