Mass-analysed Ion Kinetic Energy Spectra Research Articles

Distinction of Amino Acid Enantiomers Based on the Basicity of Their Dimers

Mixtures of several amino acid pairs, in all four chiral combinations, were studied. The protonated trimers (A(2)BH(+)) fragment, forming ABH(+) and A(2)H(+) dimers. Abundance ratios of these fragments were measured in the mass-analyzed ion kinetic energy spectra of the trimers. These were found to depend on the stereochemistry (homo- or heterochiral form) of the ABH(+) dimer. The results were evaluated using the kinetic method, and the chiral discrimination was related to a difference in gas-phase basicity (GB) between the homo- and the heterochiral dimers. Four amino acid pairs (proline-tryptophan, phenylalanine-alanine, phenylalanine-proline, and phenylalanine-valine) were studied. Chiral discriminations were observed in all cases, relating to 0.4-4 kJ/mol differences in GB. The technique described here can generally be used to study enantiomers by mass spectrometry and is capable of reliably distinguishing energy differences as small as 0.2 kJ/mol in cluster ions.

Estimation of cytidylyl cyclase activity and monitoring of side-product formation by fast-atom bombardment mass spectrometry.

The enzyme cytidylyl cyclase catalyses the conversion of cytidine 5'-triphosphate into cytidine 3',5'-cyclic monophosphate, a third naturally occurring cyclic nucleotide currently under investigation to assign a biochemical function. Quantitation of the activity of this enzyme has been carried out by the positive-ion fast-atom bombardment mass spectrometric analysis of the enzyme incubation mixture after the reaction has been terminated. The data obtained are in good agreement with those obtained from the conventional radiometric and radioimmunoassays of the same enzyme preparations. The advantage of the mass spectrometer-based assay is the facility for multiple component monitoring. Thus, the production of the cytidine diphosphates and monophosphates, and the production of four cytidine 3',5'-cyclic monophosphate analogues as side-products, were simultaneously estimated. The identities of two of the side-products, 2'-O-glutamyl- and 2'-O-aspartyl-cytidine-3',5'-cyclic monophosphate, and of the cytidine 3',5'-cyclic monophosphate product, were confirmed by mass-analysed ion kinetic energy spectra from the collision-induced dissociation of the protonated molecules.

Mass spectrometry of tert-butylnaphthalenes–a comparison with the unimolecular fragmentation of tert-butylbenzene

The unimolecular fragmentation of ionized 1- and 2-tert-butylnaphthalenes 1 and 2 have been investigated using mass analyzed ion kinetic energy (MIKE) spectrometry and specifically deuterated and 13 C labeled derivatives. Generally, the mass spectrometric fragmentations of 1 and 2 follow closely the reactions known for ionized tert-butylbenzene 3. Metastable molecular ions of 1, 2 and 3 lose exclusively a methyl radical yielding 2-naphthyl-2-propyl cations 1a and 2a or 2-phenyl-2propyl cations 3a. Further fragmentations of 1a and 2a include, as the main processes, the elimination of C 2H4 and of CH3 • . The loss of C2H4 is associated with a large kinetic energy release giving rise to a broad flat-topped peak in the MIKE spectra (as in the case of 3a) while loss of CH3 gives rise to a Gaussian shaped signal. The latter reaction is not observed for metastable 2-phenyl-2-propyl cations 3a. The fragmentations of labeled derivatives of 1 and 2 show that H/D exchange in 1a and 2a, between the methyl groups of the side chain and the naphthalene ring, preceding the losses of C 2H4 and of CH3 • , is much more extensive than in 3a. This H/D exchange decreases for metastable 1a and 2a decomposing in the second field-free region, and the interchange of the C atoms within the C 3H6 is less than for 3a. These observations are explained by H migrations between the side chain and the naphthalene rings of 1a and 2a, before the rearrangements within the side chain by the series of reversible 1,2-H shifts, of a 1,2-aryl shift, and of a 1,2-methyl shift known for the reactions of 3a. The differences observed for the reactions of 1a and 2a and of 3a are explained by the more basic aryl group of 1a and 2a. These fragmentation mechanisms are corroborated by an AM1 calculation of the minimum energy reaction pathways (MERP).

Liquid secondary ion mass spectrometry of methyl glycosides of oligosaccharides using matrices containing carboxamides.

