Thomas Schlathölter

Photodissociation of protonated leucine-enkephalin in the VUV range of 8–40 eV

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.

Cq+-induced excitation and fragmentation of uracil : effects of the projectile electronic structure

Ionization and fragmentation of the RNA base uracil (C4H4N2O2) by means of Cq+ ions (q = 1-6) has been studied for ion kinetic energies ranging from ;2 to 120 keV. Whereas for Cq+ (q = 1, 3, 4, 5, 6) very similar fragmentation yields are observed which increase with the projectile velocity v, C2+ ions not only induce a fundamentally different fragmentation pattern but also lead to a decrease of fragmentation with ion velocity v. At low velocities, even almost complete fragmentation is observed. Our findings can be explained by differences in the electronic structures of the Cq+ ions as well as by employing the electronic stopping model previously applied to ion-fullerene interactions.

Quantification of ion-induced molecular fragmentation of isolated 2-deoxy-D-ribose molecules.

Recent experiments on low energy ion-induced damage to DNA building blocks indicate that ion induced DNA damage is dominated by deoxyribose disintegration (Phys. Rev. Lett., 2005, 95, 153201). We have studied interactions of keV H+ and He(q+) with isolated deoxyribose molecules by means of high resolution time-of-flight spectrometry. Extensive statistical fragmentation of the molecules is observed. The fragment distribution is found to follow a power law dependence. The exponent can be used to characterize and quantify the molecular damage.

IONIZATION AND FRAGMENTATION OF ANTHRACENE UPON INTERACTION WITH keV PROTONS AND α PARTICLES

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+.

Interactions of neutral and singly charged keV atomic particles with gas-phase adenine molecules

KeV atomic particles traversing biological matter are subject to charge exchange and screening effects which dynamically change this particles effective charge. The understanding of the collision cascade along the track thus requires a detailed knowledge of the interaction dynamics of radiobiologically relevant molecules, such as DNA building blocks or water, not only with ionic but also with neutral species. We have studied collisions of keV H(+), He(+), and C(+) ions and H(0), He(0), and C(0) atoms with the DNA base adenine by means of high resolution time-of-flight spectrometry. For H(0) and H(+) we find qualitatively very similar fragmentation patterns, while for carbon, strong differences are observed when comparing C(0) and C(+) impact. For collisions with He(0) and He(+) projectiles, a pronounced delayed fragmentation channel is observed, which has not been reported before.

Scattering of swift molecules, H2 and CO2, from metal surfaces

Abstract Molecular ions and neutral molecules are scattered at 500 eV and at grazing incidence from metal surfaces. H 2 + and CO 2 + are subject to charge exchange processes which lead in part to dissociation. In the case of CO 2 + evidence for negative molecular ion formation is found. When using neutral H 2 and CO 2 only in the case of CO 2 is charge capture observed, i.e. negative ions are found. Potassium “poisons” the dissociation of H 2 on Pd. These findings are supported by comparison of different metals, Ni(110), Pd(110) and Pd(110) covered with a monolayer of K. The dissociation probability of H 2 + and H 2 0 decrease at the K-covered surface, in the cases of CO 2 + and CO 2 0 an increase is found. CO 2 + on Pd(110) + K dissociates completely.

Near-edge X-ray absorption mass spectrometry of a gas-phase peptide.

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.

Electron capture and loss in the scattering of oxygen atoms and ions on Mg, Al and Ag surfaces

We present the results of a study of collisions of 1 to 4 keV oxygen ions and atoms with Mg, Al and Ag surfaces. Formation of O- is in particular investigated. This is an interesting multichannel problem, since the ground state electronic configuration of oxygen 2p(4) corresponds to three states and electron capture processes involve three atom (P-3, D-1 and S-1)-metal continua. We report scattered ion fractions, measured in an angular range extending from 2 degrees to 40 degrees with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process for a large range of collision velocities normal to the surface, thus probing the charge transfer process in different atom-surface distance ranges. The ion fractions are found to increase with increasing angle and increasing energy. Similar fractions are obtained for Al and Ag, but significantly higher ones for Mg. Ionisation processes in hard collisions with surface atoms are observed. An electron spectroscopy study was performed and did not reveal any signs of autoionising state (O**2p(2)3s(2)) production.

