Hypervalent ammonium radicals. Competitive N–C and N–H bond dissociations in methyl ammonium and ethyl ammonium

Self Cite

Electron capture dissociation (ECD) was studied with doubly charged dipeptide ions that were tagged with fixed-charge tris-(2,4,6-trimethoxyphenyl)phosphonium-methylenecarboxamido (TMPP-ac) groups. Dipeptides GK, KG, AK, KA, and GR were each selectively tagged with one TMPP-ac group at the N-terminal amino group while the other charge was introduced by protonation at the lysine or arginine side-chain groups to give (TMPP-ac-peptide ϩ H) 2ϩ ions by electrospray ionization. Doubly tagged peptide derivatives were also prepared from GK, KG, AK, and KA in which the fixed-charge TMPP-ac groups were attached to the N-terminal and lysine side-chain amino groups to give (TMPP-ac-peptide-ac-TMPP) 2ϩ dications by electrospray. ECD of (TMPP-ac-peptide ϩ H) 2ϩ resulted in 72% to 84% conversion to singly charged dissociation products while no intact charge-reduced (TMPP-ac-dipeptide ϩ H) ϩ• ions were detected. The dissociations involved loss of H, formation of (TMPP ϩ H) ϩ , and N-C ␣ bond cleavages giving TMPP-CH 2 CONH 2 ϩ (c 0 ) and c 1 fragments. In contrast, ECD of (TMPP-ac-peptide-ac-TMPP) 2 g g ϩ resulted in 31% to 40% conversion to dissociation products due to loss of neutral TMPP molecules and 2,4,6-trimethoxyphenyl radicals. No peptide backbone cleavages were observed for the doubly tagged peptide ions. Ab initio and density functional theory calculations for (Ph 3 P-ac-GK ϩ H) 2ϩ and (H 3 P-ac-GK ϩ H) 2ϩ analogs indicated that the doubly charged ions contained the lysine side-chain NH 3 ϩ group internally solvated by the COOH group. The distance between the charge-carrying phosphonium and ammonium atoms was calculated to be 13.1-13.2 Å in the most stable dication conformers. The intrinsic g p g y gp p recombination energies of the TMPP ϩ -ac and (GK ϩ H) ϩ moieties, 2.7 and 3.15 eV, respectively, indicated that upon electron capture the ground electronic states of the (TMPPac-peptide ϩ H) ϩ• ions retained the charge in the TMPP group. Ground electronic state (TMPP-ac-GK ϩ H) ϩ• ions were calculated to spontaneously isomerize by lysine H-atom transfer to the COOH group to form dihydroxycarbinyl radical intermediates with the retention of the charged TMPP group. These can trigger cleavages of the adjacent N-C ␣ bonds to give rise to the c 1 fragment ions. However, the calculated transition-state energies for GK and GGK models suggested that the ground-state potential energy surface was not favorable for the formation of the abundant c 0 fragment ions. This pointed to the involvement of excited electronic states according to the Utah-Washington mechanism of ECD. (J Am Soc Mass Spectrom 2007, 18, 2146 -2161) © 2007 American Society for Mass Spectrometry E lectron-based methods of gas-phase ion chemistry rely on partial or complete neutralization of gas-phase ions to produce transient species with open electronic shells that undergo various dissociations [1]. Amongst the several electron-based methods in current use, electron capture dissociation (ECD) [2] has received much attention lately because of its potential for ...

Section: Discussionmentioning

confidence: 99%

Section: Calculationsmentioning

confidence: 99%

Electron capture in charge-tagged peptides. Evidence for the role of excited electronic states

Chamot‐Rooke

Malosse

Frison

et al. 2007

Self Cite

“…The third charge reduction channel, gas-phase adduction of a reagent anion, is typically sufficiently rare so that it can be disregarded. As before [7], we note that electron transfer followed by loss of a hydrogen atom by the protein, which has been proposed to occur to some extent in hypervalent ammonium radicals under ECD conditions [8][9][10][11], is not explicitly included in our approach, but contributes to the intensity of the PTR channel.…”

Section: Introductionmentioning

confidence: 92%

Conformational Space and Stability of ETD Charge Reduction Products of Ubiquitin

Lermyte

Łącki

Valkenborg

et al. 2016

Due to its versatility, electron transfer dissociation (ETD) has become one of the most commonly utilized fragmentation techniques in both native and non-native top-down mass spectrometry. However, several competing reactions primarily different forms of charge reduction occur under ETD conditions, as evidenced by the distorted isotope patterns usually observed. In this work, we analyze these isotope patterns to compare the stability of ETnoD products, specifically noncovalent c/z fragment complexes, across a range of ubiquitin conformational states. Using ion mobility, we find that more extended states are more prone to fragment release. We obtain evidence that for a given charge state, populations of ubiquitin ions formed either directly by electrospray ionization or through collapse of more extended states upon charge reduction, span a similar range of collision cross-sections. Products of gas-phase collapse are however less stabilized towards unfolding than the native conformation, indicating that the ions retain a memory of previous conformational states. Furthermore, this collapse of charge--collisional activation, with possible implications for the kinetics of gas-phase compaction.

“…and (2) How are they accessed upon electron capture? Loss of H from neutral primary ammonium groups in ammonium radicals is facile in the ground electronic states, where it is thermoneutral to mildly exothermic and has low TS energies (15-24 kJ mol -1 ) [67][68][69]. This is due to the nature of the ground-state electronic structure which places the unpaired electron in a symmetry adjusted Rydberg 3s ammonium orbital and weakens the N-H bonds [67,68].…”

Section: Electron Attachment Energeticsmentioning

confidence: 99%

“…To further assess the dipole effects on outer molecular wave functions, we performed a simple modeling of the well-studied methylammonium radical [49,67,69], in which we inserted a 10-D dipole consisting of ±0.208 atomic point charges that were placed along the C-N axis 10Å apart and symmetrically positioned with respect of the C and N atoms. According to the wave function analysis ( Figure S5), dipole moments of this magnitude (10 D) were able deform the 3s, 4s, 3p and 4p symmetry-adjusted molecular Rydberg orbitals by squeezing, extending, or tilting them along the C-N axis depending on the dipole orientation.…”

Section: Electronic States and Dipolar Field Effectsmentioning

confidence: 99%

Dipole-Guided Electron Capture Causes Abnormal Dissociations of Phosphorylated Pentapeptides

Moss

Chung

Wyer

et al. 2011

Self Cite

Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible. In contrast, capture of lowenergy electrons by doubly charged ions in a Penning ion trap induced loss of a hydrogen atom followed by elimination of phosphoric acid as the dominant dissociation channel. Backbone dissociations of charge-reduced ions also occurred but were accompanied by extensive fragmentation of the primary products. z-Ions that were terminated with a deaminated phosphoserine radical competitively eliminated phosphoric acid and H 2 PO 4 radicals. A mechanism is proposed for this novel dissociation on the basis of a computational analysis of reaction pathways and transition states. Electronic structure theory calculations in combination with extensive molecular dynamics mapping of the potential energy surface provided structures for the precursor phosphopeptide dications. Electron attachment produces a multitude of low lying electronic states in charge-reduced ions that determine their reactivity in backbone dissociations and H-atom loss. The predominant loss of H atoms in ECD is explained by a distortion of the Rydberg orbital space by the strong dipolar field of the peptide dication framework. The dipolar field steers the incoming electron to preferentially attach to the positively charged arginine side chain to form guanidinium radicals and trigger their dissociations.