Epimers

Epimers are a particular kind of stereoisomer, which is an isomer with the same chemical formula but a different spatial arrangement of atoms. To be more precise, epimers are diastereomers subclass that vary in how their single stereocenter (chiral center) is arranged inside the molecule. In this article, we will learn about epimers, examples, epimerization, etc.

Epimers Definition

Epimers are forms of a sugar molecule that have the same molecular formula but differ in their spatial arrangement at one chiral center.

What are Epimers?

Epimers are isomers of carbohydrates that have the same chemical formula but different structural arrangements. They are types of diastereomers that vary in configuration at only one chiral centre and do not form mirror images of each other. There can be more than one chiral centre in a molecule; hence a molecule can give rise to many epimeric forms. Some common examples of epimers include D-galactose and D-glucose, ribose and arabinose, Mannose and glucose, etc. An epimer in an epimeric pair undergoes epimerization reaction to form the other epimer.

Examples of Epimers

Some common examples of epimeric pairs are discussed below.

D-Glucose and D-Mannose

The most common epimers are D-glucose and D-mannose. These sugars are aldohexoses and have the same molecular formula C₆H₁₂O₆. But if we look at their structure, we will find that the position of hydroxyl group at C2 carbon is different in these two sugars. These two sugars are called C2 epimers of each other.

D-Glucose and D-Galactose

An other common example of epimeric pair are D-glucose and D-galactose. These two sugars have the same molecular formula but the arrangement of the hydroxyl ion in these molecule differ at fourth carbon atom. Hence they are called C4 epimers of each other.

Ribose and Arabinose

Ribose, a component of RNA and arabinose, a component of plant polysaccharides like hemicellulose are also epimers. Both of these sugars are pentoses and have the molecular formula C₅H₁₀O_. But they differ in their configuration at C2 carbon. Hence, they are called C2 epimers of each other.

What is Epimerization?

Epimerization is a chemical process in which one epimer is converted into its counterpart in the pair. The process involves enzymatic catalysis or chemical reactions, leading to the change in configuration at the specific carbon atom in a sugar molecule. Epimerization is important in biological system as it ensures conversion of basic sugar forms into their epimers that serve as important components in various metabolic and cell signaling pathways. Apart from this epimerization has served as important basis in understanding drug metabolism and drug development.

What are Diastereomers?

Diastereomers are stereoisomers that are not the mirror images of each other. Diastereomers have same molecular formula but differ in their configuration at only one or multiple chiral centers. Diastereomers may or may not be optically active. They vary in their physical and chemical properties. A examples of diastereomeric pair is tartaric acid and mesotartaric acid.

What are Enantiomers?

Enantiomers are also stereoisomers but they differ in configuration at every chiral center. Unlike diastereomers, enantiomers are non-superimposable mirror images of each other and are always optically active. Based on the way they rotate the plane polarized light, enantiomers are classified to be dextrorotatory or levorotatory. Dextrorotatory compound will rotate the light clockwise whereas levorotatory compound will rotate the light counterclockwise. D-Glucose and L-Glucose are common examples of enantiomers.

What are cis-trans Isomers?

Cis-trans isomers are also known as geometric isomers. They have the same molecular formula but differ in the orientation of atoms or groups. Cis-trans isomerism is commonly observed in compounds with double bonds and in cyclic compounds. In a cis-isomer, functional groups are on the same side of the double bond or cyclic ring. In a trans-isomer, functional groups are on opposite sides of the double bond or the ring. Consider the structural configuration of cis-2-Butene and trans-2-butene in the image given below. Cis-2- butene has the methyl groups located on same side whereas in trans-2-butene the methyl groups on opposite sides.

What are Anomers?

