Gas-phase chemistry of benzyl cations in dissociation of N-benzylammonium and N-benzyliminium ions studied by mass spectrometry

TransExION: a transformer based explainable similarity metric for comparing IONS in tandem mass spectrometry

Small molecule identification is a crucial task in analytical chemistry and life sciences. One of the most commonly used technologies to elucidate small molecule structures is mass spectrometry. Spectral library search of product ion spectra (MS/MS) is a popular strategy to identify or find structural analogues. This approach relies on the assumption that spectral similarity and structural similarity are correlated. However, popular spectral similarity measures, usually calculated based on identical fragment matches between the MS/MS spectra, do not always accurately reflect the structural similarity. In this study, we propose TransExION, a Transformer based Explainable similarity metric for IONS. TransExION detects related fragments between MS/MS spectra through their mass difference and uses these to estimate spectral similarity. These related fragments can be nearly identical, but can also share a substructure. TransExION also provides a post-hoc explanation of its estimation, which can be used to support scientists in evaluating the spectral library search results and thus in structure elucidation of unknown molecules. Our model has a Transformer based architecture and it is trained on the data derived from GNPS MS/MS libraries. The experimental results show that it improves existing spectral similarity measures in searching and interpreting structural analogues as well as in molecular networking. SCIENTIFIC CONTRIBUTION: We propose a transformer-based spectral similarity metrics that improves the comparison of small molecule tandem mass spectra. We provide a post hoc explanation that can serve as a good starting point for unknown spectra annotation based on database spectra.

Low Energy Collision-Induced Dissociation of Azepine Pictet-Spengler Adducts of Nω-Methylserotonin

Cimitrypazepines are a class of natural products produced by Pictet-Spengler condensation of N_ω-methylserotonin and aldehydes in a manner that produces a seven-membered azepine ring. In this study, the fragmentation behavior of this class of molecules under low-energy CID was investigated in detail. Proposed mechanisms of fragmentation were supported by deuterium labeling and DFT calculations. Loss of methylamine and methylenimine were dominant fragmentation pathways of analogs containing an aliphatic side chain. Loss of methylenimine was found to proceed via unusual methyl cation transfer. Fragmentation of analogs containing an aromatic side chain was strongly influenced by the nature of the substituents and proceeded via a novel retro-Pictet-Spengler pathway and involvement of ion-neutral complexes. In some cases, a gas-phase interconversion between the azepine and β-carboline ring was observed during fragmentation. Detailed analysis of fragmentation behavior provided in this study will serve as a valuable guide for the discovery of new analogs from natural sources.

Differentiation of isomeric cresols by silylation in combination with gas chromatography/mass spectrometry analysis

RATIONALE

m-Cresol is listed as a priority controlled contaminant in many countries, but it is very difficult to accurately determine isomeric cresols due to their incomplete chromatographic separation on commercially available chromatographic columns and their nearly identical mass spectra.

METHODS

Silylation of isomeric cresols was carried out by treatment with N-methyl-N-(trimethylsilyl)trifluoroacetamide. The formed trimethyl(tolyloxy)silanes were analyzed by gas chromatography/mass spectrometry (GC/MS). Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level.

RESULTS

The derivatives of three isomeric cresols and six isomeric xylenols have been completely separated on an HP-5MS capillary column within a GC run of only 10 minutes. In addition, the derivative o-cresol can be very easily differentiated from its isomers due to its characteristic base peak ion at m/z 91 in electron ionization (EI)-MS. DFT calculation results indicated that the formation of the abundant fragment ion at m/z 91 is attributed to a facile dissociation pathway involving the shift of a neighboring phenylmethyl hydrogen atom in EI-MS of trimethyl(o-tolyloxy)silane.

CONCLUSIONS

Silylation provides a promising solution for simultaneous determination of isomeric cresols and isomeric xylenols.

