ABSTRACT
Amino acids can be divided into two groups. Proteinogenic amino acids are defined as amino acids which can be incorporated into proteins.1–4 Proteogenic amino acids are differentiated from amino acids which are subject to post translational modification5,6 such as the γ-carboxylation of glutamic acid. Non proteinogenic (non protein) amino acids7 such as cyclopentenylglycine8 and l-p-hydroxyphenylglycine9 are involved in metabolism and in peptide antibiotics. There has been considerable interest in the development of unnatural amino acids. While the broad definition of an unnatural amino acid is any synthetic organic carboxylic acid which contains an amino or imino function, the definition for this section is that an unnatural amino acid must be capable of incorporation into a protein in a specific manner using a biological system.10–17
