Fragmentation Pathways of Singly- and Doubly-charged Ions in the Mass Spectra of Si-, N- and C-Substituted 1,3-Dioxa-6-aza-2-silacyclooctanes

The fragmentation pathways of singly- and doubly-charged ions in the mass spectra of Si-, N-, and C-substituted 1,3-dioxa-6-aza-2-silacyclooctanes upon electron impact have been studied. The directions for the fragmentation of the singly- and doubly-charged molecular ions differ markedly. All the doubly-charged fragmentation ions are formed by the loss of neutral molecules from M⁺⁺ and contain nitrogen and silicon atoms.     

The ion kinetic energy spectra of some aromatic hydrocarbons

The technique of ion kinetic energy spectroscopy has been applied to the study of the aromatic hydrocarbons benzene, toluene, naphthalene, 2-methyl naphthalene, biphenyl and anthracene. The method is illustrated by a complete study of naphthalene in which transitions of metastable doubly- and singly-charged ions are listed, including reactions in which singly-charged ions are formed by collision induced charge-exchange reactions of doubly-charged ions and by the double process of charge-exchange and metastable decomposition with loss of one or two hydrogen atoms. Decompositions of doubly-charged ions into two singly-charged ions, together with the kinetic energies released in these decompositions, are also given for all the compounds studied.     

Use of semiempirical models for calculation of B terms in MCD spectra—III : Pariser-Parr-Pople (PPP) predictions for dications and dianions of pentalene and heptalene

The PPP model is used to predict the MCD spectra of the doubly-charged ions of pentalene and heptalene. The excited states of the 10 π-electron ions are both related to those of naphthalene and exhibit interesting configuration pairing similar to that found in alternant hydrocarbons. Certain aspects of the results are very sensitive to the choice of parameters and to inclusion of doubly excited configurations in the PPP model. Comparison with the so far non-existent experimental data would probably be useful for testing the various proposed versions of the model.     

Doubly-charged ions in the planetary ionospheres: a review

This paper presents a review of the current knowledge on the doubly-charged atomic and molecular positive ions in the planetary atmospheres of the Solar System. It is focused on the terrestrial planets which have a dense atmosphere of N(2) or CO(2), i.e. Venus, the Earth and Mars, but also includes Titan, the largest satellite of Saturn, which has a dense atmosphere composed mainly of N(2) and a few percent of methane. Given the composition of these neutral atmospheres, the following species are considered: C(++), N(++), O(++), CH(4)(++), CO(++), N(2)(++), NO(++), O(2)(++), Ar(++) and CO(2)(++). We first discuss the status of their detection in the atmospheres of planets. Then, we provide a comprehensive review of their complex and original photochemistry, production and loss processes. Synthesis tables are provided for those ions, while a discussion on individual species is also provided. Methods for detecting doubly-charged ions in planetary atmospheres are presented, namely with mass-spectrometry, remote sensing and fine plasma density measurements. A section covers some original applications, like the possible effect of the presence of doubly-charged ions on the escape of an atmosphere, which is a key topic of ongoing planetary exploration, related to the evolution of a planet. The results of models, displayed in a comparative way for Venus, Earth, Mars and Titan, are discussed, as they can predict the presence of doubly-charged ions and will certainly trigger new investigations. Finally we give our view concerning next steps, challenges and needs for future studies, hoping that new scientific results will be achieved in the coming years and feed the necessary interdisciplinary exchanges amongst different scientific communities.     

The doubly-charged ion mass spectra of hydrocarbons

The doubly-charged ion mass spectra of some hydrocarbons, including a variety of structural types, have been obtained by a new technique in which doubly-charged ions are charge exchanged with neutral molecules and so separated from singly-charged ions. The spectra show strong similarities, independent of hydrocarbon structure; characteristic ions include [C_mH₂]⁺⁺ (m = 2 to 5), [C_nH₆]⁺⁺(n > 6), [C₁₀H₈]⁺⁺, [C₁₂H₈]⁺⁺, [C₁₁H₁₀]⁺⁺, [C₇H₇]⁺⁺·, [C₉H₇]⁺⁺· and [C₁₃H₁₁]⁺⁺·. The fragmentation pattern of 2-phenylnaphthalene has been reconstructed, based on observed reactions of metastable doubly-charged ions to give fragment doubly-charged ions. In addition, we examined metastable ion fragmentations leading to two singly-charged ions for some of the characteristic ions, using several compounds. The value of doubly-charged ion mass spectra of hydrocarbons appears to lie in the information they provide on ion structures; this information was sufficient to permit the proposal of structures for the major ions encountered in this study.     