Characterization of MS/MS Product Ions for the Differentiation of Structurally Isomeric Pesticides by High-Resolution Mass Spectrometry

Structural isomeric pesticides are used in agriculture and may be challenging to differentiate for accurate identification in pesticide monitoring programs. Due to structural similarity, isomeric pesticides are difficult to separate chromatographically, and thus, their accurate identification may rely solely on mass spectrometric analysis (MS). In this study, we challenged the ability of high-resolution quadrupole-orbitrap (Q-Orbitrap) mass spectrometry to produce and evaluate the tandem mass spectrometry (MS/MS) product ions for the selected five pairs of isomeric pesticides from different classes: Pebulate and vernolate, methiocarb and ethiofencarb, uniconazole and cyproconazole, sebuthylazine and terbuthylazine, and orbencarb and thiobencarb. The use of Q-Orbitrap instrument with a mass error <3 ppm allowed proposed elucidation of the product ion structures with consideration of the ion formulae, data interpretation, and literature searches. Product ions unique to pebulate, vernolate, methiocarb, ethiofencarb, and uniconazole were observed. Elucidation of the observed MS/MS product ion structures was conducted, and the fragmentation pathways were proposed. This information is valuable to increase selectivity in MS/MS analysis and differentiate isomeric pesticides, and thereby reduce the rates of false positives in pesticide monitoring programs.

Recent (2000-2015) developments in the analysis of minor unknown natural products based on characteristic fragment information using LC-MS

Liquid chromatography-Mass Spectrometry (LC-MS) has been widely used in natural product analysis. Global detection and identification of nontargeted components are desirable in natural product research, for example, in quality control of Chinese herbal medicine. Nontargeted components analysis continues to expand to exciting life science application domains such as metabonomics. With this background, the present review summarizes recent developments in the analysis of minor unknown natural products using LC-MS and mainly focuses on the determination of the molecular formulae, selection of precursor ions, and characteristic fragmentation patterns of the known compounds. This review consists of three parts. Firstly, the methods used to determine unique molecular formula of unknown compounds such as accurate mass measurements, MSⁿ spectra, or relative isotopic abundance information, are introduced. Secondly, the methods improving signal-to-noise ratio of MS/MS spectra by manual-MS/MS or workflow targeting-only signals were elucidated; pure precursor ions can be selected by changing the precursor ion isolated window. Lastly, characteristic fragmentation patterns such as Retro-Diels-Alder (RDA), McLafferty rearrangements, "internal residue loss," and so on, occurring in the molecular ions of natural products are summarized. Classical application of characteristic fragmentation patterns in identifying unknown compounds in extracts and relevant fragmentation mechanisms are presented (RDA reactions occurring readily in the molecular ions of flavanones or isoflavanones, McLafferty-type fragmentation reactions of some natural products such as epipolythiodioxopiperazines; fragmentation by "internal residue loss" possibly involving ion-neutral complex intermediates). © 2016 Wiley Periodicals, Inc. Mass Spec Rev 37:202-216, 2018.

Quaternary ammonium isobaric tag for a relative and absolute quantification of peptides

Isobaric labeling quantification of peptides has become a method of choice for mass spectrometry-based proteomics studies. However, despite of wide variety of commercially available isobaric tags, none of the currently available methods offers significant improvement of sensitivity of detection during MS experiment. Recently, many strategies were applied to increase the ionization efficiency of peptides involving chemical modifications introducing quaternary ammonium fixed charge. Here, we present a novel quaternary ammonium-based isobaric tag for relative and absolute quantification of peptides (QAS-iTRAQ 2-plex). Upon collisional activation, the new stable benzylic-type cationic reporter ion is liberated from the tag. Deuterium atoms were used to offset the differential masses of a reporter group. We tested the applicability of QAS-iTRAQ 2-plex reagent on a series of model peptides as well as bovine serum albumin tryptic digest. Obtained results suggest usefulness of this isobaric ionization tag for relative and absolute quantification of peptides.

Collision-induced dissociation of phenethylamides: role of ion-neutral complexes

RATIONALE

Phenethylamides are a large group of naturally occurring molecules found both in the plant and animal kingdoms. In addition, they are used as intermediates for the synthesis of pharmaceutically important dihydro- and tetrahydroisoquinolines. To enable efficient characterization of this class of molecules, a detailed mass spectrometric fragmentation study of a broad series of analogs was carried out.

METHODS

The test compounds were synthesized using standard methods for amide bond formation. Low-energy high-resolution tandem mass spectra were acquired on a hybrid quadrupole/time-of-flight mass spectrometer using positive ion electrospray ionization.

RESULTS

A total of 26 analogs were investigated in the study. Fragmentation of phenethylamides was found to proceed via intermediate ion-neutral complexes. The complexes can break down via multiple pathways including dissociation, proton transfer, Friedel-Crafts acylation, and single electron transfer. The relative contribution of each of these pathways strongly depends on the structure of the coupling amine and acid.

