A mechanism for the loss of 60 u from peptides containing an arginine residue at the C-terminus

The loss of 60 u from protonated peptide ions containing an arginine residue at the C-terminus has been investigated by means of low energy tandem mass spectrometry. The lowest energy conformation of singly charged bradykinin is thought to involve a salt-bridge structure, which may lead to the formation of two isomeric forms. It is thought that one isomer retains the ionizing proton at the C-terminal end of the peptide, leading to the formation of the [b _n?1 + H + OH]⁺ fragment ion, and the other isomer retains the charge at the N-terminus, leading to the formation of the [M + H ? 60]⁺ fragment ion. It was found that the formation of the [M + H ? 60]⁺ ion occurs only from singly charged precursor ions. In addition, the loss of 60 u occurs from peptides in which the charge is localized at the N-terminus. These results indicate that the mechanism of formation of the [M + H ? 60]⁺ ion may be driven by a charge-remote process.     

Translational energy release and stereochemistry of steroids. VII—the loss of angular methyl groups after the dehydration of molecular ions of cholesterol and related C(5)-unsaturated 3β-hydroxy steroids

The translational energy, T, released during the loss of the angular 18- and 19-methyl groups both from metastable molecular ions and metastable [M ? H₂O]⁺ and [M ? 2H₂O]⁺ ions, in C(5)-unsaturated mono-and di-hydroxy steroids, as well as in their 19-nor and deuterated analogues bearing the label in the 19-methyl group, has been measured. It was found that, while the T values for the 19-CH₃ loss, following the dehydration of the molecular ions, are increased substantially when compared to those for the same loss from the molecular ions, the T values for the 18-CH₃ loss are increased much more moderately. Nevertheless, the amounts of translational energy released in the [M ? H₂O]⁺˙ ? 18-CH₃˙ and [M ? 2 H₂O]⁺˙ ? 18-CH₃˙ transitions are still higher than those found for the respective 19-methyl loss, in accordance with the general rule established recently.     

Electron impact,chemical ionization and negative chemical ionization mass spectra,and mass-analysed ion kinetic energy spectrometry–collision-induced dissociation fragmentation pathways of some deuterated 2,4,6-trinitrotoluene (TNT) derivatives

Mass-analysed ion kinetic energy spectrometry (MIKES) with collision-induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6-trinitrotoluene (TNT) and deuterated 2,4,6-trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R₂′O and 3NO₂, whilst in TNTCI fragment ions are formed by the loss of OCI and R₂′OCI. The TNTCI derivatives did not produce molecular ions. In chemical ionization (Cl) of both groups. MH⁺ ions were observed, with [M – OR′]⁺ fragments in TNT and [M – OCI]⁺ fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M^?′, [M–R′]^?, [M–OR′]^? and [M–NO]^? ions, while TNTCI derivatives produced [M–R]^?, [M–Cl]^? and [M – NO₂]^? fragment ions without a molecular ion.     

Mass spectrometric determination of the configuration of 2-methyl-and 1,2-dimethyl-4-vinyl-ethinyl(n-butyl)-4-hydroxyperhydroquinolines

The configuration at C-2 and C-4 in the molecules of 2-methyl- and 1,2-dimethyl-4-vinylethinyl(n-butyl)-4-hydroxyperhydroquinolines was determined by mass spectrometry. The principal conclusions concerning the stereochemistry were made on the basis of differences in the values of the I_[M?15]⁺/I_[M]⁺·, I_[M?17]⁺/I_[M]⁺·, I[_M?43]⁺/I_[M]⁺· and I_[M?57]⁺/I_[M]⁺· ratios in the mass spectra of the epimeric vinylethinylic alcohols, and of the I_[M?15]⁺/I_[M]⁺· and I_[M?15]⁺/I_[M]⁺· ratios in the case of the n-butylic alcohols.     

