COORDINATION PROPERTIES OF CYCLOPEPTIDES - FORMATION AND STABILITY OFZINC(II) AND COPPER(II) COMPLEXES OF HISTIDINE-CONTAINING CYCLOPEPTIDES, OR IMIDAZOLE

In order to obtain an improved understanding of the complexing propert ies of cyclopeptides involved in biological processes, the interaction of zinc(II) with cyclo(L-histidyl-L-histidyl) was studied by potentio metric and calorimetric techniques in aqueous solution. Comparison bet ween the thermodynamic parameters of the complexes formed with those f or analogous species formed with cyclo(glycyl-L-histidyl) allowed evid ence to be obtained for the formation of chelate rings of unusual size . The thermodynamic quantities associated with copper(II) complex form ation with cyclo(-Gly-His-) were determined. The role of the different stereochemical requirements of copper(II) and zinc(II) in the formati on of large chelate rings was assessed by means of previously reported data concerning the copper(II)-cyclo(-His-His-) complexes.