CONCLUSIONS

A general scheme for the fragmentation of phenethylamides was developed. This study further extends the knowledge base of the ion-neutral complex by discovering Friedel-Crafts acylation as a novel reaction. The strong influence of minor structural modifications on the fragmentation patterns highlights the importance of testing many analogs in order to fully predict a fragmentation pattern of a particular class of molecules.

Competitive benzyl cation transfer and proton transfer: collision-induced mass spectrometric fragmentation of protonated N,N-dibenzylaniline

Collision-induced dissociation of protonated N,N-dibenzylaniline was investigated by electrospray tandem mass spectrometry. Various fragmentation pathways were dominated by benzyl cation and proton transfer. Benzyl cation transfers from the initial site (nitrogen) to benzylic phenyl or aniline phenyl ring. The benzyl cations transfer to the two different sites, and both result in the benzene loss combined with 1,3-H shift. In addition, after the benzyl cation transfers to the benzylic phenyl ring, 1,2-H shift and 1,4-H shift proceed competitively to trigger the diphenylmethane loss and aniline loss, respectively. Deuterium labeling experiments, substituent labeling experiments and density functional theory calculations were performed to support the proposed benzyl cation and proton transfer mechanism. Overall, this study enriches the knowledge of fragmentation mechanisms of protonated N-benzyl compounds. Copyright © 2017 John Wiley & Sons, Ltd.

How does a CC double bond cleave in the gas phase? Fragmentation of protonated ketotifen in mass spectrometry

In the literature, it is reported that the protonated ketotifen mainly undergoes CC double bond cleavage in electrospray ionization tandem mass spectrometry (ESI-MS/MS); however, there is no explanation on the mechanism of this fragmentation reaction. Therefore, we carried out a combined experimental and theoretical study on this interesting fragmentation reaction. The fragmentation of protonated ketotifen (m/z 310) always generated a dominant fragment ion at m/z 96 in different electrospray ionization mass spectrometers (ion trap, triple quadrupole and linear trap quadrupole (LTQ)-orbitrap). The mechanism of the generation of this product ion (m/z 96) through the CC double bond cleavage was proposed to be a sequential hydrogen migration process (including proton transfer, continuous two-step 1,2-hydride transfer and ion-neutral complex-mediated hydride transfer). This mechanism was supported by density functional theory (DFT) calculations and a deuterium labeling experiment. DFT calculations also showed that the formation of the product ion m/z 96 was most favorable in terms of energy. This study provides a reasonable explanation for the fragmentation of protonated ketotifen in ESI-MS/MS, and the fragmentation mechanism is suitable to explain other CC double bond cleavage reactions in mass spectrometry. Copyright © 2016 John Wiley & Sons, Ltd.

Ortho-hydroxyl effect and proton transfer via ion-neutral complex: the fragmentation study of protonated imine resveratrol analogues in mass spectrometry

The fragmentation pathways of protonated imine resveratrol analogues in the gas-phase were investigated by electrospray ionization-tandem mass spectrometry. Benzyl cations were formed in the imine resveratrol analogues that had an ortho-hydroxyl group on the benzene ring A. The specific elimination of the quinomethane neutral, CH2  = C6 H4  = O, from the two isomeric ions [M1 + H](+) and [M3 + H](+) via the corresponding ion-neutral complexes was observed. The fragmentation pathway for the related meta-isomer, ion [M2 + H](+) and the other congeners was not observed. Accurate mass measurements and additional experiments carried out with a chlorinated analogue and the trideuterated isotopolog of M1 supported the overall interpretation of the fragmentation phenomena observed. It is very helpful for understanding the intriguing roles of ortho-hydroxyl effect and ion-neutral complexes in fragmentation reactions and enriching the knowledge of the gas-phase chemistry of the benzyl cation. Copyright © 2016 John Wiley & Sons, Ltd.

Decarboxylative Coupling Reaction in ESI(-)-MS/MS of 4-Nitrobenzyl 4-Hydroxybenzoates: Triplet Ion-Neutral Complex-Mediated 4-Nitrobenzyl Transfer

In negative electrospray ionization mass spectrometry of 4-nitrobenzyl 4-hydroxybenzoates, a decarboxylation reaction, which was significantly promoted by the presence of a nitro group on the benzyl group, competed with radical elimination reactions. Density functional theory calculations indicated that decarboxylation of deprotonated 4-nitrobenzyl vanillate occurred via a radical route involving homolytic cleavage of the Cbenzyl-O bond to give a triplet ion-neutral complex, followed by decarboxylative coupling.

