Determination of a Binding Site of Cu and Ni Metal Ions with Oxytocin Peptide by Electrospray Tandem Mass Spectrometry and Multiple Mass Spectrometry

The structures of a [Ni^II, Cu^II—oxytocin] complex were investigated by electrospray ionization-mass spectrometry in positive mode. The fragmentation patterns of the [Ni^II, Cu^II + OT]²⁺ complex were analyzed by tandem mass spectrometry and multiple mass spectrometry in the gas-phase. Conformations of metal^II ion binding to oxytocin (OT) have been studied to explain the biological activity difference in the physiological solution. The [Ni^II + OT]²⁺ and [Cu^II + OT]²⁺ complexes were observed as the main ions in MS spectra. The Cys1–Tyr2–Ile3–Gln4 sequence of oxytocin is suggested to be a binding site for the [Ni^II + OT]²⁺ gas-phase complex and Ile3–Gln4–Asn5–Cys6 sequence for the [Cu^II + OT]²⁺ gas-phase complex. The specific binding site of Cu^II ion in the [Cu^II + OT]²⁺ complex is explained as a reason of the negligible effect on the [Cu^II—oxytocin] biological activity in aqueous solution.