Intense cluster ions corresponding to proton-bound hetero-dimers of an amide molecule and an oligosaccharide molecule are observed in the liquid secondary ion mass spectra of methyl glycosides of oligoxylans if a solution of an aliphatic carboxamide in glycerol is used as the liquid matrix. These cluster ions are particularly abundant and persist for a long period if urea (U) or thiourea (TU) is used as the matrix additive. In these cases, cluster ions containing more than one molecule of U or TU and two oligosaccharide molecules are also observed. The intense signal due to the proton-bound hetero-dimer between U or TU and the oligosaccharide can be used with advantage for a molecular weight determination. The bonding interactions between a protonated saccharide molecule and a molecule U or TU in the proton-bound hetero-dimers are so strong that the urea molecules remain attached to the fragment ions during the decay of metastable cluster ions and even during collision-induced dissociation. Thus, the mass-analysed ion kinetic energy spectra of these proton-bound hetero-dimers are dominated by abundant cluster ions [Bn+U] and [Ym+U] arising from cleavage of the glycosidic bonds within the oligosaccharides. The collisionally-activated mass spectra of the proton-bound hetero-dimers additionally contain peaks of the free ions Bn and Ym. Therefore, these spectra clearly reflect the arrangement of the monosaccharide residues in the oligosaccharide and can be used conveniently for structural analysis.

Characterization and differentiation of heterocyclic isomers. Part 2 . Mass spectrometry and molecular orbital calculations on pyrrolo[1,2-a][1,4]benzodiazepin-4-one, -6-one, and -4,6-dione

Pyrrolo[1,2-a][1,4]benzodiazepin-4-one (1), -6-one (2), and -4,6-dione (3), which are starting materials for the synthesis of pharmacologically interesting compounds that are active as neurotropic agents, have been characterized in the gas phase. The application of different mass spectrometric techniques, such as electron ionization, high-resolution, and tandem mass spectrometry, has allowed the structural characterization and differentiation of their molecular ions and most abundant fragment ions formed in the source. In particular, the two positional isomers 1 and 2 produce quite different mass spectra, and their molecular and the most intense fragment ions yield different metastable mass-analyzed ion kinetic energy spectra. Furthermore, high-resolution mass spectrometry and accurate mass measurements have revealed different elemental compositions and abundances for isobaric fragment ions produced by isomers 1 and 2. From these data and from the comparison with those relevant to compound 3, it has been possible to evaluate the influence of the position of the carbonyl group on the fragmentation pathways.Semiempirical molecular orbital calculations carried out by both the modified neglect of differential overlap and Austin 1 methods have provided useful information on the characterization of the neutrals as well as the molecular ions of compounds 1-3.

A Mass Spectral Investigation of Macroheterobicyclic Crown Ethers and their Lithium and Potassium Complexes in the Gas Phase

Mass spectral measurements of complex ions MX+ formed from a bicyclic-molecule (M), which comprises a macroheterocyclic crown ether and a polyoxyethylene chain linked with two thioamido groups on a diaza-3n-crown-n-poly-ether, together with an X+ ion (X=H, Li or K) are discussed. The use of fast-atom/ion bombardment mass spectra and particularly collision-induced dissociation and mass-analysed ion kinetic energy spectra enabled an estimate of the site of the localization of X+ and the relative bond strength inside the collisionally-activated host–guest complex ion in the gas phase to be made.

Reaction dynamics of the four-centered elimination CH2OH+→CHO++H2: Measurement of kinetic energy release distribution and classical trajectory calculation

Mass-analyzed ion kinetic energy (MIKE) spectrum of CHO+ generated in the unimolecular dissociation of CH2OH+ was measured. Kinetic energy release distribution (KERD) was evaluated by analyzing the spectrum according to the algorithm developed previously. The average kinetic energy release evaluated from the distribution was extraordinarily large, 1.63 eV, corresponding to 75% of the reverse barrier of the reaction. A global analytical potential energy surface was constructed such that the experimental energetics was represented and that various features in the ab initio potential energy surface were closely reproduced. Classical trajectory calculation was carried out with the global analytical potential energy surface to investigate the causes for the extraordinarily large kinetic energy release. Based on the detailed dynamical calculations, it was found that the strained bending forces at the transition state and strengthening of the CO bond from double to triple bond character were mainly responsible for such a significant kinetic energy release. In addition, the dissociation products H2 and CHO+ ion were found to be rotationally excited in the trajectory calculations. This was attributed to the asymmetry of the transition state and the release of asymmetric bending forces. Also, the bending vibrational modes of CHO+ and the H2 stretching mode, which are coupled with the bending coordinates, were found to be moderately excited.