Dissociation of protonated N-(3-phenyl-2H-chromen-2-ylidene)-benzenesulfonamide in the gas phase: cyclization via sulfonyl cation transfer

RATIONALE

In the tandem mass spectrometry of protonated N-(3-phenyl-2H-chromen-2-ylidene)benzenesulfonamides, the precursor ions have been observed to undergo gas-phase dissociation via two competing channels: (a) the predominant channel involves migration of the sulfonyl cation to the phenyl C atom and the subsequent loss of benzenesulfinic acid along with cyclization reaction, and (b) the minor one involves dissociation of the precursor ion to give an ion/neutral complex of [sulfonyl cation/imine], followed by decomposition to afford sulfonyl cation or the INC-mediated electron transfer to give an imine radical cation.

METHODS

The proposed reaction channels have been supported by theoretical calculations and D-labeling experiments.

RESULTS

The gas-phase cyclization reaction originating from the N- to C-sulfonyl cation transfer has been first reported to the best of our knowledge.

CONCLUSIONS

For the substituted sulfonamides, the presence of electron-donating groups (R(2) -) at the C-ring effectively facilitates the reaction channel of cyclization reaction, whereas that of electron-withdrawing groups inhibits this pathway.

Intramolecular Halogen Transfer via Halonium Ion Intermediates in the Gas Phase

The fragmentation of halogen-substituted protonated amines and quaternary ammonium ions (R(1)R(2)R(3)N(+)CH2(CH2)nX, where X = F, Cl, Br, I, n = 1, 2, 3, 4) was studied by electrospray ionization tandem mass spectrometry. A characteristic fragment ion (R(1)R(2)R(3)N(+)X) resulting from halogen transfer was observed in collision-induced dissociation. A new mechanism for the intramolecular halogen transfer was proposed that involves a reactive intermediate, [amine/halonium ion]. A potential energy surface scan using DFT calculation for CH2-N bond cleavage process of protonated 2-bromo-N,N-dimethylethanamine supports the formation of this intermediate. The bromonium ion intermediate-involved halogen transfer mechanism is supported by an examination of the ion/molecule reaction between isolated ethylenebromonium ion and triethylamine, which generates the N-bromo-N,N,N-triethylammonium cation. For other halogens, Cl and I also can be involved in similar intramolecular halogen transfer, but F cannot be involved. With the elongation of the carbon chain between the halogen (bromine as a representative example) and amine, the migration ability of halogen decreases. When the carbon chain contains two or three CH2 units (n = 1, 2), formal bromine cation transfer can take place, and the transfer is easier when n = 1. When the carbon chain contains four or five CH2 units (n = 3, 4), formal bromine cation transfer does not occur, probably because the five- and six-membered cyclic bromonium ions are very stable and do not donate the bromine to the amine.

Flavonoid derivatives as selective ABCC1 modulators: Synthesis and functional characterization

A series of chromones, bearing substituted amino groups or N-substituted carboxamide moieties in position 2, was synthesized and characterized in cellular assays for modulation of the ABC transporters ABCC1 (MDCKII-MRP1 cells), ABCB1 (Kb-V1 cells) and ABCG2 (MCF-7/Topo cells). The most potent ABCC1 modulators identified among these flavonoid-type compounds were comparable to the reference compound reversan regarding potency, but superior in terms of selectivity concerning ABCB1 and ABCG2 (2-[4-(Benzo[c][1,2,5]oxadiazol-5-ylmethyl)piperazin-1-yl]-5,7-dimethoxy-4H-chromen-4-one (51): ABCC1, IC50 11.3 μM; inactive at ABCB1 and ABCG2). Compound 51 was as effective as reversan in reverting ABCC1-mediated resistance to cytostatics in MDCKII-MRP1 cells and proved to be stable in mouse plasma and cell culture medium. Modulators, such as compound 51, are of potential value as pharmacological tools for the investigation of the (patho)physiological role of ABCC1.

