Even-electron Fragment Research Articles

Radical fragment ions in collision-induced dissociation-based tandem mass spectrometry

Collision-induced dissociation (CID) is a common fragmentation strategy in tandem mass spectrometry (MS2) analysis. A conventional understanding is that fragment ions generated in low-energy CID should follow the even-electron rule. As such, (de)protonated ([M+H]+/[M−H]−) or even-electron precursor ions should follow heterolytic cleavages and predominately generate even-electron fragment ions with very few radical fragment ions (RFIs). However, the extent to which RFIs present in MS2 spectra has not been comprehensively investigated. This work uses the annotated high-resolution MS2 spectra from the latest NIST 20 tandem mass spectral library to investigate the occurrence of RFIs in CID MS2 experiments. In particular, RFIs were recognized using integer double bond equivalent (DBE) values calculated from their annotated molecular formulas. Our study shows that 65.4% and 68.8% of MS2 spectra of even-electron precursors contain at least 10% RFIs by ion-count (total number of ions) in positive and negative electrospray ionization modes, respectively. Furthermore, we classified chemicals based on their compound classes and chemical substructures, and calculated the percentages of RFIs in each class. As expected, compounds that can stabilize the radical site via resonance, such as aromatic and conjugated double bond-containing chemicals, are more likely to form RFIs. We also found four possible patterns of change in RFI percentages as a function of CID collision energy. Finally, we demonstrate that the inadequate consideration of RFIs in most conventional bioinformatic tools might be problematic during in silico fragmentation and de novo annotation of MS2 spectra. This work provides a further understanding of CID MS2 mechanisms, and the unexpectedly large percentage of RFIs suggests that the even-electron rule seems to be challenged in numerous cases where it is disobeyed.

Structure and m/z of Singly Charged Even-Electron Fragment Ions in Organic Mass Spectrometry: "A Rule of Mass Shift" Revisited (Secondary Publication).

Typical modes of bond cleavages of organic compounds in mass spectrometry are briefly summarized. Although these fragmentation rules can be quite useful for interpreting mass spectra of simple compounds, application to structurally complex molecules that contain multiple hetero atoms such as nitrogen or oxygen becomes increasingly difficult, because the exact location of an unpaired electron or positive or negative charges becomes obscure in precursor ions.About a decade ago, we proposed “a rule of mass shift,” which correctly predicts the m/z for observed peaks corresponding to singly charged even-electron fragment ions. The basis of the rule postulates that ions observed as peaks in an ordinary mass spectrum should be sufficiently stable to survive during the flight path in a mass spectrometer.The important recognition is that each atom in a stable ion should be in an ordinary valence state, and no free valence should be allowed. Therefore, if the cleavage of a bond leads to an ion with an unstable structure, some structural changes must take place in order for the ion to be observed in the mass spectrum. Such structural changes can be the addition of hydrogen atom(s) and/or a proton for positive ions, and the addition of a hydrogen atom and/or the elimination of two hydrogen atoms in the case of negative ions. These required structural changes in each case are schematically depicted and discussed in detail.Two typical examples are shown, in which m/z’s of the observed peaks are correctly predicted. The scope and limitations, as well as the significance of the rule for analyzing fragmentations in organic mass spectrometry are also discussed.

Influence of electronic and molecular structure on the fragmentation dynamic of even-electron carbocationic triangulenes and helicenes in the gas phase.

Stable, long-lived organic cations are directly transferred by electrospray ionization (ESI) from solution into the gas phase where their collision-induced dissociations (CID) are studied by tandem mass spectrometry. Three related types of triphenyl carbenium ions are investigated, in which the meta positions are either substituted by methoxy groups or tertiary nitrogen bridges, including tetramethoxyphenylacridinium (TMPA+ ), dimethoxyquinacridinium (DMQA+ ), and triazatriangulenium (TATA+ ) cations. These ions are triangular in shape with increasing degrees of planarity. Fragmentation occurs at the periphery of the triangular molecule, involving the methoxy groups and the substituent of the nitrogen bridge. Each initial precursor cation is an even electron (EE) system and shows competing dissociations into both even (EE) and odd electron (OE) fragment ions. The latter reaction is a breach of the classic 'even-electron rule' in mass spectrometry. While the EE fragment dissociates similar to the precursor, the OE fragment ion shows a rich radical-induced fragmentation pattern. Two driving forces direct the fragmentation of the EE precursor ion toward OE fragment ions, including the release of stabilized radicals and the extension of the π-system by increasing planarization of the triangulene core. Copyright © 2017 John Wiley & Sons, Ltd.