Anomers are subtypes of epimers that vary in configuration at the anomeric carbon. They are commonly associated with cyclic sugars, where the linear form of the sugar undergoes intramolecular cyclization and its C1 carbon forms the chiral centre in the cyclic molecule. Anomers can be classified in two forms: alpha and beta. If the hydroxyl group at the anomeric carbon lies below the plane of ring then the anomer is in alpha (α) forms and if the hydroxyl group lies above the plane of ring then it is said to be in beta ( β) form. Anomers in their aqueous form exhibit mutarotation.

Difference between Epimers and Diastereomers

The difference between epimers and diastereomers are summarized in the table below.

Criteria	Epimers	Diastereomers
Definition	Epimers are a subtype of diastereomers. that differ in configuration at only one chiral centre.	Diastereomers are stereoisomers that are not mirror images of each other.
Structural configuration	Epimers exhibit difference in spatial arrangement at only one chiral carbon.	Diastereomers can exhibit difference in spatial arrangement at more than one chiral carbon.
Example	D-Glucose and D-Galactose	Tartaric acid and mesotartaric acid.

Difference between Anomers and Epimers

The table below summarizes the difference between anomers and epimers.

Criteria	Anomers	Epimers
Definition	Epimers that vary in configuration of molecule at the anomeric carbon.	Diastereomers that vary in arrangement of groups at a single chiral centre.
Example	α-D- Glucose and β-D-Glucose.	D-Mannose and D-Glucose.
Mutarotation	Exhibit mutarotation in aqueous solutions.	Mutarotation is not necessarily exhibited.

Difference between Epimers and Enantiomers

Both Epimers and enantiomers are stereoisomers. But they have significant differences which are listed in the table below.

Criteria	Epimers	Enantiomers
Classification	Epimers are subtype of diastereomers which are a type if stereoisomers.	Enantiomers are types of stereoisomer.
Configuration and molecular formula	Epimers have same molecular formula but they differ structurally at one chiral centre.	Enantiomers also have same molecular formula and their configuration differ at all chiral centres.
Structure	Epimers are not the mirror images of each other.	Enantiomers are non-superimposable mirror images of each other.
Example	D-Glucose and D-Galactose.	Dextro-lactic acid and Laevo -lactic acid.

Conclusion – Epimers

Epimers have immense significance in the biological systems because of various factors like difference in rate of their absorption and the way they participate in physiological processes. Their understanding helps in drug designing, determining the composition of nutritional compounds, understanding the underlying causes of various disorders, etc. Epimers can have several health implications like difference in cell signaling process, introducing allergic reaction, and various deficiency disorders like galactosemia. Apart from this understanding the stereoisomerism in epimers have several pharmaceutical application like development of therapeutic processes and customization of treatment.

FAQs – Epimers

What are Epimers?

Epimers are two forms of a carbohydrate molecule that differ in their configuration at one chiral centre. They are classified under diastereomers.

What are Epimers of D-Glucose?

D-mannose and D-Galactose are the epimers of D-glucose. D-mannose is an epimer of glucose at C1 carbon whereas D-Galactose is an epimer of D-Glucose at C4 carbon.

How to Identify Epimers?

Epimers can be identified by their structural configuration.Epimers will vary in configuration at only one chiral centre and rest of the structure of the molecule remains the same.

What are Diastereomers?

Diastereomers are stereoisomers that have same chemical formula but different spatial arrangements at some of their chiral centres.

What is the Difference between Epimers and Anomers?

Epimers are diastereomers that vary in their spatial arrangements at only one chiral centre. Anomers are epimers of cyclic sugars that differ in configuration at anomeric carbon.

What are Epimers and Isomers?

Epimers are type of stereoisomers that are non-mirror images and differ in spatial arrangement only at one chiral center. Isomers is a broader term that include the compounds that have same molecular formula but different structural configuration.

Why are Epimers Important?

Epimers are important due to their influence on biological processes, particularly in drug design, little changes in stereoisomer configuration can significantly impact pharmacological properties.

Are all Epimers Diastereomers?

Yes, all epimers are diastereomers . They are diastereomers that differ in their configuration at only one chiral centre.