Journal of the American Society for Mass Spectrometry

, Volume 15, Issue 1, pp 65–76

Gas-phase reactions of protonated tryptophan


DOI: 10.1016/j.jasms.2003.09.011

Cite this article as:
Lioe, H., O’Hair, R.A.J. & Reid, G.E. J Am Soc Mass Spectrom (2004) 15: 65. doi:10.1016/j.jasms.2003.09.011


The gas phase reactions of protonated tryptophan have been examined in a quadrupole ion trap using a combination of collision induced dissociation, hydrogen–deuterium exchange, regiospecific deuterium labeling and molecular orbital calculations (at the B3LYP/6-31G* level of theory). The loss of ammonia from protonated tryptophan is observed as the primary fragmentation pathway, with concomitant formation of a [M+H−NH3]+ ion by nucleophilic attack from the C3 position of the indole side chain. Hydrogen-deuterium exchange and regiospecific deuterium labeling reveals that scrambling of protons in the C2 and C4 positions of the indole ring, via intramolecular proton transfer from the thermodynamically preferred site of protonation at the amino nitrogen, precedes ammonia loss. Molecular orbital calculations have been employed to demonstrate that the activation barriers to intramolecular proton transfer are lower than that for NH3 loss.

Copyright information

© American Society for Mass Spectrometry 2004

Authors and Affiliations

  1. 1.School of ChemistryUniversity of MelbourneMelbourneAustralia
  2. 2.Joint Proteomics Laboratory, The Ludwig Institute for Cancer Research and the Walter and Eliza Hall Institute of Medical ResearchRoyal Melbourne HospitalParkvilleAustralia