According to the popular “mobile proton model” for peptide ion fragmentation in tandem mass spectrometry, peptide bond cleavage
is typically preceded by intramolecular proton transfer from basic sites to an amide nitrogen in the backbone. If the intrinsic
barrier to dissociation is the same for all backbone sites, the fragmentation propensity at each amide bond should reflect
the stability of the corresponding N-protonated isomer. This hypothesis was tested by using ab initio and force-field computations
on several polyalanines and Leu-enkephalin. The results agree acceptably with experimental reports, supporting the hypothesis.
It was found that backbone N-protonation is most favorable near the C-terminus. The preference for C-terminal N-protonation,
which is stronger for longer polyalanines, may be understood in terms of the well known “helix macrodipole” in the corresponding
helical conformations. The opposite stability trend is found for peptides constrained to be linear, which is initially surprising
but turns out to be consistent with the reversed direction of the macrodipole in the linear conformation. 相似文献
The collisionally activated dissociation mass spectra of the protonated and alkali metal cationized ions of a triazole-epothilone
analogue were studied in a Fourier transform ion cyclotron resonance mass spectrometer. The fragmentation pathway of the protonated
ion was characterized by the loss of the unit of C3H4O3. However, another fragmentation pathway with the loss of C3H2O2 was identified for the complex ions with Na+, K+, Rb+, and Cs+. The branching ratio of the second pathway increases with the increment of the size of alkali metal ions. Theoretical calculations
based on density functional theory (DFT) method show the difference in the binding position of the proton and the metal ions.
With the increase of the radii of the metal ions, progressive changes in the macrocycle of the compound are induced, which
cause the corresponding change in their fragmentation pathways. It has also been found that the interaction energy between
the compound and the metal ion decreases with increase in the size of the latter. This is consistent with the experimental
results, which show that cesiated complexes readily eject Cs+ when subject to collisions. 相似文献
Characterization of acidic peptides and proteins is greatly hindered due to lack of suitable analytical techniques. Here we present the implementation of 213 nm ultraviolet photodissociation (UVPD) in high-resolution quadrupole-Orbitrap mass spectrometer in negative polarity for peptide anions. Radical-driven backbone fragmentation provides 22 distinctive fragment ion types, achieving the complete sequence coverage for all reported peptides. Hydrogen-deficient radical anion not only promotes the cleavage of Cα–C bond but also stimulates the breaking of N–Cα and C–N bonds. Radical-directed loss of small molecules and specific side chain of amino acids are detected in these experiments. Radical containing side chain of amino acids (Tyr, Ser, Thr, and Asp) may possibly support the N–Cα backbone fragmentation. Proline comprising peptides exhibit the unusual fragment ions similar to reported earlier. Interestingly, basic amino acids such as Arg and Lys also stimulated the formation of abundant b and y ions of the related peptide anions. Loss of hydrogen atom from the charge-reduced radical anion and fragment ions are rationalized by time-dependent density functional theory (TDDFT) calculation, locating the potential energy surface (PES) of ππ* and repulsive πσ* excited states of a model amide system.
We have studied the photodissociation of gas-phase deprotonated caerulein anions by vacuum ultraviolet (VUV) photons in the
4.5 to 20 eV range, as provided by the DESIRS beamline at the synchrotron radiation facility SOLEIL (France). Caerulein is
a sulphated peptide with three aromatic residues and nine amide bonds. Electron loss is found to be the major relaxation channel
at every photon energy. However, an increase in the fragmentation efficiency (neutral losses and peptide backbone cleavages)
as a function of the energy is also observed. The oxidized ions, generated by electron photodetachment were further isolated
and activated by collision (CID) in a MS3 scheme. The branching ratios of the different fragments observed by CID as a function of the initial VUV photon energy are
found to be independent of the initial photon energy. Thus, there is no memory effect of the initial excitation energy on
the fragmentation channels of the oxidized species on the time scale of our tandem MS experiment. We also report photofragment
yields as a function of photon energy for doubly deprotonated caerulein ions, for both closed-shell ([M–2H]2–) non-radical ions and open-shell ([M–3H]2–•) radical ions. These latter ions are generated by electron photodetachment from [M–3H]3– precursor ions. The detachment yield increases monotonically with the energy with the appearance of several absorption bands.