Peptide photodissociation at 157 nm in a linear ion trap mass spectrometer

The photodissociation by 157 nm light of singly- and doubly-charged peptide ions containing C- or N-terminal arginine residues was studied in a linear ion trap mass spectrometer. Singly-charged peptides yielded primarily x- and a-type ions, depending on the location of the arginine residue, along with some related side-chain fragments. These results are consistent with our previous work using a tandem time-of-flight (TOF) instrument with a vacuum matrix-assisted laser desorption/ionization (MALDI) source. Thus, the different internal energies of precursor ions in the two experiments seem to have little effect on their photofragmentation. For doubly-charged peptides, the dominant fragments observed in both photodissociation and collisionally induced dissociation (CID) experiments are b- and y-type ions. Preliminary experiments demonstrating fragmentation of multiply-charged ubiquitin ions by 157 nm photodissociation are also presented.     

Mass spectrometry in structural and stereochemical problems—CXCIX Contrasting fragmentations of singly- and doubly-charged o-, m- and p-hydroxyalkylphenones and their trimethylsilyl ethers

High resolution mass spectrometry, metastable defocusing and deuterium labeling of trimethylsilyl (TMS) ethers have been used to study the electron-impact induced fragmentations of o-, m- and p-hydroxyalkylphenones and their TMS ether derivatives. These derivatives have proven useful in contrasting the fragmentation patterns of singly- and doubly-charged ions because of the competing fragmentations: α-cleavage and a McLafferty rearrangement from the ketone moiety and methyl cleavage from the TMS group. A proximity effect was responsible for a markedly increased methyl radical loss from the o-TMS ether. This fragmentation was minor with the m- and p-isomers. Significantly intense doubly-charged ions were formed from ketonic cleavage and by the loss of a TMS methyl radical. The sequence of fragmentation depended on the size of the alkyl group attached to the ketone carbonyl. There was no evidence found for a McLafferty rearrangement occurring from the doubly-charged molecular ion of the TMS ethers of the hydroxyalkylphenones but the rearrangement occurred from the doubly-charge molecular ion of bis-3-(1-oxopentyl)-4-hydroxy-phenyl-methane and, of course, from the singly charged [M]^+. The bis-p-hydroxyphenylmethane derivatives were studied in an effort to increase the intensity of the doubly-charged ions as it was expected that the charges would be separated by a longer distance.     

Doubly-charged gas phase cations

Unique features of doubly-charged stable organic ions are examined and the results correlated with experimental observations. Self-consistent field molecular orbital methods are used to compute structures and stabilities of C_nH ₂ ²⁺ (n=2–9) ions which are prominent in electron impact ionization of hydrocarbon molecules. A simple curve crossing model is employed to rationalize charge transfer reactions of these ions.     

Mass spectrometric behavior of trimethylsilyl derivatives of isomeric diamino-, dihydroxy- and aminohydroxynaphthalenes

The mass spectrometric behavior of the di-TMSi derivatives of diamino-,dihydroxy-and aminohydroxynaphthalenes is described. The presence or lack of doubly-charged ions in the mass spectra is correlated with substituent position. Comparison is made with the spectra of the disubstituted benzenes. Other fragmentations are discussed with supporting data from TMSi-d₉ derivatized compounds.     

Impact of proline and aspartic acid residues on the dissociation of intermolecularly crosslinked peptides