Electrospray mass analysis of chemical oxidation of 1,2-diarylcyclopropanes by Cu(II) salt

Electrospray ionization mass spectrometry was used to study chemical electron-transfer reactions of 1,2-diarylcyclopropanes by Cu(II) salt in acetonitrile. The ion [M ? H]⁺ with a hydrogen atom loss and the solvent adduct ions, [M+42]⁺, etc., were detected as the initial reaction products, where [M+42]⁺ represents the ion whose mass is 42 u greater than the parent molecule M. From the study of deuterated derivatives, the hydrogen abstraction was revealed to occur at the 3 position of the cyclopropanes, and the mechanism of the hydrogen abstraction reaction and of the solvent addition were discussed.     

Measurement of Hydroxyl-Radical Formation in the Rat Striatum by In Vivo Microdialysis and GC-MS

A GC-MS method was developed for measuring hydroxyl-radical capture products of salicylic acid, a common trapping agent for this reactive oxygen species, in samples obtained by in vivo cerebral microdialysis experiments. The assay employed liquid–liquid extraction followed by derivatization of 2,3- and 2,5-dihydroxybenzoic acid, along with 3,5-dihydroxybenzoic acid added as an internal standard. Due to their simple electron ionization mass spectra featuring [M–57]⁺ ions through the loss of tertiary alkyl group from the corresponding molecular ions, tert-butyldimethylsilyl (TBDMS) derivatives afforded straightforward method development based on selected-ion monitoring. In addition, tandem mass spectrometry probing collision-induced dissociation of [M–57]⁺ ions obtained from the isomeric tert-butyldimethylsilyl derivatives revealed characteristic differences in the resultant product-ion spectra. Our work has demonstrated the applicability of GC-MS for the assay of microdialysates for 2,3- and 2,5-dihydroxybenzoic acid by confirming that local administration of the excitotoxic glutamate into the rat striatum significantly increased in vivo hydroxyl-radical production in this brain region and that subsequent systemic administration of α-phenyl-tert-butylnitrone reversed glutamate-induced oxidative stress.     

Translational energy release and stereochemistry of steroids. VIII-The mechanism of the elimination of water from metastable ions in epimeric 3-hydroxy steroids of the 5α-series

The mechanism of water elimination from metastable molecular, [M ? CH₃˙]⁺ and [M ? ring D]⁺˙ ions of epimeric 3-hydroxy steroids of the 5α-series has been elucidated. Deuterium labelling, the measurement of the translational energy released during the loss of water, and collision-induced decomposition mass-analysed kinetic energy spectrometry were the techniques used. It was found that the mechanisms of water loss from metastable M⁺˙ and [M ? ring D]⁺˙ ions is different from that from [M ? CH₃˙]⁺ ions.     

Studies in chemical ionization mass spectrometry: 17-hydroxy steroids

The chemical ionization mass spectra of several hydroxy steroids were obtained using methane as the reactant gas. The spectra are much less complex than the electron ionization spectra and little fragmentation of the steroid nucleus is observed. The major fragment ions involve the loss of water from [M + H]⁺. A 3-keto group in the steroids was characterized by an abundant [M + C₂H₅]⁺ ion. 5α- and 5β-Dihydrotestosterone could be distinguished by their spectra, with H₂ as the reactant gas by marked differences in amounts of [M + H]⁺, [M + H ? H₂O]⁺ and [M + H ? 2H₂O]⁺. Substituted 3α-X-, 17 β-ol compounds, (X = Cl, Br) were also studied to obtain relative amounts of protonation at these sites.     

[C]+· and [CH]+ from doubly charged ions formed from malononitrile

The metastable ions [M]²⁺, [M – H]²⁺· and [M – H₂]²⁺ from malononitrile fragment by loss of [CH]⁺, [C]⁺· and [C]⁺·, respectively. The reaction of the molecular ion involves the methylene and nitrile carbon atoms in the statistical probability ratio, while that of [M – H]²⁺· involves exclusively the nitrile carbon and that of [M ? H₂]²⁺ involves an approximately equal contribution, from both sources. It is suggested that the metastable molecular ion fragments through a bipyrimidal intermediate.     

