Secondary amino acid

{{Short description|Amino acids containing a secondary amine}}

In organic chemistry, secondary amino acids are amino acids which do not contain the amino group {{chem2|\sNH2}} but is rather a secondary amine ({{chem2|>NH}}). Secondary amino acids can be classified to cyclic acids, such as proline, and acyclic N-substituted amino acids.{{cite journal |vauthors=Konar AD, Vass E, Hollósi M, Majer Z, Grüber G, Frese K, Sewald N | title =Conformational Properties of Secondary Amino Acids: Replacement of Pipecolic Acid by N-Methyl-L-alanine in Efrapeptin C | journal = Chemistry & Biodiversity | year = 2013 | volume = 10 | issue = 5 | pages = 942–951 | doi = 10.1002/cbdv.201300086| pmid =23681735 | s2cid =37926505 }}{{cite journal |author1= V V Ryakhovskii|author2= S V Agafonov| author3= Yu M Kosyrev | title =Special features of the synthesis of peptides containing secondary amino acids | journal = Russ. Chem. Rev. | year = 1991 | volume = 60|issue= 8| pages = 924–933 | doi = 10.1070/RC1991v060n08ABEH001119|bibcode= 1991RuCRv..60..924R|s2cid= 250735186}}

In nature, proline, hydroxyproline, pipecolic acid and sarcosine are well-known secondary amino acids. Proline is the only proteinogenic secondary amino acids. Other secondary amino acids are non-proteinogenic amino acids. In protein, hydroxyproline is incorporated into protein by hydroxylation of proline. Pipecolic acid, a heavier analog of proline, is found in efrapeptin. Sarcosine is a N-methylized glycine so its methyl group is used in many biochemical reactions. Azetidine-2-carboxylic acid, which is a smaller homolog of proline in plants.