Electron Impact Induced Fragmentation of Dibenzo Crown Ethers

Six structurally related crown ethers have been studied by electron impact (EI) mass spectrometry. From accurate mass measurements, metastabel ion and mass-analyzed ion kinetic energy (MIKE) spectra, plausible fragmentation pathways were deduced. All compounds showed very abundant molecular ions and some amount of the doubly charged molecular ions. The primary fragmentations always involved the catechol moiety and consequent loss of oxirane although the fragment(ation)s also showed a clear dependence on the functionalities X in one of the crown ether segments, —OCH2XCH2O— (compound/X: 1/—CH2—, 2/—C=C—, 3/trans—CH=CH—, 4/cis—CH=CH—, 5/—HC—CH—, and 6/—CH2CH2—).

Electron impact mass spectra of δ-keto enol esters

Enol esters with a remote keto function in the δ-position produce upon electron impact (EI) (70 eV) molecular ions of very low abundance (< 1%) due to two fast main fragmentations. The first one involves loss of a carboxylic acid fragment from the α-carbon and subsequent cyclisation to a stable furan ion. A second, more abundant, fragmentation leads, by loss of ketene and an acyl moiety, to a protonated α,β-enone. The stability of this ion is governed by the substituents in the βand γ-positions, which thus determine the order in which a ketene or an acyl radical is lost. Mass-analysed ion kinetic energy (MIKE) spectra of the most abundant ions were also recorded. In one case, a perfect match was found between the collisional activation (CA) mass spectrum of the most abundant ion of one enol-ester under EI conditions and the CA mass spectrum of the corresponding protonated α,β-enone obtained under chemical ionisation (CI) conditions. The fragmentation mechanisms were also inferred from the spectra of two deuterium-labelled δ-keto enol esters.

Studies in organic mass spectrometry. Part 20: a hidden ortho effect in the electron ionisation mass spectra of some 2′-alkyl substituted 2-and 3-thiophenecarboxanilides

The electron-ionisation-induced amide-bond cleavage of some 2′-methyl- and 2′-ethyl-substituted 2- and 3-thiophenecarbox-anilides, which yields formally anilylium ions having relative intensities apparently in contrast with the Stevenson–Audier rule, has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry and compared to that of the 3′- and 4′-isomers. It has been shown that, in the case of the 2′-methyl and 2′-ethyl derivatives, the amide-bond cleavage is anchimerically assisted through the hidden migration of a benzyl hydrogen to the nitrogen. Analysis of the MIKE and collision-induced decomposition (CID) MIKE spectra of model compounds indicates that this cryptic ortho effect produces a stable ortho quinoide or aminotropylium structure.

Fragmentation of organosulfur compounds upon electron impact: 2-mercaptoethanol and 1,2-ethanedithiol

The spontaneous unimolecular dissociations of 2-mercaptoethanol ( 1) and 1,2-ethanedithiol ( 2) have been investigated by mass-analyzed ion kinetic energy (MIKE) spectra, collision-induced dissociation (CID) spectra and deuterium labeling. Most of the fragmentation pathways of 1 and 2 are similar to those of ethylene glycol ( 3), but there are exceptions. The results are rationalized by thermochemical considerations. Protonated methanethiol (CH 3SH 2 +; m/ z 49) is produced from 1 and 2, and protonated methanol (CH 3OH 2 +; m/ z 33) is also generated from 1. The latter is a minor route in comparison with the case of 3. The m/z 49 ion contains both hydroxyl and mercapto hydrogens of 1 +. The former is contained in the methyl group, and one of the β-methylene hydrogens is bonded to the sulfur atom.

Radical cation adduct formation between dihydroxybenzenes and ammonia under CI conditions

AbstractUnder ammonia chemical ionization conditions, instead of the expected [M + H]+, and [M + NH4]+ and [M + N2H7]+ ions, hydroquinone gives rise to M+·, [M + NH3]+· and [M + 2NH3]+· ions, whereas resorcinol gives both series of ions. Catechol shows M+·, [M + NH3]+·, [M + NH4]+ and [M + N2H7]+ ions. The favourable ionization energy difference between ammonia and the dihydroxybenzenes leads to charge exchange and the resulting radical cations, being highly acidic, form clusters with the NH3. The effect of ion source parameters and mass‐analysed ion kinetic energy (MIKE) and collisional activation spectra confirm proton‐bound structures for [M + NH3]+· and [M + 2NH3]+ ions. Because of the proximity of the two OH groups, NH4+ attachment is favoured in catechol. The MIKE spectra of [M + H + Et2O]+ ions show that resorcinol has the highest proton affinity among the dihydroxybenzenes.