C-C and C-H bond activation in the fragmentation of the [M + Ni]+ adducts of aliphatic amino acids

The major metal-containing species formed upon fast atom bombardment of amino acid/Ni⁺² mixtures is the [M + Ni]⁺ adduct, involving reduction of the Ni⁺² to the +1 oxidation state. By contrast, electrospray ionization of amino acid/Ni⁺² mixtures produces predominantly [Ni(M ? H)M]⁺; this species, on collisional activation, produces predominantly [M + Ni]⁺ by elimination of [M - H], presumably a carboxylate radical. The unimolecular fragmentation reactions occurring on the metastable ion time scale for the [M + Ni]⁺ adducts of a variety of α-amino acids have been recorded. The adducts with phenylalanine, α-aminoisobutyric acid and α-aminobutyric acid fragment by elimination of H₂O, H₂O + CO and, to a minor extent, by elimination of CO₂. These reactions are similar to those observed for the [M + Cu]⁺ adducts of α-amino acids. A reaction distinctive for the [M + Ni]⁺ adducts involves formation of the immonium ion RCH=NH ₂ ⁺ . By contrast, the [M + Ni]⁺ adducts with leucine, isoleucine, and norleucine show extensive metastable ion fragmentation by elimination of H₂, CH₄, C₂H₄, C₃H₆, and C₄H₈, with the relative importance of the different fragmentation channels depending on the configuration of the C₄H₉ side chain. These results are interpreted in terms of C-C and C-H bond activation of the C₄H₉ side chain by the Ni⁺. The adducts with valine and norvaline fragment in a fashion similar to the adduct with phenylalanine, except that minor elimination of C₃H₆ is observed.     

Mass spectrometric monitoring of oxidation of aliphatic C6–C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n‐hexane, cyclohexane, n‐heptane, n‐octane and isooctane) and ethanol in 28 Torr O₂ or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15]⁺ and [M + 13]⁺ in addition to [M ? H]⁺ and [M ? 3H]⁺ were detected as major ions where M is the sample molecule. The ions [M + 15]⁺ and [M + 13]⁺ were assigned as oxidation products, [M ? H + O]⁺ and [M ? 3H + O]⁺, respectively. By the tandem mass spectrometry analysis of [M ? H + O]⁺ and [M ? 3H + O]⁺, H₂O, olefins (and/or cycloalkanes) and oxygen‐containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O₂ plasma was postulated as the oxidizing reagent. As an example, the reactions of C₆H₁₄^+? with O₂ and of C₆H₁₃⁺ (CH₃CH₂CH⁺CH₂CH₂CH₃) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C₆H₁₃⁺ leads to the formation of protonated ketone, CH₃CH₂C(=OH⁺)CH₂CH₂CH₃. In air plasma, [M ? H + O]⁺ became predominant over carbocations, [M ? H]⁺ and [M ? 3H]⁺. For ethanol, the protonated acetic acid CH₃C(OH)₂⁺ (m/z 61.03) was formed as the oxidation product. The peaks at m/z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m/z 89.06 as protonated ethyl acetate. Copyright © 2016 John Wiley & Sons, Ltd.     

Laser desorption mass spectrometry of squaric acid and its salts

The laser desorption mass spectrometry of the oxocarbon squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) and its salts of the form A₂C₄O₄ (A = cation) is described. Both positive and negative ion spectra were obtained. The positive ion spectrum of the acid is characterized by an ion corresponding to loss of CO from [M + H]⁺. The negative ion spectrum shows an intense [M ? H]^? peak in addition to a dimer species. The alkali salt spectra contain [M + A]⁺ in the positive mode and [M ? A]^? and an intense [C₄HO₄]^? in the negative mode. The smaller alkali salts also have an [M + H]⁺ adduct ion. Unlike the alkali squarates, the ammonium salt shows ions corresponding to losses of neutrals from the molecular adduct in the positive ion spectrum and a dimer species in the negative ion spectrum. Molecular weight information was obtained in all cases. A (bis) dicyanomethylene derivative of potassium squarate was also studied. Some field desorption mass spectrometry results are presented for comparison.     

