Spectrométrie de Masse d'Hétérocycles Azotés. IX—Structure des Ions [M?CH2CO]+⋅ Formés à Partir des Ions Moléculaires d'Acétamido-2 Benzazoles et mixage d'Hydrogěnes avant leur Fragmentation

It has been established that the loss of ketene from the molecular ions of 2-acetamidobenzothiazole and 2-acetamidobenzimidazole proceeds via a four centre mechanism and gives the corresponding 2-amino-benzazole. For metastable ions with low energy and a long lifetime, hydrogen scrambling occurs before fragmentation through departure of HNC from the 2-aminobenzazole. A mechanism is proposed.     

Studies in organic mass spectrometry. XVI—A combined approach to the structures of ions generated from the molecular ions of acyclic β-diketones through loss of small neutral molecules

The structures of ions produced from acyclic 1,3-diketones by loss of carbon monoxide, ketene or olefins are investigated. Structural inference is made by comparison of the heats of formation of these ions with data for ions of known structure generated from other precursors. All assignments are confirmed by a study of metastable transitions. Loss of carbon monoxide yields ions in the keto form. Expulsion of a ketene or olefin moiety results in enol type ions.     

Metastable ion Characteristics. XXXXIII—separation and comparison of collisional activation spectra and metastable ion spectra

Additional ion acceleration in a separate collision chamber in the field free drift region makes it possible to add extra kinetic energy to those ions undergoing decomposition in the chamber. In this way product ions formed in the collision chamber (the [collisional activation] spectrum) can be distinguished from ions formed from decompositions in the low pressure portion of the field free drift region (the [metastable ion] spectrum). This technique is used to show that often a relatively small proportion of the ions in the [pure] collisional activation spectrum arises from processes which produce the metastable ion spectrum.     

Mass spectrometry of heterocyclic compounds. VII—Evidence for common structure(s) of 1,2-benzisothiazole and benzothiazole molecular ions produced by electron-impact

1,2-Benzisothiazole and benzothiazole ion kinetic energy spectra, and metastable ion relative abundances of the same primary and secondary decomposition processes are compared. The results are interpretable as postulating slow (metastable) process(es) involving common structures and fast process(es)which are structurally dependent. Analogous indications are given by the interring H/D scrambling data, preceding the loss of DCN (or HCN) from 1,2-benzisothiazole-3-d₁ and benzothiazole-2-d₁ molecular ions, measured in either metastable or normal daughter ions at various electron beam energies.     

Metastable atom‐activated dissociation mass spectrometry: leucine/isoleucine differentiation and ring cleavage of proline residues

Extensive backbone fragmentation resulting in a‐, b‐, c‐, x‐, y‐ and z‐type ions is observed of singly and doubly charged peptide ions through their interaction with a high kinetic energy beam of argon or helium metastable atoms in a modified quadrupole ion trap mass spectrometer. The ability to determine phosphorylation‐sites confirms the observation with previous reports and we report the new ability to distinguish between leucine and isoleucine residues and the ability to cleave two covalent bonds of the proline ring resulting in a‐, b‐, x‐, y‐, z‐ and w‐type ions. The fragmentation spectra indicate that fragmentation occurs through nonergodic radical ion chemistry akin to electron capture dissociation (ECD), electron transfer dissociation (ETD) and electron ionization dissociation mechanisms. However, metastable atom‐activated dissociation mass spectrometry demonstrates three apparent benefits to ECD and ETD: (1) the ability to fragment singly charged precursor ions, (2) the ability to fragment negatively charged ions and (3) the ability to cleave the proline ring that requires the cleavage of two covalent bonds. Helium metastable atoms generated more fragment ions than argon metastable atoms for both substance P and bradykinin regardless of the precursor ion charge state. Reaction times less than 250 ms and efficiencies approaching 5% are compatible with on‐line fragmentation, as would be desirable for bottom‐up proteomics applications. Copyright © 2009 John Wiley & Sons, Ltd.     

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

The mechanism of water elimination from metastable molecular and [M ? CH₃˙]⁺ ions, as well as from ions deprived of ring D, in epimeric 3-hydroxy steroids of the 5β-series has been elucidated by deuterium labelling, by the measurements of the translational energy released during loss of water, and by collision-induced decomposition mass-analysed ion kinetic energy spectrometry. It was found that the dehydration of the metastable molecular ion in 3α-hydroxy steroids of the 5β-series occurs mostly regiospecifically as an elimination of the 3α-hydroxyl together with the 9α-hydrogen atom. The ring A in the molecular ion has to flip to the boat conformation to make this reaction possible. In the metastable molecular ion of 3β-hydroxy steroids of the 5β-series a different dehydration mechanism operates, with very little participation of the 9α-hydrogen atom. The mechanisms of water loss from metastable [M ? CH₃˙]⁺ ions and from ions deprived of ring D differ from that of the molecular ion.     

