1,721,014 research outputs found
Experimental and theoretical charge density study of an antimalarial drug
No full textMalaria, an infection caused by the Plasmodium Falciparum protozoa, is nowadays one of the most lethal parasitic disease. As the Plasmodium protozoon is becoming resistant to quinoline-based molecules, the development of new drugs and the understanding of the key chemical features for their activity and of their mechanism of action is of great importance. In this context, we carried out a thorough analysis on the antimalarial drug dihydroartemisinin (DHA, Figure 1), through the study of its experimental and theoretical charge density (CD) distributions.[1]
The experimental CD has been obtained by a single-crystal X-ray diffraction experiment at T = 100 K on a Bruker SMART APEX II diffractometer equipped with a CCD area detector, while the corresponding theoretical CD has been derived through fully periodic single point DFT calculations at the experimental geometry.
We have identified nucleophilic as well as electrophilic regions of the molecule by analyzing its electrostatic potential and investigated the crystal packing and the change in the CD distribution moving from the isolated molecule to the crystal. Several CD analysis tools, with special emphasis on the Quantum Theory of Atoms in Molecules (QTAIM) [2], have been adopted, with the aim of fully characterize the chemical nature of specific functional groups, such as the peroxide group and the polyether chain.
We have also performed geometry optimizations on deprotonated and radical anion of DHA, the latter being the intermediate species in most of the proposed antimalarial modes of action of the drug.
[1] G. Saleh, R. Soave, L. Lo Presti, R. Destro Chem. Eur. J. 2013, 19, 3490.
[2] R. F. W. Bader Atoms in Molecules: A Quantum Theory Oxford University Press, Oxford, 1990
Structural behaviour and comparison of CGF panels
No full textA CGF Panel (Concrete Glulam Framed Panel) is a concrete panel with a glued laminated timber frame. The experimentation on this new construction system at LabSCo (Laboratory of Strength of Materials) of IUAV University of Venice, inspired a wide research on buildings made of this construction system investigating in different aspect of building behaviour: particularly about mechanical property of the materials, mechanical of the system and building physics. This paper presents the results of quasi-static in-plane tests on single panel and configurations of some different panels. The tests in the laboratory are used for measuring the in-plane strength and stiffness of individual panels and wall sections consist of some panels in order to verify and measure the behavior of the connections between the various parts of the single panel and the connection between the panels. Thanks to the results obtained it was possible carry out the FE model to calibrate the characteristics in relation to experimental data. Finally, in order to compare this constructive system with the well known X-lam systems, on the basis of the calibration of the models we were able to set up a comparable FE model with those of the X-lam wall described in the publication: Quasi-Static and Pseudo-Dynamic Tests on XLAM Walls and Buildings inherent in the SOFIE project coordinated by the CNR-IVALSA (Italian National Research Council - Trees and Timber Institute). © (2014) Trans Tech Publications, Switzerland
On the interplay between CH...O and OH...O interactions in determining crystal packing and molecular conformation : an experimental and theoretical charge density study of the Fungal Secondary Metabolite Austdiol (C12H12O5)
No full textThe total exptl. electron d. r(r), its Laplacian .del.2r(r), the mol. dipole moment, the electrostatic potential j(r), and the intermol. interaction energies have been obtained from an extensive set of single-crystal X-ray diffracted intensities, collected at T= 70(1) K, for the fungal metabolite austdiol (1). The exptl. results have been compared with theor. densities from DFT calcns. on the isolated mol. and with fully periodic calcns. The crystal structure of (1) consists of zigzag ribbons extended along one cell axis and formed by mols. connected by both OH...O and CH...O interactions, while in a perpendicular direction, adjacent mols. are linked by short CH...O intermol. contacts. An extensive, quant. study of all the intra- and intermol. H...O interactions, based not only on geometrical criteria, but also on the topol. anal. of r(r), as well as on the evaluation of the pertinent energetics, allowed us (i) to assess the mutual role of OH...O and CH...O interactions in detg. mol. conformation and crystal packing; (ii) to identify those CH...O contacts which are true hydrogen bonds (HBs); (iii) to det. the relative hydrogen bond strengths. An exptl., quant. evidence is given that CH...O HBs are very similar to the conventional OH...O HBs, albeit generally weaker. The comparison between exptl. and theor. elec. dipole moments indicates that a noticeable charge rearrangement occurs upon crystn. and shows the effects of the mutual cooperation of HBs in the crystal. The total intermol. interaction energies and the electrostatic energy contribution obtained through different theor. methods are reported and compared with the exptl. results. It is found that the new approach proposed by Spackman, based on the use of the promol. charge d. to approx. the penetration contribution to intermol. electrostatic energies, predicts the correct relative electrostatic interaction energies in most of the cases
The fungal metabolite austdiol
No full textThe title compound, (7R,8S)-7,8-dihydroxy-3,7-dimethyl-6-oxo-7,8-dihydro-6H-isochromene-5-carbaldehyde, C12H12O5,
is a trans-vicinal diol. Of the two fused rings, which lie approximately in the same plane, the pyran ring is almost perfectly planar, while the cyclohexenone ring adopts a
slightly distorted half-chair conformation. The crystal packing is dictated by two strong intermolecular O-H···O interactions,
one involving hydroxy and keto groups, the other involving two hydroxy groups. Molecules are linked together through twofold axes, forming zigzag ribbons extended along the a axis
