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Metalloporphyrins as potentially prebiotic molecules and chemical biosignatures
No full textThis doctoral thesis addresses two important astrobiological aspects of metalloporphyrins. First, the possible abiotic origin of metalloporphyrins under prebiotically plausible conditions, and second, metalloporphyrins as possible molecular biosignatures in the context of the search for extraterrestrial life. This two-pronged approach allowed us to obtain a more comprehensive overview of metalloporphyrins in an astrobiological context.
In presently known living organisms, metal complexes of porphyrins and porphyrinoids occur ubiquitously, whereas free base porphyrins scarcely matter. Among the best known examples are chlorophylls (magnesium complexes), which are crucially involved in photosynthesis, and the heme group, for example as a cofactor of cytochromes, which play an important role in cel¬lular respiration. In terms of evolution, porphyrinoid cofactors appear to be very old, and indeed 1.1 billion-year-old geoporphyrins have been found in sedimentary rocks. Ancestors of current porphyrinoid cofactors, simple metalloporphyrins, may have been present in the first organisms or even earlier in abiotic protometabolisms. Furthermore, the wide distribution of porphyrinoid cofactors among known organisms, as well as their participation in basic biological functions, suggests that metalloporphyrinoids may also be present in potential life forms beyond Earth. Thus, metalloporphyrinoids could be useful as chemical biosignatures in the search for life on Mars, Europa, Enceladus and beyond.
The first objective of this work was to investigate whether complex formation with selected metals (Fe, Mg, Co, Ni, and Cu) is possible under prebiotically plausible conditions. Based on a prebiotic synthesis of octaalkylporphyrins at simulated primordial volcanic coasts, which has been previously described by our group, the influence of wet-dry cycles on octaethylporphyrin (OEP) and selected metal sources was to be studied. For this purpose, first a novel, automated apparatus had to be developed that allowed the simulation of conditions on the early Earth, especially at primordial volcanic coasts. In particular, this apparatus had to ensure strict exclu-sion of atmospheric oxygen and allow fluctuating changes between wet and dry phases (simu-lation of tides or rainfall).
Initially, experiments were conducted to test the new apparatus. It was shown that the appa-ratus worked completely automatically and reliably over a longer period of time. In further experiments, it was tested whether oxidation-sensitive substances can be handled in the apparatus. Indeed, it was shown that it is possible to strictly exclude oxygen. Thus, the apparatus proved to be suitable for investigating the formation of metal complexes from OEP and various metal sources under the influence of alternating wet-dry cycles. Metal sources used included metal(II) chlorides, metal sulfides, basalt, and iron meteorites. The focus was on iron sources. Indeed, it was shown that iron, magnesium, cobalt, nickel and copper complexes formed with the corresponding metal sources in an unusual reaction (completely water-insoluble OEP reacted with partly also insoluble metal sources), whereby wet-dry cycles turned out to be essential. Yields ranging from 20 to 78% (relative to the porphyrin) were obtained in fresh water. In addition, the influence of artificial seawater and low pH values on complex formation was investigated.
In the second part of the thesis, the stability of metalloporphyrins was investigated using the compound chlorido(octaethylporphyrinato)iron(III), [FeCl(oep)], as a model. Potentially de-structive conditions relevant to metalloporphyrins as possible chemical biosignatures were selected for the experiments performed, as well as conditions that presumably prevailed on primordial volcanic coasts. Extensive series of experiments demonstrated, among other things, that (a) the iron OEP core is stable over a pH range of 0.0 to 13.5, (b) [FeCl(oep)] is stable up to ca. 250 °C in an inert atmosphere, (c) a salt matrix protects [FeCl(oep)] from X-ray radiation but not from iron particle radiation, and (d) hypochlorite, hydrogen peroxide, chlorate, and nitric acid oxidatively decompose [FeCl(oep)] (oxidation power in descending order). On the other hand, perchlorate, which is often mentioned in connection with its occurrence in the Martian regolith, did not show any oxidation effect. Finally, the importance of metalloporphyrins as potential biosignatures is discussed in the presented work, particularly in the light of their stability and possible abiotic synthetic pathways for these compounds.Diese Doktorarbeit befasst sich mit zwei wichtigen astrobiologischen Aspekten von Metalloporphyrinen. Erstens mit der möglichen abiotischen Entstehung von Metalloporphyrinen unter präbiotisch plausiblen Bedingungen und zweitens mit Metalloporphyrinen als mögliche molekulare Biosignaturen im Zusammenhang mit der Suche nach außerirdischem Leben. Diese zweigleisige Herangehensweise ermöglichte es, einen umfassenderen Überblick über Metalloporphyrine im astrobiologischen Kontext zu gewinnen.