Metastable ion studies. XII—Molecular and fragment ion structures for isomeric C4H6O2 acids

Metastable peak characteristics, ionization and appearance energy data and isotopic labelling experiments have been applied to a study of the fragmentation behaviour of the molecular ions of the isomeric C₄H₆O₂C acids, cis and trans-crotonic acids, methacrylic acid, butenoic acid and cyclopropane carboxylic acid. Prior to the losses of H₂O and CH₃, all the metastable molecular ions rearrange to [cis-crotonic acid]⁺? ions. Loss of H₂O, which generates a composite metastable peak, is proposed to yield vinylketene and/or cyclobutenone molecular ions. Detailed mechanisms are presented for the isomerizations of the various molecular ions and for the above fragmentations. Ionized 3-butenoic and cyclopropane carboxylic acids display a major loss of CO from their metastable ions, a minor process in the other isomers. The metastable peaks consist of two components and these are ascribed to the formation of propen-1-ol and allyl alcohol as daughter ions. Some comparative data are presented for the isomeric C₅H₈O₂ acids, tiglic acid, angelic acid and senecioic acid.     

Competing metastable decompositions of [C7H14]+· ions

The relative rates of competing metastable decompositions of fourteen isomeric C7H14 monoolefins were measured and compared. In every case except one the most important metastable reaction was loss of either CH3 or C2H4, but the rates of these and the other reactions observed varied over a wide range. It was concluded that the molecular ions of these compounds probably do not isomerize to a common structure prior to metastable decay. It was found that a terminal double bond strongly enhances metastable loss of C2H4 and that the additional presence of a 2-methyl substituent favours this reaction still more. Several possible mechanisms for this transition are discussed, but none was found to explain the observed results satisfactorily.     

Half protons or doubly charged protons? The history of metastable ions

The history of metastable ions is intertwined with that of ions formed by collision-induced dissociation (CID), and frequently the genesis of the two ion types cannot be strictly separated. Originally, metastable ions were considered a curiosity or even a nuisance, being responsible for "humps" in the base line of the recorded mass spectra. In their heyday metastable ions were recognized as having importance for establishing fragmentation sequences and for distinguishing between isomeric ion structures. Today, in many respects the utility of metastable ions has been superseded by a systematic application of CID techniques; yet the evaluation of their shape is still of importance for questions of reaction energies and ion thermochemistry.     

Fragmentation des δ-Dicetones,Analogies Avec le Comportement Photochimique

The mass spectra of thirty δ-diketones are reported. Their main pathways have been rationalized with the aid of deuterium labelling, metastable ions and high resolution mass measurements. Special attention has been given to ions resulting from the expulsion of neutral molecules such as water preceded, or followed by the loss of ethylene or alkenes. The expulsion may involve an intermediate cyclobutanol ion which would be analogous to the photochemical reaction observed by Yang.     

Mass spectrometric studies of structural isomers—II: Mono-and bicyclic C6H10 molecules

The electron-impact-induced ionization and fragmentation of six C₆H₁₀ structural isomers have been studied in order to determine the effect of isomerism upon their mass spectrometric behavior. The 70 eV mass spectra, metastable transitions and appearance potentials of the principal ions are reported. Significant differences between the mass spectra of the six isomers were observed; however, metastable transition and appearance potential data indicate that the fragmentation path-ways are the same for all the C₆H₁₀ molecules. Experimentally determined ionization potentials for the structural isomers are presented and compared to ionization potentials calculated by the bond orbital method. Utilizing fragmentation pathways deduced from general features in the mass spectra and from observed metastable transitions, we calculated heats of formation (ΔH_f) for the observed principal ions and compared these values to ΔH_f values for isomeric ions from other molecules.     

Metastable transitions in the mass spectra of methyl and phenylthiohydantoin derivatives of amino acids

The mass spectra of a number of methyl- and phenylthiohydantoin amino acid derivatives have been obtained. The major metastable transitions occurring in the mass spectra of these derivatives have been identified and measured. The major fragmentation pathways associated with the metastable transitions have been outlined and are discussed for each group of compounds. Inspection of the metastable data has shown that there is at least one unique metastable transition occurring for each thiohydantoin derivative which may be used to uniquely identify that derivative in the presence of a mixture of thiohydantoin derivatives obtained from the Edman degradation of a peptide or protein. The use of metastable ions to uniquely identify thiohydantoin derivatives in mixtures has proven useful in the identification of the MTH and PTH derivatives of glycine whose molecular ions are not unique and for resolving such ambiguities as occur for example in the mixture of leucine and isoleucine.     

