1,720,967 research outputs found
The Role of Mineral Surfaces on the Reactivity of Prebiotic Chemical Species: an ab initio Study on Energetic and Spectroscopic Features
No full textThe abiotic origin of biomolecules is a fundamental question in understanding the emergence of life. Numerous models have been developed to explore how molecular complexity arose on the ancient Earth, but many aspects remain unclear. One widely recognized hypothesis is that mineral surfaces acted as catalysts, concentrators, scaffolds and protecting agents, playing a key role in abiogenesis. This thesis aims to characterize the interactions between prebiotic molecules and mineral surfaces representative of the primordial Earth's chemical inventory.N-methylformamide (NMF) and glycolaldehyde (GA) were selected as target molecules. NMF due to its similarity to formamide, which has been extensively studied as a precursor of nitrogenous compounds and intermediates of the Krebs Cycle. GA, on the other hand, is a crucial precursor of sugars as it is a key intermediated in the early steps of the formose reaction. The chosen mineral substrates, on the other hand, titanium dioxide (TiO2) and edingtonite (EDI), represent oxides and silicates—minerals abundant in the Earth's crust during the Hadean Eon.Computational simulations were performed employing both periodic and cluster approaches using the CRYSTAL17 and Gaussian 16 quantum chemical packages, respectively. Geometry optimizations and harmonic frequency calculations were carried out using the B3LYP functional combined with DFT-D3 dispersion correction. Double-ζ basis sets were employed: cc-pVDZ for GA and NMF, pob-DZVP for periodic models of EDI, and 86-51G*/8-411G for titanium and oxygen atoms in TiO2. Thermodynamically stable surfaces were modeled, specifically anatase (101), rutile (110), and edingtonite (100). Upon the thus obtained structures energetic results were refined applying higher levels of theory. The adsorption of NMF on TiO2 was investigated under various conditions. Computations revealed that the cis-NMF conformer is favored due to its potential of forming a dual interaction with the surface; both physisorption and chemisorption mechanisms were simulated on both polymorphs. Introducing co-adsorbed water molecules resulted in favoring physisorption, with cis- and trans-NMF exhibiting similar behavior. Spectroscopic features of adsorbed NMF were compared to gas-phase data, with vibrational shifts highlighting the effect of the adsorption. In addition to quantum chemical simulations, the adsorption interaction of NMF on both anatase and rutile was investigated experimentally by Diffuse Reflectance Infrared Fourier Transform Spectroscopy. Both experimental evidence and computational results point to an adsorption process that involves anchoring through both the CO and NH groups of NMF.The interaction between GA and EDI focused on the silica-mediated second step of the formose reaction, where formaldehyde (FA) adds to GA to form glyceraldehyde (GCA). The reaction mechanism was explored by quantum chemical simulations performed at various degrees of sophistication to shed light on the thermochemical and kinetic feasibility of the reaction. The same pathway was also investigated in the gas-phase, in order to disentangle the role played by the zeolitic mineral. The obtained results show that the endothermic reaction between GA and hydroxymethylene (HCOH, an active conformer of FA) yields GCA by a submerged reaction path, both in the gas-phase and on the edingtonite surface. The mineral substrate provides further stabilization, by about 20 kcal∙mol-1, of all the species involved in the reaction pathway and acts as a scaffold favoring the interaction of the two reactantsIn summary, the research carried out sheds light into the atomistic details of the interactions between prebiotic molecules and minerals present on the primordial Earth to get insights into the role they could have played in promoting molecular evolution
Mechanistic Insights into the Silica-Mediated Synthesis of Glyceraldehyde from Glycolaldehyde and Hydroxymethylene
No full textMinerals are crucial ingredients in prebiotic chemistry as they could have promoted
the evolution of simple organic molecules toward proto-biomolecules that are on the route of the
emergence of self-replicating information-rich macromolecules. In this respect, the formose
reaction, involving the sequential autocatalytic condensation of formaldehyde, is the generally
accepted pathway to sugar synthesis. Although obtained under controlled laboratory conditions,
with enhanced sugar yields promoted by the presence of silicate in the reaction medium, it presents
a number of limitations, and the underlying reaction mechanism remains an unsolved riddle. In
this work, the focus is on the second step of the formose reaction, namely the synthesis of
glyceraldehyde, which is accomplished by considering the reaction between glycolaldehyde and
hydroxymethylene taking place on the edingtonite mineral. The reaction mechanism is explored
by quantum chemical simulations performed at various degrees of sophistication to shed light on
the thermochemical and kinetic feasibility of the reaction. The same pathway is also investigated
in the gas-phase, in order to disentangle the role played by the zeolitic mineral. The obtained results
show that the endothermic reaction between glycolaldehyde and hydroxymethylene yields
glyceraldehyde by a submerged reaction path, both in the gas-phase and on the edingtonite surface.
The mineral substrate provides further stabilization, by about 20 kcal mol-1, of all the species
involved in the reaction pathway and acts as a scaffold favoring the interaction of the two reactants
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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