The peptide bond

The amino acids are joined to form peptides and proteins by a peptide bond. If the number of amino acids that make up the peptide is less than ten, it is called an oligopeptide, and if it is greater than ten, the peptide is called a polypeptide.

The peptide bond is a bond between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another amino acid. The peptide bond involves the loss of a water molecule and the formation of a CO-NH covalent bond. It is actually an amide- type covalent bond.

Animated image: Peptide bond formation.

In turn, this bond can be hydrolyzed by separating the two amino acids. 

The set of two amino acids linked by a peptide bond is called a dipeptide. If it is three amino acids linked by peptide bonds, tripeptide, and so on. 

Generically speaking of oligopeptides when it consists of less than ten amino acids and of polypeptides when there is a high number of amino acids.

By Pozitron (Own work) [CC BY-SA 4.0], via Wikimedia Commons

The two ends of a peptide are not equivalent, since it has an N-terminal and a C-terminal end. By convention, the amino terminus is considered the beginning of the peptide chain.

The peptide bond is stabilized because the carbon, oxygen and nitrogen atoms share electrons, which causes two resonant forms to appear and that the bond between carbon and nitrogen has a partial double bond character.

Due to this partial double bond character of the peptide bond, the arrangement in space of the four atoms of the peptide unit and the two alpha carbon atoms is such that they lie in the same plane (peptide plane) with fixed distances and angles; this planar arrangement is rigid. There is only freedom of rotation around the alpha carbons.

The radicals of the amino acids do not participate in the peptide bonds, being arranged up and down, alternately, in the polypeptide,

The main characteristics of the peptide bond are:

• It is an amide-type bond between the carboxyl group of one amino acid and the amino of another with the release of a water molecule, but it is shorter than other bonds.
• The peptide bond has a partial double bond character, which is why it is rigid and does not show twist, which causes the atoms involved (C and N, as well as O and H) to lie in the same plane.
• The peptide bond presents polarity with the appearance of charge densities in O and N. This property is responsible for the appearance of H bonds between peptide bonds, responsible for the secondary structure of proteins.