Spectrométrie de Masse d'Hétérocycles Azotés. X–Contribution à l'Etude du Comportement de ll'Ion Aniline et des Ions Aminopyridines avant Fragmentation par Perte de HCN

The study of the mass analysed ion kinetic energy spectra of deuterated derivatives of aniline, aminopyridines and 2-chloro-5-aminopyridine shows that prior to HCN loss, hydrogen scrambling does not occur for aminopyridines and is limited but noticeable for aniline. In the case of this last compound the extent of scrambling varies markedly for small variations in the energy of the ions studied, these variations being within the energy window corresponding to metastable ions. Furthermore, an examination of the mass analysed ion kinetic energy spectra of monodeuterated derivatives of aminopyridines leads to the rejection of the generally accepted mechanism for HCN loss from the molecular ions of these compounds.     

Profil de Pics Métastables (Pertes de NO·) en Série Nitroindazolique

The kinetic energy releases associated with the loss of NO? from nitroindazole molecular ions have been measured. The results show that, although the mass spectra are nearly identical, the corresponding metastable peak shapes can be used to differentiate the five isomers.     

Kinetic energy release in ionic fragmentations: Use as an ion structure probe

The kinetic energy released in unimolecular reactions, as measured from the width of the corresponding metastable peak, shows only a small dependence on such parameters as source temperature, ion-source residence time and ion acceleration voltage. Similarly, fragmenting ions generated from different members of an homologous series of molecular ions have been found to release the same kinetic energy and hence do not exhibit a degrees-of-freedom effect analogous to that for metastable abundances. In general, molecular ions formed by electron-impact have been found to release slightly less kinetic energy on fragmentation than the corresponding ions formed via a fragmentation sequence. These observations suggest that kinetic energy release is a useful method of structural characterization of metastable ions; while increase in the average internal energies of the ions sample lead to larger energy releases, this effect is usually small. The use of a very narrow energy resolving (β) slit and a procedure in which the metastable peak width is extrapolated to zero slit width has been found to improve the accuracy of measurement of the kinetic energy release, particularly when the metastable and main beam peak widths are of comparable magnitude.     

Spectrométrie de Masse d'Hétérocycles Azotés. VII—Phenyl-1 Triazole-1,2,3

Comparison of the ‘normal’ and mass analysed ion kinetic energy spectra of 1-phenyl-1,2,3-triazole and a few D and ¹³C labelled derivatives indicates that the ions may follow two competing decomposition pathways: a simple rupture without rearrangement is preferred by the ions of high internal energy content (normal spectrum). On the other hand, for low internal energy metastable ions, loss of HCN is more important and occurs after a complete randomization of the H atoms.     

Translational energy release and stereochemistry of steroids. XI—Determination of the configuration of α,β-unsaturated 3-keto steroid alcohols with the hydroxyl group in rings C and D

The elimination of water from metastable molecular ions of epimeric hydroxy steroids of the Δ⁴-3-keto series containing a hydroxyl group in the conformationally rigid rings C and D has been studied. The measurement of translational energy released during the loss of water and collision-induced decomposition (CID) mass-analysed ion kinetic energy (MIKE) spectrometry were the techniques used. It was found that it is possible to determine the configuration of the hydroxy steroids of this series on the basis of the CID MIKE spectra of [M ? H₂O]^+˙ ions formed by dehydration of metastable molecular ions in the first field-free region of a reversed geometry double-focusing mass spectrometer.     

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.     

The dependence of collision induced fragmentation pattern on the internal energy of the precursor ion

A Method is presented whereby product organic ions formed as the result of fragmentation of metastable ions can be selected on the basis of their internal energies. The method requires requires angular collimation of the beam of reactant ions issuing from the ion source and that the fragmentation of the metastable ions is studied in the first field free region of a reversed geometry double focusing mass spectrometer. The product ions that make up the metastable peak are allowed to fall on the intermediate resolving slit and, by adjusting the magnet current over a small range, ions contained in different regions of the peak can be allowed into the second field free region. It is shown that the position of an ion within the metastable peak correlates with its internal energy, ions near the edges of the peak being the least excited. The ions entering the second field free region can be investigated by collisional activation. This has been done for molecular ions of p-chlorophenol, methylbenzoate, benzaldehyde, m-chlorotoluene and n-butane. The in which the collision induced fragmentation pattern varies with internal energy of the ions is illustrated.