Peptides are compounds formed by linking amino acids via peptide bonds; they are intermediate products of protein hydrolysis, with molecular structures intermediate between those of proteins and amino acids. A compound formed by the dehydration condensation of two amino acid molecules is called a dipeptide; similarly, there are tripeptides, tetrapeptides, pentapeptides, and so on. Peptides formed from multiple amino acid molecules are known as polypeptides, while chains generally exceeding 50 amino acids are classified as proteins.
For example, the anti-glycation agent carnosine is a dipeptide formed by the condensation of alanine and L-histidine; the antioxidant glutathione is formed by the sequential condensation of glutamic acid, cysteine, and glycine; and the blue copper peptide GHK-Cu is a complex of a tripeptide and a copper ion.