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Chelation
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Metal-
EDTA chelate
Chelation (pronounced
/kiːˈleɪʃən/) is the formation or presence of two or more separate bindings between a
polydentate (multiple bonded) ligand and a single central atom. <sup id="cite_ref-IUPAC_0-0" class="reference">
[1]</sup> 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 χηλή,
chelè, meaning claw; the ligands lie around the central atom like the claws of a
lobster. <sup id="cite_ref-1" class="reference">
[2]</sup>
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[edit] The chelate effect
Ethylenediamine ligand, binding to a central metal ion with two bonds
Cu<sup>2+</sup>
complexes with
methylamine (left)
and ethylenediamine (right)
The chelate effect describes the enhanced affinity of chelating ligands for a metal ion compared to the affinity of a collection of similar nonchelating (monodentate) ligands for the same metal.
Consider the two equilibria, in aqueous solution, between the
copper(II) ion, Cu<sup>2+</sup> and
ethylenediamine (en) on the one hand and
methylamine, MeNH<sub>2</sub> on the other.
<dl><dd>Cu<sup>2+</sup> + en
[Cu(en)]<sup>2+</sup> (1)</dd><dd>Cu<sup>2+</sup> + 2 MeNH<sub>2</sub>
[Cu(MeNH<sub>2</sub>)<sub>2</sub>]<sup>2+</sup> (2)</dd></dl> In (1) the
bidentate ligand ethylene diamine forms a chelate complex with the copper ion. Chelation results in the formation of a five–membered ring. In (2) the bidentate ligand is replaced by two
monodentate methylamine ligands of approximately the same donor power, meaning that the
enthalpy of formation of Cu—N bonds is approximately the same in the two reactions. Under conditions of equal copper concentrations and when the concentration of methylamine is twice the concentration of ethylenediamine, the concentration of the complex (1) will be greater than the concentration of the complex (2). The effect increases with the number of chelate rings so the concentration of the
EDTA complex, which has six chelate rings, is much much higher than a corresponding complex with two monodentate nitrogen donor ligands and four monodentate carboxylate ligands. Thus, the
phenomenon of the chelate effect is a firmly established
empirical fact.
The
thermodynamic approach to explaining the chelate effect considers the
equilibrium constant for the reaction: the larger the equilibrium constant, the higher the concentration of the complex.
<dl><dd>[Cu(en)] =β<sub>11</sub>[Cu][en]</dd><dd>[Cu(MeNH<sub>2</sub>)<sub>2</sub>]= β<sub>12</sub>[Cu][MeNH<sub>2</sub>]<sup>2</sup></dd></dl> Electrical charges have been omitted for simplicity of notation. The square brackets indicate concentration, and the subscripts to the
stability constants, β, indicate the
stoichiometry of the complex. When the
analytical concentration of methylamine is twice that of ethylenediamine and the concentration of copper is the same in both reactions, the concentration [Cu(en)] is much higher than the concentration [Cu(MeNH<sub>2</sub>)<sub>2</sub>] because β<sub>11</sub> >> β<sub>12</sub>.
