Chelation (from
Greek χηλή, ''chelè'', meaning claw; pronounced ) 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.
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."
[''J. Chem. Soc.'', 1920, ''117'', 1456]
General
Relative to the aqua
complexes, ''e.g.''
M(H2O)62+, the increased stability of a chelated complex, ''e.g.''
href=/EDTA>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.
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.
[Dr. Michael Pidwirny, University of British Columbia Okanagan, http://www.physicalgeography.net/fundamentals/10r.html] 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.
[Prasad (ed). Metals in the Environment. University of Hyderabad. Dekker, New York, 2001]
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 a scientifically unverified
treatment for
autism or other conditions. There are no published peer review publications regarding the efficacy of chelation agents for the treatment of autism.
EDTA chelation can be a dangerous practice, especially when Na
2EDTA is prescribed rather than CaEDTA. The CDC reports that use of Na
2EDTA has resulted in fatalities due to
hypocalcemia.
[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]
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.