A peptide is an organic molecule consisting of two or more amino acids linked through an amide linkage called a peptide bond. Peptides are formed when the amino group (-NH₂) of one amino acid, links with the carboxyl group (-COOH) of an adjacent amino acid. An amide linkage (-CO-NH-) results. If two amino acids are linked, the compound is called a dipeptide, if three, a tripeptide, and so on. Longer chains containing a few amino acids are often called oligopeptides or polypeptides if they contain as many as 50. Still longer chains are called proteins. Peptides may have many different properties depending on the nature of the amino acids they contain, and the order in which they are linked. Some peptides function as hormones--a few as antibiotics and others as important participants in metabolism.

For example, an octapeptide with a hormone-like function, called angiotensin, is formed in the blood...