Binary and ternary complexes of cadmium(II) involving ethylenediamine-N′,N′-diacetic acid and amino acids or peptides

The present work examines the acid-base equilibria of Ethylenediamine-N,N -diacetic acid (EDDA) and the formation equilibria of binary and ternary complexes of cadmium (II) with EDDA and amino acids or peptides. Cd(II) is found to form a 1:1 complex with EDDA. The ternary complexes are formed in a stepwise mechanism, whereby the EDDA binds to Cd(II), followed by ligation of the amino acid or peptide. The amino acid, lysine, forms protonated and deprotonated ternary complexes. The stability constants of the ternary complexes have been quantified in relation to the corresponding binary complexes. The concentration distribution of the various complex species has been evaluated.