Comparing the fragmentation reactions of protonated cyclic indolyl α-amino esters in Q/Orbitrap and QTOF mass spectrometers.

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 experiments and only the aromatic indole C-H hydrogens were substituted with deuterium in regiospecific hydrogen-deuterium labeling experiments. Theoretical calculations were carried out by DFT method at B3LYP level with 6-311G (d, p) basis set in the Gaussian 03 package of programs.

RESULTS: In Q/Orbitrap MS/MS, when the added proton on N8 position of protonated cyclic indolyl α-amino esters stepwise migrated to C3 position via two sequential 1,4-H shift, an ion-neutral complex INC1 of [protonated cyclic N-sulfonyl ketimino esters/indoles] was formed by a charge-directed heterolytic cleavage of C3 -C10 bond, while an ion-neutral complex INC3 of [cyclic N-sulfonyl ketimino esters/protonated indoles] was formed when another labile hydrogen on N8 position successively migrates to C4 position. Direct decomposition of INC1 and INC3 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 hydrogen-deuterium exchange reaction with residual water in HCD cell was also observed. In QTOF MS/MS, protonated cyclic N-sulfonyl ketimino esters and protonated indoles were resulted from direct decomposition of INC1 and INC3 , respectively, rather than proton transfer.

CONCLUSIONS: Due to specific construction of Q/Orbitrap MS and QTOFMS, 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 tandem mass spectrometry technology.