Benzyl anion transfer in the fragmentation of N-(phenylsulfonyl)-benzeneacetamides: a gas-phase intramolecular S(N)Ar reaction

In this study, we report a gas-phase benzyl anion transfer via intramolecular aromatic nucleophilic substitution (SNAr) during the course of tandem mass spectrometry of deprotonated N-(phenylsulfonyl)-benzeneacetamide. Upon collisional activation, the formation of the initial ion/neutral complex ([C6H5CH2(-)/C6H5SO2NCO]), which was generated by heterolytic cleavage of the CH2-CO bond, is proposed as the key step. Subsequently, the anionic counterpart, benzyl anion, is transferred to conduct the intra-complex SNAr reaction. After losing neutral HNCO, the intermediate gives rise to product ion B at m/z 231, whose structure is confirmed by comparing the multistage spectra with those of deprotonated 2-benzylbenzenesulfinic acid and (benzylsulfonyl)benzene. In addition, intra-complex proton transfer is also observed within the complex [C6H5CH2(-)/C6H5SO2NCO] to generate product ion C at m/z 182. The INC-mediated mechanism was corroborated by theoretical calculations, isotope experiments, breakdown curve, substituent experiments, etc. This work may provide further understanding of the physicochemical properties of the gaseous benzyl anion.

The in situ gas-phase formation of a C-glycoside ion obtained during electrospray ionization tandem mass spectrometry. A unique intramolecular mechanism involving an ion-molecule reaction

RATIONALE

This study examines the electrospray ionization mass spectrometry (ESI-MS), in-source collision-induced dissociation (CID) fragmentation and low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) of a synthetic pair of β- and α-anomers of the amphiphilic cholesteryl polyethoxy neoglycolipids containing the 2-azido-2-deoxy-D-galactosyl-D-GalN3 moiety. We describe the novel and unique in situ gas-phase formation of a C-glycoside ion formed during all these gas-phase processes and propose a reasonable mechanism for its formation.

METHODS

The synthetic amphiphilic glycolipids were composed of the 2-deoxy-2-azido-D-galactosyl moiety (GalN3, the hydrophilic part) covalently attached to a polyethoxy spacer which is covalently linked to the cholesteryl moiety (hydrophobic part). The 2-azido-2-deoxy-α- and β-D-galactosyl-containing glycolipids were studied by in-time and in-space ESI-MS and CID-MS/MS in positive ion mode, with quadrupole ion trap (QIT), quadrupole-quadrupole-time-of-flight (QqTOF), and Fourier transform ion cyclotron resonance (FTICR) instruments.

RESULTS

Conventional single-stage ESI-MS analysis showed the formation of the protonated molecule. During the single-stage ESI-MS analysis and the CID-MS/MS of the [M+H](+) and [M+NH4](+) adducts obtained from both glycolipid anomers, the presence of a series of specific product ions with different intensities was observed, consistent with the [C-glycoside+H-N2](+), [cholestadiene+H](+), 2-deoxy-2-D-azido-galactosyl [GalN3](+), [GalNH](+) and [sugar-Spacer+H](+) ions.

CONCLUSIONS

The gas-phase formation of the [C-glycoside+H-N2](+) ion isolated from the glycolipid anomers was observed during both the ESI-MS of the glycolipids and the CID-MS/MS analyses of the [M+H](+) ions and it was found to occur by an intramolecular rearrangement involving an ion-molecule complex. CID-QqTOF-MS/MS and CID-FTICR-MS(2) analysis allowed the differentiation of the two glycolipid anomers and showed noticeable variation in the intensities of the product ions.

Gas-Phase Fragmentation of Protonated N,2-Diphenyl-N'-(p-Toluenesulfonyl)Ethanimidamides: Tosyl Cation Transfer Versus Proton Transfer

The gas-phase dissociation chemistry of protonated N,2-diphenyl-N'-(p-toluenesulfonyl) ethanimidamides was investigated by electrospray ionization mass spectrometry in combination with density functional theory calculation. The protonated molecules underwent fragmentation via two main competing channels: (1) migration of the tosyl cation to the anilinic N atom and the subsequent loss of 2-phenylacetonitrile to afford protonated N-phenyl p-toluenesulfonamide (m/z 248); and (2) transfer of the ionizing proton to the anilinic N atom to give an ion/neutral complex of [tosyl cation / 2-phenylacetonitrile] (m/z 272) and the subsequent decomposition to yield tosyl cation (m/z 155). To the best of our knowledge, the gas-phase tosyl cation transfer has not been reported previously. For the para-substituted sulfonamides, the presence of electron-donating groups on the anilinic ring inhibits the reaction channel of the tosyl cation migration, whereas the presence of electron-withdrawing groups favors this pathway.

