bluuebunny/crossref_metadata_2025 huggingface dataset:
In situobservation of the elastic deformation of a single epitaxial SiGe crystal by combining atomic force microscopy and micro x-ray diffraction
Authors: Rodrigues M. S. , Cornelius T. W. , Scheler T. , Mocuta C. , Malachias A. , Magalhães-Paniago R. , Dhez O. , Comin F. , Metzger T. H. , Chevrier J.
Published: December 2009

An in situ combination of atomic force microscopy and micro x-ray diffraction was developed to study the elastic behavior of nanosized objects. This technique offers the means to locally access the Young elastic moduli and Poisson ratios of individual nanostructures. Here, we investigated the elastic behavior of a single self-assembled 450 nm high SiGe island. As pressure was applied on the island, the resonance frequency of the atomic force microscope tuning fork was tracked together with the x-ray diffraction stemming from this individual crystal. The change in the tip-island contact stiffness could be derived from the variation in the resonance frequency of the tuning fork, whereas the island mean lattice parameter was inferred from the center of mass of the island’s Bragg scattering. From this information, the reduced elastic modulus of the tip-island system could be directly determined, which is in very good agreement with literature values. The pressure needed to compress the island lattice to the Si value amounts to about 3 GPa and is in good accordance with finite-element method simulations of the displacement field in the pressurized object.
Optimized, unequal pulse spacing in multiple echo sequences improves refocusing in magnetic resonance
Authors: Jenista Elizabeth R. , Stokes Ashley M. , Branca Rosa Tamara , Warren Warren S.
Published: December 2009

A recent quantum computing paper (G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007)) analytically derived optimal pulse spacings for a multiple spin echo sequence designed to remove decoherence in a two-level system coupled to a bath. The spacings in what has been called a “Uhrig dynamic decoupling (UDD) sequence” differ dramatically from the conventional, equal pulse spacing of a Carr–Purcell–Meiboom–Gill (CPMG) multiple spin echo sequence. The UDD sequence was derived for a model that is unrelated to magnetic resonance, but was recently shown theoretically to be more general. Here we show that the UDD sequence has theoretical advantages for magnetic resonance imaging of structured materials such as tissue, where diffusion in compartmentalized and microstructured environments leads to fluctuating fields on a range of different time scales. We also show experimentally, both in excised tissue and in a live mouse tumor model, that optimal UDD sequences produce different T2-weighted contrast than do CPMG sequences with the same number of pulses and total delay, with substantial enhancements in most regions. This permits improved characterization of low-frequency spectral density functions in a wide range of applications.
Flexural deformation and bending mode of magnetoelectric nanobilayer
Authors: Petrov V. M. , Bichurin M. I. , Zibtsev V. V. , Mandal S. K. , Srinivasan G.
Published: December 2009

A model is discussed for magnetoelectric (ME) coupling in a nanobilayer of ferromagnetic and piezoelectric phases on a substrate. The theory is applied to a specific case, nickel ferrite and lead zirconate titanate bilayer on strontium titanate substrate. Both low-frequency coupling and ME interactions at frequencies corresponding to bending oscillations have been considered. The influence of lattice mismatch between the substrate and piezoelectric and piezomagnetic layers on material parameters has been estimated using the Landau–Ginsburg–Devonshire phenomenological thermodynamic theory. At low frequencies, the model predicts a maximum in the ME coupling strength when the substrate is of the same thickness as the bilayer. The ME voltage coefficient drops with further increase in substrate thickness due to clamping effects. The bending oscillations in nanobilayers are expected to occur at ultralow frequencies (<5 Hz) compared to longitudinal acoustic modes at 100–200 kHz for bilayers of nominal dimensions. The mode frequency shows an increase with increasing substrate thickness, and the resonance value of the ME coefficient is predicted to show a minimum when the substrate-to-bilayer thickness ratio is 0.3.
Diffuse and doubly split atom occupation in hexagonal LiBH4
Authors: Ikeshoji Tamio , Tsuchida Eiji , Ikeda Kazutaka , Matsuo Motoaki , Li Hai-Wen , Kawazoe Yoshiyuki , Orimo Shin-ichi
Published: December 2009

