Rogalewicz Françoise, Bourcier Sophie, Hoppilliard Yannik
Laboratoire des Mécanismes Réactionnels, Unité Mixte de Recherche CNRS 7651, Ecole Polytechnique, 91128 Palaiseau Cedex, France.
Rapid Commun Mass Spectrom. 2005;19(6):743-51. doi: 10.1002/rcm.1841.
As part of a research program on neurotransmitters in a biological fluid, the fragmentations characterising catecholamines protonated under electrospray ionisation (ESI) conditions, under low collision energy in a triple-quadrupole mass spectrometer, were investigated. The decompositions of protonated noradrenaline (VH) and normetanephrine (VIH) were studied. Both precursor ions eliminate first H2O at very low collision energy, and the fragmentations of [MH-H2O]+ occur at higher collision energy. The breakdown graphs of [MH-H2O]+ ions, with collision energy varying from 0-40 eV in the laboratory frame, are presented. [VIH-H2O]+ ions lose competitively NH3 and CH3OH. For [VH-H2O]+ the loss of NH3 is dominant while H2O is eliminated at very low abundance at all collision energies. All of these secondary fragmentations are followed at higher collision energies by elimination of CO. These fragmentations are interpreted by means of ab initio calculations up to the B3LYP/6-311+G(2d,2p) level of theory. The elimination of H2O requires first the isomerisation of N-protonated forms, chosen as energy references, to O-protonated forms. The isomerisation barriers are calculated to be lower than 81 kJ/mol above the N-protonated forms. The elimination of NH3 from [MH-H2O]+ requires first the migration, via a cyclisation, of the amine function from the linear chain to the aromatic ring in order to prevent the formation of unstable disubstituted carbocations in the ring. The barriers associated with the loss of NH3 are located 220 and 233 kJ/mol above VH and 219 kJ/mol above VIH. The energy barrier for the loss of ROH is located 236 and 228 kJ/mol above VH and VIH, respectively. The absence of ions corresponding to [VH-2H2O]+ is due to a parasitic mechanism with an activation barrier lower than 236 kJ/mol that leads to a stable species unable to fragment, thus preventing the second loss of H2O. Losses of CO following the secondary fragmentations involve activation barriers higher than 330 kJ/mol.
作为生物体液中神经递质研究项目的一部分，研究了在电喷雾电离（ESI）条件下、在三重四极杆质谱仪的低碰撞能量下质子化的儿茶酚胺的碎裂情况。研究了质子化去甲肾上腺素（VH）和去甲变肾上腺素（VIH）的分解。两种前体离子在非常低的碰撞能量下首先消除H₂O，而[MH-H₂O]+的碎裂在较高碰撞能量下发生。给出了[MH-H₂O]+离子在实验室坐标系中碰撞能量从0至40 eV变化时的碎裂图。[VIH-H₂O]+离子竞争性地失去NH₃和CH₃OH。对于[VH-H₂O]+，NH₃的损失占主导，而在所有碰撞能量下H₂O的消除丰度都非常低。所有这些二级碎裂在较高碰撞能量下随后都会消除CO。这些碎裂通过高达B3LYP/6-311+G(2d,2p)理论水平的从头算计算进行解释。H₂O的消除首先需要将选为能量参考的N-质子化形式异构化为O-质子化形式。计算得出异构化势垒比N-质子化形式高不到81 kJ/mol。从[MH-H₂O]+中消除NH₃首先需要胺官能团通过环化从直链迁移到芳环，以防止在环中形成不稳定的二取代碳正离子。与NH₃损失相关的势垒位于VH上方220和233 kJ/mol以及VIH上方219 kJ/mol处。ROH损失的能量势垒分别位于VH和VIH上方236和228 kJ/mol处。对应于[VH-2H₂O]+的离子不存在是由于一种寄生机制，其活化势垒低于236 kJ/mol，导致形成稳定的无法碎裂的物种，从而阻止了H₂O的第二次损失。二级碎裂后CO的损失涉及高于330 kJ/mol的活化势垒。
