Thermodynamic evaluation of complexes of zinc and cadmium that mimetize metallic centers in transcription factors

Abstract

Structure and energy calculations of the zinc and cadmium complexes of the peptide his–cys in their various provable geometries have been carried out using density functional methods (LSDA pBP DN ∗). It is shown that, for the same complex geometry, the zinc complexes are more stable than the cadmium complexes and that, for the octahedral and square planar complexes, the cis isomers are favored over the trans isomers. It was found that the hexa-coordinated zinc complex has a distorted octahedral geometry, which, through elongation of the two H 2O–Zn bonds, tends to adopt a square planar geometry and finally evolve to the preferred tetrahedral geometry. On the other hand, the cadmium complexes tend to stay in the octahedral geometry. These results suggest that the substitution of Zn by Cd as a probe for NMR studies of the Zn coordination environment in metaloproteins should be used with caution.