有機イオンの構造とフラグメンテーション

有機イオンの構造とフラグメンテーション

Characterization of heavy petroleum fraction by positive-ion electrospray ionization FT-ICR mass spectrometry and collision induced dissociation: Bond dissociation behavior and aromatic ring architecture of basic nitrogen compounds

This paper examined the bond dissociation behavior and aromatic ring architecture of basic nitrogen compounds in Sudan heavy petroleum fraction. Both broadband and quadrupole isolation modes positive-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with collision induced dissociation (CID) techniques were used to characterize a low sulfur crude oil derived vacuum residuum (VR). The appropriate CID operating condition was selected by comparing the molecular weight distributions of the basic nitrogen compounds under various CID operating conditions. Both odd- and even-electron fragment ions were observed from the mass spectrum, indicating that the heterolytic and homolytic bond cleavages occurred simultaneously during the CID process. The odd-electron fragment ions were predominant in each class species, indicating preferential heterolytic bond cleavages. At the optimal CID condition, the alkyl groups decomposed deeply and just left the aromatic cores of the nitrogen compounds. No significant variation in double bond equivalent (DBE) value was observed between the fragment and parent ions, revealing that the domination of single core structure.

Investigation of collision‐induced dissociations involving odd‐electron ion formation under positive electrospray ionization conditions using accurate mass

Collision induced dissociation (CID) has been extensively used for structure elucidation. CID in the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes has been found to generate mostly even-electron fragment ions while it has been occasionally reported to form odd-electron free radical ions. However, the structural requirements and the fragmentation mechanisms for free-radical CIDs have not been well characterized in the literature. For this purpose, we studied a series of aromatic and non-aromatic compounds such as sulfonamides, N-aryl amides, tert-butyl-substituted aromatic compounds, aryl alkyl ethers, and O-alkyl aryl oximes using the LTQ and LTQ Orbitrap linear ion trap mass spectrometers. The accurate measurement of the fragment ion masses established the unambiguous assignment of the fragment structures resulting from the test compounds. Our results showed that free radical fragmentation is structure dependent and is to a large extent correlated with the neighboring groups in the structures that stabilize the newly formed free radical ions.

Unusual Odd-Electron Fragments from Even-Electron Protonated Prodiginine Precursors Using Positive-Ion Electrospray Tandem Mass Spectrometry

Reports of anticancer and immunosuppressive properties have spurred recent interest in the bacterially produced prodiginines. We use electrospray tandem mass spectrometry (ES-MS/MS) to investigate prodigiosin, undecylprodiginine, and streptorubin B (butyl-meta-cycloheptylprodiginine) and to explore their fragmentation pathways to explain the unusual methyl radical loss and consecutive fragment ions that dominate low-energy collision-induced dissociation (CID) mass spectra. The competition between the formation of even-electron ions and radical ions is examined in detail. Theoretical calculations are used to optimize the structures and calculate the energies of both reactants and products using the Gaussian 03 program. Results indicate that protonation occurs on the nitrogen atom that initially held no hydrogen, thus allowing formation of a pseudo-seven-membered ring that constitutes the most stable ground state [M + H](+) structure. From this precursor, experimental data show that methyl radical loss has the lowest apparent threshold but, alternatively, even-electron fragment ions can be formed by loss of a methanol molecule. Computational modeling indicates that methyl radical loss is the more endothermic process in this competition, but the lower apparent threshold associated with methyl radical loss points to a lower kinetic barrier. Additionally, this characteristic and unusual loss of methyl radical (in combination with weaker methanol loss) from each prodiginine is useful for performing constant neutral loss scans to quickly and efficiently identify all prodiginines in a complex biological mixture without any clean-up or purification. The feasibility of this approach has been proven through the identification of a new, low-abundance prodigiosin analog arising from Hahella chejuensis.

Mechanisms of photoinduced CαCβ bond breakage in protonated aromatic amino acids

Photoexcitation of protonated aromatic amino acids leads to CαCβ bond breakage among other channels. There are two pathways for the CαCβ bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak CαCβ bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed.

Unprecedented Matrix-Induced Reduction of Flavins Observed Under FAB and MALDI Conditions

