Comparative studies of electron-impact (EI) and field ionization (FI) mass spectra of pentaerythritol derivatives

The EI Mass spectra of the polyfunctional pentaerythritol derivatives show that the molecular ions [M]⁺· exhibit extensive ion fragmentation. No Information about [M]⁺·. can be obtained. In Contrast to this, the FI Mass spectra of these compounds show intense [M]⁺· and/or [M + 1]⁺, and a characteristic ion at m/e 31, which is assumed to be the oxonium ion \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_2 = \mathop {\rm O}\limits^ + {\rm H} $\end{document}. Because of surface adsorption and field attraction, FI mass spectrometry presents a serious problem in quantitative analysis of a mixture containing compounds with quite different degrees of polarization.     

Absolute electron-impact total ionization cross sections of chlorofluoromethanes

An experimental study is reported on the electron-impact total ionization cross sections (TICSs) of CCl4, CCl3F, CCl2F2, and CClF3 molecules. The kinetic energy of the colliding electrons was in the 10-85 eV range. TICSs were obtained as the sum of the partial ionization cross sections of all fragment ions, measured and identified in a linear double focusing time-of-flight mass spectrometer. The resulting TICS profiles--as a function of the electron-impact energy--have been compared both with those computed by ab initio and (semi)empirical methods and with the available experimental data. The computational methods used include the binary-encounter-Bethe (BEB) modified to include atoms with principal quantum numbers n> or =3, the Deutsch and M?rk (DM) formalism, and the modified additivity rule (MAR). It is concluded that both modified BEB and DM methods fit the experimental TICS for (CF4), CClF3, CCl2F2, CCl3F, and CCl4 to a high accuracy, in contrast with the poor accord of the MAR method. A discussion on the factors influencing the discrepancies of the fittings is presented.     

Comparative studies of the behavior of neutral and ionic ferrocene derivatives under different conditions of electrospray ionization

We have studied the behavior of ferrocene CpFeCp (FcH), ferrocenium triiodide [FcH]⁺I₃⁻, dimethylaminomethylferrocene FcCH₂NMe₂ and its trimethylammonium salt [FcCH₂NMe₃]⁺I⁻ under the conventional conditions of electrospray ionization (ESI), when the substance solution is subjected to spraying, and in two versions of desorption electrospray ionization (DESI), when the sprayed solvent bombards the surface of solid or liquid samples. In addition to these techniques, the behavior of neutral compounds under conditions of electrospray ionization of vapors of the studied compounds in a gas phase (ESI_V) has been investigated. It has been shown using the examples of ferrocene and its dimethylaminomethyl derivative that the detection limits for these compounds occurring in a gas phase are comparable within an order of magnitude with their detection limits under the ESI and DESI conditions of solid and liquid samples. The high effectiveness of ionization of analyte vapors makes it possible to use the ESI method not only in combination with liquid (conventional ESI technology) and thin layer chromatography (DESI), but also with gas liquid chromatography (ESI_V). Thus, the electrospray ionization becomes a universal method allowing studies of a compound under the natural conditions in any state of aggregation, that is, solid, liquid, and gas. With the help of statistical methods for designing experiments (complete factorial experiment), quantitative evaluation of the influence of experimental parameters on the ion-formation processes under different ESI conditions has been carried out, which makes it possible to purposefully select the optimal conditions to record the ESI mass spectra with a minimum number of experiments. Moreover, analysis of the dependences of the mass spectra on the experimental parameters can serve as an instrument for studying the details of the ion-formation mechanisms depending upon different ways of ionization.     

Indirect processes in the electron-impact ionization of Li-like ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B²⁺, C³⁺, N⁴⁺, O⁵⁺ and F⁶⁺ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s2s2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s2snln′l′ of resonances withn=3 (Δn=2) andn=4 (Δn=3) are resolved up to principle quantum numbersn′ equal to 6 or 7.     

The electron-impact induced loss of water from meconine and related derivatives

The mass spectrum of meconine (I), a constituent of raw opium, exhibits several anomalous features (e.g. pronounced loss of H₂O from the molecular ion) which are not readily explicable in terms of simple fragmentation processes. The study of the low resolution mass spectrum of meconine and its derivatives in conjunction with deuterium labeling experiments has indicated a highly specific fragmentation mechanism in which both hydrogen atoms eliminated as H₂O originate from the methoxy function ortho to the lactone carbonyl group in meconine.     

