Mass-analyzed Ion Kinetic Energy Research Articles

Dissociation Dynamics of n-Propylbenzene Molecular Ion

Production of C7H7+ from n-propylbenzene molecular ion has been studied using mass-analyzed ion kinetic energy spectrometry. The kinetic energy release distributions in photodissociation and metastable ion decomposition have been determined. The photodissociation rates have been determined also on a nanosecond time scale. All the experimental data could be explained well with the microcanonical transition-state theories when the reaction was assumed to produce only the benzylium ion. Molecular orbital calculations have been carried out to understand the reason for the dominance of the benzylium channel in this system over the tropylium channel, which is important in the dissociation of smaller alkylbenzene ions.

Decomposition of metastable methyl trifluoroacetate and ethyl trifluoroacetate upon electron impact

Fragmentation mechanism of metastable ions generated from methyl trifluoroacetate, CF 3COOCH 3 (1), and ethyl trifluoroacetate, CF 3COOCH 2CH 3 (2), upon electron impact was investigated by means of mass analyzed ion kinetic energy (MIKE) spectra and ab initio molecular orbital (MO) calculations. The metastable [1–H] + ion decomposes to the m/z 99 ion by losing CO. This fragmentation proceeds with CF 3 migration via an ion-neutral complex. The metastable [1–F] + ion decomposes to m/z 81 ion by losing CO, too. In this case, however, the fragmentation proceeds with CH 3O migration to CF 2 group via a conventional three-membered ring transition state. The kinetic energy release (KER) values for these CO loss reactions are very different from each other, and are rationalized based on the different fragmentation mechanisms. The fragmentation pathways of 2 +· are similar to those of 1 +·. However, the loss of F 2CO from [2–H] + accompanies a very large KER value compared with that from [1–H] +.

Elimination of Ethylene from Metastable Isomeric Silylenium Ions in the Gas Phase: Experiment and Theory

The slow unimolecular dissociations (k ≈ 105 s-1) of gas-phase silylenium ions, SiCnH2n+3+ ions (n = 2−4) have been studied by mass-analyzed ion kinetic energy spectroscopy. On the microsecond time...

Observation of Internal Monosaccharide Losses in the Collisionally Activated Dissociation Mass Spectra of Anthracycline Aminodisaccharides

The loss of internal monosaccharide residues from anthracycline aminodisaccharides has been studied using high and low energy collisionally activated dissociations (CAD). The mass spectra and low energy CAD spectra obtained using electrospray ionization on a quadrupole mass spectrometer were consistent with a structure in which positions of the two monosaccharide residues were reversed. Key fragment ions observed in the product ion spectra of the peracetylated derivative and the deuterated analog were used to provide evidence that a rearrangement process had occurred. Mass-analyzed ion kinetic energy spectrometry data were also consistent with a rearrangement.

Relationship between effective temperature of thermalized ions and ion source temperature

The spontaneous dissociation of mixed proton bound dimers of nitrile bases has been studied using mass-analyzed ion kinetic energy spectra in a reverse geometry double focusing instrument coupled to a high pressure ion source. A linear correlation was found between the relative ion abundance ratios of the fragments and the ion source temperature. Proton exchange equilibria between the bases were studied using the time resolved capability of the high pressure mass spectrometry system, and relative ΔG°, ΔH°, and ΔS° values were extracted from the van't Hoff plots. The proton affinities (PA) of the nitriles studied were found to be PA(MeCN) = 186.8 kcal/mol, PA(EtCN) = 189.9 kcal/mol, PA (n-PrCN) = 191.0 kcal/mol, and PA(i-PrCN) = 192.2 kcal/mol. The effective temperatures of the metastable protonated nitrile dimers at different ion source temperatures were obtained from a plot of In(I RCN/I ref.) versus ΔGB (relative gas phase basicity). From a plot of the effective temperature of the protonated metastable nitrile dimers reacting in the second field free region of the mass spectrometer versus the ion source temperature, it was found that the effective temperature decreases with increasing ion source temperature, which can be explained from qualitative considerations of the relevant thermal Boltzmann distributions.

