2,344 research outputs found
Elastic cavitation, tube hollowing, and differential growth in plants and biological tissues
Elastic cavitation is a well-known physical process by which elastic materials under stress can open cavities. Usually, cavitation is induced by applied loads on the elastic body. However, growing materials may generate stresses in the absence of applied loads and could induce cavity opening. Here, we demonstrate the possibility of spontaneous growth-induced cavitation in elastic materials and consider the implications of this phenomenon to biological tissues and in particular to the problem of schizogenous aerenchyma formation
Evidence for Skyrmion crystallization from NMR relaxation experiments
A resistively detected NMR technique was used to probe the two-dimensional
electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate
was extracted at near complete filling of the first Landau level by
electrons. The nuclear spin of As is found to relax much more
efficiently with and when a well developed quantum Hall state with
occurs. The data show a remarkable correlation between the
nuclear spin relaxation and localization. This suggests that the magnetic
ground state near complete filling of the first Landau level may contain a
lattice of topological spin texture, i.e. a Skyrmion crystal
Distinct high-T transitions in underdoped BaKFeAs
In contrast to the simultaneous structural and magnetic first order phase
transition previously reported, our detailed investigation on an
underdoped BaKFeAs single crystal unambiguously
revealed that the transitions are not concomitant. The tetragonal (:
I4/mmm) - orthorhombic (: Fmmm) structural transition occurs at
110 K, followed by an adjacent antiferromagnetic (AFM) transition
at 102 K. Hysteresis and coexistence of the and
phases over a finite temperature range observed in our NMR
experiments confirm the first order character of the structural transition and
provide evidence that both and are strongly correlated. Our
data also show that superconductivity (SC) develops in the phase
below = 20 K and coexists with long range AFM. This new observation,
, firmly establishes another similarity between the hole-doped
BaFeAs via K substitution and the electron-doped iron-arsenide
superconductors.Comment: 4 pages, 3 figure
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Self-assembly of Fmoc-tetrapeptides based on the RGDS cell adhesion motif
Self-assembly in aqueous solution has been investigated for two Fmoc [Fmoc ¼ N-(fluorenyl)-9-methoxycarbonyl] tetrapeptides comprising the RGDS cell adhesion motif from fibronectin or the scrambled sequence GRDS. The hydrophobic Fmoc unit confers amphiphilicity on the molecules, and
introduces aromatic stacking interactions. Circular dichroism and FTIR spectroscopy show that the self-assembly of both peptides at low concentration is dominated by interactions among Fmoc units, although Fmoc-GRDS shows b-sheet features, at lower concentration than Fmoc-RGDS. Fibre X-ray diffraction indicates b-sheet formation by both peptides at sufficiently high concentration. Strong
alignment effects are revealed by linear dichroism experiments for Fmoc-GRDS. Cryo-TEM and smallangle
X-ray scattering (SAXS) reveal that both samples form fibrils with a diameter of approximately 10 nm. Both Fmoc-tetrapeptides form self-supporting hydrogels at sufficiently high concentration. Dynamic shear rheometry enabled measurements of the moduli for the Fmoc-GRDS hydrogel, however syneresis was observed for the Fmoc-RGDS hydrogel which was significantly less stable to shear. Molecular dynamics computer simulations were carried out considering parallel and antiparallel b-sheet configurations of systems containing 7 and 21 molecules of Fmoc-RGDS or Fmoc-GRDS, the results being analyzed in terms of both intermolecular structural parameters and energy contributions
Genetic Variation in North American Black Flies in the Subgenus \u3ci\u3ePsilopelmia\u3c/i\u3e (\u3ci\u3eSimulium\u3c/i\u3e: Diptera: Simuliidae)
Resolution of the genetic heterogeneity of closely related insect species depends on the selection of reliable genetic markers derived from representative specimens. We report the results of a survey of genetic variability in nine species of black flies in the subgenus Psilopelmia Enderlein. Three regions of the mitochondrial genome and an amplicon including the internal transcribed spacer 1 of the nuclear ribosomal RNA gene cluster (ITS1) were amplified using the polymerase chain reaction (PCR), and the amplicons were examined for intraspecific and interspecific polymorphisms. Six of the seven Psilopelmia species that yielded PCR products in the ITS1 PCR reaction were found to generate products that were indistinguishable on the basis of size. Similarly, little interspecific variation was noted in the 16S rRNA amplicon among nine species of Psilopelmia assayed by heteroduplex analysis. In contrast, the remaining regions of the mitochondrial genome exhibited both intra- and inter-specific variation when analyzed by heteroduplex analysis. Information collected from the five amplicons could be employed to develop a classification scheme capable of distinguishing the nine species of Psilopelmia examined
