Hydrogen atom transfer in the radical cations of tryptophan-containing peptides AW and WA studied by mass spectrometry, infrared multiple-photon dissociation spectroscopy, and theoretical calculations. Issue 1 (February 2019)

• Record Type: Journal Article
• Authors: Piatkivskyi, Andrii; Lau, Justin Kai-Chi; Berden, Giel; Oomens, Jos; Hopkinson, Alan C; Siu, KW Michael; Ryzhov, Victor
• Is Part Of: European journal of mass spectrometry. Volume 25:Issue 1(2019)
• Page Start: 112
• Page End: 121
• Publication Date: 2019-02
• Subjects: Peptide radical cations -- tryptophan radicals -- infrared multiple-photon dissociation -- ion–molecule reactions -- density functional theory calculations; Mass spectrometry -- Periodicals; Mass Spectrometry; Mass spectrometry; Periodicals; Periodicals; 543.6505
• DOI: 10.1177/1469066718802547
• Languages: English
• ISSNs: 1469-0667

Abstract

Two types of radical cations of tryptophan—the π-radical cation and the protonated tryptophan-N radical—have been studied in dipeptides AW and WA. The π-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–NH3 ] +, 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 π-radical cation. These findings are supported by the density functional theory calculations that suggest a relatively high barrier for the radical rearrangement (N to π) 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 π-radical cation. Gas-phase ion–molecule reactions with n -propyl thiol have also been used to differentiate between the π-radical cations (react by hydrogen abstraction) and the tryptophan-N species (unreactive) of AW.