251 research outputs found
Structure, Energetics, and Dynamics of Screw Dislocations in Even n-Alkane Crystals
Spiral hillocks on n-alkane crystal surfaces were observed immediately after Frank recognized the importance of screw dislocations for crystal growth, yet their structures and energies in molecular crystals remain ill-defined. To illustrate the structural chemistry of screw dislocations that are responsible for plasticity in organic crystals and upon which the organic electronics and pharmaceutical industries depend, molecular dynamics was used to examine heterochiral dislocation pairs with Burgers vectors along [001] in n-hexane, n-octane, and n-decane crystals. The cores were anisotropic and elongated in the (110) slip plane, with significant local changes in molecular position, orientation, conformation, and energy. This detailed atomic level picture produced a distribution of strain consistent with linear elastic theory, giving confidence in the simulations. Dislocations with doubled Burgers vectors split into pairs with elementary displacements. These results suggest a pathway to understanding the mechanical properties and failure associated with elastic and plastic deformation in soft crystals
Dichroism in helicoidal crystals
Accounting for the interactions of light with heterogeneous, anisotropic, absorbing, optically active media is part of the characterization of complex, transparent materials. Stained biological structures in thin tissue sections share many of these features, but systematic optical analyses beyond the employ of the simple petrographic microscopes have not be established. Here, this accounting is made for polycrystalline, spherulitic bundles of twisted d-mannitol lamellae grown from melts containing light-absorbing molecules. It has long been known that a significant percentage of molecular crystals readily grow as helicoidal ribbons with mesoscale pitches, but a general appreciation of the commonality of these non-classical crystal forms has been lost. Helicoidal crystal twisting was typically assayed by analyzing refractivity modulation in the petrographic microscope. However, by growing twisted crystals from melts in the presence of dissolved, light-absorbing molecules, crystal twisting can be assayed by analyzing the dichroism, both linear and circular. The term "helicoidal dichroism" is used here to describe the optical consequences of anisotropic absorbers precessing around radii of twisted crystalline fibrils or lamellae. d-Mannitol twists in two polymorphic forms, α and Ύ. The two polymorphs, when grown from supercooled melts in the presence of a variety of histochemical stains and textile dyes, are strongly dichroic in linearly polarized white light. The bis-azo dye Chicago sky blue is modeled because it is most absorbing when parallel and perpendicular to the radial axes in the respective spherulitic polymorphs. Optical properties were measured using Mueller matrix imaging polarimetry and simulated by taking into account the microstructure of the lamellae. The optical analysis of the dyed, patterned polycrystals clarifies aspects of the mesostructure that can be difficult to extract from bundles of tightly packed fibrils
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Intracellular trafficking, localization, and mobilization of platelet-borne thiol isomerases
OBJECTIVE:
Thiol isomerases facilitate protein folding in the endoplasmic reticulum, and several of these enzymes, including protein disulfide isomerase and ERp57, are mobilized to the surface of activated platelets, where they influence platelet aggregation, blood coagulation, and thrombus formation. In this study, we examined the synthesis and trafficking of thiol isomerases in megakaryocytes, determined their subcellular localization in platelets, and identified the cellular events responsible for their movement to the platelet surface on activation.
APPROACH AND RESULTS:
Immunofluorescence microscopy imaging was used to localize protein disulfide isomerase and ERp57 in murine and human megakaryocytes at various developmental stages. Immunofluorescence microscopy and subcellular fractionation analysis were used to localize these proteins in platelets to a compartment distinct from known secretory vesicles that overlaps with an inner cell-surface membrane region defined by the endoplasmic/sarcoplasmic reticulum proteins calnexin and sarco/endoplasmic reticulum calcium ATPase 3. Immunofluorescence microscopy and flow cytometry were used to monitor thiol isomerase mobilization in activated platelets in the presence and absence of actin polymerization (inhibited by latrunculin) and in the presence or absence of membrane fusion mediated by Munc13-4 (absent in platelets from Unc13dJinx mice).