A theoretical study has been performed to explain problems in the structural analysis of LiBH4 and its recently discovered superionic conductance. First-principles molecular dynamics simulations for the high temperature (hexagonal) phase show doubly split and diffuse occupation in the c-direction at Li and B sites, respectively. Li hopping within the split sites and libration of H atoms are also found. These dynamics are supported by the Rietveld analysis showing atomic displacement ellipsoids for Li and B atoms.
Thermophysical properties of highly doped Si and Ge melts under microgravity
Authors: Chathoth S. M. , Damaschke B. , Samwer K. , Schneider S.
Published: December 2009

We have investigated thermal expansion and surface tension of highly doped Si and Ge melts under microgravity conditions. The experiments were conducted in the TEMPUS facility on-board of a Zero-G aircraft. The thermophysical properties were quantified by analyzing the images of levitated droplets. Both Si and Ge are metallic in their solid state at the doping (P and Sb) of 1×1019 atoms cm−3. However, thermal expansion and surface tension of highly doped Si and Ge melts did not show significant changes in comparison with undoped samples.
Photoabsorption spectra of small cationic xenon clusters from time-dependent density functional theory
Authors: Oliveira Micael J. T. , Nogueira Fernando , Marques Miguel A. L. , Rubio Angel
Published: December 2009

Upon ionization, rare-gas (like Ar and Xe) clusters shift their absorption spectrum from the ultraviolet to the visible. This happens as bonding becomes much stronger due to the removal of an electron from a strongly antibonding orbital. In this article, we study the absorption spectrum of small cationic xenon clusters (Xen+, with n=3,…,35) by means of time-dependent density functional theory. These calculations include relativistic effects through the use of relativistic j-dependent pseudopotentials in a two-spinor formulation of the Kohn–Sham equations. The peak positions in our calculated spectra are in fairly good agreement with experiment and confirm that absorption is mainly due to a charged linear core composed of 3, 4, or 5 Xe atoms where the positive charge is localized. However, we find large deviations concerning the oscillator strengths, which can be partially explained by the unsatisfactory treatment of exchange in common density functionals. Furthermore, we find that adequate ground-state geometries are necessary for the correct prediction of the qualitative features of the spectra.
Laser-induced erasure and reversal of the permanent magnetization in a ferromagnet-dilute magnetic semiconductor hybrid structure
Authors: Fan Y.-H. , Puls J. , Halm S. , Sadofev S. , Seifert F. , Schuster E. , Bacher G. , Keune W. , Henneberger F.
Published: December 2009

Manipulation of the magnetizations via laser pulse heating is studied for a hybrid structure, where the fringe field of a metallic ferromagnet controls remotely the carrier and magnetic ion spins in a diluted magnetic semiconductor quantum well. A single nanosecond pulse with an energy density of 160 pJ/μm2 is found to be sufficient to erase the ferromagnet magnetization. Applying a reversed external bias field about five times below the ferromagnet coercive field, a complete reversal of the magnetization via optical excitation is demonstrated.
Effective medium approach for heterogeneous reaction-diffusion media
Authors: Alonso Sergio , Bär Markus , Kapral Raymond
Published: December 2009

An effective medium theory that can be used to calculate effective diffusion and reaction rate coefficients in random heterogeneous reaction-diffusion systems is described. The predictions of the theory are compared with simulations of spatially distributed media with different types of heterogeneity. The magnitude of the front velocity in bistable media is used to gauge the accuracy of the theoretical predictions. Quantitative agreement is found if the diffusion length in the heterogeneities is large compared to the characteristic width of the front. However, for small diffusion lengths the agreement depends on the type of heterogeneity. The effective medium predictions are also compared with simulations on systems with regular or temporal disorder.
Phase separations in mixtures of a liquid crystal and a nanocolloidal particle
Authors: Matsuyama Akihiko
Published: December 2009

We present a mean field theory to describe phase separations in mixtures of a liquid crystal and a nanocolloidal particle. By taking into account a nematic, a smectic A ordering of the liquid crystal, and a crystalline ordering of the nanoparticle, we calculate the phase diagrams on the temperature-concentration plane. We predict various phase separations, such as a smectic A-crystal phase separation and a smectic A-isotropic-crystal triple point, etc., depending on the interactions between the liquid crystal and the colloidal surface. Inside binodal curves, we find new unstable and metastable regions, which are important in the phase ordering dynamics. We also find a crystalline ordering of the nanoparticles dispersed in a smectic A phase and a nematic phase. The cooperative phenomena between liquid-crystalline ordering and crystalline ordering induce a variety of phase diagrams.
Localness of energy cascade in hydrodynamic turbulence. I. Smooth coarse graining
Authors: Eyink Gregory L. , Aluie Hussein
Published: December 2009

