Where Does the Electron Go? Electron Distribution and Reactivity of Peptide Cation Radicals Formed by Electron Transfer in the Gas phase

Abstract

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.

Electron transfer dissociation: Effects of cation charge state on product partitioning in ion/ion electron transfer to multiply protonated polypeptides

“…[38] For example, ET,noD has been noted to be maximized when protonation takes place at a histidine residue, which has been attributed to the exothermic isomerization of the His (3H)-imidazoline radical to the more stable (2H)-imidazoline isomer under sterically favorable conditions. [61,62] For a given charge state, the histidine containing model peptides all show higher contributions from ET,noD than the peptide ions without histidine. Lysine was noted as giving rise to the least ET,noD and this is consistent with the near absence of the charge reduced electron transfer product for the triply protonated RGAGGKGAGGRL peptide.…”

Electron Transfer Dissociation of Doubly Charged Ions with Different Cationizing Agents

“…ESI mass spectrum of the tetradecapeptide bombesin. 25 and is probably the one neutralized and transferred as a hydrogen atom to an uncharged amide oxygen (Cornell mechanism), 1 or the one migrating as a proton to a negatively charged (reduced) amide oxygen (Utah-Washington mechanism), 26,27 during the ETD process to form the observed sequence ions. In view of these facts, Arg is the most likely charge carrier in the final fragments.…”

Renaissance of Cation-Radicals in Mass Spectrometry

“…The common feature of the valence excited states is that their energies form a very dense manifold containing many (10-15) electronic states within 1 eV of the ground electronic state. 45) The excited states differ in the electron and spin density distribution, which is mainly delocalized over π-electron "electrophores" such as peptide amide groups and side-chain imidazole and guanidine groups. Calculations on small peptide radicals indicated that these electrophores have very similar electron affinities (recombination energies, RE), typically ranging between 3.1-3.6 eV.…”

“…Figure 5 shows two generic pathways that have been dubbed "egg" and "chicken" to depict the dilemma of deciding which of the following steps, proton transfer or bond dissociation, occurs first in the charge-reduced ion. 45) The "egg" pathway posits that the aminoketolate anion-radical first undergoes N-C α bond cleavage to form a complex of the incipient c and z fragments. The intermediate c fragment is formed as an imino enolate anion and must receive a proton to presumably form the more stable amide tautomer of the c type fragment with the experimentally observed elemental composition.…”