1,720,966 research outputs found
Looking for indirect correlations among charge density and NLO properties: the case of a simple pyridinium tetrazolate
No full textThe development of novel second-order nonlinear optical (NLO) organic materials in the solid state is an important and growing research field, as it is attractive for laser industry, electro-optic devices, data safety issues, and so on. In recent decades, some experimental electron charge density studies have been performed on solid organic NLO-phores, aimed at extracting from the ground-state charge density the information on the hyperpolarizability tensor [1]. However, other groups [2] claimed that the one-particle charge density does not contain sufficient information to determine the NLO properties. Even though it is impossible to obtain reliable quantitative approximations of the beta tensor elements directly from the charge density, it is also true that the former are influenced by the molecular structure and intermolecular interactions. In this work, we look for possible indirect correlations between topological properties of the charge density and non-linear optical properties in a suitable, simple test-case. We focus on N-methyl pyridinium tetrazolate (PYRTE) [3], a high optical gap chromophore exhibiting high thermal, chemical and photochemical stabilities, whose NLO are observable only in solution due to the centrosymmetric crystal structure. First, we investigate how the crystal field affects molecular dipole moment, and therefore optical properties, of the solid. As expected, the in-crystal molecular dipole moment undergoes a significant enhancement, determining an important change in the second-order tensorial parameters of PYRTE. We then compare the predicted hyperpolarizability of the title compound with that of suitable chemical derivatives. Possible correlations among non-local integral topological descriptors, such as the Source Function, and the computed second order hyperpolarizabilities are tentatively established. Eventually, we look for possible PYRTE derivatives that, while maintaining good predicted NLO properties, are also potentially able to crystallize in non-centrosymmetric space groups. To this end, the CLP model [4] is applied to rank in silico generated structures from a thermodynamic viewpoint.
Acknowledgments: F. Beghi gratefully acknowledges travel support by AIC (Italian Crystallographic Association).
[1] T. S. Koritsanszky et al. Chem.Rev. 2001, 101, 1583
[2] A. E. Whitten et al. J. Chem. Phys. 2006, 125, 174505
[3] Beverina et al. Chem. Comm. 2011, 47, 292
[4] Gavezzotti, The Coulomb-London-Pauli (CLP) model of intermolecular interaction: Description and User’s Manual v. 3.6.2013; Milano, 2013
On the interplay among non-covalent interactions and activity of 4-aminoquinoline antimalarials: a crystallographic and spectroscopic study
Full text linkMalaria is due to protozoa of the genus Plasmodium, which infect human red blood cells and digest the host hemoglobin. Degradation of the latter in the acidic food vacuole (pH ~ 5) releases free hematin (hydroxylated Fe protoporphyrin-IX, FePPIX(OH)), which is toxic to the parasite [1]. Therefore, Plasmodium deactivates hematin by promoting its crystallization into harmless pale yellow P1bar crystals of beta-hematin. 4-aminoquinoline drugs (AQ), such as chloroquine (CQ) and piperaquine (PQ), interfere with this detoxification process, either by coordinating free heme in solution [2], or by poisoning fastest-growing crystal faces of beta-hematin [3]. However, there is no general consensus on the structure of the AQ/heme complex [4], which depends on various chemical variables (aqueous/lipidic environment, pH).
We here aim at quantitatively disclosing the chemical physics underlying the pharmacophoric features of CQ and PQ in the context of predicting which chemical modifications should be applied on the AQ scaffold to enhance the drug functionality against the biochemical resistance mechanism evolved by Plasmodium [5,6].
EXAFS spectroscopy in solution across the Fe Kalpha absorption edge (~ 7.1 keV) explored the first shell coordination geometry of iron in hematin, both in the presence and in the absence of AQ systems. Differences in the signal were related to the possible occurrence of a direct Fe–N coordinative bond involving the quinoline nitrogen atom, which might coexist with other possible (e.g. pi···pi stacked) adduct geometries [5] (Fig. 1). Quantum mechanical DFT calculations showed that an aliphatic tertiary NH+ amino group might also be a crucial part of the pharmacophore (Fig. 1), as it is able to set up strong charge-assisted hydrogen bonds with proprionate groups of hematin. This complies well with single-crystal X-ray diffraction outcomes on the CQ dihydrogen phosphate salt at 103 K[6], where H2PO4– ions form hydrogen-bonded pillars which strongly interact with positively charged chloroquine molecules. Comparison of the CQ crystal structure with those of various hydrated salts of PQ (NO3–, SO42–, H2PO4–), grown by advanced sol-gel methods, disclosed subtle analogies and differences in the non-covalent interaction networks of the two drugs, which are also related to their solubilities.