We introduce a novel approach to scale decomposition of the fluid kinetic energy (or other quadratic integrals) into band-pass contributions from a series of length scales. Our decomposition is based on a multiscale generalization of the “Germano identity” for smooth, graded filter kernels. We employ this method to derive a budget equation that describes the transfers of turbulent kinetic energy both in space and in scale. It is shown that the interscale energy transfer is dominated by local triadic interactions, assuming only the scaling properties expected in a turbulent inertial range. We derive rigorous upper bounds on the contributions of nonlocal triads, extending the work of Eyink [Physica D 207, 91 (2005)] for low-pass filtering. We also propose a physical explanation of the differing exponents for our rigorous upper bounds and for the scaling predictions of Kraichnan [Phys. Fluids 9, 1728 (1966); J. Fluid Mech. 47, 525 (1971)]. The faster decay predicted by Kraichnan is argued to be the consequence of additional cancellations in the signed contributions to transfer from nonlocal triads after averaging over space. This picture is supported by data from a 5123 pseudospectral simulation of Navier–Stokes turbulence with phase-shift dealiasing.
Backward stimulated Bragg scattering in multiphoton active CdTexSe1−x quantum dots system
Authors: He Guang S. , Zhu Jing , Yong Ken-Tye , Hu Rui , Cui Yiping , Prasad P. N.
Published: December 2009

The backward stimulated Bragg scattering (SBgS) of CdTexSe1−x quantum dots in chloroform is investigated at three pump laser wavelengths (532, 816, and 1064 nm) in nanosecond regime. The spectral and temporal structures of the backward stimulated scattering and pump threshold dependence on the concentration are presented in this paper. The energy conversion efficiency from input pump pulse to SBgS pulse was measured to be ≥14%. In addition, the samples exhibit multi- (two-, three-)photon absorption capability over the spectral range we investigated. More importantly, both mechanisms of SBgS and multiphoton absorption provided an enhanced optical limiting performance. The measured nonlinear transmissivity was changed from ∼0.73 to ∼0.17 for 532 nm laser pulses and from ∼0.9 to ∼0.35 for 816 nm laser pulses when the input pulse energy was changed from 10 to ∼1500 μJ.
Streamwise oscillation of spanwise velocity at the wall of a channel for turbulent drag reduction
Authors: Viotti Claudio , Quadrio Maurizio , Luchini Paolo
Published: December 2009

Steady forcing at the wall of a channel flow is studied via direct numerical simulation to assess its ability of yielding reductions in turbulent friction drag. The wall forcing consists of a stationary distribution of spanwise velocity that alternates in the streamwise direction. The idea behind the forcing builds on the existing technique of the spanwise wall oscillation and exploits the convective nature of the flow to achieve an unsteady interaction with turbulence. The analysis takes advantage of the equivalent laminar flow, which is solved analytically to show that the energetic cost of the forcing is unaffected by turbulence. In a turbulent flow, the alternate forcing is found to behave similarly to the oscillating wall; in particular an optimal wavelength is found which yields a maximal reduction in turbulent drag. The energetic performance is significantly improved, with more than 50% of maximum friction saving at large intensities of the forcing, and a net energetic saving of 23% for smaller intensities. Such a steady, wall-based forcing may pave the way to passively interacting with the turbulent flow to achieve drag reduction through a suitable distribution of roughness, designed to excite a selected streamwise wavelength.
Localness of energy cascade in hydrodynamic turbulence. II. Sharp spectral filter
Authors: Aluie Hussein , Eyink Gregory L.
Published: December 2009

