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Comparing the fragmentation reactions of protonated cyclic indolyl α‐amino esters in quadrupole/orbitrap and quadrupole time‐of‐flight mass spectrometers
Author(s) -
Cao Xiaoji,
Cai Xue,
Mo Weimin
Publication year - 2018
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.8063
Subject(s) - chemistry , orbitrap , protonation , hydrogen–deuterium exchange , mass spectrometry , deuterium , collision induced dissociation , dissociation (chemistry) , indole test , sulfonyl , electrospray ionization , tandem mass spectrometry , computational chemistry , photochemistry , ion , stereochemistry , organic chemistry , chromatography , physics , quantum mechanics , alkyl
Rationale The comparative study of higher‐energy collisional dissociation (HCD) and collision‐induced dissociation (CID) mechanisms for protonated cyclic indolyl α‐amino esters in quadrupole/orbitrap (Q/Orbitrap) and quadrupole time‐of‐flight (QTOF) mass spectrometers, respectively, is helpful to study the structures and properties of biologically active indole derivatives using tandem mass spectrometry (MS/MS) technology. Methods HCD and CID experiments were carried out using electrospray ionization Q/Orbitrap MS and QTOFMS in positive ion mode, respectively. Only the labile hydrogens were exchanged with deuterium in hydrogen/deuterium exchange (HDX) experiments and only the aromatic indole C‐H hydrogens were substituted with deuterium in regiospecific hydrogen‐deuterium labeling experiments. Theoretical calculations were carried out using the density functional theory (DFT) method at the B3LYP level with the 6‐311G(d,p) basis set in the Gaussian 03 package of programs. Results In Q/Orbitrap MS/MS, when the added proton on the N 8 position of protonated cyclic indolyl α‐amino esters migrated in a stepwise fashion to the C 3 position via two sequential 1,4‐H shifts, an ion‐neutral complex INC 1 of [protonated cyclic N‐sulfonyl ketimino esters/indoles] was formed by a charge‐directed heterolytic cleavage of the C 3 –C 10 bond, while an ion‐neutral complex INC 3 of [cyclic N‐sulfonyl ketimino esters/protonated indoles] was formed when another labile hydrogen on the N 8 position successively migrated to the C 4 position. Direct decomposition of INC 1 and INC 3 resulted in protonated cyclic N‐sulfonyl ketimino esters and protonated indoles, respectively, while proton transfer led to protonated indoles and protonated cyclic N‐sulfonyl ketimino esters. The HDX reaction with residual water in the HCD cell was also observed. In QTOF‐MS/MS, protonated cyclic N‐sulfonyl ketimino esters and protonated indoles resulted from direct decomposition of INC 1 and INC 3 , respectively, rather than proton transfer. Conclusions Due to the specific construction of the Q/Orbitrap and QTOF mass spectrometers, different fragmentation mechanisms medicated by ion‐neutral complexes of protonated cyclic indolyl α‐amino esters were proposed. This study is desirable for qualitative and quantitive investigation of indole derivatives using MS/MS technology.