Structural variations in metal ion complexes of the ligand EGTA⁴-
Structural studies of several metal ion complexes with the tetraanion of the octadentate ligand, H^4EGTA (3,12-bis(carboxymethyl)-6, 9-dioxa-3, 12-diazatetradecanedioic acid), as well as the structure of H^4EGTA, have been performed by single crystal X-ray diffraction. Of particular interest was the structural basis for the large preference for EGTA^4- to bind calcium ion rather than (K(CaL^2-) ~= 10^6 magnesium ion = 10^6 (K(MgL^2-)), a preference which is similar to that exhibited by intracellular calcium binding proteins. The alkaline earth compounds, Ca[Ca(EGTA)]·(22/3)H2o, Sr[Ca(EGTA)] ·6H2O, Mg[Sr(EGTA)(OH2 )]·7H2o, Mg(Ba-(EGTA)]·(8/3)H2O·(1/3) (CH3) 2CO, and (Mg2(EGTA)(OH2 )6]·5H2O, have been structurally characterized. [Ca(EGTA)]2- is eight-coordinate and utilizes the full octadentate chelating capability of the EGTA^4- ligand. The ether oxygen atoms are bound at a shorter distance than the amine nitrogen atoms. EGTA^4 - is octadentate toward both the strontium and barium ions, which are nine- and ten-coordinate, respectively. The magnesium complex is dinuclear, utilizing each end of the EGTA^4- ligand as a tridentate iminodiacetate ligand; the ether oxygen atoms are not involved in coordination to the metal ion. Structures of EGTA^4 - chelates of metal ions that are commonly used as spectroscopic probes for calcium ion binding sites have also been determined. The cadmium chelate in Sr[Cd(EGTA)]·7H2 o is eight-coordinate, li k e [Ca(EGTA)]^2 -, but the amine nitrogen atoms are bound at shorter distances than the ether oxygen atoms. The metal ions in the structures of tripositive lanthanide ion complexes, Ca[Er(EGTA)(OH2)]2·12H2O and Ca[Nd(EGTA)- (OH 2 )]2·9H2O, are nine- and ten-coordinate, respectively. To further explore coordination modes of the EGTA^4 - ligand with smaller metal ions, where the ligand is not likely to be octadentate, structures of manganese and copper complexes of were determined. Sr[Mn(EGTA)] ·7H2O is isomorphous with the cadmium compound. As a result, the Mn(II) ion is eight-coordinate. The copper complex crystallizes as a dinuclear species, [CU2(EGTA)(OH2)2] ·2H2O, in which each end of the EGTA^4- ligand binds a copper(II) ion in a tetradentate fashion; the ether oxygen atom is bound in the apical position of the square pyramidal coordination sphere.
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