Equilibrium studies on metal-peptide systems-I: polarographic determination of association constants of cadmium(II), zinc(II) and lead(II) complexes of glycylgycine

The equilibria involved in the association of Cd2+, Zn2+ and Pb2+ with glycylglycine have been investigated polarographically (at 25°C). As the nature of binding depends on solution pH, studies have been made at pH < 4 (participation of C-terminal end glycine residue takes place) and at pH > 7 (participation of both the N-terminal amino group and the carbonyl group of the peptide linkage occurs). In the lower range of pH, comparatively weaker (1:1) complexes are formed with Cd2+ and Pb2+ ions (that of Zn2+ could not be detected due to the overlapping hydrogen wave) whereas in the higher range, more stable complexes (both 1:1 and 1:2) are formed for all the metal ions. Stepwise formation constants were determined by DeFord and Hume's method. Distribution of different species formed in solution at different pH values (for the cadmium system) and also at the different peptide concentrations studied (for the lead system) is shown graphically