The dissociation of intermolecularly crosslinked peptides was evaluated for a series of peptides with proline or aspartic acid residues positioned adjacent to the crosslinking sites (lysine residues). The peptides were crosslinked with either disuccinimidyl suberate (DSS) or disuccinimidyl L-tartrate (DST), and the influence of proline and aspartic acid residues on the fragmentation patterns were investigated for precursor ions with and without a mobile proton. Collisionally activated dissociation (CAD) spectra of aspartic acid-containing crosslinked peptide ions, doubly-charged with both protons sequestered, were dominated by cleavage C-terminal to the Asp residue, similar to that of unmodified peptides. The proline-containing crosslinked peptides exhibited a high degree of internal ion formation, with the resulting product ions having an N-terminal proline residue. Upon dissociation of the doubly-charged crosslinked peptides, twenty to fifty percent of the fragment ion abundance was accounted for by multiple cleavage products. Crosslinked peptides possessing a mobile proton yielded almost a full series of b- and y-type fragment ions, with only proline-directed fragments still observed at high abundances. Interestingly, the crosslinked peptides exhibited a tendency to dissociate at the amide bond C-terminal to the crosslinked lysine residue, relative to the N-terminal side. One could envision updating computer algorithms to include these crosslinker specific product ions--particularly for precursor ions with localized protons--that provide complementary and confirmatory information, to offer more confident identification of both the crosslinked peptides and the location of the crosslink, as well as affording predictive guidelines for interpretation of the product-ion spectra of crosslinked peptides.     

Pyridine N-oxide and pyridine-d5 N-oxide: an electrospray/tandem mass spectrometric study carried out at high mass resolution

A mass spectrometric study of pyridine N-oxide and pyridine-d5 N-oxide was carried out with a hybrid quadrupole/time-of-flight (TOF) mass spectrometer coupled with an electrospray (ES) source. In addition to the observation of protonated, sodiated, and proton-bound dimers of pyridine N-oxide and pyridine-d5 N-oxide, mass scans revealed the presence of several doubly-charged ion species. Doubly-charged ions of m/z 191 were identified as diprotonated tetramers of pyridine N-oxide; a structure has been proposed for the diprotonated tetramer and its energy relative to that of protonated pyridine N-oxide has been obtained from geometry optimizations. The principal ion species observed were subjected to collision-induced dissociation; accurate mass measurements were made of each fragment ion so as to determine its elemental composition. On the basis of mass spectrometric evidence, it is suggested that dissociation of pyridine N-oxide may occur during the ES process and the resulting fragments become embedded in doubly-charged ions. The proton affinity for both pyridine N-oxide and pyridine-d5 N-oxide was calculated; the difference between these proton affinities was compared with an experimentally determined difference between the proton affinities of pyridine N-oxide and pyridine-d5 N-oxide.     

Electronic spectroscopy and relaxation of gas phase molecular ions

Techniques for obtaining electronic spectra of singly charged cations are described. Jahn-Teller effects in the spectra of polyhalobenzene cations are discussed. Experimental and theoretical methods for studying radiationless transitions in molecular ions are presented. Recent work on the spectroscopy and intramolecular relaxation of doubly-charged cations is reviewed.     

Mass Spectrometric Studies of Mono- and Di-Haloacetylenes

The mass spectra of the four monohaloacetylenes X-C?C-H with X = F, Cl, Br, I and the six dihaloacetylenes X-C?C-Y with X, Y = Cl, Br, I have been recorded. The dissociation energies of the carbon-carbon triple bonds of these compounds have been determined from the appearance potentials of the CH⁺ and CX⁺ ions. The appearance potentials of the singly- and doubly-charged molecular ions are also reported.     

Charge localization—I The ion kinetic energy spectra of ortho-, meta- and para-phenylenediamine

The ion kinetic energy spectra of o-,m-and p-phenylendiamine are presented. Decompositions of doubly-charged ions are given for the three isomers. From the width of the peaks, the energy released is measured and the equivalent intercharge distance is calculated. The results are discussed in terms of possible structures of the various ions. Also, metastable transitions which could give rise to peaks corresponding to energies below that of the mainion beam are listed and the results discussed. Significant differences were observed among the energy spectra of the three isomers.     

Spark-source mass-spectrometric sensitivity factors for elements in a graphite matrix

Relative sensitivity factors for determination of 41 elements by spark-source mass-spectrometry have been measured. The samples were pressed into graphite electrodes and ionized with a radiofrequency spark. The mass spectra were recorded on a photoplate and the resulting data processed by a computer. Indium was used as standard and the relative sensitivity factors for both singly- and doubly-charged ions were determined with reference to the singly-charged indium ion, with an overall error of 30%. The mean analysis precision was 16%.     

Electron-impact studies—LXXIV: A survey of rearrangement processes in the ‘doubly-charged ion’ mass spectra of aromatic compounds

The technique of Beynon² has been used to determine the ‘doubly-charged ion’ mass spectra of a variety of aromatic compounds which contain pronounced rearrangement peaks in their conventional mass spectra. Many, but not all, of the ‘doubly-charged ion’ spectra contain similar rearrangement peaks to those observed in conventional spectra.     

