High-energy electron transfer dissociation of protonated amino acids

High-energy electron transfer dissociation (HE-ETD) of singly protonated amino acids was investigated using charge inversion mass spectrometry with alkali metal targets in which dissociations of charge-reduced neutrals were induced from the excited neutral intermediates. The dominant trapezoidal peaks of the 17 u loss in the charge inversion spectra of the protonated aliphatic amino acids were assigned to NH3 loss caused by NCα bond cleavage from hypervalent ammonium radical intermediates via a repulsive state; this assignment was based on the peak shape and kinetic energy release (KER) values. The peak of the non-dissociated ion observed in the proline spectra was explained by the same bond cleavage. The triangular peaks of the 2 u loss, which were dominantly observed in all of the spectra, were assigned to H2 elimination, which involved H atom migration in the neutral radical. Neutral intermediates that induced the H2 loss were estimated to have a lower internal energy than that of the NH3 loss by comparing the charge inversion spectra of the different target atoms. Differences in the isobaric ions between leucine and isoleucine were found in the bond dissociation of CβCγ in their side chains following the NH3 loss and in the intensity of the NH3 loss relative to that of the H2 loss. The charge inversion spectra of the protonated amino acids, with side chains that are subject to post-translational modification, provided intense peaks resulting from the consecutive and competitive loss of neutral molecules following the NH3 loss and H2 loss. Along with these losses, side chain losses caused by H atom migration were also observed as sharp peaks with KER values smaller than that of the consecutive loss.