Chelation

Chelation (from Greek χηλή, chelè, meaning claw; pronounced [ˌki:ˈleɪʃən]) 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, although the bonds may be any combination of coordination or ionic bonds.

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History

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. [M(H₂O)₆]²⁺, the increased stability of a chelated complex, e.g. [M(EDTA]^2- 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. Such chelating agents include the porphyrin rings in hemoglobin or chlorophyll and the Fe³⁺-chelating siderophores secreted by microorganisms. Histidine, malate and phytochelatin are typical chelators used by plants to avoid having poisonous metal ions in a free form.^[2][3][4]

In geology

In earth science, chemical weathering is attributed to organic chelating agents, e.g. peptides and sugars, that have the ability to solubilize the metal ions in minerals and rocks.^[5] 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.^[6]

In microbiology

Many microbial species produce water-soluble pigments that are fluorescent under UV light. These pigments serve as chelating agents, termed siderophores. Among species of Pseudomonas, they are known to secrete pycocyanin and pyoverdin that bind to Iron.

Uses

Chelators are used in chemical analysis, as water softeners, and are ingredients in many commercial products such as shampoos and food preservatives. A commonly used synthetic chelator is EDTA. The term is used in water treatment programs and specifically in steam engineering, to describe a 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 describes 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. Chelation is also used as an unscientific treatment for autism or other conditions. There are no published peer review publications regarding the efficacy of chelation agents for the treatment of autism.^[7]

EDTA chelation can be a dangerous practice, especially when Na₂EDTA is prescribed rather than CaEDTA. The CDC reports that use of Na₂EDTA has resulted in fatalities due to hypocalcemia.^[8]

EDTA is also used in root canal treatment as a way to irrigate the canal. EDTA is used as a chelating agent to either soften the dentin facilitating access to the entire canal length and to remove the smear layer formed during instrumentation.

See also

• Bioremediation
• Chelation therapy
• Electron counting

References

1. ^ J. Chem. Soc., 1920, 117, 1456
2. ^ 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.
3. ^ Jurandir Vieira Magalhaes (2006). "Aluminum tolerance genes are conserved between monocots and dicots". Proc Natl Acad Sci U S A 103 (26).
4. ^ 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.
5. ^ Dr. Michael Pidwirny, University of British Columbia Okanagan, http://www.physicalgeography.net/fundamentals/10r.html
6. ^ Prasad (ed). Metals in the Environment. University of Hyderabad. Dekker, New York, 2001
7. ^ Doja A, Roberts W (2006). "Immunizations and autism: a review of the literature". Can J Neurol Sci 33 (4): 341–46. PMID 17168158.
8. ^ 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