Multiply protonated peptides and proteins in the gas phase can respond to near edge X-ray absorption in three different ways: (i) non dissociative ionization and ionization accompanied by loss of ...small neutrals, both known to dominate for proteins with masses in the 10 kDa range. (ii) Formation of immonium ions, dominating for peptides in the 1 kDa range. (iii) Backbone scission leading to sequence ions which is typically weaker and has mainly been observed for peptides in the 1 kDa range. We have studied carbon 1s photoexcitation and photoionization for a series of peptides and proteins with masses covering the range from 0.5 kDa to more than 10 kDa. The gas phase protonated molecules were trapped in a radiofrequency ion trap and exposed to synchrotron radiation. Time of flight mass spectrometry was employed for investigation of the photoionization and photofragmentation processes. A smooth transition from the photofragmentation regime to the non-dissociative photoionization regime is observed. Mass spectra are most complex in the few kDa regime, where non-dissociative ionization, backbone scission and immonium ion formation coexist. The observed correlation between protein size and fragmentation, i.e. radiation damage, is of relevance for soft X-ray microscopy.
Near-Edge X‑ray Absorption Mass Spectrometry of a Gas-Phase Peptide González-Magaña, O; Reitsma, G; Tiemens, M ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
11/2012, Letnik:
116, Številka:
44
Journal Article
Recenzirano
We have studied the dissociation of the gas-phase protonated peptide leucine enkephalin YGGFL+H+ upon X-ray absorption in the region of the C K-edge. The yield of photodissociation products was ...recorded as a function of photon energy. The total photoabsorption yield is qualitatively similar to near-edge X-ray absorption fine structure (NEXAFS) spectra recorded from condensed phase peptides and proteins. Fragment specificity reveals distinct quantitative differences between spectra obtained for different masses. Fragmentation channels can be assigned to specific electronic transitions some of which are site specific. For instance, C 1s → π★ excitations in the leucine enkephalin aromatic side chains lead to relatively little fragmentation, whereas such excitations along the peptide backbone induce strong fragmentation.
We have investigated the response of superhydrogenated gas-phase coronene cations upon soft x-ray absorption. Carbon (1s)⟶π^{⋆} transitions were resonantly excited at hν=285 eV. The resulting core ...hole is then filled in an Auger decay process, with the excess energy being released in the form of an Auger electron. Predominantly highly excited dications are thus formed, which cool down by hydrogen emission. In superhydrogenated systems, the additional H atoms act as a buffer, quenching loss of native H atoms and molecular fragmentation. Dissociation and transition state energies for several H loss channels were computed by means of density functional theory. Using these energies as input into an Arrhenius-type cascade model, very good agreement with the experimental data is found. The results have important implications for the survival of polyaromatic hydrocarbons in the interstellar medium and reflect key aspects of graphene hydrogenation.
In modern rare isotope facilities, ion cooling and bunching lies at the heart of the ion transfer along a low-energy beam line that consists of several differential pumping stages. We present a ...conceptual design of an ion guide as an alternative to the conventional linear Radio-Frequency Quadrupole (RFQ) for cooling and bunching rare isotopes. The ion guide is composed of stacked ring electrodes of varying apertures, to which a confining RF potential following a rectangular waveform is applied. The thicknesses of the rings and the gaps in between are varied accordingly to maximize the confining volume and to reduce ion losses. Ion transport within the ion guide is facilitated by a lower-frequency wave traveling on top of the higher-frequency confining field. The former is induced by locally adjusting the duty cycle of the rectangular waveform of the confining potential. Design parameters are first calculated by analytical studies and then optimized by ion trajectory simulations with SIMION®. The results show that the ion guide enables high ion transmission and produces well focused ion bunches. It will be used in the NEXT project—an experimental study of atomic masses of Neutron-rich EXotic nuclei produced in multi-nucleon Transfer reactions.
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•A stacked-ring ion guide is proposed to cool and bunch ions as an alternative to RFQs.•It consists of ring electrodes of varying apertures, thicknesses, and in-between gaps.•It is driven by a rectangular-wave RF to offer the deepest pseudopotential barrier.•The duty cycles of the rectangular waveform are locally tuned to induce bias voltages.•Ion transport amid buffer gas is assisted by a traveling wave of bias voltages.
Until now, photodissociation studies on free complex protonated peptides were limited to the UV wavelength range accessible by intense lasers. We have studied photodissociation of gas-phase ...protonated leucine-enkephalin cations for vacuum ultraviolet (VUV) photons energies ranging from 8 to 40 eV. We report time-of-flight mass spectra of the photofragments and various photofragment-yields as a function of photon energy. For sub-ionization energies our results are in line with existing studies on UV photodissociation of leucine-enkephalin. For photon energies exceeding 10 eV we could identify a new dissociation scheme in which photoabsorption leads to a fast loss of the tyrosine side chain. This loss process leads to the formation of a residual peptide that is remarkably cold internally.
The interaction of keV ions with polyaromatic hydrocarbons is dominated by charge exchange and electronic stopping. We have studied the response of the polyaromatic hydrocarbon anthracene (C14H10) ...upon keV H+ and He2+ impact using high-resolution time-of-flight mass spectrometry. Extensive fragmentation into small C n H q+ m as well as formation of up to triply charged parent ions is observed. Ab initio electron densities are used to calculate the molecular excitation due to electronic stopping. Fragment yields increase with the increase of electronic stopping as a function of projectile velocity. For equal electronic stopping, He2+ is found to induce more fragmentation than H+. The difference in fragmentation is concluded to be due to two electron processes, which are relevant channels only for He2+.
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