Spectra for radical and non-radical ions are quite similar in terms of observed bands; however, the VUV fragmentation yield
is enhanced by the presence of a radical in caerulein peptides. 相似文献
Structural changes in SiO2 and TiO2 gel films were investigated using ultraviolet (UV) and vacuum ultraviolet (VUV) irradiations. A significant compaction with dehydration of SiO2 gel films was induced by irradiation of photons in the range of 9–18 eV. The refractive index and the shrinkage of the irradiated SiO2 gel films were comparable to those obtained by sintering at 1000°C. Densification of TiO2 gel films was also observed with irradiation of 5–14 eV photons. However, effects of the irradiation on TiO2 gel were smaller that those on SiO2 gel. The structural changes in the gel films are attributed to electronic excitations which are induced by irradiation with photons having higher energies than the bandgap of the oxides. The photo-induced effects are presumed to depend on the optical properties and structure of the gels. 相似文献
The interest in incoherent sources for wavelength-selective photochemistry has increased lately, but little is still known about the behavior of polymers when exposed to far UV and vacuum UV (VUV) radiation. The same dearth of information exists regarding UV (VUV) radiation emitted by low-pressure plasmas during polymer treatment. In order to study VUV-UV effects on several polymers (polyethylene - PE, polystyrene - PS, hexatriacontane - HTC, and poly(methyl methacrylate) - PMMA), we have used the well-characterized emissions from hydrogen (broad-band emission) and hydrogen/argon mixture (near-monochromatic radiation) plasmas as light sources. During irradiation, samples were kept under vacuum or in a flow of pure oxygen at low pressure; in both cases the radiation fluxes at the sample position have been precisely determined by careful spectroscopic calibration experiments. We have employed a quartz crystal microbalance (QCM) to measure in-situ any possible mass change of the various polymers. Following irradiation, samples were analysed by ellipsometry (for thickness and refractive index), X-ray photoelectron spectroscopy (XPS, to evaluate the near-surface composition and content of various functional groups), and atomic force microscopy (AFM, for surface topography and roughness measurements). 相似文献
We studied the optical properties of gas-phase polysaccharides (maltose, maltotetraose, and maltohexaose) ions by action spectroscopy using the coupling between a quadrupole ion trap and a vacuum ultraviolet (VUV) beamline at the SOLEIL synchrotron radiation facility (France) in the 7 to 18 eV range. The spectra provide unique benchmarks for evaluation of theoretical data on electronic transitions of model carbohydrates in the VUV range. The effects of the nature of the charge held by polysaccharide ions on the relaxation processes were also explored. Finally the effect of isomerization of polysaccharides (with melezitose and raffinose) on their photofragmentation with VUV photons is presented.
Differential ion mobility spectrometry (DIMS) has the ability to separate gas phase ions based on their difference in ion mobility in low and high electric fields. DIMS can be used to separate mixtures of isobaric and isomeric species indistinguishable by mass spectrometry (MS). DIMS can also be used as a filter to improve the signal-to-background of analytes in complex samples. The resolving power of DIMS separations can be improved several ways, including increasing the dispersion field and increasing the amount of helium in the nitrogen carrier gas. It has been previously demonstrated that the addition of helium to the DIMS carrier gas provides improves separations when the dispersion field is the kept constant as helium content is varied. However, helium has a lower breakdown voltage than nitrogen. Therefore, as the percent helium content in the nitrogen carrier gas is increased, the highest dispersion field accessible decreases. This work presents the trade-offs between increasing dispersion fields and using helium in the carrier gas by comparing the separation of a mixture of isobaric peptides. The maximum resolution for a separation of a mixture of three peptides with the same nominal molar mass was achieved by using a high dispersion field (~72 kV/cm) with pure nitrogen as the carrier gas within the DIMS assembly. The conditions used to achieve the maximum resolution also exhibit the lowest ion transmission through the assembly, suggesting that it is necessary to consider the trade-off between sensitivity and resolution when optimizing DIMS conditions for a given application. Figure
The products of reaction of C3F6 with CH3OH, initiated by vacuum ultraviolet, were determined. The kinetics scheme of the process was proposed. The parameters of the kinetic processes were calculated. The feasibility of photochemical preparation of 2,2,3,4,4,4-hexafluoro-1-butanol was analyzed. 相似文献
Ion mobility spectrometry-mass spectrometry (IMS-MS) techniques are used to study the general effects of phosphorylation on peptide structure. Cross sections for a library of 66 singly phosphorylated peptide ions from 33 pairs of positional isomers, and unmodified analogues were measured. Intrinsic size parameters (ISPs) derived from these measurements yield calculated collision cross sections for 85% of these phosphopeptide sequences that are within ±2.5% of experimental values. The average ISP for the phosphoryl group (0.64 ± 0.05) suggests that in general this moiety forms intramolecular interactions with the neighboring residues and peptide backbone, resulting in relatively compact structures. We assess the capability of ion mobility to separate positional isomers (i.e., peptide sequences that differ only in the location of the modification) and find that more than half of the isomeric pairs have >1% difference in collision cross section. Phosphorylation is also found to influence populations of structures that differ in the cis/trans orientation of Xaa–Pro peptide bonds. Several sequences with phosphorylated Ser or Thr residues located N-terminally adjacent to Pro residues show fewer conformations compared to the unmodified sequences.