Gas-phase Smiles rearrangement reactions of deprotonated N-phenylbenzamides studied by electrospray ionization tandem mass spectrometry

RATIONALE

Electrospray ionization tandem mass spectrometry (ESI-MS(n)) is an invaluable tool for the study of gas-phase reactions. When N-phenylbenzamide is analyzed in negative ion mode, the nucleophilic deprotonated site of nitrogen or oxygen, together with the adjacent electrophilic phenyl carbon in the same molecule, provides a useful opportunity to study the intramolecular nucleophilic reaction in the gas phase.

METHODS

All MS(n) experiments of deprotonated N-phenylbenzamides were conducted on an ion trap mass spectrometer using ESI in negative ion mode. The accurate masses of fragments were measured on an ESI quadrupole time-of-flight mass spectrometer in negative ion mode. Theoretical calculations were conducted at the B3LYP/6-31++G(d,p) level of density functional theory using the Gaussian 03 program.

RESULTS

When the polarity of the substituent on the aniline ring was changed, gas-phase Smiles rearrangement reactions could be initiated by different atoms in the anionic center. Upon collisional activation, loss of CO from deprotonated N-phenylbenzamides could be observed, which can be interpreted as a nitrogen anion triggering the Smiles rearrangement reaction through a three-membered ring transition state. As the aniline ring was substituted by a strong electron-withdrawing group (e.g., NO(2), COCH(3), or CF(3)) at the para position, a characteristic phenolate anion was obtained, which was derived from the Smiles rearrangement reaction initiated by the oxygen anion through a four-membered ring transition state.

CONCLUSIONS

In the fragmentation of deprotonated N-phenylbenzamides, the gas-phase Smiles rearrangement reaction initiated by either the nitrogen or the oxygen atom can proceed. The findings in this study have not only enriched knowledge on the gas-phase Smiles rearrangement reactions, but also provided valuable information for understanding the rearrangements of deprotonated aromatic amides in gas phase.

Proton-bound complex mediating retro-Michael-type fragmentation of protonated 3-substituted oxindoles in the Orbitrap high-energy collisional dissociation cell

RATIONALE

Oxindole derivatives are valuable building blocks for indole chemistry. Systematically exploring the fragmentation behavior of the protonated 3-pyrazole-substituted oxindoles by kinetic methods combined with density functional theory (DFT) calculations is useful for further understanding their basic properties, and might provide some insights into their reactivity trends in synthesis and metabolism.

METHODS

All high-resolution high-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) experiments were carried out using electrospray ionization hybrid Quadrupole-Orbitrap mass spectrometry in positive ion mode. Theoretical calculations were carried out by the DFT method at the B3LYP level with the 6-311G (d, p) basis set in the Gaussian 03 package of programs.

RESULTS

In the fragmentation of protonated 3-pyrazole-substituted oxindoles, the characterized protonated 3-(3-methyl-5-oxo-1H-pyrazol-4(5H)-ylidene)indolin-2-one derivatives and the protonated 5-methylpyrazolone were observed, which were proposed from the cleavage of the C(β)-C(γ) bond in a retro-Michael reaction. With the kinetic plot, a linear correlation was established between the intensities of this two competitive product ions and the difference in proton affinities of the corresponding neutral molecules, which demonstrated that the retro-Michael reaction was mediated by a proton-bound complex.

CONCLUSIONS

Using the kinetic method combined with theoretical calculations, a proton-bound complex mediating retro-Michael reaction was proposed for the fragmentation of protonated 3-pyrazole-substituted oxindoles in the high-energy collisional dissociation tandem mass spectrometry for the first time, which provided potential evidence to further understand their intrinsic bioactivities.

Intramolecular electrophilic aromatic substitution in gas-phase fragmentation of protonated N-benzylbenzaldimines

In this study, the gas-phase fragmentations of protonated N-benzylbenzaldimines were investigated by electrospray ionization tandem mass spectrometry (ESI-MS(n)). Upon collisional activation, several characteristic fragment ions are produced and their fragmentation mechanisms are rationalized by electrophilic aromatic substitution accompanied by benzyl cation transfer. (1) For N-(p-methoxybenzylidene)-1-phenylmethanimine, concomitant with a loss of HCN, a product ion at m/z 121 was observed. It is proposed to be generated from electrophilic substitution at the ipso-position by transferring benzyl cation rather than cleavage of the C-N double bond. (2) For N-(m-methoxybenzylidene)-1-phenylmethanimine, a product ion at m/z 209 was obtained, corresponding to the elimination of NH(3) carrying two hydrogens from the two aromatic rings respectively. This process can be rationalized by two sequential electrophilic substitutions and cyclodeamination reaction based on the benzyl cation transfer. Deuterium-labeled experiments, density functional theory (DFT) calculation and substituent effect results also corroborate the proposed mechanism.

