Nickel(II), zinc(II) and cadmium(II) complexes of peptides containing separate aspartyl and cysteinyl residues

Abstract Nickel(II), zinc(II) and cadmium(II) complexes of two hexapeptides ADAAAC-NH 2 and AADAAC-NH 2 containing terminal amino, separate aspartyl carboxylate and cysteinyl thiolate donor functions have been studied by potentiometric and spectroscopic techniques. For nickel(II), the amino terminus and the aspartyl residues are the primary metal binding sites. At high pH values, thiolate groups can also coordinate to nickel(II) and this interaction results in the co-existence of various coordination isomers. In the case of AADAAC-NH 2 , even dinuclear complexes can be formed with separate (NH 2 , N − , N − , COO − ) and (S − , 3N − ) binding modes. On the contrary, the thiolate functions are the primary binding site for zinc(II) and especially cadmium(II) ions. The formation of macrochelate complexes is the most characteristic with these metal ions including the terminal amino, the internal carboxylate and thiolate donor functions. None of the side chain donors can, however, induce the deprotonation and zinc(II) or cadmium(II) coordination of the amide functions of these peptides.