We investigate the scale-locality of subgrid-scale (SGS) energy flux and interband energy transfers defined by the sharp spectral filter. We show by rigorous bounds, physical arguments, and numerical simulations that the spectral SGS flux is dominated by local triadic interactions in an extended turbulent inertial range. Interband energy transfers are also shown to be dominated by local triads if the spectral bands have constant width on a logarithmic scale. We disprove in particular an alternative picture of “local transfer by nonlocal triads,” with the advecting wavenumber mode at the energy peak. Although such triads have the largest transfer rates of all individual wavenumber triads, we show rigorously that, due to their restricted number, they make an asymptotically negligible contribution to energy flux and log-banded energy transfers at high wavenumbers in the inertial range. We show that it is only the aggregate effect of a geometrically increasing number of local wavenumber triads which can sustain an energy cascade to small scales. Furthermore, nonlocal triads are argued to contribute even less to the space-average energy flux than is implied by our rigorous bounds, because of additional cancellations from scale-decorrelation effects. We can thus recover the −4/3 scaling of nonlocal contributions to spectral energy flux predicted by Kraichnan’s abridged Lagrangian-history direct-interaction approximation and test-field model closures. We support our results with numerical data from a 5123 pseudospectral simulation of isotropic turbulence with phase-shift dealiasing. We also discuss a rigorous counterexample of Eyink [Physica D 78, 222 (1994)], which showed that nonlocal wavenumber triads may dominate in the sharp spectral flux (but not in the SGS energy flux for graded filters). We show that this mathematical counterexample fails to satisfy reasonable physical requirements for a turbulent velocity field, which are employed in our proof of scale locality. We conclude that the sharp spectral filter has a firm theoretical basis for use in large-eddy simulation modeling of turbulent flows.
Antiproton, positron, and electron imaging with a microchannel plate/phosphor detector
Authors: Andresen G. B. , Bertsche W. , Bowe P. D. , Bray C. C. , Butler E. , Cesar C. L. , Chapman S. , Charlton M. , El Nasr S. Seif , Fajans J. , Fujiwara M. C. , Gill D. R. , Hangst J. S. , Hardy W. N. , Hayano R. S. , Hayden M. E. , Humphries A. J. , Hydomako R. , Jørgensen L. V. , Kerrigan S. J. , Kurchaninov L. , Lambo R. , Madsen N. , Nolan P. , Olchanski K. , Olin A. , Povilus A. P. , Pusa P. , Sarid E. , Silveira D. M. , Storey J. W. , Thompson R. I. , van der Werf D. P. , Yamazaki Y. , Collaboration ALPHA
Published: December 2009

A microchannel plate (MCP)/phosphor screen assembly has been used to destructively measure the radial profile of cold, confined antiprotons, electrons, and positrons in the ALPHA experiment, with the goal of using these trapped particles for antihydrogen creation and confinement. The response of the MCP to low energy (10–200 eV, <1 eV spread) antiproton extractions is compared to that of electrons and positrons.
A portable microevaporator for low temperature single atom studies by scanning tunneling and dynamic force microscopy
Authors: Rust H.-P. , König T. , Simon G. H. , Nowicki M. , Simic-Milosevic V. , Thielsch G. , Heyde M. , Freund H.-J.
Published: December 2009

Here, we present a microevaporator setup for single adatom deposition at low temperature, which is a prerequisite for most single atom studies with scanning probe techniques. The construction of the microevaporator is based on the tungsten filament of a modified halogen lamp, covered with the required adsorbate. Very stable evaporation conditions were obtained, which were controlled by the filament current. The installation of this microevaporator on a manipulator enabled its transportation directly to the sample at the microscope kept at 5 K. In this way, the controlled deposition of Li onto Ag(100), Li, Pd, and Au onto MgO/Ag(001) as well as Au onto alumina/NiAl(110) at low temperature has been performed. The obtained images recorded after the deposition show the presence of single Li/Au atoms on the sample surfaces as a prove for successful dispersion of single atoms onto the sample surface using this technique.
Improved two-dimensional electron gas transport characteristics in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor with atomic layer-deposited Al2O3 as gate insulator
Authors: Liu Z. H. , Ng G. I. , Arulkumaran S. , Maung Y. K. T. , Teo K. L. , Foo S. C. , Sahmuganathan V.
Published: December 2009