Facile synthesis of reduced graphene oxide/MWNTs nanocomposite supercapacitor materials tested as electrophoretically deposited films on glassy carbon electrodes
This paper reports on a facile synthesis method for reduced graphene oxide (rGO)/multi-walled carbon nanotubes (MWNTs) nanocomposites. The initial step involves the use of graphene oxide to disperse the MWNTs, with subsequent reduction of the resultant graphene oxide/MWNTs composites using l-ascorbic acid (LAA) as a mild reductant. Reduction by LAA preserves the interaction between the rGO sheets and MWNTs. The dispersion-containing rGO/MWNTs composites was characterized and electrophoretically deposited anodically onto glassy carbon electrodes to form high surface area films for capacitance testing. Pseudo capacitance peaks were observed in the rGO/MWNTs composite electrodes, resulting in superior performance with capacitance values up to 134.3 F g−1 recorded. This capacitance value is higher than those observed for LAA-reduced GO (LAA-rGO) (63.5 F g−1), electrochemically reduced GO (EC-rGO) (27.6 F g−1), or electrochemically reduced GO/MWNTs (EC-rGO/MWNTs) (98.4 F g−1)-based electrodes.© 2013, Springer Science+Business Media Dordrecht
Dynamical derivation of Bode's law
In a planetary or satellite system, idealized as n small bodies in initially
coplanar, concentric orbits around a large central body, obeying Newtonian
point-particle mechanics, resonant perturbations will cause dynamical evolution
of the orbital radii except under highly specific mutual relationships, here
derived analytically apparently for the first time. In particular, the most
stable situation is achieved (in this idealized model) only when each planetary
orbit is roughly twice as far from the Sun as the preceding one, as observed
empirically already by Titius (1766) and Bode (1778) and used in both the
discoveries of Uranus (1781) and the Asteroid Belt (1801). ETC.Comment: 27 page
A screen for nuclear transcripts identifies two linked noncoding RNAs associated with SC35 splicing domains
BACKGROUND: Noncoding RNA species play a diverse set of roles in the eukaryotic cell. While much recent attention has focused on smaller RNA species, larger noncoding transcripts are also thought to be highly abundant in mammalian cells. To search for large noncoding RNAs that might control gene expression or mRNA metabolism, we used Affymetrix expression arrays to identify polyadenylated RNA transcripts displaying nuclear enrichment.
RESULTS: This screen identified no more than three transcripts; XIST, and two unique noncoding nuclear enriched abundant transcripts (NEAT) RNAs strikingly located less than 70 kb apart on human chromosome 11: NEAT1, a noncoding RNA from the locus encoding for TncRNA, and NEAT2 (also known as MALAT-1). While the two NEAT transcripts share no significant homology with each other, each is conserved within the mammalian lineage, suggesting significant function for these noncoding RNAs. NEAT2 is extraordinarily well conserved for a noncoding RNA, more so than even XIST. Bioinformatic analyses of publicly available mouse transcriptome data support our findings from human cells as they confirm that the murine homologs of these noncoding RNAs are also nuclear enriched. RNA FISH analyses suggest that these noncoding RNAs function in mRNA metabolism as they demonstrate an intimate association of these RNA species with SC35 nuclear speckles in both human and mouse cells. These studies show that one of these transcripts, NEAT1 localizes to the periphery of such domains, whereas the neighboring transcript, NEAT2, is part of the long-sought polyadenylated component of nuclear speckles.
CONCLUSION: Our genome-wide screens in two mammalian species reveal no more than three abundant large non-coding polyadenylated RNAs in the nucleus; the canonical large noncoding RNA XIST and NEAT1 and NEAT2. The function of these noncoding RNAs in mRNA metabolism is suggested by their high levels of conservation and their intimate association with SC35 splicing domains in multiple mammalian species
Periodic orbits in the restricted three-body problem and Arnold's -invariant
We apply Arnold's theory of generic smooth plane curves to Stark-Zeeman
systems. This is a class of Hamiltonian dynamical systems that describes the
dynamics of an electron in an external electric and magnetic field, and
includes many systems from celestial mechanics. Based on Arnold's
-invariant, we introduce invariants of periodic orbits in planar
Stark-Zeeman systems and study their behaviour.Comment: 36 Pages, 16 Figure
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