An equilibrium constant,
K, is related to the standard
Gibbs free energy, Δ
G<sup>
</sup> by
<dl><dd>Δ
G<sup>
</sup> = −RT ln
K = Δ
H<sup>
</sup> − TΔ
S<sup>
</sup></dd></dl> where
R is the
gas constant and
T is the temperature in
kelvins. Δ
H<sup>
</sup> is the standard
enthalpy change of the reaction and Δ
S<sup>
</sup> is the standard
entropy change. It has already been posited that the enthalpy term should be approximately the same for the two reactions. Therefore the difference between the two stability constants is due to the entropy term. In equation (1) there are two particles on the left and one on the right, whereas in equation (2) there are three particles on the left and one on the right. This means that less
entropy of disorder is lost when the chelate complex is formed than when the complex with monodentate ligands is formed. This is one of the factors contributing to the entropy difference. Other factors include solvation changes and ring formation. Some experimental data to illustrate the effect are shown in the following table.<sup id="cite_ref-GE_2-0" class="reference">
[3]</sup>
<dl><dd> <table class="wikitable"> <tbody><tr> <th>Equilibrium</th> <th>log β</th> <th>ΔG<sup>
</sup></th> <th>Δ
H<sup>
</sup> /kJ mol<sup>−1</sup></th> <th>−
TΔ
S<sup>
</sup> /kJ mol<sup>−1</sup></th> </tr> <tr> <td>Cd<sup>2+</sup> + 4 MeNH<sub>2</sub>
Cd(MeNH<sub>2</sub>)<sub>4</sub><sup>2+</sup></td> <td>6.55</td> <td>-37.4</td> <td>-57.3</td> <td>19.9</td> </tr> <tr> <td>Cd<sup>2+</sup> + 2 en
Cd(en)<sub>2</sub><sup>2+</sup></td> <td>10.62</td> <td>-60.67</td> <td>-56.48</td> <td>-4.19</td> </tr> </tbody></table> </dd></dl> These data show that the standard enthalpy changes are indeed approximately equal for the two reactions and that the main reason why the chelate complex is so much more stable is that the standard entropy term is much less unfavourable, indeed, it is favourable in this instance. In general it is difficult to account precisely for thermodynamic values in terms of changes in solution at the molecular level, but it is clear that the chelate effect is predominantly an effect of entropy.
Other explanations, Including that of Schwarzenbach,<sup id="cite_ref-3" class="reference">
[4]</sup> are discussed in Greenwood and Earnshaw (
loc.cit).
[edit] In nature
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 plants.<sup id="cite_ref-4" class="reference">
[5]</sup><sup id="cite_ref-5" class="reference">
[6]</sup><sup id="cite_ref-6" class="reference">
[7]</sup>
[edit] In biochemistry and microbiology
Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups.<sup id="cite_ref-7" class="reference">
[8]</sup> 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.
[edit] 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.<sup id="cite_ref-8" class="reference">
[9]</sup> 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 <sup>137</sup>Cs from radioactive waste.<sup id="cite_ref-9" class="reference">
[10]</sup>
[edit] 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.
[edit] Heavy metal detoxification
Main article:
Chelation therapy
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 was approved by the
U.S. Food and Drug Administration in 1991. Chelation is also used as a
treatment for
autism, though this practice is controversial due to weak scientific support for its efficacy and its occasionally-deadly side-effects.<sup id="cite_ref-10" class="reference">
[11]</sup>
Though they can be beneficial in cases of heavy metal poisoning, chelating agents can also be dangerous. The U.S. CDC reports that use of disodium EDTA instead of calcium EDTA has resulted in fatalities due to
hypocalcemia.<sup id="cite_ref-11" class="reference">
[12]</sup>
[edit] Other medical applications
Antibiotic drugs of the
tetracycline family are chelators of
Ca<sup>2+</sup> and
Mg<sup>2+</sup> ions.
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.
Chelate complexes of
gadolinium are often used as
contrast agents in
MRI scans.
[edit] Chemical applications
Homogeneous catalysts are often chelated complexes. A typical example is the
ruthenium(II) chloride chelated with
BINAP (a bidentate
phosphine) used in e.g.
Noyori asymmetric hydrogenation and asymmetric isomerization. The latter has the practical use of manufacture of synthetic
(–)-menthol.
[edit] References
- ^ IUPAC definition of chelation.
- ^ 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."
Morgan, Gilbert T.; Drew, Harry D. K. (1920). "CLXII.—Researches on residual affinity and co-ordination. Part II. Acetylacetones of selenium and tellurium". J. Chem. Soc., Trans. 117: 1456. doi:10.1039/CT9201701456. (nonfree access)
- ^ Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0080379419 p 910
- ^ Schwarzenbach, G (1952). "Der Chelateffekt". Helv. Chim. Acta 35: 2344–2359. doi:10.1002/hlca.19520350721.
- ^ 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 USA 103 (26): 9749. doi:10.1073/pnas.0603957103. PMID 16785425. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1502523.
- ^ 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. http://www.plantcell.org/cgi/content/full/11/6/1153?ck=nck.
- ^ 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
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