jjmirujo at unav.es
Sat Apr 1 03:20:57 EST 2000
I agree with Tim Fitzwater in that a single cation is bound per EDTA
molecule. The matter is not how many charges EDTA has, but what makes EDTA
so efficient in chelating cations. EDTA behaves like an octopus: each
nitrogen forms a dative bond with the cation and oxygens from the carboxylic
groups forms additional coordination bonds. As a result, the cation remains
tightly bound at the center of the structure (and only a cation can be bound
in this way).
It is of interest that immobilized iminodiacetic acid (IDA, half and EDTA
molecule), commonly used in the purification of his-tagged proteins, is
also able to chelate cations. In this case, the remaining coordination bonds
are formed by water, or by histidines if the protein is bound. As reported
in this newsgroup, a method to elute these proteins is by adding EDTA, that
is able to easily remove cations from IDA and histidines.
Incidentally , EDTA was sometimes used to titrate calcium solutions, in the
absence of other divalent cations, in the way proposed by Wolfgang.
Eriochrome black T is used as indicator in these titrations (blue in the
free form, dark red when calcium is bound), and, of course, a 1:1 proportion
is used to calculate calcium concentration.
Juan J. Martinez Irujo
Departamento de Bioquimica
Universidad de Navarra
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