Interactions of the Hormone Oxytocin with Divalent Metal Ions

The interaction of the cyclic nonapeptide oxytocin (OT) with a number of alkaline earth and divalent transition metal ions (X2+) was examined employing mass spectrometry (MS) and ion mobility spectrometry (IMS) techniques in combination with molecular dynamics (MD) and density functional theory (DFT) calculations. Under acidic conditions it was found that OT exhibits an exceptionally strong affinity for all divalent metal ions resulting in strong [OT + X]2+ peaks in the mass spectrum. Under basic conditions only Cu2+ and Ni2+−OT complexes were detected and these were singly, doubly, triply, or quadruply deprotonated. Collision-induced dissociation of the [OT – 3H + Cu]− complex yielded exclusively C-terminal Cu2+-containing fragments (Cu2+fragment3−), suggesting that the Cu2+ ligation site includes deprotonated C-terminal backbone amide nitrogen atoms and the N-terminal amino nitrogen atom in [OT − 3H + Cu]−. MD and DFT calculations indicate a square-planar complex is consistent with these observations and with experimental collision cross sections. MD and DFT calculations also indicate either an octahedral or trigonal-bipyramidal complex between Zn2+ and OT is lowest in energy with carbonyl oxygens being the primary ligation sites. Both complexes yield cross sections in agreement with experiment. The biological impact of the structural changes induced in OT by divalent metal ion coodination is discussed.

本研究针对环状九肽催产素（oxytocin, OT）与多种碱土金属离子及二价过渡金属离子（X²⁺）的相互作用展开探究，采用质谱（mass spectrometry, MS）、离子迁移谱（ion mobility spectrometry, IMS）技术，并结合分子动力学（molecular dynamics, MD）与密度泛函理论（density functional theory, DFT）计算进行分析。在酸性条件下，催产素对所有二价金属离子均表现出极强的结合亲和力，质谱中可观测到强度显著的[OT + X]²⁺峰信号。在碱性条件下，仅能检测到Cu²⁺、Ni²⁺与催产素形成的复合物，且这些复合物分别发生了单质子、双质子、三质子及四质子脱除。对[OT – 3H + Cu]⁻复合物进行碰撞诱导解离实验，仅得到含Cu²⁺的C端片段（Cu²⁺fragment3⁻），这表明在[OT – 3H + Cu]⁻复合物中，Cu²⁺的配位位点包含去质子化的C端主链酰胺氮原子与N端氨基氮原子。分子动力学与密度泛函理论计算结果显示，平面正方形配位构型的复合物与上述实验观测结果及实验测得的碰撞截面均相符。分子动力学与密度泛函理论计算还表明，Zn²⁺与催产素形成的复合物中，以羰基氧为主要配位位点的八面体或三角双锥构型复合物能量最低，且这两种构型对应的碰撞截面均与实验结果一致。本研究还讨论了二价金属离子配位诱导催产素结构变化所带来的生物学影响。