Flavin-derived compounds such as riboflavin, riboflavin 5-phosphate (flavin mononucleotide, FMN) and flavin-adenine dinucleotide (FAD), under matrix-assisted laser desorption ionization (MALDI) and fast atom bombardment (FAB) conditions produce unprecedented reduced species that can be attributed to [M+2H]+. and [M+3H]+ in addition to the anticipated [M+H]+ ions. On the other hand, electrospray ionization (ESI) conditions generate only the expected [M+H]+ as the sole molecular-related ion. A protonation from the matrix accompanied by a concomitant electron transfer, rather than the transfer of hydrogen atom(s), [H. of CH groups], is proposed to explain the observed reduction process. The reduction site expands to N(1)=C(10a)-C(4a)=N(5), a 1,4-diaza-1,3-butadiene system, in the isoalloxazine ring, resembling the behavior of prosthetic groups of aerobic dehydrogenases. Interestingly, CID-MS / MS of most abundant molecular-related ions generated by the FAB and ESI modes, namely [M+2H]+. for FAB and [M+H]+ for ESI, produce the identical even-electron fragment ion of isoalloxazine moiety, indicating a radical loss vs. the evenelectron molecule loss from the individual precursor ions. The observed phenomena are compared with those previously reported for FABMS, thermospray mass spectrometry (TSPMS) and ESIMS. Keywords: Matrix-Induced Reduction, Flavins, thermospray, prosthetic groups, aerobic dehydrogenases

Fragmentation pathways of organoarsenical compounds by electrospray ion trap multiple mass spectrometry (MS 6)

With its detection limit well below 30 pg μl −1 LC–MS–MS has become a sensitive and thus popular analytical technique for organoarsenical compounds. Collision induced dissociation (CID) is a valuable tool for speciation and facilitates a positive identification of the species detected. However, it is not straightforward to understand the fragmentation pathways of organoarsenical compounds when only CID-MS–MS data is available. In the present paper we have investigated multiple mass spectrometry (MS n , n=1, 2, 3, 4, 5, 6) with electrospray CID fragmentation for a number of organoarsenical compounds likely to occur in the environment. The investigated compounds were tetramethylarsonium, trimethylarsinoxide, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, arsenocholine, and dimethylarsinoylethanol. By CID of (protonated) organoarsenical cations mostly even-electron fragments are produced after neutral loss processes such as elimination of H 2, H 2O, CH 4, C 2H 2, C 2H 4, C 2H 6, HCHO, CH 3OH, C 2H 5OH, C 2H 4O, and CH 2CO. However, abundant odd-electron fragments are also formed after elimination of radical species. Evidence for reduction of As(V) to As(III) as a driving force in the odd-electron ion formation is obtained.

Effects of ionization mode on charge-site-remote and related fragmentation reactions of long-chain quaternary ammonium ions

Comparison of collisionally activated fragment spectra of long-chain quaternary ammonium ions, formed by liquid-assisted secondary ion mass spectrometry (LSIMS) and electrospray ionization (ESI), shows the latter are dominated by radical cations while the former yield mainly even-electron charge-site-remote (CSR) fragments, similar to the report for different precursors by Cheng et al., J. Am. Soc. Mass Spectrom. 1998, 9, 840. Here, mixed-site fragmentation products (formal loss of a radical directly bonded to the nitrogen plus a radical derived from the long chain) are of comparable importance for both ionization techniques. These observations are difficult to understand if the CSR ions are formed by a concerted rearrangement-elimination reaction, since precollision internal energies of the ESI ions are much lower than those of the ions from LSIMS. Alternatively, if one discards the concerted mechanism for high-energy CA, and assumes that the even-electron fragments are predominantly formed via homolytic bond cleavage, the colder radical cations from ESI survive to the detector while the more energized counterparts from LSIMS preferentially lose a hydrogen atom to yield the CSR ions, as proposed by Wysocki and Ross ( Int. J. Mass Spectrom. Ion Processes 1991, 104, 179). The present work also attempts to reconcile discrepancies involving critical energies and known structures for neutral fragments.

Mass spectrometric study of ?-aryl-?,�?-diethoxy-?, ?-butenolides

Electron impact mass spectra of eight of the title compounds are reported. Abundant fragment ions were produced under electron impact (EI) conditions and, with one exception, the ($¿hbox¿ArC¿¿equiv$ O) ions were the base peaks. The EI fragmentation mechanisms of two representative compounds were studied with the aid of high-resolution and mass-analyzed ion kinetic energy spectrometry (MIKES) data. The M(+) ions fragment to give both an odd-electron ion and an even-electron fragment ion. Two H-atom rearrangements proceeding via four-membered ring intermediates and three losses of CO through i- and alpha-fragmentations were observed under EI. On comparing fragmentations under EI conditions with those under FAB conditions for two of the compounds, the fragmentation mechanisms were reasonably similar, with additional fragmentations rationalized in terms of the ionization proton being located on the oxygen atom of the beta-ethoxy group.

A rule to account for mass shifts of even-electron fragment ions in fast-atom bombardment mass spectrometry

A rule is proposed for prediction of correct mass numbers of fragment ions in fast-atom bombardment (FAB) ionization mass spectrometry. The term “mass shift” in the rule is defined as the increment...

Applications of tandem mass spectrometry to the characterization of derivatized glutathione conjugates. Studies with S-(N-methylcarbamoyl)glutathione, a metabolite of the antineoplastic agent N-methylformamide.

