%0 Journal Article
%T Hydrogen atom transfer in the radical cations of tryptophan
%A Alan C Hopkinson
%A Andrii Piatkivskyi
%A Giel Berden
%A Jos Oomens
%A Justin Kai-Chi Lau
%A KW Michael Siu
%A Victor Ryzhov
%J European Journal of Mass Spectrometry
%@ 1751-6838
%D 2019
%R 10.1177/1469066718802547
%X Two types of radical cations of tryptophan¡ªthe ¦Ð-radical cation and the protonated tryptophan-N radical¡ªhave been studied in dipeptides AW and WA. The ¦Ð-radical cation produced by removal of an electron during collision-induced dissociation of a ternary Cu(II) complex was only observed for the AW peptide. In the case of WA, only the ion corresponding to the loss of ammonia, [WA¨CNH3] £¿+, was observed from the copper complex. Both protonated tryptophan-N radicals were produced by N-nitrosylation of the neutral peptides followed by transfer to the gas phase via electrospray ionization and subsequent collision-induced dissociation. The regiospecifically formed N£¿ species were characterized by infrared multiple-photon dissociation spectroscopy which revealed that the WA tryptophan-N£¿ radical remains the nitrogen radical, while the AW nitrogen radical rearranges into the ¦Ð-radical cation. These findings are supported by the density functional theory calculations that suggest a relatively high barrier for the radical rearrangement (N£¿ to ¦Ð) in WA (156.3£¿kJ£¿mol£¿1) and a very low barrier in AW (6.1£¿kJ£¿mol£¿1). The facile hydrogen atom migration in the AW system is also supported by the collision-induced dissociation of the tryptophan-N radical species that produces fragments characteristic of the tryptophan ¦Ð-radical cation. Gas-phase ion¨Cmolecule reactions with n-propyl thiol have also been used to differentiate between the ¦Ð-radical cations (react by hydrogen abstraction) and the tryptophan-N£¿ species (unreactive) of AW
%K Peptide radical cations
%K tryptophan radicals
%K infrared multiple-photon dissociation
%K ion¨Cmolecule reactions
%K density functional theory calculations
%U https://journals.sagepub.com/doi/full/10.1177/1469066718802547