1,720,958 research outputs found
Biosynthesis of fluoroacetate and 4-fluorothreonine in Streptomyces cattleya
Full text linkThis thesis describes results on the biosynthesis of fluoroacetate and 4-fluorothreonine by the bacterium Streptomyces cattleya. Exploratory (^14)C-radiolabelling experiments, designed to determine the relative incorporation of a range of metabolites into fluoroacetate, demonstrated that substrates such as glucose, glycine and serine were efficient precursors. [2-(^13)C]-Glycolate, [l,2-(^13)C(_2)]-glycolate, [2,2-(^2)H(_2)]-glycolate, DL-[3,4,4,4-(^2)H(_4)]-threonine, DL-[4,4,4-(^2)H(_3)]-threonine, [3,3,3-(^2)H(_3)]-alanine, [3-(^2)H]-fluoropyruvate and [3-(^2)H]-3-fluorolactate were synthesised and characterised. Incorporations of synthesised and commercially available putative precursors, [l-(^13)C]-glycine, [2-(^13)C]-glycine, [1,2-(^13)C(_2) glycine, [3-(^13)C]-serine, [l-(^13)C]-pyruvate, [2-(^13)C]-pyruvate, [3-(^13)C]-pyruvate, [2-(^13)C]-acetate, [3-(^13)C]-alanine, [2,3,3-(^2)H(_3)]- aspartate, [2,2,3,3-(^2)H(_4)]-succinate and [2-(^13)C]-glycerol into fluoroacetate and 4-fluorothreonine were assessed. [2-(^13)C]-Glycerol appeared to be the highest incorporated precursor of those tested. The (^13)C label from [2-(^13)C]-glycerol predominated at G-1 (-56%) of fluoroacetate and C-3 (57%) of 4-fluorothreonine. (^19)F-NMR emerged as a powerful analytical tool by which to determine the regiospecific incorporation of '^G and into the fluorometabolites. A conclusion to emerge from this study is that a glycolytic intermediate in S. cattleya appears to be a subsfrate for the bio-fluorination process. Incubation studies with [3-(^2)H]-fluoropyruvate and [3-(^2)H]-3-fluorolactate, tested as possible initial fluorination products, showed low incorporation of deuterium atoms into the fluorometabolites which did not reinforce a role for 2-phosphoglycerate,3- phosphoglycerate and glycerol-3-phosphate as substrates for the fluorinating enzyme. The incorporations which are almost identical into the two fluorometabolites strongly indicate that the assembly of the carbon skeleton of fluoroacetate and C-3 and C-4 of4- fluorothreonine involves a common biosynthetic intermediate. Thus the close metabolic relationship between the two fluorometabolites indicates the presence of a single fluorinating enzyme in S. cattleya. A three step route to 4-(25;55)-fluorothreonine was developed by a modification of Seebach's imidazolidinone methodology for the preparation of L-threonine and is the subject of Chapter 4. An X-ray crystal structure of the synthetic 4-(2S,3S)- fluorothreonine assisted in assignment and confirmation of the absolute stereochemistry of the natural product. This modified methodology is amenable to isotope labelling by employing NaB(^2)H(_4) to generate [3-(^2)H]-4-fluorothreonine. Moreover it offers an alternative route to threo-amino acids and has the advantage of using acid chlorides in place of aldehydes when the required aldehyde is unavailable
Sensing Applications of Nanomechanical Resonators
No full textNanomechanical resonators are explored as mass sensors and molecular sensors due to their high sensitivity and real time label-free detection ability, respectively. One downside of mechanical molecular sensing is the lack of chemical information carried in the signal. For this reason, a secondary technique such as functionalized surface, gas chromatography, or photothermal spectroscopy, must be combined with mass sensing for chemical specificity. As an alternative to this approach, we have developed a metallized nanomechanical resonator, capable of electrical local heating. Here, we explore the use of local temperature as a secondary molecular indicator similar to thermogravimetric analysis. In the present study, we explore the desorption of a polymer from these electrical nanostring sensors above room temperature and the adsorption of acetone below room temperature
Determination of the Energy Levels of Eu2+ for Next-Generation LED Phosphors
Full text linkThe structural-property relationship of any novel materials, whether nanoscaled or in the bulk form must be understood for their successful applications. Soft X-ray spectroscopy is used to probe the electronic structures of materials. Furthermore, density functional theory calculations are used to interpret the experimental observations, which allows linking the electronic properties to the underlying atomic arrangements and luminescence parameters. In this thesis, InN and phosphor-converted light emitting diodes (pc-LEDs) are studied using these experimental techniques and theoretical methods.
InN is a key material for technological applications in opto-electronic devices because of its relatively small band gap; however, there is a persistent and active debate about its varying band gap values and their origin. The electronic structure and the band gap of ammonothermal InN powder samples are studied as a first part of this thesis. Further the origin of the measured band gap is discussed in terms of the presence of oxygen impurities and the impurity phases
of InN.
pc-LEDs can significantly reduce global energy consumption and are expected to dominate the lighting market. In pc-LEDs, the energetic position of activator ions with respect to the conduction band (CB) and valence band (VB) of the host lattice determines the electronic and luminescence properties including multicolor emission, adjustable bandwidth, and thermal quenching under application conditions. The second part of my study involves the direct measurements of the energy levels of activator Eu2+ ions for three highly efficient color-sensitive Eu2+-doped phosphors such as ultranarrow band blue-emitting oxoberyllates AELi2[Be4O6]:Eu2+ (AE = Ba, Sr), red-luminescence SrLi2Al2O2N2:Eu2+, and blue-luminescence nitridoberyllates MBe20N14:Eu2+ (M = Ba, Sr). New characterization methods with existence techniques such as soft X-ray resonant inelastic X-ray scattering (RIXS) and X-ray excited optical luminescence (XEOL) measurements are used for directly determining the energy level of Eu2+ ions. Modern phosphors use the 4f65d1 to 4f7 transition of Eu2+, where 4f65d1 is an excited state. We directly determine the energetic separation between the Eu2+ 4f65d1 state and the CB of host lattice using RIXS measurements. Thus, the location of the Eu2+} 4f65d1 states is below the CB of the corresponding energy separation. The 4f7 energy level is determined from the 4f65d1 4f7 transition of Eu2+ ions with respect to the 4f65d1 states using XEOL techniques. Therefore, we determine all participating energy levels of Eu2+ ions (4f65d1 and 4f7 states) of the studied phosphors. Thus, we identify all radiative processes involved in the design of next generation pc-LEDs. These experimental techniques allow direct access to the intragap states of the Eu2+-dopants, which are the source of the luminescence of Eu2+-doped phosphors
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
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