Daughter ion spectra are reported for [M + H]+ ions generated by fast atom bombardment mass spectrometry of S-(N-methylcarbamoyl)glutathione (1) and a series of alkoxycarbonyl methyl ester derivatives thereof. Structurally informative, even-electron fragment ions, which serve to define the nature of both the xenobiotic and peptide components of the conjugate, are observed in the collisionally activated dissociation (CAD) spectra of 1 and its ethoxy- and benzyloxycarbonyl methyl esters. Studies with the t-butyloxycarbonyl (tBOC) methyl ester derivative, on the other hand, indicated that the tBOC group exerts a powerful directing influence on the CAD process, and that the major daughter ions in this case are associated with cleavage of the tBOC functionality itself and are of little diagnostic value. Of the derivatives examined, the benzyloxycarbonyl congener, which may be generated readily from 1 in aqueous media, is judged to be the most useful from the standpoints of ease of formation, desirable high-performance liquid chromatographic properties, and informative mass spectral fragmentation characteristics under CAD conditions.

Mass spectrometric study of cyclotriphosphazenes with 1,2-di(oxymethylene)-o-carborane spiro substituents

1. The major directions for the fragmentation of di(oxymethylene)-o-carborane spiro derivatives of cyclotriphosphazene upon electron impact involve the cleavage of the P-O and P-Cl bonds exocyclic relative to the Ps3N3 ring with the formation of even-electron fragment ions. 2. The cleavage of the P-O bonds is accompanied by hydrogen rearrangements with the dissociation of not less than three initial bonds. 3. The reactions leading to the decomposition of the carborane and cyclotriphosphazene systems are uncharacteristic for molecular and other phosphorus-containing ions.

Fragmentation of the adducts of perfluoromethyl vinyl ether with halo- and nitronium fluorosulfates under electron impact

The mass spectra of CF3OCF(OSO2F)CF2X (X=halogen, NO2) and some model compounds have been studied. It has been established that the molecular cation-radicals of these compounds are not stable. The main breakdown processes of the even-electron fragment ions occur with migration of halogen atoms or fluorosulfate groups to neighboring carbon atoms.

Mass Spectral Studies of Binuclear Metal Complexes: Copper(I) Carboxylates

The thermal decomposition of twenty-five cupric carboxylates was studied by mass spectrometry. In eleven cases volatile cuprous carboxylates (many of which have not been previously reported) were detected amongst the decomposition products. The cuprous carboxylates from acetic, propionic, n-butyric, isobutyric, difluoroacetic, trifluoroacetic, benzoic, p-fluorobenzoic, p-chlorobenzoic, o-chlorobenzoic, and pentafluorobenzoic acids were all found to be dimeric in the vapor phase. Two basically different fragmentation pathways can be proposed depending upon whether the copper salt is formed from an alkyl- or aryl-carboxylic acid. For the former, the spectra are dominated by even-electron fragment ions formed by initial loss of RCO2•from the molecular ion. For the latter, a parallel fragmentation pathway initiated by loss of CO2 from the molecular ion and migration of the aryl group to the metal is also present.

Structural effects in the mass spectra of some chloro-nitro steroids and steroidal oximes

Fragmentation of steroids of the 5-chloro-6β-nitro-5α-cholestane type is initiated by loss of heteroatom-containing substituents, followed by further radical loss to form even-electron fragments, with preservation of the steroid nucleus. In the presence of a Δ 5 double bond, retention and rearrangements of the nitrogen function are observed. Steroidal oximes retain nitrogen in their major fragments while undergoing extensive rearrangements involving the oxime function. The fragmentation patterns suggest the determination of decomposition by charge localization on heteroatom functions and stabilization by charge delocalization in fragmentation products.

Charge Migration in Odd- and Even-Electron Fragment Ions. The Mass Spectrum of a Bisaziridinone

Charge Migration in Odd- and Even-Electron Fragment Ions. The Mass Spectrum of a Bisaziridinone

Mass spectra of organo-stannanes and -plumbanes

The fragmentation behaviour of organostannanes [SnPhnEt4–n(n= 0–4), SnPh3(CH2·CH2·Ph), spiro-[CH2]4 > Sn < [CH2]4, SnPh3X (X = F, Cl, Br, or I), Sn2R6(R = Et, Ph)], organoplumbanes (PbEt4, PbPh4), and Ph4C have been examined under high resolution at 70 eV. Parent ions are always of low abundance and decompose mainly by elimination of an odd-electron neutral fragment, whereas MR3+ ions are of high abundance and lose even-electron fragments. Most of the ion current is carried by metal-containing species. Appearance-potential measurements on organostannanes have been used to evaluate dissociation-energy differences and dissociation energies in molecular ions.