Silver (Ι)‐assisted enantiomeric analysis of ginsenosides using electrospray ionization tandem mass spectrometry

For identification of ginsenoside enantiomers, electrospray ionization mass spectrometry (ESI‐MS) was used to generate silver complexes of the type [ginsenoside + Ag]⁺. Collision induced dissociation of the silver‐ginsenoside complexes produced fragment ions by dehydration, allowing differentiation of ginsenoside enantiomers by the intensity of [M + Ag ? H₂O]⁺ ion. In the meanwhile, an approach based on the distinct profiles of enantiomer‐selective fragment ion intensity varied with collision energy was introduced to refine the identification and quantitation of ginsenoside enantiomers. Five pairs of enantiomeric ginsenosides were distinguished and quantified on the basis of the distribution of fragment ion [M + Ag ? H₂O]⁺. This method was also extended to the identification of other type of ginsenoside isomers such as ginsenoside Rb2 and Rb3. For demonstrating the practicability of this novel approach, it was utilized to analyze the molar ratio of 20‐(S) and 20‐(R) type enantiomeric ginsenosides in enantiomer mixture in red ginseng extract. The generation of characteristic fragment ion [M + Ag ? H₂O]⁺ likely results from the reduction of potential energy barrier of dehydration because of the catalysis of silver ion. The mechanism of enantiomer identification of ginsenosides was discussed from the aspects of computational modeling and internal energy. Copyright © 2012 John Wiley & Sons, Ltd.     

A mass spectral study of 1-(aryldimethylsilyl)-2-propanones and their isomeric 2-(aryldimethylsiloxy)-1-propenes

The mass spectra of a series of β-ketosilanes, p-Y? C₆H₄Me₂SiCH₂C(O)Me and their isomeric silyl enol ethers, p-Y? C₆H₄Me₂SiOC(CH₃)?CH₂, where Y = H, Me, MeO, Cl, F and CF₃, have been recorded. The fragmentation patterns for the β-ketosilanes are very similar to those of their silyl enol ether counterparts. The seven major primary fragment ions are [M? Me·]⁺, [M? C₆H₄Y·]⁺, [M? Me₂SiO]⁺˙, [M? C₃H₄]⁺˙, [M? HC?CCF₃]⁺˙, [Me₂SiOH]⁺˙ and [C₃H₆O]⁺˙ Apparently, upon electron bombardment the β-ketosilanes must undergo rearrangement to an ion structure very similar to that of the ionized silyl enol ethers followed by unimolecular ion decompositions. Substitutions on the benzene ring show a significant effect on the formation of the ions [M? Me₂SiO]⁺˙ and [Me₂SiOH]⁺˙, electron donating groups favoring the former and electron withdrawing groups favoring the latter. The mass spectral fragmentation pathways were identified by observing metastable peaks, metastable ion mass spectra and ion kinetic energy spectra.     

Provitamin D and vitamin D isomerizations in the mass spectrometer. Translational energy release and collision-induced dissociation studies

Collision-induced dissociation mass-analysed ion kinetic energy spectrometry and translational energy release studies have established that provitamin D₃ (7-dehydrocholesterol), provitamin D₂ (ergosterol), vitamin D₃ and vitamin D₂ isomerizations in the mass spectrometer occur at the fragment ion level leading to [M? H₂O? CH₃]⁺ ions of identical structure. It was found that the reaction, [M]⁺˙→[M? H₂O]⁺˙, plays a central role in these isomerizations.     

Rearrangement reactions of 2-nitrothiobenzamides upon electron impact

The decomposing molecular cations derived from (substituted) 2-nitrothiobenzamides fragment by complex rearrangement reactions. When the alkyl substituents (R) attached to N are methyl, the major fragmentations are [M]⁺˙ → [M? SO]^+˙ and [M? SO]^+˙ → [(M? SO)^+˙–R˙]⁺. This remains a basic pathway when R ? Et, but other rearrangements are also observed. For example, when R=Et, additional competitive processes are [M]^+˙ → [M? HO˙]⁺ and [M]^+˙ → [M? C₂H₄O]⁺˙.     