CONCLUSIONS:
Platelet-borne thiol isomerases are trafficked independently of secretory granule contents in megakaryocytes and become concentrated in a subcellular compartment near the inner surface of the platelet outer membrane corresponding to the sarco/endoplasmic reticulum of these cells. Thiol isomerases are mobilized to the surface of activated platelets via a process that requires actin polymerization but not soluble N-ethylmaleimide-sensitive fusion protein attachment receptor/Munc13-4-dependent vesicular-plasma membrane fusion
The cell motility modulator Slit2 is a potent inhibitor of platelet function.
Vascular injury and atherothrombosis involve vessel infiltration by inflammatory leukocytes, migration of medial vascular smooth muscle cells to the intimal layer, and ultimately acute thrombosis. A strategy to simultaneously target these pathological processes has yet to be identified. The secreted protein, Slit2, and its transmembrane receptor, Robo-1, repel neuronal migration in the developing central nervous system. More recently, it has been appreciated that Slit2 impairs chemotaxis of leukocytes and vascular smooth muscle cells toward diverse inflammatory attractants. The effects of Slit2 on platelet function and thrombus formation have never been explored. We detected Robo-1 expression in human and murine platelets and megakaryocytes and confirmed its presence via immunofluorescence microscopy and flow cytometry. In both static and shear microfluidic assays, Slit2 impaired platelet adhesion and spreading on diverse extracellular matrix substrates by suppressing activation of Akt. Slit2 also prevented platelet activation on exposure to ADP. In in vivo studies, Slit2 prolonged bleeding times in murine tail bleeding assays. Using intravital microscopy, we found that after mesenteric arteriolar and carotid artery injury, Slit2 delayed vessel occlusion time and prevented the stable formation of occlusive arteriolar thrombi. These data demonstrate that Slit2 is a powerful negative regulator of platelet function and thrombus formation. The ability to simultaneously block multiple events in vascular injury may allow Slit2 to effectively prevent and treat thrombotic disorders such as myocardial infarction and stroke
Steepening of waves at the duskside magnetopause
Surface waves at the magnetopause flanks typically feature steeper, i.e., more inclined leading (antisunward facing) than trailing (sunward facing) edges. This is expected for KelvinâHelmholtz instability (KHI) amplified waves. Very rarely, during northward interplanetary magnetic field (IMF) conditions, anomalous/inverse steepening has been observed. The smallâscale tetrahedral configuration of the Magnetospheric Multiscale spacecraft and their high time resolution measurements enable us to routinely ascertain magnetopause boundary inclinations during surface wave passage with high accuracy by fourâspacecraft timing analysis. At the dusk flank magnetopause, 77%/23% of the analyzed wave intervals exhibit regular/inverse steepening. Inverse steepening happens during northward IMF conditions, as previously reported and, in addition, during intervals of dominant equatorial IMF. Inverse steepening observed under the latter conditions may be due to the absence of KHI or due to instabilities arising from the alignment of flow and magnetic fields in the magnetosheath.Key PointsThe MMS spacecraft configuration, orbits, and data resolution enable us to ascertain magnetopause (wave) inclinations with high accuracyInverse wave steepening (steeper trailing edges) occurs also when the IMF is in the GSM xây plane, not only during mainly northward IMFInverse steepening may be associated to the absence of KHI or to instabilities from the alignment of flow and magnetic fields in the sheathPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134254/1/grl54723.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134254/2/grl54723_am.pd
Human platelets contain a pool of free zinc in dense granules