Tryptic <Emphasis Type="Italic">y</Emphasis><Superscript>++</Superscript> Fragment Ion Distributions Are Guided by Coulombic Repulsion

Ideal tryptic peptides contain only a single basic residue, located at the C-terminus. Collisional fragmentation of their doubly- or triply-protonated ions generates doubly-charged y ⁺⁺ fragment ions with modest intensities. The size distribution of the y ⁺⁺ fragments, when averaged over many spectra, corresponds closely to the expectations from charge-directed backbone cleavage and a Coulomb-Boltzmann distribution of mobile protons. This observation should be helpful in developing mechanistic models for y ⁺⁺ formation.     

The effect of phosphorylation on the electron capture dissociation of peptide ions

The effect of site and frequency of phosphorylation on the electron capture dissociation of peptide ions has been investigated. The ECD of a suite of synthetic peptides (APLSFRGSLPKSYVK; one unmodified, three singly-phosphorylated, three-doubly phosphorylated, and one triply-phosphorylated); two tryptic phosphopeptides (YKVPQLEIVPN(p)SAEER, alpha-casein and FQ(p)SEEQQQTEDELQDK, beta-casein) and their unmodified counterparts, were determined over a range of ECD cathode potentials. The results show that, for doubly-charged precursor ions, the presence of phosphorylation has a deleterious effect on ECD sequence coverage. The fragmentation patterns observed suggest that for peptides with multiple basic residues, the phospho-groups exist in their deprotonated form and form salt-bridges with protonated amino acid side chains. The fragmentation observed for the acidic tryptic peptides suggested the presence of noncovalent interactions, which were perturbed on phosphorylation. Increasing the ECD electron energy significantly improves sequence coverage. Alternatively, improved sequence coverage can be achieved by performing ECD on triply-charged precursor ions. The findings are important for the understanding of gas-phase fragmentation of phosphopeptides.     

Gas-phase fragmentation characteristics of benzyl-aminated lysyl-containing tryptic peptides

The fragmentation characteristics of peptides derivatized at the side-chain ε-amino group of lysyl residues via reductive amination with benzaldehyde have been examined using collision-induced dissociation (CID) tandem mass spectrometry. The resulting MS/MS spectra exhibit peaks representing product ions formed from two independent fragmentation pathways. One pathway results in backbone fragmentation and commonly observed sequence ion peaks. The other pathway corresponds to the unsymmetrical, heterolytic cleavage of the C_ζ-N_ε bond that links the benzyl derivative to the side-chain lysyl residue. This results in the elimination of the derivative as a benzylic or tropylium carbocation and a (n − l)⁺-charged peptide product (where n is the precursor ion charge state). The frequency of occurrence of the elimination pathway increases with increasing charge of the precursor ion. For the benzylmodified tryptic peptides analyzed in this study, peaks representing products from both of these pathways are observed in the MS/MS spectra of doubly-charged precursor ions, but the carbocation elimination pathway occurs almost exclusively for triply-charged precursor ions. The experimental evidence presented herein, combined with molecular orbital calculations, suggests that the elimination pathway is a charge-directed reaction contingent upon protonation of the secondary ε-amino group of the benzyl-derivatized lysyl side chain. If the secondary ε-amine is protonated, the elimination of the carbocation is observed. If the precursor is not protonated at the secondary ε-amine, backbone fragmentation persists. The application of appropriately substituted benzyl analogs may allow for selective control over the relative abundance of product ions generated from the two pathways.     

Differentiation of ionised benzimidazole from its isomeric alpha-distonic ion by collision-induced dissociation and neutralisation-reionisation mass spectrometry

Ionised benzimidazole and its isomeric alpha-distonic ion (or ionised ylid) have been examined by recording their metastable ion, collision-induced dissociation and neutralisation-reionisation mass spectra. These tautomers may be distinguished by careful consideration of key features of the collision-induced dissociation spectra, with or without prior neutralisation and reionisation. Formation of doubly-charged ions by charge stripping occurs preferentially when the alpha-distonic ion is subjected to collision. This alpha-distonic ion survives neutralisation and reionisation, thus establishing that the corresponding ylid is stable on the microsecond time frame. The effects of benzannulation on the ease of differentiation of classical and distonic radical cations derived from biologically important heterocycles are considered.