Co-ordination properties of cyclopeptides. Formation and stability of zinc(II) and copper(II) complexes of histidine-containing cyclopeptides, or imidazole
In order to obtain an improved understanding of the complexing properties of cyclopeptides involved in biological processes, the interaction of zinc(II) with cyclo(L-histidyl-L-histidyl) was studied by potentiometric and calorimetric techniques in aqueous solution. Comparison between the thermodynamic parameters of the complexes formed with those for analogous species formed with cyclo(glycyl-L-histidyl) allowed evidence to be obtained for the formation of chelate rings of unusual size. The thermodynamic quantities associated with copper(II) complex formation with cyclo(-Gly-His-) were determined. The role of the different stereochemical requirements of copper(II) and zinc(II) in the formation of large chelate rings was assessed by means of previously reported data concerning the copper(II)–cyclo(-His-His-) complexes.