Mass spectrometry study on five‐substituted pyrroles and the related N‐methylene‐ 1,3,4‐thiodiazoles

Electron impact mass spectra of nine new substituted pyrroles are reported. Apart from compounds 1a and 1b, the other seven compounds produce R1--Ph--C≡O+ ions as base peaks. Except for compounds 2a–c, each compound may produce [M−EtOH]∔, [M−OEt]+ and other fragment ions. The proposed fragmentation mechanisms of the six new compounds giving M∔ ions have been supported by mass analysed ion kinetic energy spectrometry and accurate mass data. No molecular ion peaks of compounds 2a–c are observed under electron impact conditions. However, very intense protonated molecule peaks of compounds 2a–c were observed under fast atom bombardment conditions together with [M+H − EtOH]+ ions. By D-exchange experiments, it was verified that the proton is localized on the N atom between the carbonyl and thiocarbonyl groups during the formation of protonated molecules. © 1998 John Wiley & Sons, Ltd.

Fast-atom bombardment tandem mass spectrometry of cyclic nucleotide analogues used as site-selective activators of cyclic nucleotide-dependent protein kinases.

The mass spectrometric behaviour of six cyclic nucleotide analogues which activate cyclic AMP-dependent protein kinase was studied by positive-ion fast-atom bombardment (FAB) and collision-induced dissociation (CID) mass-analysed ion kinetic energy (MIKE) spectrometry. The compounds studied were 1,N6-ethenoadenosine-3',5'-cyclic monophosphate, (epsilon-cyclic AMP) and 2'-aza-1,N6-ethenoadenosine-3',5'-cyclic monophosphate, which each activate both isoforms of cyclic AMP-dependent protein kinase and have similar affinity for both the 'fast' and the 'slow' regulatory site of each isoform, N6-phenyl-cyclic AMP, which is selective for the 'fast' regulatory site of each isoform, and 6-chloropurine riboside-3',5'-cyclic monophosphate, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3',5'-cyclic monophosphate and 8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphate, which are each selective for the 'slow' regulatory site and preferentially activate isoform II. The FAB- and CID/MIKE spectra of the analogues are discussed in relation to their use in studies of the regulation of protein kinase activity by quantitative FAB mass spectrometry.

Electron ionization-induced loss of SO2 from 2-nitrodiaryl sulfides

Electron ionization-induced loss of SO2 from 2-nitrodiphenyl sulfide leads to the same ionic structure, or mixture of structures, as loss of N2 from the molecular ion of N1-phenylbenzotriazole. Ab initio calculations are in favor of the [C6H4—N—C6H5]+• distonic nitrenium radical cation instead of the N-phenylbenzo-Δ2-azirinium structure for the product ion. Collisionally activated dissociation mass-analyzed ion kinetic energy (CAD-MIKE) spectra of the four chloro isomers [3- or 4-Cl—C6H3—N—C6H5]+• and [C6H4—N—C6H4-3 or 4-Cl]+• show that these ions are non-interconverting structures and eliminate regioselectively their nitrene moiety. Comparison with the CAD-MIKE spectra of the product ions of the elimination of N2 from N1–3(or 4)-chlorophenylbenzotriazoles demonstrates that the loss of SO2 from 2-nitrophenyl aryl sulfides occurs in the gas phase via an ipso rearrangement involving a [1,4] migration of the aryl group. © 1998 John Wiley & Sons, Ltd.

A tandem mass spectrometry study of trifluoroacetic acid using chemical ionization with methanol and water

Mass analyzed ion kinetic energy spectra (MIKES) were scanned for the following ion species generated with water or methanol as reagent gases under chemical ionization conditions: [CF3CO2CH3H]+, [CF3CO2HH]+, [CF2CO2CH3]+, [CF3CO]+, [CF2CO2H]+, [CF3OH2]+ and [CF3OH]+·. Unimolecular decompositions of the [CF3CO2CH3H]+, [CF3CO2H2]+ and [CF3COH2]+ ions were all associated with a dominant loss of an HF molecule. In labeling experiments, an isotopic kinetic effect was evident for the unimolecular loss of HF from the [CF3CO2HD]+ species, in that the intensity of the peak produced by the HF loss exceeded that pertaining to DF loss by a factor of 10. Further unimolecular transitions were found for protonated trifluoroacetic acid at 87 → 67 and 86 → 67. In the MIKE spectrum of [CF2CO2CH3]+ ions derived from protonated trifluoroacetic acid methyl ester, a dish-shaped peak corresponding to 15 kcal/mol kinetic energy release was found for the loss of CO. On introduction of collision gas, additional characteristic product ions were identified, and their compositions were established using deuterium labeled isotopic variants of the parent ions. © 1998 John Wiley & Sons, Ltd.

