Peptide bond

A peptide bond is an amide type linkage between the carboxyl group of an amino acid and the amino group of the α - carbon atom ( α -C - atom ) of a second amino acid. Two amino acids can be (formally ) condense with elimination of water to a dipeptide. During translation, this reaction is catalyzed by the ribosomes.

Formal react for example, two molecules of the proteinogenic amino acid alanine elimination of water to the dipeptide alanyl -alanine:

Since the amino group is too weak nucleophilic to react directly with the carboxyl group or protonated as NH3 ( ) may be present, the equilibrium lies under standard conditions on the left. The chemical reaction is endergonic.

Both in the laboratory synthesis of peptides, as well as in the biological synthesis of peptides and proteins, the reactive groups must first be activated. This happens in biological systems mostly by enzymes.

By multiple condensation to tripeptides, tetrapeptides, polypeptides and proteins, finally, chain-like macromolecules composed of amino acids form. Polypeptide (or protein chains ) form the primary structural element of proteins.

To primary, secondary and tertiary structures of peptides and proteins, see protein structure.

The crystal structure analysis of amino acids and dipeptides show that the amide group is planar, i.e., all the constituent atoms are located in one plane. The dihedral angle ( HNCO ) is 180 ° and the atoms can not be rotated relative to each other due to resonance stabilization. In the native proteins are predominantly trans peptide bonds, cis peptide bonds are found mainly in cyclic dipeptides ( diketopiperazines ) and cyclic tripeptides (example: Cyclotriprolyl ).

The bond lengths between the nitrogen and the carbonyl carbon 133 pm, between nitrogen and α -carbon atom of 146 pm, between the carbonyl carbon and α -carbon atom of 151 pm and the carbonyl carbon and oxygen between 124 pm. The shorter length of the CN bond in the amide bond in comparison to the normal CN bond indicates that it has double-bond character; this type of chemical bond is also called partial double bond. This feature is explained by the amide iminol tautomerism of the peptide bond.

Peptides and amides

By reaction of the carboxyl group of an amino acid and the amino group of a second amino acid forms a peptide bond with elimination of water. Each peptide bond is an amide bond.

Condition for the formation of a peptide bond, the condensation reaction of the terminal carboxyl group, carries an amino group on the α - carbon atom, with the amino group on the α -C - atom of the second amino acid. Any other condensation between carboxyl group and amino group also leads to an amide bond, but which is not a peptide bond.

Peptides are extended by means of peptide bonds react the terminal carboxyl or amino groups on the α - carbon atom with other amino acids.