Physicochemical properties of zwitterionic L- and DL-alanine crystals from their experimental and theoretical charge densities
No full textThe total experimental electron density distributions p(r) of zwitterionic L- and DL-alanine crystals, as derived from extensive sets of X-ray diffracted intensities collected at 23 and 19 K, are compared to gain an insight into the different physical properties of the two related chiral compounds in the solid state and to explore the extent of the p(r) transferability. Relevant parameters that characterize the two crystal forms are obtained, showing differences and similarities in terms of (i) geometric descriptors, (ii) topological indexes, (iii) molecular electrostatic potential Φ(r) distributions, (iv) atomic volumes and charges, (v) molecular electric moments, and (vi) electrostatic interaction energies. To assess the relative stability of the racemate with respect to the pure enantiomer, the crystal lattice energies, as obtained through DFT fully periodic calculations, are also discussed and compared with the experimental sublimation enthalpies after correction for the proton-transfer energies. In-crystal group charges, evaluated with the quantum theory of atoms in molecules, are found to be transferable between the racemic and the pure enantiomer, at variance with group volumes. Similarly, molecular first and third moments are not strictly transferable and indicate that for the zwitterionic alanine molecule the molecular charge distribution in the DL-crystal is more polarized in the c direction by about 10%. By contrast, quantitative agreement is observed for second and fourth moments. Significant differences arise from (1) the crystal packing of the dipole vectors, which are aligned in an antiparallel fashion in the L-crystal, to be compared with a parallel alignment in the racemate, due the polar space group Pna21 of the latter, (2) the strongly attractive electrostatic energy of a homochiral pair in the L-crystal, which is opposed to the corresponding heterochiral pair in the DL-crystal form. The difference between these Ees values amounts to 135-150 kJ mol-1. Despite this, the two crystal forms are predicted as equally thermodynamically favored by the theoretical P-B3LYP estimates of the crystal lattice energies. Finally, the necessity of an upgrading of the dispersion and exchange-repulsion terms currently adopted within the experimental charge density approach to intermolecular interactions is recognized and discussed
The crystal structure of 10-bromoanthrone
No full textCrystals of 10-bromoanthrone are triclinic, space group PT, with a=7"304 (1), b=7-768 (1), c=
11.358 (1)~, e=90.33 (1), ,8=104.58 (1), y= 116.83 (1) °, Z=2. Approximately 2300 intensities were
collected on a Pailred diffractometer from a crystal mounted along a. The structure was derived by direct
phasing methods and refined by full-matrix least-squares calculations. Final refinement led to an R of
0.061 and standard deviations in the range 0.003-0.006 ,~ in the positions of the heavy atoms. The molecule,
whose symmetry is very close to m, exhibits a 'butterfly' shape: the planes of the two outer rings
make a dihedral angle of 164 °. The most salient details of the molecular geometry are the long C(10)-Br
distance of 2.032 (4) ,~, and the boatlike configuration of the central ring, the atoms C(10) and
Approximate anisotropic displacement parameters for H atoms in molecular crystals
No full textAnisotropic displacement parameters for H atoms in molecular crystals are calculated with a simplified model that uses approximate vibrational modes, neglecting correlations between internal and external contributions to the mean square motions. These calculated values agree within a few esds with the ones obtained from neutron diffraction; used in charge density multipolar refinements against X-ray diffraction data, they yield electron densities and derived electrostatic properties that are more accurate than the ones obtainable with the traditional isotropic description. For example, electric field gradients at the H nuclei of L-Alanine are in quantitative agreement with Nuclear Quadrupole Resonance results
Experimental and theoretical charge density distribution of the colossal magnetoresistive transition metal sulfide FeCr2S4
No full textThe total charge density distribution rho(r) of the colossal magnetoresistive transition metal sulfide FeCr2S4 was evaluated through a multipole formalism from a set of structure factors obtained both experimentally, by means of single crystal high-quality x-ray diffraction data collected at T=23 K, and theoretically, with an extended-basis unrestricted Hartree-Fock periodic calculation on the experimental geometry. A full topological analysis, followed by the calculation of local energy
density values and net atomic charges, was performed using the quantum theory of atoms in molecules. The experimental and theoretical results were compared. Good agreement was found for the topological properties of the system, as well as for the atomic net charges and the nature of the chemical bonds. An analysis of the electron density rho(r), its Laplacian L(r), and the total energy density H(r) at the bond critical points was employed to classify all the interactions that resulted as predominantly closed shell (ionic) in nature. The topological indicators of the bonded interactions for Fe are distinct from those for Cr. The Fe–S bond distances were found to be 0.145 Å shorter than the ideal values computed on the basis of Shannon’s crystal radii, much shorter than the Cr–S distances with respect to their ideal Shannon lengths. Concomitantly, rho(r) and H(r) at the bond critical points are greater for Fe–S interactions, indicating that the local concentration of charge density in the internuclear region is larger for the tetrahedrally coordinated iron than for the octahedrally coordinated chromium. The isosurface in the real space for L(r)=0 was plotted for both iron and chromium, pointing out the local zones of valence shell charge concentration and relating them to the partial d-orbital occupancy of the two transition metal atoms
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
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