Pulsed Q collision induced dissociation (PQD) was introduced for isobaric tag quantification on linear ion traps to circumvent
the problem of the low-mass cut-off for collision induced dissociation (CID). Unfortunately, fragmentation efficiency is compromised
and PQD has found limited use for identification as well as quantification. We demonstrate that PQD has a comparable peptide
identification performance to CID on dual-pressure linear ion traps, opening the potential for wider use of isobaric tag quantification
on this new generation of linear ion traps. 相似文献
Differential or field asymmetric waveform ion mobility spectrometry (FAIMS) operating at high electric fields fully resolves isotopic isomers for a peptide with labeled residues. The naturally present isotopes, alone and together with targeted labels, also cause spectral shifts that approximately add for multiple heavy atoms. Separation qualitatively depends on the gas composition. These findings may enable novel strategies in proteomic and metabolomic analyses using stable isotope labeling.
Various K-, L- and M-shell X-ray production cross sections are measured for heavy ion impact on elements in the range Z2 = 13 to 83. The ion species range from Z1 = 10 to 36, and ion energies from 1 to 16 MeV are used. Enhanced cross sections are observed when the projectile K- or L-
binding energy is similar to the energy of the target K-, L- or M-shell. This effect is used to improve the analysis sensitivity
for selected elements. As an example trace analysis of Fe in glass with V, Mn, Co and Ni ions is investigated. Results are
compared with proton induced X-ray emission analysis on the same samples. In these samples Fe-Kα X-ray production is similar for irradiation with 3 MeV protons and 14 MeV Ni ions. However the signal to background ratio
is four times higher for the irradiation with Ni ions as compared to irradiation with protons. Advantages and drawbacks of
heavy ion induced X-ray emission for quantitative analysis compared to proton induced X-ray emission analysis are discussed. 相似文献
Abstract A simple method involving ion chromatography with ultraviolet detection (IC-UV) is applied to the determination of benzoic acid in atmospheric samples. The detection limit is 10 nanograms. Validation and interference studies are described. The application of the IC-UV method to other aromatic acids including nitrophenols is briefly discussed. 相似文献
Recently, dozens of new psychoactive substances have appeared on the European drug market every year. The most abundant group of these compounds is synthetic cannabinoids. In the first few years of the “legal highs” phenomenon, JWH (John W. Huffman) compounds were especially popular among drug users. However, the group of synthetic cannabinoids is constantly expanding, as new compounds are created by replacing known structural elements with different chemical groups. The problem with the identification of novel substances in forensic laboratories results from the structural similarity of the compounds and the rapid introduction of newer designer drugs on the black market. In this study, the fragmentation patterns of 29 new-type synthetic cannabinoids using electrospray ionization were investigated. The analysis was performed using quadrupole time-of-flight mass spectrometry. Based on measurements carried out under various conditions, the way of fragmentation of the tested compounds that were divided into groups due to their chemical structure was established. The study showed that the bond between the carbon atom of the carbonyl group and the ring or NH group attached to the ring was mainly cleaved. This mechanism was adequate for the fragmentation of first-generation synthetic cannabinoids. This paper presents characteristic ions formed by synthetic cannabinoids (i.e., ions originating from an indole/indazole ring and an adamanyl/naphthalene/quinoline ring) using electrospray ionization. Knowledge of these specific fragments can be used in forensic laboratories to determine the structure of novel compounds from the group of synthetic cannabinoids.
