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22-AUGUST-2008 06:13:22 - Chelation Metal-EDTA chelate. Metal-EDTA chelate. Chelation is the binding or complexation of a bi- or multidentate ligand. These ligands, which are often organic compounds, are called chelants, chelators, chelating agents, or 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. Contents 1 History and etymology 2 General 3 Chelation in nature 3.1 In biochemistry and microbiology 3.2 In geology 4 Applications 4.1 In medicine 5 See also 6 References History and etymology Chelate is from Greek χηλή, chelè, meaning claw; pronounced /kiË?ˈleɪʃən/. 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 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.1 General Relative to the aqua complexes, e.g. MH2O62+, the increased stability of a chelated complex, e.g. MEDTA2- 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.234 In biochemistry and microbiology Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups.5 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. peptides and sugars, that extract metal ions from minerals and rocks.6 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 137Cs from radioactive waste.7 Applications Chelators are used in chemical analysis, as water softeners, and are ingredients in many commercial products such as shampoos and food preservatives. Citric acid is used to soften water in soaps and laundry detergents. A common synthetic chelator is EDTA. Phosphonates are also well known chelating agents. Chelators are used in water treatment programs and specifically in steam engineering, e.g., boiler water treatment system: Chelant Water Treatment system. In medicine Antibiotic drugs of the tetracycline family are chelators of Ca2+ and Mg2+ ions. Chelation therapy is the use of chelating agents to detoxify poisonous metal agents such as mercury, arsenic, and lead by converting them to a chemically inert form that can be excreted without further interaction with the body, and has been approved by the FDA in 1991. Despite being approved by the FDA, there are no peer-reviewed studies that show Chelation to be an effective treatment for any condition. Chelation is also used as a treatment for autism or other conditions. There are, however, no peer-reviewed publications regarding the efficacy of chelation agents for the treatment of autism.8 In addition to being beneficial, some chelating agents can be dangerous. The U.S. CDC reports that use of Na2EDTA instead of CaEDTA has resulted in fatalities due to hypocalcemia.9 EDTA is also used in 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. GadoliniumIII chelates are often used as contrast agents in MRI scans. See also Bioremediation Chelation therapy References ^ J. Chem. Soc., 1920, 117, 1456 ^ U Krämer, J D Cotter-Howells, J M Charnock, A H J M Baker, J A C Smith 1996. Free histidine as a metal chelator in plants that accumulate nickel. Nature 379: 635-638. doi:10.1038/379635a0. ^ Jurandir Vieira Magalhaes 2006. Aluminum tolerance genes are conserved between monocots and dicots. Proc Natl Acad Sci U S A 103 26: 9749. doi:10.1073/pnas.0603957103. PMID 16785425. ^ Suk-Bong Ha, Aaron P. Smith, Ross Howden, Wendy M. Dietrich, Sarah Bugg, Matthew J. O'Connell, Peter B. Goldsbrough, and Christopher S. Cobbett 1999. Phytochelatin synthase genes from arabidopsis and the yeast Schizosaccharomyces pombe. Plant Cell 11: 1153-1164. doi:10.1105/tpc.11.6.1153. PMID 10368185. ^ S. J. Lippard, J. M. Berg Principles of Bioinorganic Chemistry University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3. ^ Dr. Michael Pidwirny, University of British Columbia Okanagan, http://www.physicalgeography.net/fundamentals/10r.html ^ Prasad ed. Metals in the Environment. University of Hyderabad. Dekker, New York, 2001 ^ Doja A, Roberts W 2006. Immunizations and autism: a review of the literature. Can J Neurol Sci 33 4: 341-46. PMID 17168158. ^ 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 Retrieved from http://en..org/wiki/Chelation Categories: Coordination chemistry | Chelating agents Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Dansk Deutsch Eesti Español Ù?ارسی Français í•œêµì–´ Ido Italiano עברית Lietuvių Nederlands 日本語 Polski Português РуÑ?Ñ?кий Suomi ä¸æ–‡ This page was last modified on 22 August 2008, at 03:15. of the GNU Free Documentation License. ® , Inc., a U.S.
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