Fragmentation reactions of N-benzyltetrahydroquinolines in electrospray ionization mass spectrometry: the roles of ion/neutral complex intermediates

RATIONALE

Electrospray ionization mass spectrometry (ESI-MS) combined with the collision-induced dissociation (CID) technique has assumed increasing importance as an invaluable tool for the structural analysis of organic and biological molecules. However, general rules for elucidating the fragmentation behaviors of charged molecules in the gas phase are still lacking. Therefore, explorations on the mechanistic information are desirable at all times.

METHODS

CID experiments of protonated N-benzyltetrahydroquinolines were carried out on ESI ion trap mass spectrometer and accurate mass measurements were performed on a high-resolution ESI quadrupole time-of-flight (Q-TOF) mass spectrometer in positive ion mode.

RESULTS

An ion/neutral complex, [RC6H4CH2(+)/tetrahydroquinoline], resulting from cleavage of the C-N bond induced by the positive charge brought in by protonation, was proposed to be the intermediate to elucidate the fragmentation reactions. For all the compounds investigated, benzyl cation transfer, electron transfer and hydride transfer reactions mediated by the complex were observed. Moreover, for the compound substituted by a methyl group at the para-position of the benzylic phenyl ring, proton transfer reaction via the complex also occurs.

CONCLUSIONS

This study is a case for better understanding the intriguing roles of ion/neutral complexes in gas-phase fragmentation reactions and enriching the knowledge about the gas-phase chemistry of the benzyl cation. In addition, it provides useful information for researchers working on analysis or structural elucidation of complicated compounds which contain the N-benzyltetrahydroquinoline substructure.

Identifying the proton transfer reaction mechanism via a proton-bound dimeric intermediate for esomeprazoles by a kinetic method combined with density functional theory calculations

RATIONALE

Esomeprazole analogs are a class of important proton pump inhibitors for the treatment of gastro-esophageal reflux diseases. Understanding the fragmentation reaction mechanism of the protonated esomeprazole analogs will facilitate the characterization of their complex metabolic fate in humans. In this paper, the kinetic method and theoretical calculations were applied to evaluate the fragmentation of protonated esomeprazole analogs.

METHODS

All collision-induced dissociation (CID) mass spectrometry experiments were carried out using electrospray ionization (ESI) ion trap mass spectrometry in positive ion mode. Also the accurate masses of fragments were measured on by ESI quadrupole time-of-flight (QTOF) MS in positive ion mode. Theoretical calculations were carried out by the density functional theory (DFT) method with the 6-31G(d) basis set in the Gaussian 03 program.

RESULTS

In the fragmentation of the protonated esomeprazole analogs, C-S bond breakage is observed, which gives rise to protonated 2-(sulfinylmethylene)pyridines and protonated benzimidazoles. DFT calculations demonstrate that the nitrogen atom of the pyridine part is the thermodynamically most favorable protonation site, and the C-S bond cleavage is triggered by the transfer of this ionizing proton from the nitrogen atom of the pyridine part to the carbon atom of the benzimidazole part to which the sulfinyl is attached. Moreover, with the kinetic plot, the intensity ratios of two protonated product ions yield a linear relationship with the differences in proton affinities of the corresponding neutral molecules, which provides strong experimental evidence that the reaction proceeds via proton-bound 2-(sulfinylmethylene)pyridine/benzimidazole complex intermediates.

CONCLUSIONS

The kinetic method combined with theoretical calculations was successfully applied to probe the proton transfer reaction by proton-bound 2-(sulfinylmethylene)pyridine/benzimidazole complexes in the fragmentation of protonated esomeprazole analogs by ESI CID MS, which is a strong evidence that the kinetic method can be applied in identifying a proton-bound dimeric intermediate in the fragmentation of protonated ions.

