Gas-phase Protonation of Pyridine. A Variable-time Neutralization-Reionization andAb InitioStudy of Pyridinium Radicals

Electron capture dissociation was studied with tetradecapeptides and pentadecapeptides that were capped at N-termini with a 2-(4=-carboxypyrid-2=-yl)-4-carboxamide group (pepy), e.g., pepy-AEQLLQEEQLLQEL-NH 2 , pepy-AQEFGEQGQKALKQL-NH 2 , and pepy-AQEGSE-QAQKFFKQL-NH 2 . Doubly and triply protonated peptide cations underwent efficient electron capture in the ion-cyclotron resonance cell to yield charge-reduced species. However, the electron capture was not accompanied by backbone dissociations. When the peptide ions were preheated by absorption of infrared photons close to the dissociation threshold, subsequent electron capture triggered ion dissociations near the remote C-terminus forming mainly (b 11-14 ϩ 1) ϩ· fragment ions that were analogous to those produced by infrared multiphoton dissociation alone. Ab initio calculations indicated that the N-1 and N-1= positions in the pepy moiety had topical gas-phase basicities (GB ϭ 923 kJ mol Ϫ1 ) that were greater than those of backbone amide groups. Hence, pepy was a likely protonation site in the doubly and triply charged ions. Electron capture in the protonated pepy moiety produced the ground electronic state of the charge-reduced cation-radical with a topical recombination energy, RE ϭ 5.43-5.46 eV, which was greater than that of protonated peptide residues. The hydrogen atom in the charge-reduced pepy moiety was bound by Ͼ160 kJ mol Ϫ1 , which exceeded the hydrogen atom affinity of the backbone amide groups (21-41 kJ mol Ϫ1 ). Thus, the pepy moiety functioned as a stable electron and hydrogen atom trap that did not trigger radical-type dissociations in the peptide backbone that are typical of ECD. Instead, the internal energy gained by electron capture was redistributed over the peptide moiety, and when combined with additional IR excitation, induced proton-driven ion dissociations which occurred at sites that were remote from the site of electron capture. This example of a spin-remote fragmentation provided the first clear-cut experimental example of an ergodic dissociation upon ECD. . Mechanistic details of peptide cation dissociations and the product ion structures have been elucidated by quantum chemical calculations [2][3][4] and appear to provide a coherent model of the peptide gas-phase ion chemistry [5]. Inherent to the mechanistic model of peptide ion fragmentations is the concept of efficient internal vibrational energy redistribution (IVR) in the dissociating peptide ion [6], so that the dissociation can be treated as an ergodic process by appropriate kinetic schemes, such as the RiceRamsperger-Kassel-Marcus [7] or transition-state theories [8]. The concept of IVR has received experimental support by time-resolved photodissociation studies [9], and is consistent with the mode of excitation upon collisional activation at low kinetic energies (Ͻ100 eV) or upon absorption of infrared photons [10].The current understanding is less definite concerning the dissociations of peptide cation-radicals produced by electron capture or transfer [11]. In electron ...

Section: Sociated (Bpy ϩ H)mentioning

confidence: 90%

mentioning

confidence: 99%

Electron capture in spin-trap capped peptides. An experimental example of ergodic dissociation in peptide cation-radicals

Jones

Sasaki

Goodlett

et al. 2007

“…* marks the electronic noise peak, 2 marks the first harmonic peak of the precursor ion, peaks marked with "-amino acid residue" resulting from cleavage at the C ␣ OC ␤ bond, and partial side-chain losses were represented by the molecular formulas of the departing group(s). tion energy of the pyridinium ion (4.71 eV) [56] is much greater than that of the N-terminus protonated peptide (3.71 eV) [57]. After the initial electron capture in a high-lying Rydberg state, the pyridinium is far more likely to be the eventual neutralization site than the protonated N-terminal amino group.…”

Section: Backbone Cleavagesmentioning

confidence: 99%

“…It was previously shown in a neutralization--reionization study of gas-phase pyridinium ion that the pyridinium radical preferentially lost the N-bound H or D atom to re-form the aromatic ring [56], which competed favorably over the hydrogen rearrangement within the pyridinium ring. For the doubly TMP-tagged peptides studied here, this implied that loss of the whole tag should be a favored process upon electron capture (Scheme 4).…”

Section: Tag and Alkyl Group Lossesmentioning

confidence: 99%

The effect of fixed charge modifications on electron capture dissociation

Cournoyer

Lin

et al. 2008

Electron capture dissociation (ECD) studies of two modified amyloid {3 peptides (20-29 and 25-35) were performed to investigate the role of H" radicals in the ECD of peptide ions and the free-radical cascade (FRC) mechanism. 2,4,6-Trimethylpyridinium (TMP) was used as the fixed charge tag, which is postulated to both trap the originally formed radical upon electron capture and inhibit the H" generation. It was found that both the number and locations of the fixed charge groups influenced the backbone and side-chain cleavages of these peptides in ECD. In general, the frequency and extent of backbone cleavages decreased and those of side-chain cleavages increased with the addition of fixed charge tags. A singly labeled peptide with the tag group farther away from the protonated site experienced a smaller abundance decrease in backbone cleavage fragments than the one with the tag group closer to the protonated site. Despite the nonprotonated nature of all charge carriers in doubly labeled peptide ions, several c and z" ions were still observed in their ECD spectra. Thus, although H" transfer may be important for the N--C", bond cleavage, there also exist other pathways, which would require a radical migration via H" abstraction through space or via an amide superbase mechanism. Finally, internal fragment ions were observed in the ECD of these linear peptides, indicating that the important role of the FRC in backbone cleavages is not limited to the ECD of cyclic peptides. aAm

“…), the rarefied gas phase offers an inherently inert medium in which highly reactive species can be studied in an isolated state. We have previously reported experimental studies that used neutralization-reionization (NR) mass spectrometry [16] to generate heterocyclic [17][18][19], nucleobase [20 -22], and phosphate [23] radicals and elucidate their unimolecular dissociations, as reviewed [24]. In the preceding paper in this issue [25], we report on the generation of 2-hydroxyoxolan-2-yl radical which is a simplified model for deoxyribose radicals following hydrogen abstraction at C-1.…”

mentioning

confidence: 99%

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

Shetty

Sadı́lek

Chen

et al. 2004

Self Cite

The title radical (1) is generated in the gas-phase by collisional neutralization of carbonylprotonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (ϳ56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one (2). The competing ring cleavages by O™C-2 and C-4™C-5 bond dissociations combined account for ϳ42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C 2 H 3 O and C 2 H 4 O cannot be explained as occurring competitively on the doublet ground (X) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-(1H)-oxolanium cation (3 ؉ ) which upon collisional neutralization gives hypervalent 3-oxo-(1H)-oxolanium radical (3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at ϳ590 and