Unimolecular Decompositions of 2,2,2-Trifluoroethyl Acetate, 2,2,2-Trifluoroethyl Trifluoroacetate, and S-Ethyl Trifluorothioacetate upon Electron Impact.

The unimolecular decompositions of the metastable ions produced from 2,2,2-trifluoroethyl acetate, CH3COOCH2CF3, 1, (Mw: 142), 2,2,2-trifluoroethyl trifluoroacetate, CF3COOCH2CF3, 2, (Mw: 196), and S-ethyl trifluorothioacetate, CF3COSCH2CH3, 3, (Mw: 158) upon electron ionization have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. The metastable ions 1+ decompose exclusively to give the ion at m/z 122 by losing HF via an 1,2-elimination.In the normal mass spectrum of 2, no molecular ion is observed, while the ion corresponding to the loss of CF3 gives rise to an intense peak at m/z 127 (about 90% of the base peak) and the ion corresponding to the loss of F gives the peak at m/z 177 with moderate intensity (about 20%). Both ions consist of two isomeric ions. Some interesting rearrangement processes, such as CF3 or OCH2CF3 migration followed by the loss of a CO molecule, occur during the metastable fragmentations of these ions.The molecular ion of 3, which is a base peak, decomposes into the ions at m/z 89 and 61 by losing CF3 and CF3CO, respectively, in the metastable time region. The relative abundance of ions in the MIKE spectrum are rationalized with the thermochemical data estimated by the PM 3 method.

Mass spectrometric fragmentation reactions of aliphatic carboxamides. A reinvestigation using metastable ions reveals new rearrangements

The mass spectrometric fragmentations of aliphatic carboxamides have been reinvestigated by using caproamide 2 and its N-methyl- and N,N-dimethyl derivatives 3 and 4 as model compounds and by studying the unimolecular reactions of metastable molecular ions. Whereas the EI (70 eV) mass spectra of caproic acid 1 and the amides 2-4 suggest a very similar behavior of the unstable molecular ions of these compounds, the mass analyzed ion kinetic energy(MIRE) spectra of metastable molecular ions indicate distinct differences. Of the three fragmentation pathways observed in the MIKE spectra, by far the most important for the molecular ions of the acid 1 is the loss of an ethyl radical which results in the formation of protonated methacrylic acid by a 1,2-shift of the protonated carboxyl group in intermediate distonic ions. The metastable molecular ions of the amides 2-4 also fragment to a significant extent by loss of a propyl radical and by the McLafferty rearrangement, the latter in particular for the tertiary molecular ions of 4. An even more significant difference is the formation of protonated vinylacetamides instead of protonated methacrylamides from the molecular ions of the amides 2-4. An analysis of the MIKE spectra of specifically deuterated derivatives of 2-4 shows that first, in contrast to the reactions of metastable molecular ions of the acid 1, the McLafferty rearrangement of the amide ions 2-4 occurs without significant H/D exchange reactions involving D atoms at the gamma-position. This implies that the McLafferty rearrangement of the amide ions is energetically more favored. Second, it is shown that If-shifts from the alkyl chain to the amide group are followed by 1,5- and 1,6-shifts (long-distance shifts) of the protonated amide group in the intermediate distonic ions. Thus, the metastable molecular ions of aliphatic carboxamides are mainly distinguished from those of the corresponding carboxylic acids and methyl esters by a preference for a fragmentation by the McLafferty rearrangement and for long-distance shifts of the protonated functional group in intermediate distonic ions.

