1,721,183 research outputs found
Einfluss von antibakteriellen Peptiden auf die Struktur von Lipidmembranen
No full textAntimicrobial peptides are used by organisms to kill invading bacteria by interacting with the bacterial membrane. At the same time, the membrane of the host organism remains intact. The exact mechanism of action is not known. Here, we studied the structure of lipid membranes mixed with antibacterial peptides by X-ray diffraction at beamline A2 in Hasylab, Hamburg. We used two lipids differing in the headgroup; DOPC is a typical mammalian cell membrane lipid, while DOPE is abundant in bacterial membranes. We used two model antibacterial peptides, mellitin and alamethicin. Both peptides act in similar ways on each lipid. With DOPC, the membrane structure remains lamellar in the studied temperature range. With DOPE, new phases form at the onset of the hexagonal phase. We observed two cubic phases, Im3m and Pn3m. All observed phases were related by epitaxial relationships which suggests their coexistence within one multilamellar vesicle, the so-called onion vesicle.Antibakterielle Peptide sind ein Baustein des Immunsystems. Diese Peptide
können Bakterien abtöten, ohne dabei die Zellen des Organismus zu
beschädigen. Der genaue Mechanismus der Wechselwirkung mit der Zellmembran ist nicht bekannt. Wir haben die Struktur von zwei Lipidarten, DOPC und DOPE, mit zwei Peptiden, Melittin und Alamethicin, mit Hilfe der Kleinwinkelstreuung untersucht. Die Messungen wurden an der Beamline A2 in Hasylab, Hamburg durchgeführt. DOPC kommt vor allem in Säugetierzellmembranen vor, DOPE in bakteriellen Membranen. Beide Peptide haben die Struktur von DOPC lamellaren Membranen nicht wesentlich verändert, während sich in DOPE Membranen zwei neue Strukturen mit kubischer Symmetrie gebildet haben, Im3m und Pn3m. Zwischen den koexistierenden Lipidphasen in DOPE treten mehrere Epitaxien auf. Der Grund hierfür ist der Einschluß aller Phasen in demselben Liposom, dem sogenannten Zwiebelliposom (onion vesicle). Dieses enthält mehrere Schichten aus unterschiedlichen Phasen
Kohärente Röntgenbeugungsstudien der mesoskopischen Materialien
No full textThis thesis is devoted to three separate projects, which can be considered as independent. First, the dynamical scattering effects in the Coherent X-ray Diffractive Imaging (CXDI) method are discussed. Based on the simulation results, a straightforward method for correction for the refraction and absorption artifacts in the Bragg CXDI reconstruction is suggested. The second part summarizes the results of an Coherent X-ray Diffractive Imaging experiment with a single colloidal crystal grain. A remarkable result is that positions of individual particles in the crystal lattice have been resolved in three dimensions. The third project is devoted to X-ray diffraction experimental studies of structural evolution of colloidal crystalline films upon incremental heating. Based on the results of the analysis a model of structural evolution of a colloidal crystal upon heating on nanoscopic and mesoscopic length scales is suggested.Diese Arbeit widmet sich drei in sich abgeschlossenen Projekten. Zuerst werden Effekte dynamischer Streuungtheorie in Verbindung mit kohärenter Röntgenbeugungsbildgebung (Coherent X-ray Diffractive Imaging, CXDI) vorgestellt. Im zweiten Teil sind die Ergebnisse
eines CXDI Experiments an einem einzelnen kolloidialen Kristallkorn zusammengefasst. Ein wichtiges Resultat stellt die Auflösung von einzelnen Partikeln im drei-dimensionalen Kristallgitter dar. Der dritte Teil umfasst Röntgenbeugungsstudien zur Strukturentwicklung von dünnen
kolloidialen Kristallschichten unter steigender Temperatureinwirkung. Auf Basis dieser Studien wird ein physikalisches Model zur Beschreibung von nano- und mesoskopischer Strukturentwicklung unter Temperatureinfluss vorgeschlagen
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
Diffraction anomalous fine structure in Laue geometry
No full textThis thesis deals with the further development of the method of Borrmannspectroscopy and the development of the novel method of diffractionanomalous fine structure in Laue geometry.Borrmann spectroscopy utilizes the Borrmann effect in order to enhancethe quadrupol absorption, which enables studying weak quadrupoltransitions. In the present work a model is developed, which describes thedependence of the quadrupol enhancement on temperature and anisotropyof the chemical environment. This model is used to describe the Borrmannspectroscopy results obtained from a strontium titanate single crystal.Furthermore, the influence of the mosaicity of real crystals on thequadrupole enhancement is studied by means of simulations, which arebased on the dynamical theory of diffraction and a modified structuremodel of barium titanate.The method of diffraction anomalous fine structure (DAFS) is based onthe combination of X-ray diffraction and absorption spectroscopy and allowsinvestigating the local atomic structure of centrosymmetric crystalsutilising the additional selectivity due to diffraction. In the present workthe novel method of diffraction anomalous fine struture in Laue geometry(LDAFS) is developed. It utilizes the intensity of the Laue geometrydiffracted beam, which experiences an altered absorption due to the Borrmanneffect. Its experimental applicability is verified by experiments ona barium titanate single crystal and comparison with conventional X-rayabsorption spectroscopy results
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
A New Fixed-Target Approach for Serial Crystallography at Synchrotron Light Sources and X-ray Free Electron Lasers
No full textIn the framework of this thesis, a new method for high-speed fixed-target serial crystallography experiments and its applicability to biomacromolecular crystallography at both synchrotron light sources and X-ray free electron lasers (XFELs) is presented. The method is based on a sample holder, which can carry up to 20,000 microcrystals and which is made of single-crystalline silicon. Using synchrotron radiation, the structure of Operophtera brumata cytoplasmic polyhedrosis virus type 18 polyhedrin, lysozyme and cubic insulin was determined by collecting X-ray diffraction data from multiple microcrystals. Data collection was shown to be possible at both cryogenic and ambient conditions. For room-temperature measurements, both global and specific indications of radiation damage were investigated and characterized. Due to the sieve-like structure of the chip, the microcrystals tend to arrange themselves according to the micropore pattern, which allows for efficient sampling of the sample material. In combination with a high-speed scanning stage, the sample holder was furthermore shown to be highly suitable for serial femtosecond crystallography experiments. By fast raster scanning of the chip through the pulsed X-ray beam of an XFEL, structure determination of a virus, using the example of bovine enterovirus type 2, has been demonstrated at an XFEL for the first time. Hit rates of up to 100% were obtained by the presented method, which refers to a reduction in sample consumption by at least three orders of magnitude with respect to common liquid-jet injection methods used for sample delivery. In this way, the typical time needed for data collection in serial femtosecond crystallography is significantly decreased. The presented technique for sample loading of the chip is easy to learn and results in efficient removal of the surrounding mother liquor, thereby reducing the generated background signal. Since the chip is made of single-crystalline silicon, in principle no diffuse background contribution is caused by the chip itself
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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