Gas-phase arylmethyl transfer and cyclodeamination of argentinated N-arylmethyl-pyridin-2-ylmethanimine

In collisional activation of argentinated N-arylmethyl-pyridin-2-ylmethanimine, a neutral molecule of AgNH2 is eliminated, carrying one hydrogen from the methylene and the other one from the ortho position (relative to the ipso carbon) of the aryl ring. Taking argentinated N-benzyl-pyridin-2-ylmethanimine for example, the proposition that the AgNH2 loss results from intramolecular arylmethyl transfer combined with cyclodeamination is rationalized by deuterium labeling experiments, blocking experiments, and theoretical calculations. The structure of the final product ion from loss of AgNH2 was confirmed further by multistage mass spectrometry.

Single electron redox via an ion-neutral complex in the fragmentation of protonated benzoylferrocenes

RATIONALE

Ferrocene derivatives have become very popular molecules for biological applications. Although considerable experimental and theoretical calculation studies have demonstrated that ferrocene derivatives are easily oxidized during electrospray ionization (ESI), the details of the single electron redox reaction for protonated benzoylferrocenes in collision-induced dissociation (CID) mass spectrometry (MS) has not been obtained. Characterizing this mechanism is useful for further understanding the properties of ferrocene-containing biomaterials.

METHODS

All CID MS experiments were carried out using ESI ion trap mass spectrometry in positive ion mode. In addition, the accurate mass of fragments was measured on a ESI quadrupole time-of-flight (QTOF) mass spectrometer in positive ion mode. Theoretical calculations were carried out by the density functional theory (DFT) method with a hybrid basis set consisting of 6-31G (d) and ECP LanL2DZ in the Gaussian 03 program.

RESULTS

In the fragmentation of protonated benzoylferrocenes, the characterized ferrocinium cation was observed, which was proposed from the cleavage of the bond between the ipso-carbon atom and the carbonyl carbon followed by a single electron redox in [substituted benzoyl/ferrocene] complexes. Moreover, when the complex contained an oxidant (substituted benzoyl cation) with higher electron affinity, the single electron redox reaction was more efficient. A correlation was established between the intensities of the two competitive product ions and the electron affinities of substituted benzoyl cations.

CONCLUSIONS

The single electron redox reaction by the [substituted benzoyl/ferrocene] complexes was proposed by CID MS and theoretical calculations, which provided potential evidence to further understand the reversible reduction characteristics of ferrocene-containing biomaterials.

Gas-phase fragmentation of the protonated benzyl ester of proline: intramolecular electrophilic substitution versus hydride transfer

In this study, the gas phase chemistry of the protonated benzyl esters of proline has been investigated by electrospray ionization mass spectrometry and theoretical calculation. Upon collisional activation, the protonated molecules undergo fragmentation reactions via three primary channels: (1) direct decomposition to the benzyl cation (m/z 91), (2) formation of an ion-neutral complex of [benzyl cation + proline](+), followed by a hydride transfer to generate the protonated 4,5-dihydro-3H-pyrrole-2-carboxylic acid (m/z 114), and (3) electrophilic attack at the amino by the transferring benzyl cation, and the subsequent migration of the activated amino proton leading to the simultaneous loss of (H2O + CO). Interestingly, no hydrogen/deuterium exchange for the fragment ion m/z 114 occurs in the d-labeling experiments, indicating that the transferring hydride in path-b comes from the methenyl hydrogen rather than the amino hydrogen. For para-substituted benzyl esters, the presence of electron-donating substituents significantly promotes the direct decomposition (path-a), whereas the presence of electron-withdrawing ones distinctively inhibits that channel. For the competing channels of path-b and path-c, the presence of electron-donating substituents favors path-b rather than path-c, whereas the presence of electron-withdrawing ones favors path-c rather than path-b.

Elimination of benzene from protonated N-benzylindoline: benzyl cation/proton transfer or direct proton transfer?

Collision-induced dissociation (CID) of protonated N-benzylindoline and its derivatives was investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Elimination of benzene was observed besides hydride transfer and electron transfer reactions. D-labeling experiments and accurate mass determinations of the product ions confirm that the external proton is retained in the fragment ion, and the elimination reaction was proposed to be initiated by benzyl cation transfer rather than proton transfer. Benzyl cation transfer from the nitrogen atom to one of the sp(2)-hybridized carbon atoms in the indoline core is the key step, and subsequent proton transfer reaction leads to the elimination of benzene. Density functional theory (DFT)-based calculations were performed and the computational results also support the benzyl cation/proton transfer mechanism. Figure ᅟ