Stability of secondary structural elements in a solvent-free environment. II: The β-pleated sheets

The stability of single β-strands and multistrand β-pleated sheets as elements of secondary structure is examined in the absence of intermolecular interactions. Such experimental conditions (e.g., complete removal of solvent molecules and counterions) are achieved by placing the peptide ions in the gas phase. The metastable multiply- charged peptide ions produced by electrospray ionization undergo unimolecular dissociation. Intercharge repulsion within the precursor ions gives rise to the elevated kinetic energy of fragment ions, which is measured using Mass-analyzed Ion Kinetic Energy (MIKE) spectrometry. Intercharge distances calculated based on these measurements are compared to the numbers derived from molecular mechanics calculations with charge site assignments based on relative proton affinities. Evidence is presented suggesting that single β-strands form collapsed structures in the absence of solvents, while multistrand β-pleated sheets are likely to retain “native-like” secondary structures under the same conditions. These results indicate that intramolecular hydrogen bonds are the major factor determining the three-dimensional arrangements of polypeptides in the gas phase, compensating both long- and short-range electrostatic repulsions. This is in good agreement with our earlier findings (Proteins 27:165–170, 1997) concerning stability of helical conformation of melittin in the absence of solvent. Proteins Suppl. 2:22–27, 1998. © 1998 Wiley-Liss, Inc.

The gas-phase reactions of the methoxymethyl cation with aldehydes and ketones

The results of both mass-analysed ion kinetic energy spectroscopy and Fourier transform ion cyclotron resonance experiments involving reactions of the methoxymethyl cation CH3OCH2+ with a variety of aldehydes and ketones are reported. With ketones, the reaction yields a covalent complex whose dissociation either gives back the CH3OCH2+ cation or leads to a methyl cation transfer to the ketone. Except for CH2O, a third pathway is open with aldehydes. A hydride transfer to CH3OCH2+ yields a RCO+ acylium product ion. The branching ratio of these three pathways strongly depends on the structure of the aldehyde. Ab initio calculations confirm that the results can be explained by the interconversion of covalent structures and electrostatic complexes on the potential energy surface.

Participation of the nucleobases in the regioselective backbone fragmentation of nucleic acids. A molecular dynamics and tandem mass spectrometric investigation on a model dinucleoside phosphotriester

The anions (I–III) obtained from O-methyl 5′-O-(5′-deoxythymidine) 3′-O-(2′,3′-dideoxyuridine) phosphate by the competitive removal of the 3-N-H protons of the nucleobases and of the methyl group from the phosphotriester bond, assume in the gas phase stable conformations as a function of their charge site. The mass-analyzed ion kinetic energy (MIKE) spectra of I and III show that the regioselective backbone cleavage of the internucleotidic linkage is controlled by the 2′-H proton transfer to the nucleobase within the 5′-end nucleoside. Similar pathways are taken by species II when the nucleobase is eliminated as neutral from the 5′-end nucleoside.

Elimination of the multiple collision effect from a mass-analyzed ion kinetic energy profile in collision-induced dissociation of high mass ions

It has been found that the multiple collisions distort and broaden a daughter ion kinetic energy profile significantly in the collision-induced dissociation (CID) of high mass ions. Their contribution to a daughter ion profile, both the relative yield and the peak position, has been analyzed using the probability theory. Based on this, an algorithm has been developed to eliminate the multiple collision effect from an experimental spectrum, which is essentially a deconvolution technique utilizing the parent ion beam profile. The scattering angular distribution calculated with the deconvoluted daughter ion profile has been virtually independent of the collision gas pressure, indicating a successful elimination of the multiple collision effect.

Fast atom bombardment of some asymmetric compartmental [1 +1] Schiff bases

New compartmental ligands (L), containing a N2O2 or N3O2 Schiff base and an O2O4, O2O5 or O2O6 ‘crown like’ coordination site, have been prepared by reaction of the appropriate diformyl- and polyamine precursors. The related amine derivatives have been obtained by reduction of the imine analogues with NaBH4. The mass spectrometric behaviour of these compounds has been studied by fast atom bombardment mass spectrometry, which mainly leads to the facile production of protonated [MH]+ and cationized [MNa]+ molecules. The mass analysed ion kinetic energy spectra of these ionic species show that the primary fragmentation patterns strongly depend on the crown dimensions and on the imine–amine structure of the nitrogen-containing chain. Macrocyclic rare earth complexes Me(L)(H2O)4(Cl)3 (Me = Ce, Y), prepared by a template procedure, have been also studied under the same experimental conditions, proving that the presence of the metal ion leads to a strong reduction of the number of primary decomposition pathways. © 1997 John Wiley & Sons, Ltd.

