1,720,979 research outputs found
FUEL-DRIVEN SYNTHESIS OF DYNAMIC COVALENT CATALYSTS
Get PDFIn this thesis, I focus my attention on applying different ratchet mechanisms to the synthesis of a molecule, thus filling the gap between non-equilibrium self-assembly and molecular machines.
In the second chapter, an energy ratchet mechanism is exploited for the synthesis of an hydrazone bond between an aldehyde and an hydrazide bearing a TACN·Zn(II) unit. In the presence of ATP hydrazone-bond formation is accelerated and the composition at thermodynamic equilibrium is shifted towards the product. Enzymatic hydrolysis of ATP installs a kinetically stable state at which hydrazone is present at a higher concentration compared to the composition at thermodynamic equilibrium in the presence of the degradation products of ATP. Initial thermodynamic equilibrium is restored by heating the sample. It is shown that the kinetic state has an enhanced catalytic activity in the hydrolysis of HPNPP, an RNA model compound.
In the third chapter, the discovery that TACN*Zn(II) compounds can hydrolyse phosphodiester bonds at higher temperatures allowed us to explore the possibility of making the same system presented in Chapter 2 autonomous. At 70 °C, the formation of hydrazone was favoured both kinetically and thermodynamically by the presence of ADP in the solution. Hydrolysis of ADP into AMP+Pi catalysed by hydrazone push the system in a state far from equilibrium in which its concentration is higher than the thermodynamic. Since the cycle is performed at high temperature equilibration of hydrazone occurs spontaneously.
In the fourth chapter, light is used to form a dimer starting from a TACN*Zn(II)-functionalized styryl pyrene which can react through [2+2] cycloaddition. Reaction passes first through the formation of a cis- isomer that dimerizes. Although this kind of reaction is well known to be reversible, in this case, it occurs only at concentrations below 200uM. The dimer has a CAC which is 30 times lower than the cac of the trans-isomer and 200 times the cac of the cis-isomer. When irradiation is performed in a range of concentrations where styryl pyrene molecule doesn’t aggregate but the dimer does, the multivalent properties of the dimer-aggregate can be exploited to have an increased catalytic activity towards HPNPP hydrolysis
Measurements of the polarization properties of foam materials useful for mm-wave polarimeters windows
No full textWe have measured in the W-band, using a custom setup, the absorption and polarization properties in transmission of foam materials (elyfoama"double dagger, styrodura"double dagger, plastazotea"double dagger, and propozotea"double dagger) useful for windows of mm-wave photometers and polarimeters. The levels of the induced polarization degree and of the absorption are very small, and difficult to measure accurately. We find induced polarization degrees lower than 0.6 %, and transmissions higher than 97 % for few centimeter thicknesses of our samples. We describe the instrumental setup, the measurements, and the impact of our findings in the design of precision polarimeters for Cosmic Microwave Background measurements. All these materials, with the exception of black plastazotea"double dagger, feature transmissions higher than 99 %, and induced polarizations lower than similar to 1 % for sample thicknesses around 2-3 cm
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
ATP Drives the Formation of a Catalytic Hydrazone through an Energy Ratchet Mechanism
Get PDF: An energy ratchet mechanism is exploited for the synthesis of a molecule. In the presence of adenosine triphosphate (ATP), hydrazone-bond formation between an aldehyde and hydrazide is accelerated and the composition at thermodynamic equilibrium is shifted towards the hydrazone. Enzymatic hydrolysis of ATP installs a kinetically stable state, at which hydrazone is present at a higher concentration compared to the composition at thermodynamic equilibrium in the presence of the degradation products of ATP. It is shown that the kinetic state has an enhanced catalytic activity in the hydrolysis of an RNA-model compound
Sequence-selective duplex formation and template effect in recognition-encoded oligoanilines
No full textA new family of duplex-forming recognition encoded oligomers, capable of sequence selective duplex formation and template directed synthesis, was developed. Monomers equipped with both amine and aldehyde groups were functionalized with 2-trifluoromethylphenol or phosphine oxide as H-bond recognition units. Duplex formation and assembly properties of homo- and hetero-oligomers were studied by F-19 and H-1 NMR experiments in chloroform. The designed backbone prevents the undesired 1,2-folding allowing sequence-selective duplex formation, and the stability of the antiparallel duplex is 3-fold higher than the parallel arrangement. Dynamic combinatorial chemistry was exploited for the templated synthesis of complementary oligomers, showing that an aniline dimer can template the formation of the complementary imine. The key role of the H-bond recognition confers to the system the ability to discriminate a mutated donor monomer incapable of H-bonding. Sequence selective duplex formation combined with the template effect makes this system an attractive target for further studies
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