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Peptide Bond

A peptide bond, also called an eupeptide bond, is a chemical bond that is formed by joining the carboxyl group of one amino acid to the amino group of another. A peptide bond is basically an amide-type of the covalent chemical bond. This bond links two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another. This linkage is found along a peptide or protein chain.

During the formation of this bond, there is a release of water (H2O) molecules. A peptide bond is usually a covalent bond (CO-NH bond), and since the water molecule is eliminated, it is considered a dehydration process. Generally, this process occurs mostly between amino groups.

Peptide is a Greek word that means “digested”. A peptide is a short polymer of amino acid monomers linked by an amide bond.

Table of Contents

Peptide Bond Formation or Synthesis

A peptide bond is formed by a dehydration synthesis or reaction at a molecular level.  This reaction is also known as a condensation reaction which usually occurs between amino acids.

As depicted in the figure given below, two amino acids bond together to form a peptide bond by dehydration synthesis. During the reaction, one of the amino acids gives a carboxyl group to the reaction and loses a hydroxyl group (hydrogen and oxygen).

Peptide Bond Formation

The other amino acid loses hydrogen from the NH2 group. The hydroxyl group is substituted by nitrogen, thus forming a peptide bond. This is one of the primary reasons for peptide bonds being referred to as substituted amide linkages. Both amino acids are covalently bonded to each other.

The newly formed amino acids are also called a dipeptide.

Let’s have a look at a simpler diagram depicting the formation of the peptide bond.

Formation of peptide bond

During the reactions that occur, the resulting CO-NH bond is the peptide bond, and the resulting molecule is an amide. The four-atom functional group -C(=O)NH- is called an amide group or a peptide group.

Characteristics of Peptide Bonds

1. Peptide bonds are strong with partial double bond character:

  • They are not broken by heating or high salt concentration.
  • They can be broken by exposing them to strong acids or bases for a long time at elevated temperature, and also by some specific enzymes (digestive enzymes).

2. Peptide bonds are rigid and planar bonds; therefore, they stabilise protein structure.

3. Peptide bond contains partial positive charge groups (polar hydrogen atoms of amino groups) and partial negative charge groups (polar oxygen atoms of carboxyl groups).

Different Forms of Peptide Bond

  • Dipeptide = contains 2 amino acid units.
  • Tripeptide = contains 3 amino acid units.
  • Tetrapeptide = contains 4 amino acid units.
  • Oligopeptide = contains not more than 10 amino acid units.
  • Polypeptide = contains more than 10 amino acid units, up to 100 residues.
  • Macropeptides = made up of more than 100 amino acids.

Degradation of Peptide Bond

The degradation of peptide bonds involves a reaction in which the breaking of the peptide bonds between the molecules occurs. Hydrolysis (addition of water) is the reaction used for the degradation of the peptide bond. During the reaction, they will emit Gibbs energy in an amount of 8-16 kJ/mol. But generally, this is a very slow process having a half-life of 350 to 600 years per bond at a temperature of 25oC. Enzymes like proteases are used as the catalysts for this process.

Degradation of Peptide Bonds Using Proteases Enzymes as a Catalyst

Peptide Bond Structure

A peptide bond is a planar, trans and rigid configuration. It also shows a partial double bond character. The coplanarity of the peptide bond denotes the resonance or partial sharing of two pairs of electrons between the amide nitrogen and carboxyl oxygen.

The atoms C, H, N, and O of the peptide bond lie in the same plane, like the hydrogen atom of the amide group and the oxygen atom of the carboxyl group, which are trans to each other.

Linus Pauling and Robert Corey are the scientists who found that the peptide bonds are rigid and planar.

Peptide Bond Features

Some key features of this bond include the following:

Writing of the Peptide Bond Structure

Generally, these bonds are written in a form where free amino acids are at the left and the free carboxyl on the right side. The left side is the N-terminal residue, and the right side is the C-terminal residue. This amino acid sequence is read from the N-terminal to the C-terminal. And also, protein biosynthesis starts in the same direction.

Representation of the Peptide Bond

Rattlesnake moving representation is used for the peptide bond representation from left to right of the page. The N-terminal residues to its rattle and C-terminal residues are considered as the fangs.

Shorthand to Read the Peptide Bond

The peptide or protein of the amino acid is represented by the 3 letters or one-letter abbreviation.

The Naming of the Peptide Bond

To name the peptides, we should know the suffixes of the amino acids. -ine for glycine, -an for tryptophan, -ate for glutamate, are changed to -yl, except in the case of the C-terminal of the amino acid.

Stereochemistry of the Peptide Bond

We know that every protein is made of simpler units of amino acids with L-configuration. The steric arrangement of the alpha carbon is fixed by that configuration.


The wavelength of absorption for a peptide bond is 190–230 nm. With such a reading, it means that the bond is easily susceptible to UV radiation.


As a result of its resonance stabilization, a peptide bond is almost unreactive under physiological conditions. It is even less than that of esters. However, in some cases, peptide bonds can undergo chemical reactions, but this is mainly due to an attack from an electronegative atom on the carbonyl carbon. This results in the breaking of the carbonyl double bond and the formation of a tetrahedral intermediate.

Some Important Questions

1. What do you mean by the peptide bond?

2. What is a peptide bond? Which molecule has this bond? Explain the strength of this bond.

3. What is the reaction that caused the formation of the peptide bond? What catalysts are used for such a reaction?

4. Write down some examples of compounds having peptide bonds.

6. Are all proteins bonded by peptide bonds?

7. Comment on the partial double bond nature of the peptide bond. How is it possible?

8. What is polypeptide?

9. Explain the synthesis of the peptide bond in detail with neat diagrams.

10. How is the degradation of the peptide bond possible? Write down the equations.

11. List out the catalysts used for the synthesis and degradation of the peptide bond.

12. A peptide bond is rigid and planar. Discuss.

13. What effect is caused by the coplanarity of the peptide bond?

14. Explain the representation of the peptide bond.

15. Write a note on the cis-trans isomerism of the peptide bond.

Frequently Asked Questions (FAQs)


Who found that the peptide bond is planar and rigid?

Linus Pauling and Robert Corey found that the peptide bond is rigid and planar.

What do peptide bonds hold together?

Peptide bonds link and hold the amino acids together.

How is the peptide bond broken?

The peptide bond is broken by hydrolysis.

Do peptide bonds rotate?

No, peptide bonds do not rotate.
Test Your Knowledge on Peptide Bond


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