Water-Catalyzed Interconversion of Conventional and Distonic Radical Cations:  Methanol and Methyleneoxonium Radical Cations

Gauld, James W.; Audier, H. E.; Fossey, Jacques; Radom, Leo

doi:10.1021/ja960924f

Cited by 89 publications

(72 citation statements)

References 16 publications

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“…However, with 6 and more H 2 O molecules, the Nprotonated tautomer (i.e., the more stable tautomer in protic liquid) was more stable and abundant. This interconversion via the proton shuttling efforts of polar molecules (e.g., H 2 O, CH 3 OH) has also been suggested for many other small ionic isomer systems (e.g., distonic and conventional radical cations) [7][8][9][10][11][12]36], although none were separated prior to analysis in the gas-phase as DMS has provided here.…”

Section: Resultssupporting

confidence: 60%

“…In rare cases, orthogonal separation of tautomers prior to MS analysis could be afforded by gas chromatography [4], but the vast majority of cases involve MS analysis of convolved mixtures of tautomers. As such, the probing of tautomeric systems with MS has consisted of observing the relative ratios of tautomers and their fragment ions by adjusting ionization conditions [5], adjusting the energy of a tandem mass spectrometry (MS/MS) event [6], using diagnostic ion/molecule reactions [7][8][9][10][11][12], probing with ion spectroscopy [13,14], or through the assistance of computational modeling to confirm the presence and behavior of a given tautomer [15].…”

Section: Introductionmentioning

confidence: 99%

“…Depending upon that environment, the mechanism of gas-phase HDX [35,36] should also be considered. In addition, while HDX generally probes static noninterconvertible structures [37][38][39], it can also serve to catalyze interconversion (tautomerization) [7][8][9][10][11][12], thereby complicating the characterization of subtle structural features by MS. However, in the gas-phase HDX studies cited here, the only mechanism for tautomer identification was by observation of changes in ion reactivity within convolved populations; no mechanism for separation of these tautomers before or after identification was available.…”

Section: Introductionmentioning

confidence: 99%

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Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry

Campbell

Yang

Melo

et al. 2016

J. Am. Soc. Mass Spectrom.

125

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Abstract. In this study, we report on the use of differential mobility spectrometry (DMS) as a tool for studying tautomeric species, allowing a more in-depth interrogation of these elusive isomers using ion/molecule reactions and tandem mass spectrometry. As an example, we revisit a case study in which gas-phase hydrogendeuterium exchange (HDX)-a probe of ion structure in mass spectrometry-actually altered analyte ion structure by tautomerization. For the N-and O-protonated tautomers of 4-aminobenzoic acid, when separated using DMS and subjected to subsequent HDX with trace levels of D 2 O, the anticipated difference between the exchange rates of the two tautomers is observed. However, when using higher levels of D 2 O or a more basic reagent, equivalent and almost complete exchange of all labile protons is observed. This second observation is a result of the interconversion of the N-protonated tautomer to the Oprotonated form during HDX. We can monitor this transformation experimentally, with support from detailed molecular dynamics and electronic structure calculations. In fact, calculations suggest the onset of bulk solution phase properties for 4-aminobenzoic acid upon solvation with eight CH 3 OH molecules. These findings also underscore the need for choosing HDX reagents and conditions judiciously when separating interconvertible isomers using DMS.

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Section: Resultssupporting

confidence: 60%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry

Campbell

Yang

Melo

et al. 2016

J. Am. Soc. Mass Spectrom.

125

View full text Add to dashboard Cite

show abstract

“…An alternative explanation is that the radical may rearrange via ion-molecule catalytic processes involving background ESI solvents present in high concentrations in the skimmer-cone ("in source") region. A possible mechanism that might operate involves proton transport catalysis [57] observed for a large number of systems including enol/ keto radical cation tautomers [58] and conventional radical cations/distonic ions [59].…”

Section: Resultsmentioning

confidence: 99%

Structure and Reactivity of theN-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

Osburn

Berden

O’Hair

et al. 2011

J. Am. Soc. Mass Spectrom.

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The structure and reactivity of the N-acetyl-cysteine radical cation and anion were studied using ion-molecule reactions, infrared multi-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The radical cation was generated by first nitrosylating the thiol of N-acetyl-cysteine followed by the homolytic cleavage of the S-NO bond in the gas phase. IRMPD spectroscopy coupled with DFT calculations revealed that for the radical cation the radical migrates from its initial position on the sulfur atom to the α-carbon position, which is 2.5 kJ mol -1 lower in energy. The radical migration was confirmed by time-resolved ion-molecule reactions. These results are in contrast with our previous study on cysteine methyl ester radical cation (Osburn et al., Chem. Eur. J. 2011, 17, 873-879) and the study by Sinha et al. for cysteine radical cation (Phys. Chem. Chem. Phys. 2010, 12, 9794-9800) where the radical was found to stay on the sulfur atom as formed. A similar approach allowed us to form a hydrogen-deficient radical anion of N-acetyl-cysteine, (M -2H)•-. IRMPD studies and ion-molecule reactions performed on the radical anion showed that the radical remains on the sulfur, which is the initial and more stable (by 63.6 kJ mol -1 ) position, and does not rearrange.

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“…Initial configurations were created using a locally modified version of the stochastic search method. [62][63][64] Different mechanisms for water-catalyzed reactions (denoted as A, B, or C in ESI †) have been considered, 65 but only the most favorable are presented in the text (for selected examples see Fig. 1).…”

Section: Organic and Biomolecular Chemistry Papermentioning

confidence: 99%

The chemical fate of paroxetine metabolites. Dehydration of radicals derived from 4-(4-fluorophenyl)-3-(hydroxymethyl)piperidine

et al. 2013

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Quantum chemical calculations have been used to model reactions which are important for understanding the chemical fate of paroxetine-derived radicals in the environment. In order to explain the experimental observation that the loss of water occurs along the ( photo)degradation pathway, four different mechanisms of radical-induced dehydrations have been considered. The elimination of water from the Ncentered radical cation, which results in the formation of an imine intermediate, has been calculated as the most feasible process. The predicted energy barrier (ΔG # 298 = 98.5 kJ mol −1 ) is within the barrier limits set by experimental measurements. All reaction intermediates and transition state structures have been calculated using the G3(MP2)-RAD composite procedure, and solvent effects have been determined using a mixed (cluster/continuum) solvation model. Several new products, which comply with the available experimental data, have been proposed. These structures could be relevant for the chemical fate of antidepressant paroxetine, but also for biologically and environmentally related substrates.

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Water-Catalyzed Interconversion of Conventional and Distonic Radical Cations: Methanol and Methyleneoxonium Radical Cations

Cited by 89 publications

References 16 publications

Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry

Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry

Structure and Reactivity of theN-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

The chemical fate of paroxetine metabolites. Dehydration of radicals derived from 4-(4-fluorophenyl)-3-(hydroxymethyl)piperidine

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