Editing Amino acid (section)

===Chirality===
The [[carbon]] atom next to the [[carboxyl group]] is called the [[alpha carbon|α–carbon]]. In proteinogenic amino acids, it bears the amine and the R group or [[Substituent|side chain]] specific to each amino acid, as well as a hydrogen atom. With the exception of glycine, for which the side chain is also a hydrogen atom, the α–carbon is [[stereogenic]]. All [[chiral]] proteogenic amino acids have the <small>L</small> configuration. They are "left-handed" [[enantiomer]]s, which refers to the [[stereoisomers]] of the alpha carbon.

A few <small>D</small>-amino acids ("right-handed") have been found in nature, e.g., in [[bacterial envelope]]s, as a [[Neuromodulation|neuromodulator]] (<small>D</small>-[[serine]]), and in some [[antibiotic]]s.<ref>{{cite book | title = Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology | publisher = Wiley-Blackwell | year = 2012 | isbn = 978-0-470-14684-2 | location = Oxford | veditors = Michal G, Schomburg D | page = 5 | edition = 2nd }}</ref><ref name="Creighton">{{Cite book | vauthors = Creighton TH |title=Proteins: structures and molecular properties |publisher=W. H. Freeman |location=San Francisco |year=1993 |chapter=Chapter 1 |isbn=978-0-7167-7030-5 |chapter-url-access=registration |chapter-url=https://archive.org/details/proteinsstructur0000crei }}</ref> Rarely, [[D-Amino acid|<small>D</small>-amino acid residues]] are found in proteins, and are converted from the <small>L</small>-amino acid as a [[post-translational modification]].<ref>{{cite journal | vauthors = Genchi G | title = An overview on D-amino acids | journal = Amino Acids | volume = 49 | issue = 9 | pages = 1521–1533 | date = September 2017 | pmid = 28681245 | doi = 10.1007/s00726-017-2459-5 | s2cid = 254088816 }}</ref>{{efn|The <small>L</small> and <small>D</small> convention for amino acid configuration refers not to the optical activity of the amino acid itself but rather to the optical activity of the isomer of [[glyceraldehyde]] from which that amino acid can, in theory, be synthesized (<small>D</small>-glyceraldehyde is dextrorotatory; <small>L</small>-glyceraldehyde is levorotatory).

An alternative convention is to use the [[Cahn–Ingold–Prelog priority rules|(''S'') and (''R'') designators]] to specify the ''absolute configuration''.<ref name=Cahn>{{Cite journal | vauthors = Cahn RS, Ingold C, Prelog V | author-link = Robert Sidney Cahn | author2-link = Christopher Kelk Ingold | author3-link = Vladimir Prelog | title = Specification of Molecular Chirality | journal = [[Angewandte Chemie International Edition]] | volume = 5 | issue = 4 | pages = 385–415 | year = 1966 | doi = 10.1002/anie.196603851}}</ref> Almost all of the amino acids in proteins are (''S'') at the α carbon, with [[cysteine]] being (''R'') and glycine non-[[Chirality (chemistry)|chiral]].<ref>{{cite web | vauthors = Hatem SM | year = 2006 | url = http://geb.uni-giessen.de/geb/volltexte/2006/3038/index.html | title = Gas chromatographic determination of Amino Acid Enantiomers in tobacco and bottled wines | publisher = University of Giessen | access-date = 17 November 2008 | archive-url = https://web.archive.org/web/20090122104055/http://geb.uni-giessen.de/geb/volltexte/2006/3038/index.html | archive-date = 22 January 2009 | url-status = dead }}</ref> Cysteine has its side chain in the same geometric location as the other amino acids, but the ''R''/''S'' terminology is reversed because [[sulfur]] has higher atomic number compared to the carboxyl oxygen which gives the side chain a higher priority by the [[Cahn–Ingold–Prelog priority rules|Cahn-Ingold-Prelog sequence rules]].}}