[1] L. Kořený, M. Oborník, J. Lukeš, PLoS Pathog. 2013, 9(1), e1003088.
[2] D.C. Warhurst J.C. Craig, K.S. Raheem, Biochem. Pharmacol. 2007, 73, 1910.
[3] M.S.Walczak, K. Lawniczak-Jablonska, A. Wolska, A. Sienkiewicz, L. Suarez, A.J. Kosar, D.S. Bohle J. Phys. Chem. B 2011, 115, 1145.
[4] J. Gildenhuys, T. Roex, T.J. Egan, K.A. De Villiers, J. Am. Chem. Soc. 2013, 135, 1037.
[5] G. Macetti, S. Rizzato, F. Beghi, L. Silvestrini, L. Lo Presti, Physica Scripta 2016, 91, 023001.
[6] G. Macetti, L. Loconte, S. Rizzato, C. Gatti, L. Lo Presti, Crystal Growth Des. 2016. 16, 6043
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
Unveiling interactions of the antimalarial drug chloroquine with haeme in aqueous solutions through spectroscopic and quantum mechanical methods
No full textMalaria is one of the most worldwide spread parasitic disease. It is caused by Plasmodium protozoa, which eventually infect human erythrocytes and digest the host haemoglobin. This process releases free haeme (Fe-protoporphyrin-IX), which is toxic to the parasite as it produces reactive oxygen species (ROS), the cause of oxidative stress. The protozoon deactivates haeme by promoting its crystallization into solid pale-yellow hemozoin, that gives the characteristic skin color of malaria-infected people. Aminoquinoline-type (AQ) drugs interfere with this detoxification process either by directly hampering haeme- haeme self-recognition in solution [1] or by preventing the growth of hemozoin crystals [2] (Figure 1). The nature of the specific AQ compound-haeme scaffold interactions is not yet understood, even though it is a necessary requirement to explicate antiplasmodial activity. We report here on an experimental and theoretical study of the AQ- type antimalarial chloroquine (CQ)-free haeme interactions in aqueous solutions. Extended X-ray Absorption Fine Structure (EXAFS) experiments at the Fe Kα absorption edge (7.1 keV) were performed at the BM26A station of the ESRF facility in Grenoble (FR) on various haeme-containing solutions, both in the presence and in the absence of CQ. The effect of pH was monitored through the addition of suitable buffers in the 4–7 range at variable pH interval. A tensioactive (sodium dodecyl sulfate) at its critical micellar concentration was also employed to model lipidic nanodroplets in the parasite food vacuole, as their presence was reported [3] to favor hemozoin crystallization (Figure 1). EXAFS results were complemented by accurate UV absorption measurements of the same solutions and DFT B3LYP 6-311G(d,p) simulations of possible haeme:chloroquine adduct geometries. We found evidence that, at least in the experimental conditions here employed, CQ does not set stacking π···π interactions with the protoporphyrin scaffold, even though this geometry was proposed as the most probable one through molecular mechanics simulations [4,5] and previous EXAFS studies of mesohaematin anhydride in dimethylsulfoxyde [6]. Rather, our DFT calculations point out that CQ and haeme seem to recognize each other through electrostatic interactions among lateral charged groups. If proven true, this would have obvious implications on the engineering of novel antimalarials able to thwart the parasite adaptability against classical AQ-based therapies. Acknowledgments: This work has been supported by Unimi Development Plan – Line B1 and CINECA – [1] Slater et al., Proc. Natl. Acad. Sci. USA 1991, 88, 2, [2] Gildenhuys et al., J. Am. Chem. Soc. 2013, 135, 3, [3] Pisciotta et al. Biochem. J., 2007, 402, 1, 197. [6] Walczac et al., J. Phys. Chem. B, 2011, 115, 5, 11 [4] Warhurst et al, Biochem. Pharmacol. 2007, 73, 12, 1910. [5] Otelo et al., Bioorganic & Medicinal Chemistry Letters 2011, 21, 1, 250
Crystal growth in gelled solution : applications to coordination polymers
Full text linkSome critical issues concerning the complex synthesis and crystallization of designed coordination polymers are highlighted by using gels as diffusion media. The systematic use of gel methods in a variety of different crystallization configurations has been proven useful in revealing the field of existence and the stability hierarchy of competitive and concomitant crystalline phases, and also in discovering some important effects due to gel matrices. The most interesting phenomena observed are the modification and alteration of the macro- and microscopic morphology of crystals due to gel incorporation, which are responsible for changes in many properties and functionalities of these materials, opening up new possibilities for their applications in a variety of technological fields. In addition, a kinetic study of a gel grown polymeric system has also been conducted, demonstrating the continuous evolution of crystal morphology and its connection with the crystal growth process and growth mechanism
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
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