Electron ionization mass spectral studies of bridgehead-substituted norbornan-2-ones: camphor derivatives

The electron ionization (EI) mass spectra of a series of bridgehead-substituted 3,3-dimethylnorbornan-2-ones, derived from natural (1R)-(+)-camphor, have been studied and their cleavage mechanisms rationalized on the basis of the substituent shifts as well as on the identification of relevant peaks through accurate mass measurements and collision-induced dissociation (CID) tandem mass spectrometric experiments. The fragmentation patterns are very dependent on both the structural nature and the electronic properties of the bridgehead substituent. The driving force for the main fragmentation pathways are competitive cleavages of the C(1)-C(2) and C(2)-C(3) bonds directed by the bridgehead substituent and either the gem-dimethyl or carbonyl groups. These cleavages lead to distonic ions in which the charge is preferentially located either at the C(1), C(2) or C(3) positions depending on the electronic character and structural nature of the bridgehead substituent. This charge distribution determines the subsequent rearrangements and fragmentations.     

Model studies on protease-catalysed peptide synthesis using 9-fluorenylmethoxycarbonyl protected amino acid derivatives

Summary Fmoc-Phe-OMe,Fmoc-Ala-OMe andFmoc-Gly-OH were coupled with H-Leu-NH₂ under catalytic action of chymotrypsin, papain and thermolysin, respectively. The influence of different reaction media and several reaction parameters, such as reactants and enzyme concentrations as well as reaction time, on the peptide bond formation was investigated. Abbreviations: IUPAC-IUB rules for peptides are followed, see (1984) Eur. J. Biochem.138: 9. All amino acids except of Gly are ofL-configuration.DMF=N,N-dimethylformamide,DTE=dithioerythritol,EDTA=ethylenediaminetetraacetic acid, disodium salt,N/C=ratio nucleophile component/carboxyl component,-OMe=methyl ester,TLC=thin layer chromatography.     

A study of the fragmentation of caprolactam and its methyl derivatives under electron-impact

A study of the low and high-resolution mass spectra of caprolactam (I), 1-methylcaprolactam (II), 3-methylcaprolactam (III), 4-methylcaprolactam (IV), 5-methylcaprolactam (V), 6-methylcaprolactam (VI), 7-methylcaprolactam (VII) and deuterium labelled compounds has facilitated the proposal of some principal fragmentation mechanisms of ionized molecules of these compounds. The base peaks in the spectra of all compounds studied (except 3-methylcaprolactam) are the ions m/e 30 and m/e 44 respectively formed through the cleavage of the C6–C7 bond and the C2–N bond with simultaneous transfer of a hydrogen atom. It has been proved that the fragmentation of molecular ions leading to ions with the structure of cyclopentanone 2-methylcyclopentanone and 3-methylcyclopentanone respectively (according to the position of methyl group) is the general feature of the fragmentation of caprolactam and its methyl derivatives.     

Terpenoids—CXXXVII: Fragmentation of constunolide and its derivatives under electron-impact

An attempt has been made to correlate the fragmentation modes and structure of costunolide and some lactones derived from it, containing a ten-membered carbocyclic system. The possible origins for the major fragments have been determined, using high resolution mass spectrometry and deuterium labelling.     

The electron-impact ionization of Ar and Kr revisited: a critical analysis of double-to-single ionization cross section ratio measurements using the fast-atom-beam technique

