Similar co-ordination characteristics of oxytocin (OT) towards CuII, NiII, MnII and ZnII 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 NiII and MnII. For PdII, 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 NiII, MnII or PdII.
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