1,721,048 research outputs found
Monocarbon cationic cluster yields from N-2/CH4 mixtures embedded in He nanodroplets and their calculated binding energies
No full textThe formation of monocarbon cluster ions has been investigated by electron ionization mass spectrometry of cold helium nanodroplets doped with nitrogen/methane mixtures. Ion yields for two groups of clusters, CHmN2+ or CHmN4+, were determined for mixtures with different molecular ratios of CH4. The possible geometrical structures of these clusters were analyzed using electronic structure computations. Little correlation between the ion yields and the associated binding energies has been observed indicating that in most cases kinetic control is more important than thermodynamic control for forming the clusters. (C) 2014 AIP Publishing LLC
Collision Studies with Electro-Sprayed Biomolecules
Get PDFIn this PhD thesis, the fragmentation of prototype radiosensitizer molecules for cance radio- and chemo-therapy is investigated through gas-phase collision experiments. The main goal is to extend the current knowledge on the radiosensitization mechanism by assessing the formation of reactive species, ions and neutral radicals, which may lead to DNA damage during the early stages of radiation damage.
In the first part, a collision-induced dissociation study with protonated ronidazole carried out with a home-built electrospray ionization source coupled to a double-focusing mass spectrometer is presented. The main fragmentation pathway results from an intramolecular proton transfer reaction followed by release of neutral – NH2CO2H fragment.
This reaction was demonstrated in low- and high-energy CID experiments with partially deuterated ronidazole supported by DFT quantum chemical calculations. The second part of the thesis focuses on low-energy electron interactions with 5-trifluoromethanesulfonyluracil (OTfU), a modified pyrimidine, and benzaldehyde, a compound used in anti-cancer clinical trials. Crossed electron-molecular beam setups coupled with a quadrupole mass spectrometer were employed to identify the formed fragment anions, and to measure anion
efficiency curves as function of the incident electron energy. In dissociative electron attachment (DEA) to OTfU, the triflate anion (OTF-), along with the reactive uracil-5-yl radical was identified as the dominant anionic fragment. The efficient decomposition of OTfU into reactive species upon electron attachment endorses its potential as a radiosensitizer. For benzaldehyde, in addition to the molecular anion detection, further nine fragment anions were observed with modest DEA cross sections of about 10−24 − 10−23 m2. The study with partially deuterated benzaldehyde showed that the dehydrogenation of benzaldehyde is selective with respect to the incident electron energy. The formation of resonances was also theoretically investigated by electron scattering calculations, and a quantum chemical study predicted the thermochemical thresholds for the observed fragments.
The insights gained in this PhD thesis may contribute for a better understanding on radiation damage, which is of paramount importance for the design of new radiosensitizer drugs, as well as for the development of more efficient cancer treatments
Low-Energy Electron Interactions with Radiosensitisers and Hydrated Biomolecular Clusters
Get PDFInteractions of free electrons with kinetic energies between 0 eV and 100 eV with isolated
biomolecules, (hydrated) radiosensitisers and doped neon clusters were studied,
with focus on associative and dissociative electron attachment (AA and DEA, respectively)
processes and electron ionisation. Radiosensitisers are applied in radiotherapy
to enhance the ratio of damage to malignant compared to healthy cells. The principle
of action on a physico-chemical stage is yet unknown and was studied within this thesis.
Clusters are an intermediate state of matter between gaseous and solid state. In
this thesis, cluster size dependencies and effects in doped clusters were studied. Two
different mass spectrometry setups were used. Data from electron interactions with
both gas-phase biomolecules and doped neon clusters was taken at the experimental
apparatus in Innsbruck. A hemispherical electron monochromator enables high energy
resolution measurements and is combined with a quadrupole mass filter. For the acquisition
of hydrated radiosensitiser data, the setup in Prague was used. There, an
electron gun is combined with a time-of-flight mass analyser. The clusters were in
both cases produced via supersonic expansion. The radiosensitisers studied include
5-selenocyanato-2’-deoxyuridine (SeCNdU), nimorazole and misonidazole. All of them
exhibit efficient electron capturing characteristics. SeCNdU is a potential radiosensitiser
for highly proliferating cells and exhibits (SeU-yl) and CN as strongest fragment anions
upon DEA, formed already at virtually 0 eV kinetic energy of the incoming electron.
