3,529 research outputs found
X-ray induced persistent photoconductivity in Si-doped AlGaAs
We demonstrate that X-ray irradiation can be used to induce an
insulator-metal transition in Si-doped AlGaAs, a
semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX}
centers into their shallow donor state was revealed by studying the
photoconductance along with fluorescence. The photoconductance as a function of
incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying
that core-hole excitation of Ga and As are efficient primary steps for the
excitation of {\it DX} centers. A high quantum yield () suggests that
the excitation is indirect and nonlocal, due to secondary electrons, holes, and
fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we
corrected i
Self-gravitating spheres of anisotropic fluid in geodesic flow
The fluid models mentioned in the title are classified. All characteristics
of the fluid are expressed through a master potential, satisfying an ordinary
second order differential equation. Different constraints are imposed on this
core of relations, finding new solutions and deriving the classical results for
perfect fluids and dust as particular cases. Many uncharged and charged
anisotropic solutions, all conformally flat and some uniform density solutions
are found. A number of solutions with linear equation among the two pressures
are derived, including the case of vanishing tangential pressure.Comment: 21 page
Membrane lipid order of sub-synaptic T cell vesicles correlates with their dynamics and function
During an immune response, T cells survey antigen presenting cells for antigenic peptides via the formation of an interface known as an immunological synapse. Among the complex and dynamic biophysical phenomena occurring at this interface is the trafficking of sub-synaptic vesicles carrying a variety of proximal signalling molecules. Here, we show that rather than being a homogeneous population, these vesicles display a diversity of membrane lipid order profiles, as measured using the environmentally sensitive dye di-4-ANEPPDHQ and multi-spectral TIRF microscopy. Using live-cell imaging, vesicle tracking and a variety of small molecule drugs to manipulate components of the actin and tubulin cytoskeleton, we show that the membrane lipid order of these vesicles correlate with their dynamics. Furthermore, we show that the key proximal signalling molecule Linker for Activation of T cells (LAT) is enriched in specific vesicle populations as defined by their higher membrane order. These results imply that vesicle lipid order may represent a novel regulatory mechanism for the sorting and trafficking of signalling molecules at the immunological synapse, and, potentially, other cellular structures
Notes on the Herbaceous plants of Ayer Hitam Forest Reserve, Puchong, Selangor
A survey conducted on the plants of Ayer Hitam Forest Reserve recorded 27 species of herbaceous plants which
represented 19 families. From this total, 8 species are recorded for the first time from this forest. Five spesies endemic to Peninsular Malaysia are also found here. This finding has increased the current herb checklist of Ayer Hitam Forest Reserve to 86 species
First principles investigation of ferroelectricity in epitaxially strained PbTiO
The structure and polarization of the as-yet hypothetical Ruddlesden-Popper
compound PbTiO are investigated within density-functional theory. Zone
enter phonons of the high-symmetry KNiF-type reference structure, space
group , were calculated. At the theoretical ground-state lattice
constants, there is one unstable infrared-active phonon. This phonon freezes in
to give the ferroelectric state. As a function of epitaxial strain, two
additional ferroelectric phases are found, with space groups and
at compressive and tensile strains, respectively.Comment: 4 pages, 4 figure
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Magnetic Levitation as a Platform for Competitive Protein–Ligand Binding Assays
This paper describes a method based on magnetic levitation (MagLev) that is capable of indirectly measuring the binding of unlabeled ligands to unlabeled protein. We demonstrate this method by measuring the affinity of unlabeled bovine carbonic anhydrase (BCA) for a variety of ligands (most of which are benzene sulfonamide derivatives). This method utilizes porous gel beads that are functionalized with a common aryl sulfonamide ligand. The beads are incubated with BCA and allowed to reach an equilibrium state in which the majority of the immobilized ligands are bound to BCA. Since the beads are less dense than the protein, protein binding to the bead increases the overall density of the bead. This change in density can be monitored using MagLev. Transferring the beads to a solution containing no protein creates a situation where net protein efflux from the bead is thermodynamically favorable. The rate at which protein leaves the bead for the solution can be calculated from the rate at which the levitation height of the bead changes. If another small molecule ligand of BCA is dissolved in the solution, the rate of protein efflux is accelerated significantly. This paper develops a reaction-diffusion (RD) model to explain both this observation, and the physical-organic chemistry that underlies it. Using this model, we calculate the dissociation constants of several unlabeled ligands from BCA, using plots of levitation height versus time. Notably, although this method requires no electricity, and only a single piece of inexpensive equipment, it can measure accurately the binding of unlabeled proteins to small molecules over a wide range of dissociation constants (Kd values within the range from 10 nM to 100 μM are measured easily). Assays performed using this method generally can be completed within a relatively short time period (20 min–2 h). A deficiency of this system is that it is not, in its present form, applicable to proteins with molecular weight greater than approximately 65 kDa.Chemistry and Chemical Biolog
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