Compositions and conformations of several transition metal complexes with a nonapeptide hormone oxytocin

Similar co-ordination characteristics of oxytocin (OT) towards Cu-II, Ni-II, Mn-II and Zn-II at different pH values have been demonstrated by electrospray mass spectrometry (ESMS) and rationalized by molecular mechanics simulation. At ca. pH 2 oxytocin does not interact with the metal ions; at pH 5 species with metal bound oxytocin were detected, including [OT+H+](+), [M+OT](2+), [M+OT-H+](+), [M+OT+ClO4-+H+](2+) and [M+OT+ClO4-](+) and only stable 4N complexes were found at pH approximate to9. Molecular modelling studies using the Universal force field (UFF) showed that the four N-donor centers of oxytocin prefer the square planar geometry in complexes of Ni-II and Mn-II. For Pd-II, the S gamma (1) co-ordinated conformer was found to be more stable than the S gamma (6) co-ordinated one. Dramatic conformational changes occur upon oxytocin co-ordinating to Ni-II, Mn-II or Pd-II.