Chelation

Chelation is the formation or presence of two or more separate bindings between a polydentate (multiple bonded) ligand and a single central atom.  Usually these ligands are organic compounds, and are called chelants, chelators, chelating agents, or sequestering agents. The ligand forms a chelate complex with the substrate. Chelate complexes are contrasted with coordination complexes with monodentate ligands, which form only one bond with the central atom. Chelants, according to ASTM-A-380, are "chemicals that form soluble, complex molecules with certain metal ions, inactivating the ions so that they cannot normally react with other elements or ions to produce precipitates or scale." The word chelation is derived from Greek χηλ?, chelè, meaning claw; the ligands lie around the central atom like the claws of a lobster. Virtually all biochemicals exhibit the ability to dissolve certain metal cations. Thus, proteins, polysaccharides, and polynucleic acids are excellent polydentate ligands for many metal ions. In addition to these adventitious chelators, several biomolecules are produced to specifically bind certain metals (see next section). Histidine, malate and phytochelatin are typical chelators used by plantsVirtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups. 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, species of Pseudomonas are known to secrete pycocyanin and pyoverdin that bind iron. Enterobactin, produced by E. coli, is the strongest chelating agent known. In earth science, chemical weathering is attributed to organic chelating agents, e.g. peptides and sugars, that extract metal ions from minerals and rocks. Most metal complexes in the environment and in nature are bound in some form of chelate ring, e.g. with a 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.

Further information:

1. Статьи Химическая энциклопедия. Т. 5 в 5 т. - М.: Большая Российская Энциклопедия, 1998 - с.224-225.
2. Стид Дж.В., Этвуд Дж.Л.Супрамолекулярная химия. Т. 1. - М.: Академкнига, 2007 - с. 38-41.
3. Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0080379419 p 910 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.
4. Jurandir Vieira Magalhaes (2006). "Aluminum tolerance genes are conserved between monocots and dicots". Proc Natl Acad Sci USA 103 (26): 9749.
5. Article Chelation from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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