Factors controlling metal-ion selectivity in the binding sites of calcium-binding proteins. The metal-binding properties of amide donors. A crystallographic and thermodynamic study.

The metal-ion complexing properties of the ligand EDTAM (ethylenediamine-N,N,N‘,N‘-tetraacetamide) are investigated as a model for the role of amide oxygen donors in the binding sites of Ca-binding proteins. The structures of the complexes [Ca(EDTAM)NO3]NO3 (1), [La(EDTAM)(H2O)4](NO3)3·H2O (2), and [Cd(EDTAM)(NO3)]NO3 (3) are reported:  1 monoclinic, P21/c, a = 10.853(2) A, b = 12.893(3) A, c = 13.407(3) A, β = 103.28(3)°, Z = 4, R = 0.0281; 2 triclinic, P1, a = 8.695(2) A, b = 9.960(2) A, c = 16.136(3) A, α = 95.57(3)°, β = 94.84(3)°, γ = 98.72(3)°, Z = 2, R = 0.0394; 3 monoclinic, P21/c, a = 10.767(2) A, b = 12.952(2) A, c = 13.273(2) A, β = 103.572(3)°, Z = 4, R = 0.0167. Compounds 1 and 3 are isostructural, and the EDTAM binds to the metal ion through its two N-donors and four O-donors from the amide groups. Ca(II) in 1 is 8-coordinate with a chelating NO3- group, while Cd(II) in 3 may possibly be 7-coordinate, with an asymmetrically coordinated NO3- that is best regarded as unidentate. The La(III) i...