Some aspects of the mass spectra of triptycene and tri- and diphenylmethanes

Mass spectra of isotope-labeled triptycenes, triphenylmethanes and diphenylmethanes rule out the bulk of postulated decomposition mechanisms and fragment-ion structures. The formation of [M ? H]⁺ and [M ? 2H]²⁺ from triptycene, of [M ? H]⁺, [M ? CH₃]⁺ and [M ? CH₄]⁺ from triphenylmethane, and of [M ? H]²⁺ and [M ? 2H]²⁺-as well as the previously reported [M ? H]⁺ and [M ? CH₃]⁺-from diphenylmethane all seem to be preceded or accompanied by complete loss of position identity of the α and ring hydrogens in the original molecules. A statistical preference for loss of α hydrogens is found in the process leading to [M ? 2H]⁺ and [M ? H]²⁺ from triptycene, as in the formation of [M ? H]²⁺ from toluene.     

Collisional activation study of cyclic polysulfides

Cyclic polysulfides isolated from higher plants, model compounds and their electron impact induced fragment ions have been investigated by various mass spectrometric methods. These species represent three sets of sulfur compounds: C₃H₆S_x (x=1?6), C₂H₄S_x (x=1?5) and CH₂S_x (x=1?4). Three general fragmentation mechanisms are discussed using metastable transitions: (1) the unimolecular loss of structural parts (CH₂S, CH₂ and S_x); (2) fragmentations which involve ring opening reactions, hydrogen migrations and recyclizations of the product ions ([M? CH₃]⁺, [M? CH₃S]⁺, [M? SH]⁺ and [M? CS₂]^+˙); and (3) complete rearrangements preceding the fragmentations ([M? S₂H]⁺ and [M? C₂H₄]^+˙). The cyclic structures of [M]^+˙ and of specific fragment ions have been investigated by comparing the collisional activation spectra of model ions. On the basis of these results the cyclic ions decompose via linear intermediates and then recyclizations of the product ions occur. The stabilities of the fragment ions have been determined by electron efficiency vs electron energy curves.     

Sequential ortho effects: characterization of novel [M - 35]+ fragment ions in the mass spectra of 2-alkyl-4, 6-dinitrophenols

High-resolution mass spectrometry (HRMS), hybrid tandem mass spectrometry (MS/MS) (EBqQ), and photoelectron-photoion coincidence (PEPICO) experiments were conducted to examine a possible ortho-ortho effect resulting in a novel [M - 35]⁺ fragment ion in 2-alkyl-4, 6-dinitrophenols. For compounds having ethyl or larger alkyl substituents, [M35]⁺ was observed only when [M - 18]⁺ ions were present, with the ortho nitro group being involved in the reaction to [M- 35]⁺. For [M - 18]⁺ and [M - 35]⁺, HRMS results were consistent with losses of H₂O and H₂O + OH, respectively, whereas MS/MS results indicated a sequential reaction due to metastable dissociations. The appearance energy determined by PEPICO for [M - 35]⁺ was found to be greater than the appearance energy for [M - 18]⁺, thus supporting a sequential reaction. 69–75).     

Fragmentation of certain substituted azadispiro(5.1.5.2)pentadec-9-ene and azadispiro(4.1.4.2)tridec-8-ene derivatives under electron ionization

A study of the electron ionization mass spectra of certain azadispiro(5.1.5.2)pentadec-9-ene-7,15-diones and azadispiro(4.1.4.2)tridec-8-ene-6,13-diones and their derivatives has revealed that these molecules undergo fragmentation primarily by two routes, viz. loss of CO and elimination of the substituent on the pyrrolidine nitrogen. Under positive ionization conditions loss of CO is the predominant process in the diones as it releases the ring strain, while in the 6- or 7-ols loss of the substituent on nitrogen is the favoured pathway. The further decomposition pathways of these primary fragments [M ? CO]⁺˙ and [M ? OR³]⁺ have been delineated with the help of high-resolution mass measurements, D₂O exchange and metastable spectra, These compounds give very simple negative ion spectra showing only [M ? OR³]^? and [NCO]^? ions except the N-hydroxy compounds which show [M ? H]^? ions as well.