In den heute bekannten lebenden Organismen kommen Metallkomplexe von Porphyrinen und Porphyrinoiden ubiquitär vor; freie Porphyrinbasen spielen dagegen kaum eine Rolle. Zu den bekanntesten Beispielen zählen die Chlorophylle (Mg-Komplexe), die entscheidend an der Photosynthese beteiligt sind, und die Häm-Gruppe, z. B. als Cofaktor der Cytochrome, welche eine bedeutende Rolle bei der Zellatmung spielen. Evolutiv scheinen Porphyrinoid-Cofaktoren sehr alt zu sein, und tatsächlich wurden 1,1 Milliarden Jahre alte Geoporphyrine in Sedimentgestein gefunden. Vorfahren der heute bekannten Porphyrinoid-Cofaktoren, einfache Metalloporphyrine, könnten schon in den ersten Lebewesen oder noch früher in abiotischen Protometabolismen vorgekommen sein. Zudem legt die weite Verbreitung von Porphyrinoid-Cofaktoren in den bekannten Organismen sowie ihre Beteiligung an grundlegenden biologischen Funktionen nahe, dass auch in möglichen Lebensformen außerhalb der Erde Metalloporphyrinoide vorkommen könnten. Somit könnten Metalloporphyrinoide als chemische Biosignaturen für die Suche nach Leben auf Mars, Europa, Enceladus und darüber hinaus geeignet sein.
Das erste Ziel dieser Arbeit war es, zu untersuchen, ob eine Komplexbildung mit ausgewählten Metallen (Fe, Mg, Co, Ni und Cu) unter präbiotisch plausiblen Bedingungen möglich ist. Ausgehend von einer zuvor von unserer Arbeitsgruppe beschriebenen präbiotischen Synthese von Octaalkylporphyrinen an simulierten urzeitlichen Vulkanküsten sollte der Einfluss von Nass-Trocken-Zyklen (NTZ) auf Octaethylporphyrin (OEP) und ausgewählte Metallquellen untersucht werden. Hierzu musste zunächst eine neuartige, automatisierte Apparatur entwickelt werden, die die Simulation der Bedingungen auf der frühen Erde, speziell an urzeitlichen Vulkanküsten, ermöglichte. Insbesondere musste diese Apparatur einen strikten Ausschluss von Luftsauerstoff sicherstellen und fluktuierende Wechsel zwischen Nass- und Trockenphasen ermöglichen.
Zunächst wurden Experimente durchgeführt, um die neue Apparatur zu testen. Dabei konnte gezeigt werden, dass die Apparatur über einen längeren Zeitraum vollständig automatisch und zuverlässig arbeitete. In weiteren Versuchen wurde geprüft, ob oxidationsempfindliche Sub-stanzen in der Apparatur gehandhabt werden können. Hierbei konnte gezeigt werden, dass in der Tat ein strikter Ausschluss von Sauerstoff möglich ist. Damit erwies sich die Apparatur als geeignet für die Untersuchung der Bildung von Metall¬komplexen aus OEP und diversen Metallquellen unter dem Einfluss von wechselnden NTZ. Als Metallquellen wurden unter anderem Metall(II)-chloride, Metallsulfide, Basalt und Eisenmeteorite eingesetzt. Hierbei lag der Fokus auf Eisenquellen. Tatsächlich konnte gezeigt werden, dass sich mit den ent¬sprechenden Metallquellen Eisen-, Magnesium-, Cobalt , Nickel- und Kupfer-Komplexe in einer ungewöhnlichen Reaktion bildeten (vollständig wasserunlösliches OEP reagierte mit teilweise ebenfalls unlöslichen Metallquellen), wobei sich NTZ als essenziell erwiesen. In Süßwasser konnten Ausbeuten zwischen 20 und 78 % (bezogen auf das Porphyrin) erhalten werden. Außerdem wurde der Einfluss von künstlichem Meerwasser und niedrigen pH-Werten auf die Komplexbildung untersucht.
Im zweiten Teil der Arbeit wurde der Frage nach der Stabilität von Metalloporphyrinen modellhaft anhand der Verbindung Chlorido(octaethylporphyrinato)eisen(III), [FeCl(oep)], nachgegangen. Für die durchgeführten Experimente wurden potentiell destruktive Bedingungen ausgewählt, die für Metalloporphyrine als mögliche chemische Biosignaturen relevant sind, aber auch Bedingungen, wie sie vermutlich an urzeitlichen Vulkanküsten herrschten. In umfangreichen Versuchsreihen konnte unter anderem gezeigt werden, dass (a) der Eisen-OEP-Kern über einen pH-Bereich von 0,0 bis 13,5 stabil ist, (b) [FeCl(oep)] bis ca. 250 °C in inerter Atmosphäre stabil ist, (c) eine Salzmatrix [FeCl(oep)] vor Röntgenstrahlung schützt, aber nicht vor Eisen-Partikelstrahlung, und (d) Hypochlorit, Wasserstoffperoxid, Chlorat und Salpetersäure [FeCl(oep)] oxidativ zersetzen (Oxidationswirkung in absteigender Reihenfolge). Perchlorat, das häufig im Zusammenhang mit seinem Vorkommen im Mars-Regolith genannt wird, zeigte dagegen keine Oxidationswirkung. Schließlich wird in der vorgelegten Arbeit die Bedeutung der Metalloporphyrine als potentielle Biosignaturen diskutiert, speziell vor dem Hintergrund ihrer Stabilität und möglicher abiotischer Synthesewege für diese Verbindungen
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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