Chelation
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2008-07-16T20:32:17Z
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/* In biochemistry and microbiology */
'''Chelation''' is the [[binding (molecular)|binding]] or [[complex (chemistry)|complexation]] of a bi- or multidentate [[ligand]]. These ligands, which are often [[organic compound]]s, are called chelants, chelators, chelating agents, or [[sequestration|sequestering agent]]. The ligand forms a '''chelate complex''' with the substrate. The term is reserved for complexes in which the metal ion is bound to two or more atoms of the chelating agent.
[[Image:Medta.png|thumb|right|Metal-[[EDTA]] chelate.]]
==History and etomology==
Chelate is from [[Greek language|Greek]] χηλή, ''chelè'', meaning claw; {{pronEng|kiːˈleɪʃən}}. The term ''chelate'' was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew, who stated: "The adjective chelate, derived from the great claw or ''chele'' ([[Greek language|Greek]]) of the [[lobster]] or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce [[heterocyclic]] rings."<ref>''J. Chem. Soc.'', '''1920''', ''117'', 1456</ref>
==General==
Relative to the aqua [[complex (chemistry)|complexes]], ''e.g.'' [M(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup>, the increased stability of a chelated complex, ''e.g.'' [M([[EDTA]]]<sup>2-</sup> is called the chelate effect. Because chelating agents bind to metals through more than one coordination site, such ligands bind more tenaciously than unidentate ligands (like water). If a chelate were replaced by several monodentate ligands (such as [[water]] or [[ammonia]]), the total number of molecules would decrease, whereas if several monodentate ligands were replaced by a chelate, the number of free molecules increases. The effect is therefore [[entropic]] in that more sites are used by fewer ligands and this leaves more unbonded molecules: a total increase in the number of molecules in solution and a corresponding increase in entropy.
==Chelation in nature==
Virtually all biochemicals exhibit the ability to dissolve metal cations. Thus proteins, polysaccharides, and polynucleic acids are excellent polydentate ligands for many of the metal ions. In addition to these adventitious chelators, several are produced to specifically bind certain metals (see next section). [[Histidine]], [[malate]] and [[phytochelatin]] are typical chelators used by plants.<ref>{{cite journal | author=U Krämer, J D Cotter-Howells, J M Charnock, A H J M Baker, J A C Smith | title=Free histidine as a metal chelator in plants that accumulate nickel| journal=Nature| year=1996| volume=379| doi= 10.1038/379635a0| pages=635–638}}</ref><ref>{{cite journal | author=Jurandir Vieira Magalhaes| title=Aluminum tolerance genes are conserved between monocots and dicots| journal=Proc Natl Acad Sci U S A| year=2006| volume=103| issue=26| page=9749-9750| url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1502523 | pages=9749 | doi=10.1073/pnas.0603957103 <!--Retrieved from URL by DOI bot-->| pmid=16785425}}</ref><ref>{{cite journal | author=Suk-Bong Ha, Aaron P. Smith, Ross Howden, Wendy M. Dietrich, Sarah Bugg, Matthew J. O'Connell, Peter B. Goldsbrough, and Christopher S. Cobbett |title=Phytochelatin synthase genes from arabidopsis and the yeast Schizosaccharomyces pombe | journal=Plant Cell| year=1999| volume=11| url=http://www.plantcell.org/cgi/content/full/11/6/1153?ck=nck| pages=1153–1164 |doi=10.1105/tpc.11.6.1153 |pmid=10368185}}</ref>
===In biochemistry and microbiology===
Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups.<ref>S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3.</ref> Such chelating agents include the [[porphyrin]] rings in [[hemoglobin]] and [[chlorophyll]]. Many microbial species produce water-soluble pigments that serve as chelating agents, termed [[siderophores]]. For example, among species of ''Pseudomonas,'' they are known to secrete pycocyanin and pyoverdin that bind iron. [[Enterobactin]], produced by [[E. coli]], is the strongest chelating agent known.
===In geology===
In earth science, chemical [[weathering]] is attributed to organic chelating agents, ''e.g.'' [[peptide]]s and [[sugar]]s, that extract metal ions from minerals and rocks.<ref>Dr. Michael Pidwirny, University of British Columbia Okanagan, http://www.physicalgeography.net/fundamentals/10r.html</ref> Most metal complexes in the environment and in nature are bound in some form of chelate ring, ''e.g.'' with "[[humic acid]]" or a protein. Thus, metal chelates are relevant to the mobilization of [[metals]] in the [[soil]], the uptake and the accumulation of [[metals]] into [[plants]] and [[micro-organisms]]. Selective chelation of [[heavy metals]] is relevant to [[bioremediation]], ''e.g.'' removal of <sup>137</sup>Cs from radioactive waste.<ref>Prasad (ed). Metals in the Environment. University of Hyderabad. Dekker, New York, 2001</ref>
==Applications==
Chelators are used in [[chemical analysis]], as [[water softener]]s, and are ingredients in many commercial products such as [[shampoos]] and food [[preservative]]s. [[Citric acid]] is used to [[Citric_acid#Water_softening|soften water]] in [[soap]]s and laundry [[detergent]]s. A commonl synthetic chelator is [[EDTA]]. Clelators are used in water treatment programs and specifically in [[steam engineering]], e.g., [[boiler water treatment system]]: ''Chelant Water Treatment system''.
===In medicine===
[[Antibiotic]] [[medication|drugs]] of the [[tetracycline]] family are chelators of [[Calcium|Ca<sup>2+</sup>]] and [[Magnesium|Mg<sup>2+</sup>]] ions. [[Chelation therapy]] describes the use of chelating agents to detoxify [[poison]]ous metal agents such as [[mercury poisoning|mercury]], [[arsenic]], and [[lead]] by converting them to a chemically inert form that can be excreted without further interaction with the body. Chelation is also used as an unscientific [[Autism therapies#Chelation therapy|treatment]] for [[autism]] or other conditions. There are no peer-reviewed publications regarding the efficacy of chelation agents for the treatment of autism.<ref>{{cite journal|journal=Can J Neurol Sci|date=2006|volume=33|issue=4|pages=341–46|title=Immunizations and autism: a review of the literature|author=Doja A, Roberts W|pmid=17168158}}</ref>
In addition to being beneficial, some chelating agents can be dangerous. The U.S. CDC reports that use of Na<sub>2</sub>EDTA instead of CaEDTA has resulted in fatalities due to [[hypocalcemia]].<ref>U.S. Centers for Disease Control, "Deaths Associated with Hypocalcemia from Chelation Therapy" (March 3, 2006), http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5508a3.htm</ref>
EDTA is also used in [[root canal|root canal treatment]] as a way to irrigate the canal. EDTA softens the dentin facilitating access to the entire canal length and to remove the smear layer formed during instrumentation.
[[Gadolinium]](III) chelates are often used as contrast agents in [[MRI|MRI scans]].
==See also==
*[[Bioremediation]]
*[[Chelation therapy]]
==References==
{{reflist|2}}
[[Category:Coordination chemistry]]
[[Category:Chelating agents|*]]
[[da:Chelat]]
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[[es:Ligando quelato]]
[[fa:کیلات]]
[[fi:Kelaatio]]
[[fr:Chélation]]
[[he:קלאציה]]
[[io:Kel-ionuro]]
[[it:Chelazione]]
[[ja:キレート]]
[[ko:킬레이트]]
[[lt:Chelatas]]
[[nl:Chelatie]]
[[pl:Chelacja]]
[[pt:Quelato]]
[[ru:Хелаты]]
[[zh:螯合物]]