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Decomposition of protonated noradrenaline and normetanephrine assisted by NH 2 migration studied by electrospray tandem mass spectrometry and molecular orbital calculations
Author(s) -
Rogalewicz Françoise,
Bourcier Sophie,
Hoppilliard Yannik
Publication year - 2005
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.1841
Subject(s) - chemistry , protonation , isomerization , mass spectrometry , ion , carbocation , computational chemistry , tandem mass spectrometry , medicinal chemistry , analytical chemistry (journal) , organic chemistry , chromatography , catalysis
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 H 2 O at very low collision energy, and the fragmentations of [MH–H 2 O] + occur at higher collision energy. The breakdown graphs of [MH–H 2 O] + ions, with collision energy varying from 0–40 eV in the laboratory frame, are presented. [VIH–H 2 O] + ions lose competitively NH 3 and CH 3 OH. For [VH–H 2 O] + the loss of NH 3 is dominant while H 2 O 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 H 2 O 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 NH 3 from [MH–H 2 O] + 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 NH 3 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–2H 2 O] + 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 H 2 O. Losses of CO following the secondary fragmentations involve activation barriers higher than 330 kJ/mol. Copyright © 2005 John Wiley & Sons, Ltd.