On the charge partitioning between c and z fragments formed after electron-capture induced dissociation of charge-tagged Lys-Lys and Ala-Lys dipeptide dications

Abstract

Here we report on the charge partition between c and z fragments formed after femtosecond collisional electron-transfer from Cs atoms to charge-tagged peptide dications. Peptides chosen for study were Ala-Lys (AK) and Lys-Lys (KK) where one or both of the lysine -amino groups were trimethylated to provide one or two fixed charges. For peptides with only one charge tag, the other charge was obtained by protonation of an amino group. In some experiments the ammonium group was tagged by 18-crown-6-ether (CE). Since recombination energies decrease in the order:, it is possible to change the probability for the transferred electron to end up at either the N-terminal or the C-terminal residue by CE attachment. We find, however, that the individual recombination energies have little influence on the relative ratio between the yield of c and z ions as long as there are no mobile protons that can be transferred between the two fragments. Our results can be accounted for by the Utah-Washington model where the electron is captured into an amide * orbital that weakens the N-C ␣ bond and causes its breakage, followed by proton, electron, or hydrogen transfer between the c and z fragments that stay together as an ion-molecule complex for some time. The data are also in accordance with the notion that an amide group competes with the charged groups for the electron. Electron capture by charged groups results in loss of small neutrals such as hydrogen and ammonia. (J Am Soc

Unusual Fragmentation of β-Linked Peptides by ExD Tandem Mass Spectrometry

“…Although charge neutralization on the quaternary ammonium may not lead to backbone cleavages, 67 electron capture at the protonated N-terminal amine of the doubly charged Q 06 peptide should result in backbone fragmentations. Further, the recombination energy of the protonated N-terminal amino group is higher than that of the quaternary ammonium as calculated for the singly charged model cations by Jensen et al (4.3 eV (MeNH 3 + ) vs. 3.1 eV (NMe 4 + )), 68 making it the preferred electron capture site. However, ECD type fragments were lacking in the ECD spectrum of the tagged Q 06 peptide, which was dominated by b 5 and y fragments which could be merely the result of the residual vibrational excitation.…”

Formation of Peptide Radical Cations (M+·) in Electron Capture Dissociation of Peptides Adducted with Group IIB Metal Ions

“…An important experimental parameter is the nature of the charge carriers. Several researchers have studied the impact of replacing the mobile protons with fixed charges using functionalities such as quaternary amino groups [20][21][22][23]. Our group and other researchers have focused on the use of divalent and trivalent metal ions as charge carriers [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38].…”

The Early Life of a Peptide Cation-Radical. Ground and Excited-State Trajectories of Electron-Based Peptide Dissociations During the First 330 Femtoseconds

“…Peptide bond cleavages occur mainly, although not exclusively, between the amide nitrogens and adjacent C α atoms (N-C α cleavage for short) to form series of N-terminal (c) and C-terminal (z) fragment ions that are utilized for peptide and protein sequencing by mass spectrometry [1,4]. The nature of ExD has been investigated by experiment and theory with aims that ranged from statistical evaluation of the observed dissociations over large ad hoc sets of peptide ions [5,6] to analysis of fragmentations of selected model peptide ions to formulate plausible mechanisms [7][8][9][10][11][12][13][14][15][16][17][18].…”