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

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.