The formation of highly reactive species reinforce SeCNdU as a promising candidate.
Nimorazole and misonidazole both exhibit an intense ion signal for the parent anion
upon electron attachment. The absolute cross section at 0 eV electron energy was determined
for nimorazole and is in the order of 3 1018 m2. For misonidazole, the absolute
cross section is estimated to be of the same order of magnitude. The fragmentation
channels upon DEA are at least one order of magnitude weaker, with NO
2 being the
most intense among them. They are further quenched upon hydration of the agent.
It is suggested that the radiosensitising action is caused by the associative attachment
channel and the DEA products only play a minor role, opposing previous assumptions.
In case of the doped neon clusters, indications for the formation of a conduction band
were found in the form of an energy barrier for incoming electrons: Comparing previous
results of electron attachment to pure carbon dioxide clusters with neon clusters doped
with CO2 results in a blue-shift of the resonance positions by up to 0.8 eV. The effect depends
on the size of the neon cluster. For molecular oxygen as the dopant, evidence was
found that an incoming electron can first react with the neon cluster via an excitation
event and subsequently attach to the dopant cluster. Both the study of radiosensitisers
and of neon clusters should be continued. Further radiosensitisers should be studied
in order to implement a model in which the effects on a physico-chemical stage are
investigated and compared to radiosensitisers already in use. From such a study, potential
new radiosensitisers can be derived. In the study of doped neon clustes, the
solvation of different complexes with neon was investigated. Particularly, the formation
of a conduction band, neon cluster - dopant interactions, and quenching of molecular
processes in dopants by neon as a collision partner are analysed. The results require
further investigations of additional dopants to unambiguously explain those effects
Monocarbon cationic cluster yields from N2/CH4 mixtures embedded in He nanodroplets and their calculated binding energies
No full textThe formation of monocarbon cluster ions has been investigated by electron ionization mass spectrometry of cold helium nanodroplets doped with nitrogen/methane mixtures. Ion yields for two groups of clusters, CHmN2+ or CHmN4+, were determined for mixtures with different molecular ratios of CH4. The possible geometrical structures of these clusters were analyzed using electronic structure computations. Little correlation between the ion yields and the associated binding energies has been observed indicating that in most cases kinetic control is more important than thermodynamic control for forming the clusters. (C) 2014 AIP Publishing LLC
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Reactive electron scattering from biomolecules and technologically relevant molecules
Get PDFThe role of a set of gases relevant within the context of biomolecules and technologically relevant molecules under the interaction of low-energy electrons was studied in an effort to contribute to the understanding of the underlying processes yielding negative ion formation. The results are relevant within the context of damage to living material exposed to energetic radiation, to the role of dopants in the ion-molecule chemistry processes, to Electron Beam Induced Deposition (EBID) and Ion Beam Induced Deposition (IBID) techniques. The research described in this thesis addresses dissociative electron attachment (DEA) and electron transfer studies involving experimental setups from the University of Innsbruck, Austria and Universidade Nova de Lisboa, Portugal, respectively.
This thesis presents DEA studies, obtained by a double focusing mass spectrometer, of dimethyl disulphide (C2H6S2), two isomers, enflurane and isoflurane (C3F5Cl5) and two chlorinated ethanes, pentachloroethane (C2HCl5) and hexachloroethane (C2Cl6), along with quantum chemical calculations providing information on the molecular orbitals as well as thermochemical thresholds of anion formation for enflurane, isoflurane, pentachloroethane and hexachloroethane. The experiments represent the most accurate DEA studies to these molecules, with significant differences from previous work reported in the literature. As far as electron transfer studies are concerned, negative ion formation in collisions of neutral potassium atoms with N1 and N3 methylated pyrimidine molecules were obtained by time-of-flight mass spectrometry (TOF). The results obtained allowed to propose concerted mechanisms for site and bond selective excision of bonds
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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