Background: Activated platelets release procoagulant factors that include Ca2+ and Zn2+. Releasable Ca2+ stores have been identified in platelet dense granules and the dense tubular system, but similar stores of free Zn2+ have not been identified. Objectives: Guided by studies of platelet Ca2+, we employed minimally disruptive methods to identify and localize concentrated free Zn2+ in human platelets. Methods: Resting platelets from normal donors (NDs), patients with gray platelet syndrome (GPS) lacking α-granules, and patients with Hermansky-Pudlak syndrome (HPS) deficient in dense granules were loaded with cell-permeant fluorescent probes specific to free Ca2+ or Zn2+. Ion concentrations were detected in fixed cells as bright puncta via high-resolution confocal microscopy. Ions were also directly detected via transmission electron microscopy and energy dispersive X-ray analysis. Levels of total platelet Ca, Zn, and Mg were measured via inductively coupled plasma optical emission spectroscopy. Results: Fluorescent Zn2+ puncta counts were similar in ND and GPS platelets and markedly lower in HPS platelets, pointing to dense granules as likely reservoirs of free Zn2+. This localization was supported by direct detection of Ca2+, Zn2+, and Na+ in platelet dense granules via transmission electron microscopy and energy dispersive X-ray analysis. Measurements of total platelet Ca, Zn, and Mg via inductively coupled plasma optical emission spectroscopy indicated that free Zn2+ represents a small proportion of total platelet zinc, consistent with the strong affinity of Zn2+ for binding proteins, including several abundant in platelet α-granules. Conclusion: We conclude that normal human platelets contain a pool of free Zn2+ concentrated in dense granules that is available for secretion upon platelet activation and potentially contributes to hemostasis
Der GegenĂŒbertragungstraum â oder: Das Ping-Pong-Spiel der beiderseitigen Unbewussten
In einem Brief an Freud hatte Lou Andreas-SalomĂ© wegen des ihr unverstĂ€ndlichen Stillstands einer Analyse um Rat gefragt. Freud antwortete: >>Was man nicht deklinieren kann, das sehe man als - Ăbertragung an<< (Freud und Andreas-SalomĂ©, 1966/1980, S. 133, Brief vom 23.3.1923).
Er variiert damit die alte Grammatikregel: Was man nicht deklinieren kann, das sehe man als ein Neutrum an. Es handelt sich um keine feste grammatische GesetzmĂ€Ăigkeit wie >>Alle Substantiva mit der Endung -um sind Neutra<<, sondern um eine mehr pragmatische Regel zum Einsortieren.
Diese ĂuĂerung mag belegen, wie Freud die Ăbertragung auffasste: nicht als etwas faktisch Gegebenes, sondern als etwas zu Unterstellendes und - vor allem modischen Konstruktivismus - in der Analyse zu >>Konstruierendes<< (vgl. Freud, 1937d). Es ist nĂŒtzlich, sich immer wieder Stellen wie diese in Freuds Schriften vor Augen zu halten, um nicht dem verbreiteten trĂŒgerischen Begriffsrealismus zu verfallen: Wenn es das Wort gibt (Ăbertragung, Ădipuskomplex oder was auch immer), dann gibt es auch den damit bezeichneten Sachverhalt
Impact of tunable oligophosphonates on barium sulfate crystallization
Calixarenes can be used as well-defined scaffolds for investigating structureâactivity relationships of additives and their impact on crystallization. In this work, we present the crystal growth modification of barium sulfate by p-phosphonic acid calix[n]arenes that vary in size (n = 4, 5, 6, and 8) and thus vary in the size of the internal cavity for the same functionality in the upper rim. The tetrameric, hexameric, and octameric macrocycles induce nanoparticle formation with clear superstructure. In the case of the hexameric calix[6]arene, the initial mesocrystalline superstructure fuses over time to form almost hollow spheres, while the mesocrystals formed in the presence of the tetramer and octamer are stable over an extended period. The pentameric calix[5]arene forms more disordered aggregates of single crystals. Thermogravimetric data shows that a significant proportion of the mass of the barium sulfate-containing solid is the macrocycle, regardless of the choice of macrocycle
Molecular mass determination of saturated hydrocarbons: reactivity of η 5-cyclopentadienylcobalt ion (CpCo·+) and linear alkanes up to C-30
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