is the binding
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.
History and etymology
Chelation is from Greek
, meaning claw; . 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
) 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
Relative to the aqua complexes
, the increased stability of a chelated complex, e.g.
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
), 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 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
are typical chelators used by plants.
In biochemistry and microbiology
Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups. Such chelating agents include the porphyrin
rings in hemoglobin
. 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
, 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 "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
. Selective chelation of heavy metals
is relevant to bioremediation
removal of 137
Cs from radioactive waste.
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
and laundry detergents
. A common synthetic chelator is EDTA
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.
of the tetracycline
family are chelators of Ca2+
ions. Chelation therapy
is the use of chelating agents to detoxify poisonous
metal agents such as mercury
, and lead
by converting them to a chemically inert form that can be excreted without further interaction with the body, and has been approved by the U.S. FDA
in 1991. Chelation is also used but unproven as a treatment
In addition to being beneficial, chelating agents can also be dangerous. The U.S. CDC reports that use of Na2EDTA instead of CaEDTA has resulted in fatalities due to hypocalcemia.
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.
Gadolinium(III) chelates are often used as contrast agents in MRI scans.