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Crystal engineering of pharmaceutical co-crystals from polymorphic active pharmaceutical ingredients
Temperature induced structural changes in even-odd nylons with long polymethylene segments
Structural transitions of nylons 8 9 and 12 9 heating and cooling processes were investigated using calorimetric, spectroscopic during and real time X-ray diffraction data. These even-odd nylons had three polymorphic forms related to structures where hydrogen bonds were established in two planar directions. Heating processes showed a first structural transition at low temperature where the two strong reflections related to the packing mode of the low temperature structure (form I) disappeared instead of moving together and merging into a single reflection, as observed for conventional even-even nylons. The high temperature structure corresponded to a typical pseudohexagonal packing (form III) attained after the named Brill transition temperature. Structural transitions were not completely reversible since an intermediate structure (form II) became clearly predominant at room temperature in subsequent cooling processes. A single spherulitic morphology with negative birefringence and a flat-on edge-on lamellar disposition was obtained when the two studied polyamides crystallized from the melt state. Kinetic analyses indicated that both nylons crystallized according to a single regime and a thermal nucleation. Results also pointed out a secondary nucleation constant for nylon 12 9 higher than that for nylon 8 9, suggesting greater difficulty in crystallizing when the amide content decreased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2494–2506Peer ReviewedPostprint (author's final draft
Physics of Polymorphic Transitions in CeRuSn
We report a detailed study of the polymorphic transitions in ternary stannide
CeRuSn on high quality single crystals through a combination of X-ray
diffraction experiments conducted at 300, 275 and 120 K, and measurements of
the thermal expansion, magnetization, and resistivity, along main
crystallographic axes. In addition, the transition was followed as a function
of pressure up to 0.8 GPa. The present X-ray diffraction data show that the
room temperature polymorph consists of the lattice doubled along the c axis
with respect to the CeCoAl-type structure consistent with previous reports.
Upon cooling, the compound undergoes two successive transitions, first to a
quintuple (290 K) and than to a triple CeCoAl superstructure at 225 K. The
transitions are accompanied by a tremendous volume change due to a strong
shrinking of the lattice along the c axis, which is clearly observed in thermal
expansion. We advance arguments that the volume collapse originates from an
increasing number of crystallographically inequivalent Ce sites and the change
of ratio between the short and long Ce-Ru bonds. The observed properties of the
polymorphic transition in CeRuSn are reminiscent of the transition in
elementary Cerium, suggesting that similar physics, i.e., a Kondo influenced
transition and strong lattice vibrations might be the driving forces
Reflections on God and Evil in the Krishna Bhakti Theology of Caitanya
The story that forms the short prologue at the start of Valmiki’s Rāmāyana expresses something of how the bhakta deals with tragic loss and senseless violence. In the briefest of words, the story goes something like this: As Valmiki is performing his morning ablutions in the Tamasa River, he witnesses two cranes sweetly cooing at one another lovingly on a branch in a nearby tree. Suddenly, a hunter comes along and shoots an arrow through the chest of the male crane, who immediately drops to the ground. The female, herself alighting on the ground, sees her beloved mate writhing in agony with his mortal wound, and thus she begins to weep, drowning herself in the tears of her sorrow. Valmiki, upon seeing this senseless killing, in his anger curses the hunter from that day on to wander all the days of his life homelessly. Who can mortally wound such an innocent loving creature? What kind of person does this? What kind of senseless act is this? Where is God in all this? Questions such as these are easily elicited from those who hear this story. We shall return to this story following a brief sketch of the relationship between God and evil in a bhakti theology
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Functional plasticity of antibacterial EndoU toxins.
Bacteria use several different secretion systems to deliver toxic EndoU ribonucleases into neighboring cells. Here, we present the first structure of a prokaryotic EndoU toxin in complex with its cognate immunity protein. The contact-dependent growth inhibition toxin CdiA-CTSTECO31 from Escherichia coli STEC_O31 adopts the eukaryotic EndoU fold and shares greatest structural homology with the nuclease domain of coronavirus Nsp15. The toxin contains a canonical His-His-Lys catalytic triad in the same arrangement as eukaryotic EndoU domains, but lacks the uridylate-specific ribonuclease activity that characterizes the superfamily. Comparative sequence analysis indicates that bacterial EndoU domains segregate into at least three major clades based on structural variations in the N-terminal subdomain. Representative EndoU nucleases from clades I and II degrade tRNA molecules with little specificity. In contrast, CdiA-CTSTECO31 and other clade III toxins are specific anticodon nucleases that cleave tRNAGlu between nucleotides C37 and m2 A38. These findings suggest that the EndoU fold is a versatile scaffold for the evolution of novel substrate specificities. Such functional plasticity may account for the widespread use of EndoU effectors by diverse inter-bacterial toxin delivery systems
Structure of Equilenin at 100 K: an estrone-related steroid
The structure of the estrone-related steroid, Equilenin, C18H18O2 (systematic name 3-hy-droxy-13-methyl-11,12,13,14,15,16-hexa-hydro-cyclo-penta-[a]phen-anthren-17-one), has been determined at 100 K. The crystals are ortho-rhom-bic, P212121, and the absolute structure of the mol-ecule in the crystal has been determined by resonant scattering [Flack parameter = -0.05 (4)]. The C atoms of the A and B rings are almost coplanar, with an r.m.s. deviation from planarity of 0.0104 Å. The C ring has a sofa conformation, while the D ring has an envelope conformation with the methine C atom as the flap. The keto O atom and the methyl group are translated 0.78 and 0.79 Å, respectively, from the equivalent positions on 17β-estrone. In the crystal, mol-ecules are linked by O-H⋯O hydrogen bonds, forming chains parallel to the c-axis direction
XAS Study of the High Pressure Behaviour of Quartzlike Compounds
EXAFS spectroscopy experiments have been carried out on quartz-like GaAsO4
and AlAsO4 at high pressure and room temperature. It has been shown that these
materials exhibit two structural phase transitions; the first transition to a
high pressure crystalline form occurs at 9 GPa and is reversible upon
decompression, whereas the second transition occurs at higher pressures and is
irreversible. In GaAsO4, EXAFS measurements agree with the predicted transition
from four- to six-fold coordination of oxygen atoms around the cations, but the
two local coordination transformations are not dissociated; in fact, both As
and Ga atoms exhibit a coordination change at the onset of the first phase
transition, the rate of transformation being significantly higher for Ga atoms.
In both cases, the average bond length increases very rapidly with pressure
thus yielding the first compression stage after the transition. In the second
stage, the average bond lengths increase slowly, ultimately reaching six-fold
coordination above 28 GPa and 24 GPa for As and Ga respectively. The behaviour
of the As K-edge EXAFS is the same for both compounds, and enables us to link
the behaviour of Ga and Al atoms. The local transformations are well described
and a direct link with phosphate berlinites seems timely.Comment: 5 pages, 3 figures, LaTeX2e, J. de Physique (in press
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