We report new measurements of the absolute electron-impact double ionization cross sections for Ar and Kr and of the ratios of double-to-single ionization for impact energies from threshold to 200 eV using the crossed electron-beam — fast-atom-beam technique. The work was motivated by the recently highlighted spread of about 30% in the Ar²⁺/Ar⁺ ionization cross section ratios obtained by several groups using different experimental techniques. Such a spread is inconsistent with statistical uncertainties of typically 3% or less that were quoted for the various reported ratios. A similar situation exists for Kr where the spread among the recently published Kr²⁺/Kr⁺ ionization cross section ratios is about 15%. We made an attempt to identify all potential systematic errors inherent to the fast-beam technique that could affect the measurement of cross section ratios with special emphasis on those systematic errors that could influence the detection of singly and doubly charged product ions differently. We found Ar²⁺/Ar⁺ and Kr²⁺/Kr⁺ cross section ratios of, respectively 0.066 ±0.007 and 0.087 ±0.008 at 100 eV which confirm earlier measurements using the same experimental technique. The error limits on cross sections ratios measured in our fast-beam apparatus were determined to be at least ±9% for cross section ratios of multiple-to-single ionization for the same target atom and at least ±10% for ratios of single ionization cross sections for different target species. Our error limits are dominated by systematic uncertainties of the apparatus which do not cancel when cross section ratios are measured, since the ratios are obtained under similar, but not identical experimental conditions.     

Comparative studies on drug-receptor binding of phenylethylamine derivatives using calculated group contribution constants

Group contribution constants of phenylethylamine derivatives were calculated using a modified version of the Free-Wilson approach and data published on group contributions were also collected. The biological activity indices used to extract these contributions are the inhibition of phenylethanolamine N-methyl transferase, inhibition of norepinephrine uptake, dopamine β-hydroxylase substrate activity, phenylethanolamine N-methyl transferase substrate activity, and antagonism to the pressor activity of epinephrine. The different sets of calculated group contributions were compared. Correlation could be demonstrated between sets of group contributions, except those extracted from the antipressor activity data. It is concluded that the interaction mechanism between the receptor site and the aromatic ring might be similar in the systems considered. Methylation of the nitrogen atom appearently does not cause any change in specificity at the molecular level.     

Labelling studies of hydrogen rearrangements in the chemical ionization mass spectra of cyclohexanone and its derivatives

Deuterium labelling studies indicate that loss of water from the protonated molecular ion of cyclohexanone involves competitive site-specific eliminations. Loss of alcohol from the protonated molecular ions of 4-alkoxycyclohexanones involves competition between a direct cleavage, a 1,3-hydrogen rearrangement and consecutive losses of alkene and water.     

Effects of peptide acetylation and dimethylation on electrospray ionization efficiency

Peptide acetylation and dimethylation have been widely used to derivatize primary amino groups (peptide N‐termini and the ε‐amino group of lysines) for chemical isotope labeling of quantitative proteomics or for affinity tag labeling for selection and enrichment of labeled peptides. However, peptide acetylation results in signal suppression during electrospray ionization (ESI) due to charge neutralization. In contrast, dimethylated peptides show increased ionization efficiency after derivatization, since dimethylation increases hydrophobicity and maintains a positive charge on the peptide under common LC conditions. In this study, we quantitatively compared the ESI efficiencies of acetylated and dimethylated model peptides and tryptic peptides of BSA. Dimethylated peptides showed higher ionization efficiency than acetylated peptides for both model peptides and tryptic BSA peptides. At the proteome level, peptide dimethylation led to better protein identification than peptide acetylation when tryptic peptides of mouse brain lysate were analyzed with LC‐ESI‐MS/MS. These results demonstrate that dimethylation of tryptic peptides enhanced ESI efficiency and provided up to two‐fold improved protein identification sensitivity in comparison with acetylation. Copyright © 2016 John Wiley & Sons, Ltd.     

Matrix-assisted laser desorption ionization hydrogen/deuterium exchange studies to probe peptide conformational changes

Hydrogen/deuterium (H/D) exchange chemistry monitored by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry is used to study solution phase conformational changes of bradykinin, alpha-melanocyte stimulating hormone, and melittin as water is added to methanol-d4, acetonitrile, and isopropanol-d8 solutions. The results are interpreted in terms of a preference for the peptides to acquire more compact conformations in organic solvents as compared to the random conformations. Our interpretation is supported by circular dichroism spectra of the peptides in the same solvent systems and by previously published structural data for the peptides. These results demonstrate the utility of MALDI-TOF as a method to monitor the H/D exchange chemistry of peptides and investigations of solution-phase conformations of biomolecules.