Compositions and conformations of several transition metal complexes with a nonapeptide hormoneoxytocin

Abstract

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 ≈ 9. 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 Sγ(1) co-ordinated conformer was found to be more stable than the Sγ(6) co-ordinated one. Dramatic conformational changes occur upon oxytocin co-ordinating to Ni^II, Mn^II or Pd^II.