Chelation
The term chelate (/ˈkiːleɪt/) 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 crab 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.
"[3] Chelation is useful in applications such as providing nutritional supplements, in chelation therapy to remove toxic metals from the body, as contrast agents in MRI scanning, in manufacturing using homogeneous catalysts, in chemical water treatment to assist in the removal of metals, and in fertilizers.
The thermodynamic principles underpinning the chelate effect are illustrated by the contrasting affinities of copper(II) for ethylenediamine (en) vs. methylamine.
[4] These data confirm that the enthalpy changes are approximately equal for the two reactions and that the main reason for the greater stability of the chelate complex is the entropy term, which is much less unfavorable.
Thus, proteins, polysaccharides, and polynucleic acids are excellent polydentate ligands for many metal ions.
In addition to these adventitious chelators, several biomolecules are specifically produced to bind certain metals (see next section).
[6][7][8][9] Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups.
Many microbial species produce water-soluble pigments that serve as chelating agents, termed siderophores.
[10][11][12] In earth science, chemical weathering is attributed to organic chelating agents (e.g., peptides and sugars) that extract metal ions from minerals and rocks.
[13] 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).
Selective chelation of heavy metals is relevant to bioremediation (e.g., removal of 137Cs from radioactive waste).
[16] Dentin adhesives were first designed and produced in the 1950s based on a co-monomer chelate with calcium on the surface of the tooth and generated very weak water-resistant chemical bonding (2–3 MPa).
Chelating agents convert these metal ions into a chemically and biochemically inert form that can be excreted.
Chelation using sodium calcium edetate has been approved by the U.S. Food and Drug Administration (FDA) for serious cases of lead poisoning.
[19] Disodium EDTA is not approved by the FDA for any use,[18] and all FDA-approved chelation therapy products require a prescription.
[21][22] Bifunctional chelate complexes of zirconium, gallium, fluorine, copper, yttrium, bromine, or iodine are often used for conjugation to monoclonal antibodies for use in antibody-based PET imaging.
[26][27] Chelation in the intestinal tract is a cause of numerous interactions between drugs and metal ions (also known as "minerals" in nutrition).
Most fertilizers contain phosphate salts that, in the absence of chelating agents, typically convert these metal ions into insoluble solids that are of no nutritional value to the plants.