Identification of novel nucleotides found in the red seaweed Porphyra umbilicalis

Extracts of the red seaweed Porphyra umbilicalis are a valuable constituent of herbal medicines. The nucleotide complement of such an extract was purified and compounds identified by chromatographic behaviour, UV absorbance/pH profile, base-to-phosphate ratio, acid hydrolysis, and by fast-atom bombardment mass spectrometry with mass-analysed ion kinetic energy spectrum scanning. In addition to eighteen common nucleotides, and two cyclic nucleotides, fifteen novel nucleotides were identified, comprising eight deoxynucleotides and two cyclic deoxynucleotides, ten aminoacylnucleotides and two nucleoside trisphosphates together with 2'-phosphoadenosine-3',5'-cyclic pyrophosphate, 5'-phosphoadenosine-2',3'-cyclic monophosphate and N6,N6-dimethyladenosine-5'-monophosphate. The possible origin and potential actions of these novel compounds are discussed.

Determination of artefact peaks in mass‐analysed ion kinetic energy spectra using a new linked scan for ebe geometry magnetic sector instruments

AbstractThe possible presence of artefact peaks in mass‐analyzed ion kinetic energy (MIKE) spectra has been recognized when such scans are performed using a two‐sector instrument of reverse (BE) configuration. Since the presence of an artefact peak can be misleading when interpreting a spectrum, a new scan is proposed which allows these artefacts to be identified. This scan involves linking and scanning the two electrostatic sectors of an E1BE2 geometry instrument, while maintaining the magnet at the mass selected for the MIKE scan. This selected mass‐analyzed ion kinetic energy (SMIKE) scan at E1=E2 results in a spectrum containing only artefact peaks that can be present in the corresponding MIKE spectrum.

Fragmentation processes of 1,2‐diphenoxyethane and 2‐phenoxyethanol by electron impact

AbstractThe unimolecular fragmentation processes of 1,2‐diphenoxyethane (1) and 2‐phenoxyethanol (2) upon electron impact were investigated by a combination of mass‐analyzed ion kinetic energy (MIKE) spectrometry, deuterium‐labelling, high resolution mass spectrometry, and kinetic energy release (KER) measurement. The MIKE spectrum and the KER measurement indicate that the predominant fragment ion at m/z 120 was generated from metastable 1+· ions via loss of a phenol molecule (C6H6O, 7), and that it has the same structure as the molecular ion of 2,3‐dihydrobenzofuran (C8H8O, 3). The metastable 2+· ion eliminates C2H4O and C2H3O to produce product ions at m/z 94 (C6H6O+·, 7+·) and 95 (C6H7O+) with single and double hydrogen atom transfer, respectively. These reactions occur through some complex rearrangement reactions.

A study of protonated molecules of crown ethers containing two thiourea segments by collision‐induced dissociation mass analysed ion kinetic energy spectrometry

AbstractFast atom/ion bombardment mass spectrometry was used to investigate the structures of protonated molecules of macrocyclic nitrogen‐containing crown ethers with two polyoxyethylene chains linked by two thiourea groups. The collision‐induced dissociation (CID) of their MH+ ions, using the mass analysed ion kinetic energy (MIKE) spectrometry method reveals four clearly defined competitive fragmentations. The product‐ion intensities clearly show that, in the MH+ ions of asymmetric crown ethers, the proton is preferentially located in the region of the longer polyoxyethylene chain. The characteristics of CID MIKE spectra of isomeric crown ethers are discussed.

