Peptide Bond

A peptide bond is a covalent bond between two amino acids. It links the carboxyl group of one to the amino group of another.

This bond forms by dehydration, where water is removed. It is an amide bond, also called an eupeptide bond. Peptide bonds help form proteins and polypeptides.

Peptide Bond Formation

Peptide bonds form by condensation, also called dehydration synthesis. One amino acid loses a hydroxyl group from the carboxyl end.

Another amino acid loses hydrogen from the amino group. A water molecule is released, and a CO-NH bond is formed. This forms a dipeptide of two amino acids.

Structure and Characteristics

Peptide bonds are planar, rigid, and show partial double-bond character. The bond restricts rotation, giving the chain a stable shape.

Atoms in the bond like carbon, nitrogen, oxygen, hydrogen lie in the same plane.

Peptide bonds have polarity, with partial charges on oxygen and hydrogen atoms. They are strong bonds, not easily broken by heat or salt.

Types of Peptide Chains

Chains vary in length depending on the number of amino acids:

Name	Number of Amino Acids
Dipeptide	2 amino acids
Tripeptide	3 amino acids
Tetrapeptide	4 amino acids
Oligopeptide	Up to 10 amino acids
Polypeptide	10 to 100 amino acids
Proteins	Over 100 amino acids

Polypeptides are common and form the primary structure of proteins.

Degradation of Peptide Bonds

Peptide bonds break through hydrolysis, which adds water to the bond. This process is naturally very slow, taking hundreds of years per bond. Enzymes called proteases act as catalysts to speed up hydrolysis.

Degradation releases Gibbs energy, roughly −10 to −15 kJ/mol. Protease enzymes are commonly involved in digestion and protein breakdown.

Peptide Bond Structure

The bond has a trans configuration, making it more stable. The peptide bond usually adopts a trans configuration, where the α-carbons of adjacent amino acids lie on opposite sides of the bond. This reduces steric hindrance and stabilises protein folding.

The bond is coplanar due to electron sharing (resonance) between atoms. Linus Pauling and Robert Corey discovered this structure.

Peptide Bond Features

Peptide chains are written from the N-terminal to the C-terminal. Each amino acid is shown by a one-letter or three-letter code.

For example, glycine is “Gly” or “G”; alanine is “Ala” or “A”. In names, suffixes like -ine, -an, or -ate become -yl, except for the last amino acid, which keeps its full name.

Stereochemistry of Peptide Bonds

Proteins are made from L-amino acids with a fixed alpha carbon configuration. This arrangement defines how proteins fold into their 3D structure. Peptide bonds maintain this configuration across the polypeptide chain.

Spectral Properties

Peptide bonds absorb ultraviolet light between 190 and 230 nm. This property is used to measure proteins in lab experiments.

Chemical Reactivity

Due to resonance, peptide bonds are stable and react slowly. They show less reactivity than esters in most biological environments.

However, under certain conditions, strong nucleophiles can break the bond. This forms a tetrahedral intermediate, leading to peptide bond cleavage.

Summary

A peptide bond is a strong covalent bond between two amino acids. It forms by joining a carboxyl group to an amino group. This bond forms during a dehydration reaction and builds polypeptide chains. Peptide bonds are planar, stable, and break slowly by hydrolysis.