Precise Determination of 2‐Deoxy‐D‐Ribose Internal Energies after keV Proton Collisions

The interaction of keV protons with building blocks of DNA is of particular biological relevance in view of the increasing number of facilities employing MeV proton irradiation for tumor treatment. When ions traverse tissue and are decelerated to MeV and sub-MeV energies, the Bragg peak is reached. At ion energies in the Bragg peak region, the induced damage is highest due to maximum linear energy transfer and relative biological effectiveness. The volume selectivity given by the existence of a well-localized Bragg peak region renders proton therapy such a promising tool for cancer treatment. Furthermore, biological consequences of irradiation with energetic protons from galactic cosmic rays and solar particle events are a limiting factor for human space exploration. This issue is of particular importance for future manned missions outside low earth orbit, for example, lunar or Mars missions. It is well-known that biological radiation damage is the ultimate result of ionization and fragmentation of cellular DNA. To explore the molecular mechanisms underlying radiation-induced DNA damage, numerous recent studies focused on ionization and fragmentation of DNA building blocks upon irradiation with slow electrons, photons and ions. In their pioneering studies, Sanche and co-workers showed that low-energy (secondary) electrons can cause singleand doublestrand breaks of plasmid DNA. Huels and coworkers investigated interactions of hyperthermal ions with DNA building blocks and concluded that heavy ions have the potential to induce particularly complex damage to DNA in the Bragg peak region. Ionization and fragmentation of 2-deoxy-d-ribose (dR, C5H10O4) molecules upon impact of keV and sub-keV ions has recently been investigated for isolated molecules as well as for the condensed phase. In both studies, almost complete disintegration of the molecule was observed and it was concluded that direct ion-induced DNA damage—for example in heavy ion or proton therapy of malignant tumors—may be dominated by deoxyribose fragmentation. Furthermore, Deng and coworkers have recently reported evidence for reactive scattering damage of 2-deoxy-d-ribose thin films by hyperthermal ions. 9] In the gas-phase studies, the mass spectrum of the fragment ions largely follows a power-law distribution with a characteristic exponent t of the order of 2, depending on the ion velocity, charge state and the atomic number of the impinging ion. From this, it was concluded that the fragmentation is to a large extent statistical. t was found to be an ideal parameter for the quantification of damage inflicted upon a molecule whose fragmentation yield is very close to 100%, but this quantification was purely phenomenological, since the fragmentation channels and the characteristic exponent could not be directly related to the amount of energy initially deposited into the deoxyribose molecules. In a study on MeV Si + collisions with C60, Itoh et al. have already concluded that t carries information about the energy deposition Eexc into the target molecule. In their work, the target excitation energies Eexc were calculated and a simple exponential dependence of Eexc on the characteristic exponent t was found. However, the value of Eexc was not experimentally obtained in their study. Very recently Chen et al. presented a new experimental approach to measure directly the amount of internal energy present in the transient C 60 * formed by double electron transfer to keV protons. Using this technique, different fragmentation channels could be assigned a corresponding excitation energy. We have now applied the same technique to double electron capture processes in keV proton collisions with 2-deoxy-d-ribose (H +deoxyribose!H +deoxyribose*). To our knowledge, this is the first report on the fragmentationchannel-specific experimental determination of the excitation energy present in a transient dicationic biomolecular complex. The concept of the applied experimental technique is based on the idea of double-charge transfer (DCT) spectroscopy (introduced by Durup and coworkers in the early 1970s) but in an event-by-event mode with coincident detection of all ionic molecular fragments. In a keV ion collision with an isolated molecule, the target can be subject to ionization, electron transfer and excitation. For an accurate determination of the internal energy of the transient excited molecular ion, a process is chosen for which the complete energy balance is experimentally accessible. The target internal energy is equal to the sum of projectile kinetic energy loss, the projectile excitation energy and the difference in binding energies of the electrons. Since only the projectile kinetic energy is easily accessible experimentally, an ideal outgoing projectile has no stable excited states. This is the case for H . Since it is experimentally very challenging to measure kinetic energies of emitted electrons while keeping the electron detection efficiency high, only H formation by double electron capture from deoxyribose is considered [Eq. (1)]: [a] Dr. F. Alvarado, Prof. Dr. R. Hoekstra, Dr. T. Schlathçlter KVI Atomic Physics University of Groningen, Zernikelaan 25 9747AA Groningen (The Netherlands) Fax: (+ 31) 50 363 4003 E-mail : tschlat@kvi.nl [b] Dr. J. Bernard, B. Li, Dr. R. Br:dy, Dr. L. Chen, Dr. S. Martin Universit: Lyon 1, CNRS, LASIM UMR 5579 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne (France)

H2 formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

Aims: Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H2 forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H2 formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods: We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H2 and a H2 formation mechanism on PAHs. Since H2 formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T> 200 K). At these temperatures the conventional route of H2 formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H2 formation is possible. Results: We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H2 forms very efficiently on PAHs, and that this process can reproduce the high H2 formation rates derived in several PDRs. Appendix A is available in electronic form at http://www.aanda.org