Dissociation of proton-bound complexes and proton affinity of benzamides

Proton-bound heterodimers of substituted benzamides 1–15 and N,N-dimethyl benzamides 16–30, respectively, with a series of reference bases were generated under chemical ionization conditions. Their dissociation into the protonated amide AH + and protonated reference base BH + was studied by metastable ion techniques and by collision-induced dissociation (CID) to examine substituent effects on the proton affinity (PA) of the benzamides and to elucidate some aspects of the dissociation dynamics of proton-bound clusters. The PAs of the substituted benzamides were determined by bracketing the amide by a pair of reference bases to give rise to more and less abundant signals of the protonated base in the mass-analyzed ion kinetic energy (MIKE) spectra of the proton-bound heterodimers. The substituent effects observed agree with O-protonation in both the primary and the tertiary benzamides. However, the susceptibility of the benzamide to polar substituent effects is remarkably small, which indicates a “resonance saturation” of the amide group. The relative abundances of AH + and BH + in the MIKE and collisional activation (CA) mass spectra depend strongly on the pressure of the collision gas during CID, and in certain cases a reversal of the relative abundances with increasing pressure that favors the formation of BH + from a less basic reference base is observed. Although this effect underlines the limited possibilities of the “kinetic method” for PA determination by CID of proton-bound heterodimers, it uncovers important kinetic effects during the dissociation of proton-bound heterodimers and of proton transfer reactions in the gas phase. In the case of the protonated amide clusters, the observed intensity effects in the CA mass spectra are explained by a double-well potential energy surface caused by solvation of the protonated base by the polar amide in the protonated heterodimer.

Study of two diastereomeric pairs of regiosomeric 2‐isoxazolines by tandem mass spectrometry with electron impact ionization

AbstractOn electron impact (EI) ionization, two cis/trans pairs of 4‐methyl‐5‐phenyl and 4‐phenyl‐5‐methyl regioisomeric 3‐carbethoxy‐2‐isoxazolines showed normal mass spectra and mass‐analysed ion kinetic energy (MIKE) spectra of metastable (MI) and collision‐activated (CA) molecule ions, allowing unequivocal differentiation of the regioisomers. The cis/trans stereoisomers of each regioisomer showed very similar normal mass spectra. Very interestingly, the cis‐ and trans‐4‐phenyl‐5‐methyl stereoisomers appeared reasonably differentiated by the molecule ion MIKE spectra, whereas the 4‐methyl‐5‐phenyl regioisomeric pair of stereoisomers did not. The influence of the phenyl substituent to the fragmentation processes was notable. Some fragments of interest were studied by comparison of their MIKE spectra with those of model ions, generated by EI from suitable substrates, including (i) the isomeric α,β‐unsaturated oxime, namely ethyl (Z)‐2‐(hydroxymino)‐3‐methyl‐4‐phenylbut‐3‐enoate, a by‐product of importance for the mechanism(s) of the addition/cycloaddition reactions of nitrile oxides to alkenes and (ii) trans‐β‐methylstyrene, a dipolarophilic reactant in the same reactions. The favoured heterocyclic C(5)–O(1) bond cleavage occurred only for the ionized 4‐methyl‐5‐phenyl 2‐isoxazoline pair, leading to a distonic ion of relevance, as it can represent either a reasonable precursor for both the isomerization to the ionized α,β‐unsaturated oxime and the EI‐induced cycloreversion yielding ionized β‐methylstyrene, or the ionized form of a zwitterionic intermediate, which had been proposed previously for the addition/cycloaddition mechanism(s) in the solution phase, currently under study.

Electron impact mass spectrometry of some 5‐amino salicylic acid derivatives

AbstractThe electron impact‐induced fragmentation processes of eleven differently substituted 5‐amino salicylic acids were studied by means of mass‐analyzed ion kinetic energy (MIKE) spectroscopy, parent‐ion spectroscopy and accurate mass measurements. Most of the fragmentation processes are related to the cleavages of the 5‐amino substituents. Unusual behaviour is that related to the CO2 loss from the molecular ion, quite significant in the normal electron ionization spectra and completely undetectable in the MIKE spectra.

The addition of NF +2 to H 2O as a route to gaseous protonated F 2NOH

NF + 2 ions from the ionization of NF 3 efficiently add to H 2O (F 2NOH)H + under chemical ionization conditions. In keeping with the formation process employed, collisionally activated dissociation mass spectrometry indicates the exclusive formation of the oxygen-protonated isomer of F 2NOH. Ab initio calculations, at the gaussian-1 level of theory, indicate that this ion is the least stable among the investigated (F 2NOH)H + isomers. However, it is trapped in a deep potential well, which prevents extensive isomerization to the nitrogen-protonated and to the fluorine-protonated isomers, the latter one being by far the most stable protomer. The gaussian-1 potential energy profile accords well with the experimental features of the unimolecular decomposition of the (F 2NOH 2) + ions, probed by mass analyzed ion kinetic energy (MIKE) spectrometry. Only the loss of HF was detected, characterized by a dish-topped peak in the MIKE spectrum. Consistent with the ab initio calculations, the measured kinetic energy release is as large as 1.36 eV. The emerging picture has been compared with the previously reported results concerning the gas-phase protonation of NF 3.