Editing Racemization (section)

== Biological significance ==
In general, most [[biochemical reaction]]s are stereoselective, so only one stereoisomer will produce the intended product while the other simply does not participate or can cause side-effects. Of note, the <small>L</small> form of amino acids and the <small>D</small> form of sugars (primarily glucose) are usually the biologically reactive form. This is due to the fact that many biological molecules are chiral and thus the reactions between specific enantiomers produce pure stereoisomers.<ref name="Voet_2013">{{cite book| vauthors = Voet D, Voet JG, Pratt CW |author-link1 = Donald Voet|author-link2 = Judith G. Voet|title = Fundamentals of Biochemistry: Life at the Molecular Level|year = 2013|publisher = [[John Wiley & Sons]]|location = Hoboken, NJ|isbn = 978-0-470-54784-7|edition = 4th}}</ref> Also notable is the fact that all amino acid residues exist in the <small>L</small> form. However, bacteria produce <small>D</small>-amino acid residues that polymerize into short polypeptides which can be found in bacterial cell walls. These polypeptides are less digestible by peptidases and are synthesized by bacterial enzymes instead of mRNA translation which would normally produce <small>L</small>-amino acids.<ref name="Voet_2013"/>

The stereoselective nature of most biochemical reactions meant that different enantiomers of a chemical may have different properties and effects on a person. Many psychotropic drugs show differing activity or efficacy between isomers, e.g. [[amphetamine]] is often dispensed as racemic salts while the more active [[dextroamphetamine]] is reserved for refractory cases or more severe indications; another example is [[methadone]], of which one isomer has activity as an opioid agonist and the other as an [[NMDA antagonist]].<ref>{{cite journal | vauthors = Arnold LE, Wender PH, McCloskey K, Snyder SH | title = Levoamphetamine and dextroamphetamine: comparative efficacy in the hyperkinetic syndrome. Assessment by target symptoms | journal = Archives of General Psychiatry | volume = 27 | issue = 6 | pages = 816–822 | date = December 1972 | pmid = 4564954 | doi = 10.1001/archpsyc.1972.01750300078015 }}</ref>

Racemization of [[pharmaceutical drugs]] can occur ''in vivo''. [[Thalidomide]] as the (''R'') enantiomer is effective against [[morning sickness]], while the (''S'') enantiomer is [[teratology|teratogenic]], causing birth defects when taken in the first trimester of pregnancy. If only one enantiomer is administered to a human subject, both forms may be found later in the blood serum.<ref>{{cite journal | vauthors = Teo SK, Colburn WA, Tracewell WG, Kook KA, Stirling DI, Jaworsky MS, Scheffler MA, Thomas SD, Laskin OL | display-authors = 6 | title = Clinical pharmacokinetics of thalidomide | journal = Clinical Pharmacokinetics | volume = 43 | issue = 5 | pages = 311–327 | year = 2004 | pmid = 15080764 | doi = 10.2165/00003088-200443050-00004 | s2cid = 37728304 }}</ref> The drug is therefore not considered safe for use by women of child-bearing age, and while it has other uses, its use is tightly controlled.<ref name="nyt-fda">{{cite news|url = https://www.nytimes.com/1998/07/17/us/thalidomide-approved-to-treat-leprosy-with-other-uses-seen.html|title = Thalidomide Approved to Treat Leprosy, With Other Uses Seen|newspaper = [[The New York Times]]| vauthors  = Stolberg SG |access-date = 8 January 2012|date = 17 July 1998}}</ref><ref>{{cite web |url =https://www.who.int/lep/research/thalidomide/en/index.html|archive-url =https://web.archive.org/web/20061110083549/http://www.who.int/lep/research/thalidomide/en/index.html|url-status =dead|archive-date =November 10, 2006|title = Use of thalidomide in leprosy|work = WHO:leprosy elimination|publisher = [[World Health Organization]]|access-date = 22 April 2010}}</ref> Thalidomide can be used to treat [[multiple myeloma]].<ref>{{cite journal | vauthors = Moehler TM, Hillengass J, Glasmacher A, Goldschmidt H | title = Thalidomide in multiple myeloma | journal = Current Pharmaceutical Biotechnology | volume = 7 | issue = 6 | pages = 431–440 | date = December 2006 | pmid = 17168659 | doi = 10.2174/138920106779116919 }}</ref>

Another commonly used drug is [[ibuprofen]] which is only anti-inflammatory as one enantiomer while the other is biologically inert. Likewise, the (''S'') stereoisomer is much more reactive than the (''R'') enantiomer in [[citalopram]] (Celexa), an antidepressant which inhibits serotonin reuptake, is active.<ref name="Lehninger_2013">{{cite book | vauthors = Nelson DL, Cox MM |title=Lehninger Principles of Biochemistry |publisher=[[W. H. Freeman]] |year=2013 |isbn=978-1-4292-3414-6 |edition=6th |location=New York}}</ref><ref name="Voet_2013"/><ref>{{cite journal | vauthors = Jacquot C, David DJ, Gardier AM, Sánchez C | title = [Escitalopram and citalopram: the unexpected role of the R-enantiomer] | journal = L'Encephale | volume = 33 | issue = 2 | pages = 179–187 | year = 2007 | pmid = 17675913 | doi = 10.1016/s0013-7006(07)91548-1 }}</ref> The configurational stability of a drug is therefore an area of interest in pharmaceutical research.<ref name="Reist_2003">{{cite book | vauthors = Reist M, Testa B, Carrupt PA |chapter=Drug Racemization and Its Significance in Pharmaceutical Research | veditors = Eichelbaum MF, Testa B, Somogyi A  |title=Stereochemical Aspects of Drug Action and Disposition|volume =153| series =Handbook of Experimental Pharmacology  |chapter-url=https://books.google.com/books?id=0U_xCAAAQBAJ&pg=PA91 |pages= 91–112 |year =2003|doi= 10.1007/978-3-642-55842-9_4 |isbn =978-3-642-62575-6}}</ref> The production and analysis of enantiomers in the pharmaceutical industry is studied in the field of chiral organic synthesis.