The stability constants of the binary Cu 2+ complexes of glycine amide, diglycine, diglycine amide, triglycine, and tetraglycine were determined, as were those of the mixed-ligand Cu 2+ systems containing 2,2'-bipyridyl and one of the mentioned oligoglycines. The results evidence that all these complexes have the same structure and, therefore, the binding sites of the ligands have to be the terminal amino group and the oxygen of the neighbored amide group. The stability differences between the ternary and the binary complexes are in agreement with this interpretation. It is of interest to note that these ternary complexes are significantly more stable than expected on statistical reasons. With increasing pH, the amide groups in the binary complexes are successively deprotonated. Thus, with tetraglycine finally all three amide protons are displaced, and the amide nitrogens are bound to the square-planar coordination sphere of Cu 2+ . As in the Cu 2+ -2,2'-bipyridyl 1 : 1 complex, only two coordination positions are left for the binding of the oligoglycine, in the tenary complexes, only one amide group can be deprotonated. An increase in pH with deprotonation of other amide groups leads to a displacement of 2,2'-bipyridyl, i. e. the simple binary complexes result. No evidence could be observed for the coordination of a deprotonated amide group to an apical position of the coordination sphere of Cu 2+ . Additionally, while the displacement of the first amide proton in the several binary Cu 2+ oligoglycine complexes occurs over a large pH range (4 to 7), the deprotonation in all the mixed-ligand complexes takes place at pH approximately 8.In peptides, and probably also in proteins, the deprotonated amide group is one of the important binding sites for the coordination of Cu 2+ 2 . In fact, quite a number of Cu 2+ -peptide complexes, including those with oligoglycines 3-8 , were investigated, and it was shown that the amide proton is lost by the coordination of this metal ion to the amide nitrogen.The aim of the present study was to learn how the coordination tendency between oligoglycines and Cu 2+ is influenced, if two positions are occupied by another ligand in the coordination sphere of the metal ion. In this way, a situation which is analogous to conditions present in biological systems may be simulated. Additionally, it seems important to understand the mutual influence of two ligands bound to the same metal ion, as such mixed-ligand complexes can be considered as models for enzymemetal ion-substrate complexes 9-12 . For example, in galactose oxidase action, the role of Cu 2+ has been suggested to lie in the formation of a ternary complex with the substrates 13 .Recently, it was shown 14 that in the mixed-ligand 2,2'-bipyridyl-Cu 2+ -glycine amide complex, the Requests for reprints should be sent to Priv.-Doz. Dr. HELMUT SIGEL, Institute of Inorganic Chemistry, Spitalstrasse 51, University of Basel, CH-4000 Basel (Switzerland) .amide group may still be deprotonated in the physiological pH range. However, the un...