Peptide Bonds

Amino acids covalently bond to each other through peptide bonds. A peptide bond links the carboxyl group of one amino acid to the amino group of the other. Peptides are small chains of amino acids ranging from two to fifty monomers.

A peptide bond is an amide bond linking the carboxyl group of one amino acid to the amino group of another. The peptide bond has a rigid planar structure and shows some characteristics of a double bond.

This results from resonance involving delocalization of the nitrogen lone pair into the carbonyl group, giving the carbon–nitrogen bond partial double-bond character. This resonance shortens the carbon–nitrogen bond, lengthens the carbon–oxygen bond, and restricts rotation around the peptide bond.

Peptide bonds can exist in cis and trans configurations. In the cis configuration, the alpha carbons are on the same side of the peptide bond, and in the trans configuration, they are on opposite sides of the bond. Peptide bonds mostly occur in the trans configuration except when proline contributes its amino group to bond formation.

Amino acids linked through peptide bonds have an N-terminal and a C-terminal. The N-terminal is the end where the amino group is not involved in the formation of a peptide bond, and the C-terminal is the end where the carboxyl group is not involved in the formation of a peptide bond.

The sequence of amino acids in a peptide or protein always starts with the N-terminal and ends with the C-terminal.

When two amino acids link together through a peptide bond, they are known as dipeptides.

When three and four amino acids link together through peptide bonds, they are known as tripeptides and tetrapeptides, respectively. And when several amino acids link together through peptide bonds, they are known as polypeptides.

Peptide bonds can be hydrolyzed under acidic or basic conditions or by enzymes known as proteases. Breaking a peptide bond involves the addition of a water molecule.

Alternatively, these bonds can break spontaneously through a slow process in the presence of water.