Peptide Bond Formation
A peptide bond also termed as amide bond, is a covalent chemical bond formed when the carboxyl group of one molecule reacts with the amino group of the other molecule, which causes the release of a molecule of water (H2O). This process is a dehydration synthesis reaction also known as a condensation reaction (Figure below) The resulting C(O)NH bond is called a peptide bond, and the resulting molecule is an amide. The four-atom functional group -C(=O)NH- is called a peptide link. Oligopeptides are shorter than polypeptides, although no defined transition exists between the two forms.
Note that two amino acids can form a dipeptide (a peptide composed of two units) while three can form tripeptide and so on. No matter what the arrangement is, each dipeptide, tripeptide or polypeptide they will have one free amino group and one free carboxyl group.
Peptide Nomenclature
Peptide chains have two different ends, the amino terminus (N terminus) and carboxyl terminus (C terminus). The amino terminus of a peptide has a free ammonium group, while the carboxyl terminus is formed by the carboxylate group of the last amino acid. In peptides and proteins, the amino acid components are usually linked in linear fashion due to which peptide sequences are written in the direction from the amino to the carboxyl end. For expressing the sequence of a peptide, it is appropriate to combine the three-letter or single-letter abbreviations for the amino acid residues. For example, the peptide hormone angiotensin II has the sequence Asp-Arg-Val-Tyr-Ile-His-Pro-Phe, or DRVYIHPF.
