The unusual co-ordination ability of vasopressin-like peptides; potentiometric and spectroscopic studies of some copper(II) and nickel(II) complexes

The results are reported of a potentiometric and spectroscopic study of the H+, Cu2+, and Ni2+ complexes of [Arg8]vasopressin, [Arg8]vasotocin and some synthetic analogues containing the D-valyl residue in place of the glutamine residue at 25 °C and an ionic strength 0.10 mol dm–3(KNO3). The complexes of vasopressin and vasotocin with CuII are the most stable Cu-peptide complexes with 4N co-ordination yet reported. This results from the favourable conformation of the binding site within the ring formed by the disulphide bridge of the peptide. The high stability is lost when a non-co-ordinating residue in the ring (Gln) is replaced by a residue of opposite chirality (D–Val) as a result of steric hindrance between the α-carbon atom of the Val side chain and the neighbouring carbonyl oxygen.