The Greek origin of the word chelate signifies the plier-like claws of a crab. A chelate is a water-soluble complex between a metal ion and a complexing agent. It usually does not dissociate easily in solution, but forms an inert complex. In labile complexes, however, the metal ion can be readily exchanged. Metal complexes of transition elements are well known here, chelation occurs within a much wider range of elements. Chelating agents yielding soluble metal complexes are also called sequestering agents.1 A chelating agent has at least two functional groups which donate a pair of electrons to the metal, such as = O, -NH2 or -COO¯. Furthermore, these groups must be located so as to allow ring formation with the metal. Chelating agents are widely found in living systems and are of importance in cellular metabolism.
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Chelating agents have various uses in biochemistry:
The term complexane has been recommended by IUPAC for EDTA and other aminopolycarboxylic acids of related structure. The pK values of some complexanes are given below.
In the table above, the absolute stability constants of various metal complexes of these complexanes are reported. As mentioned above these data permit calculation the apparent stability constants of these complexes at any pH. Data were taken from reference.6
|EDTA||Ethylenediamine-tetraacetic acid Disodium salt|
|EGTA||Ethyleneglycol-O, O'-bis(2-aminoethyl)-N, N, N', N'-tetraacetic acid|
|HEDTA||N-(2-Hydroxyethyl)ethylenediamine-N, N', N'-triacetic acid Trisodium saIt|