1,720,962 research outputs found
Modelling the dependence on medium, ionic strength and temperature of N-acetyl-L-cysteine acid-base properties, and its interactions with Na+, Mg2+, Ca2+ and Zn2+
No full textN-acetyl-L-cysteine (NAC) is the acetylated precursor of L-cysteine, and it is strictly related to glutathione activity. NAC is used in medicine since more than 30 years, primarily as a mucolytic, though its properties are exploited in the treatment of many diseases, such as acute poisoning by acetaminophen (restoring protective levels of glutathione), HIV, contrast-induced nephropathy, type-2 DM (diabetes mellitus), and it also looks promising in the treatment of several psychiatric disorders [1]. This resulted in a huge number of scientific contributions published during the years on NAC (searching in some scientific databases, more than 10000 papers appear).
As expected, most of them are focused on the biological and therapeutic activity of NAC, while relatively few data are reported on the solution chemistry of this molecule, despite the thorough knowledge of its acid-base and coordination behaviour are of fundamental importance for the understanding of its properties in aqueous solution, as biological fluids are. Moreover, the relatively few thermodynamic data available (necessary to assess the speciation of this ligand in the system under study) are reported in single, specific conditions, while it is well known that the most of biological fluids (and natural waters) are, from a chemico-physical perspective, multielectrolyte aqueous solutions of very variable composition, ionic strength, and temperature [2].
Therefore, in this contribution we report the results of an investigation on the modelling of the acid-base properties of NAC in different ionic media (sodium and tetramethylammonium chlorides and tetraethylammonium iodide) at different ionic strengths (0 < I / mol dm-3 ≤ 3.0) and temperatures (283.15 ≤ T / K ≤ 318.15). Due to their importance from a biological perspective, we also report the results on the binding ability of NAC towards Na+, Mg2+, Ca2+ and Zn2+ in NaClaq at T = 298.15 K and different ionic strengths (0 < I / mol dm-3 ≤ 2.0). Surprisingly, despite the importance of these cations and their involvement in many biological mechanisms strictly connected with NAC activity, to our knowledge almost no data (only some for Zn2+) are present in literature on these interactions. The dependence on medium, ionic strength and temperature of the protonation and complex formation constants obtained has been modelled by classical approaches, such as the Extended Debye-Hückel (EDH), Specific ion Interaction Theory (SIT), Pitzer, and by the van’t Hoff equation
Polynuclear EDTA Complexes in Aqueous Solution: A Proof of Concept
Full text linkEthylenediaminetetraacetic acid (EDTA) is, so far, the most known and widely used chelating agent. It is well known to form very stable mononuclear complexes with several metal ions in aqueous solution. However, its multidentate nature opens the possibility to simultaneously bind more than one cation forming polynuclear species, rarely observed for EDTA, but common for its analogues like, e.g., DTPA and TTHA. If formed, these species could significantly affect the speciation of both EDTA and metal ions in solution. As such, in this work we report the result of a potentiometric investigation on the chemical speciation of three model systems, namely: Sn2+/Zn2+/EDTA, Sn2+/Fe2+/EDTA and Zn2+/Fe3+/EDTA, at T = 298.15 K and I = 0.15 mol⋅dm‒3 (in NaNO3(aq) or NaClO4(aq)), as a proof of concept that simple and mixed polynuclear species can be also formed by EDTA. In fact, experimental data analysis evidenced the formation of not only the simple Sn2(EDTA), Zn2(EDTA) and [Fe2(EDTA)]2+ species, but also the mixed SnZn(EDTA), SnFe(EDTA) and [ZnFe(EDTA)]+. The formation of latter species was then analyzed in terms of thermodynamic extra stability with respect to simple species, and their influence on the speciation of EDTA and the investigated cations was evaluated. Furthermore, quantum mechanical calculations were also performed for a better insight on the binding mode of EDTA when forming dinuclear species, and to support the evidence concerning the extra stability of mixed ones. We could conclude that, when forming dinuclear species, EDTA behaves as a tridentate ligand, binding cations through two carboxylates and one amino group (MIDA-like mode)
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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