Mass spectrometric study on methyl 5-methyl-2-oxo-3-[2-(4-R-phenyl)-1-ethyl]cyclopentanecarboxylates and methyl 2-oxo-5-(4-R-phenyl)-3-propylcyclopentanecarboxylates

The mechanisms of mass spectrometric fragmentation of eight new compounds, including methyl 5-methyl-2-oxo-3-[2-(4-R-phenyl)-1-ethyl]cyclopentanecarboxylates (Group I, R = substituent) and methyl 2-oxo-5-(4-R-phenyl)-3-propylcyclopentanecarboxylates (Group II, R = substituent), were studied using mass analyzed ion kinetic energy spectrometry. The chemical compositions of these compounds and some of their important fragment ions were verified by high-resolution MS data. There were two types of H-rearrangement as regards the route of fission. In one type, the γ-H migration was influenced by both the electron effect and the steric hindrance of substituents, whereas on the other α-H migration losing methanol was due to the α-position active hydrogen located between two carbonyls, and this was verified by the method of deuterium labelling. © 1997 John Wiley & Sons, Ltd.

3-Aroyl-5-hydroxyflavones: synthesis and mechanistic studies by mass spectrometry

The synthesis and mass spectra of three 3-aroyl-5-hydroxyflavones are reported. The interpretation of the mechanistic pathways for the fragmentation of the metastable molecular ions of these compounds was achieved through the analysis of their mass-analysed ion kinetic energy (MIKE) spectra and of the B 2 /E spectra of a few fragment ions. Labelling of the hydroxyl proton with deuterium and the analysis of the MIKE spectra of a model compound with chlorine atoms in the 2',6'- and 2,6-positions led to a mechanism for the losses of OH . and HCO . which involves hydrogen migration from the 2- or 6-position to the 4-carbonyl oxygen atom. A mechanism for the loss of a neutral molecule of anisole from the molecular ion of the 3-aroyl-5-hydroxyflavone with a methoxyl group in the 4'- and 4-positions is also suggested. For the flavones with hydrogen or chlorine substituents at these positions, loss of a phenyl (or chlorophenyl) radical occurs instead.

Fragmentation of organosulfur compounds upon electron impact. Part III. Metastable decomposition of the molecular ions of methyl thioglycolate and ethyl thioglycolate

The metastable decompositions of the molecular ions of methyl thioglycolate (1) and ethyl thioglycolate (2) were investigated by means of mass analyzed ion kinetic energy (MIKE) spectra and deuterium labeling. The loss of methanol is the only metastable decomposition of 1+·. This fragmentation occurs via two distinct pathways. The molecular ions of 2 decompose in a variety of ways, i.e., the losses of water, ethene, ethanol or Ċ2H3O2. All of these decompositions, except the loss of ethene, occur through two distinct mechanisms. During the loss of Ċ2H3O2, the ethyl group or ethene migrates from the oxygen to the sulfur atom. The loss of HĊS, which corresponds to the loss of HĊO with a concomitant double hydrogen transfer observed in the case of methyl glycolate (3), does not participate in the metastable decomposition of 1+· and 2+·. This is due to the energetic favorableness of the loss of methanol.

Routes of Formation and Decomposition of them/z59 Ions with Elemental Composition C3H7OGenerated upon Electron Impact Ionization of Methyl Propanoate

It is shown that the peaks at m/z 31, 29 and 15 in the mass-analyzed ion kinetic energy spectrum of the m/z 59 ions generated from methyl propanoate upon electron impact ionization originate from decompositions of m/z 59, i.e. C2H3O2+ ions having the O=C=O+CH3 structure, and of m/z 59, i.e. C3H7O+ ions having the CH3CH=O+CH3 structure. The latter ions contribute <1% to the total abundance of the m/z 59 ions in the standard mass spectrum. However, these m/z 59 C3H7O+ ions are responsible for the relatively significant amounts of the m/z 31 and 29 ions generated in the microsecond time frame. Based upon deuterium labelling and supported by calculations at the ROHF/6–31G* level of theory, the m/z 59 C3H7O+ ions are suggested to be formed via a complex skeletal rearrangement of the molecular ions of methyl propanoate and subsequent loss of a formyl radical. © 1997 by John Wiley & Sons Ltd.