New evidence for H/D scrambling of tryptophan and its analogues in the gas phase

• MS3 experiment was firstly used to reveal the H/D scrambling prior to fragmentation.
• Protonation sites lead to the fragmentation difference of isomers.
• Amino plays a key role for the H/D scrambling by firstly experiment of comparison.
• The H/D scrambling involves alkyl hydrogen and the mechanism was firstly proposed.
• The transfer of methoxyl was firstly reported using ESI-CID-MS/MS.

Tryptophan and its analogues were analyzed using electrospray ionization quadrupole-time of flight (ESI-QTOF) combined with hydrogen–deuterium exchange. The similar profiles of MS3 spectra of the product ions at m/z 190, 191, and 192 from [M (D)+D]+ for tryptophan by the losses of NH(D)3 imply the same fragmentation patterns including the characteristic transfer of hydroxyl and very similar structures of these ions. This reveals the H/D scrambling prior to fragmentation. Compared the MS/MS spectrum of D-labelled compound 2 to the MS3 spectrum of m/z 191, it was found that amino group plays an important role for the H/D scrambling. The MS2 and MS3 spectra for D-labelled compound 4 indicate that the H/D scrambling involves the hydrogen on methyl linked to indole ring. In addition, the fragmentation of transfer of hydroxyl or methoxyl is confirmed by comparing the spectra of compounds 1–4.

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