The effects of Al2O3 gate insulator grown by atomic layer deposition (ALD) system on the two-dimensional electron gas (2DEG) transport characteristics in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MISHEMT) were investigated. The shape of the electron lateral distribution in the quantum well at AlGaN/GaN interface was found to be slightly influenced by the Al2O3 thin layer. The drift mobility (μd) of the electrons in ALD-Al2O3/AlGaN/GaN MISHEMT is increased due to the surface passivation effects of the included dielectric layer. The higher dynamic channel current of the MISHEMT indicates that the electron saturation velocity (vsat) is also increased. These results show the improvement of the transport characteristics of 2DEG in Al2O3/AlGaN/GaN MISHEMT by the excellent properties of the Al2O3 grown by ALD.
Graphite based Schottky diodes formed on Si, GaAs, and 4H-SiC substrates
Authors: Tongay S. , Schumann T. , Hebard A. F.
Published: December 2009

We demonstrate the formation of semimetal graphite/semiconductor Schottky barriers where the semiconductor is either silicon (Si), gallium arsenide (GaAs), or 4H-silicon carbide (4H-SiC). Near room temperature, the forward-bias diode characteristics are well described by thermionic emission, and the extracted barrier heights, which are confirmed by capacitance voltage measurements, roughly follow the Schottky–Mott relation. Since the outermost layer of the graphite electrode is a single graphene sheet, we expect that graphene/semiconductor barriers will manifest similar behavior.
Realization of a single and closed Λ-system in a room-temperature three-level coherently prepared resonant medium with narrow D1 hyperfine splittings
Authors: Li K. , Deng L. , Payne M. G.
Published: December 2009

A single, closed three-state Λ-system in room-temperature rubidium is investigated experimentally using a copropagating beam parallel-polarization configuration under weakly driven electromagnetically induced transparency (EIT) condition. Compare with the widely used orthogonal-polarization beam EIT configuration where multiple nonclosed Λ-schemes coexist, the parallel-polarizations configuration can completely eliminate the leakage light from the control field. Our state preparation and laser polarization lead to a single and closed Λ-scheme that eliminates the detrimental absorption due to nearby states in atomic species with hyperfine separations smaller than Doppler-broadened line widths, a case where the conventional EIT configuration fails completely.
High power broad area quantum cascade lasers
Authors: Bai Y. , Slivken S. , Darvish S. R. , Haddadi A. , Gokden B. , Razeghi M.
Published: December 2009

Broad area quantum cascade lasers (QCLs) are studied with ridge widths up to 400 μm, in room temperature pulsed mode operation at an emission wavelength around 4.45 μm. The peak output power scales linearly with the ridge width. A maximum total peak output power of 120 W is obtained from a single 400-μm-wide device with a cavity length of 3 mm. A stable far field emission characteristic is observed with dual lobes at ±38° for all tested devices, which suggests that these broad area QCLs are highly resistant to filamentation.
Nanoparticle image velocimetry at topologically structured surfaces
Authors: Parikesit Gea O. F. , Guasto Jeffrey S. , Girardo Salvatore , Mele Elisa , Stabile Ripalta , Pisignano Dario , Lindken Ralph , Westerweel Jerry
Published: December 2009

Nanoparticle image velocimetry (nano-PIV), based on total internal reflection fluorescent microscopy, is very useful to investigate fluid flows within ∼100 nm from a surface; but so far it has only been applied to flow over smooth surfaces. Here we show that it can also be applied to flow over a topologically structured surface, provided that the surface structures can be carefully configured not to disrupt the evanescent-wave illumination. We apply nano-PIV to quantify the flow velocity distribution over a polydimethylsiloxane surface, with a periodic gratinglike structure (with 215 nm height and 2 μm period) fabricated using our customized multilevel lithography method. The measured tracer displacement data are in good agreement with the computed theoretical values. These results demonstrate new possibilities to study the interactions between fluid flow and topologically structured surfaces.
Influence of spot size on extreme ultraviolet efficiency of laser-produced Sn plasmas
Authors: Harilal S. S. , Coons R. W. , Hough P. , Hassanein A.
Published: December 2009

We have investigated the spot size effects on the extreme ultraviolet conversion efficiency (CE) of CO2 laser-produced Sn plasmas. The estimated CE of the laser to 13.5 nm radiation, within a 2% bandwidth, using a 10.6 μm CO2 laser with various pulse widths (25–55 ns) showed a double hump structure during a target-lens scan, where the CE is nearly 25% lower at the best focal position. Density analysis of the CO2 laser-produced plasma showed steep density gradients at the best focal position, and a reduction in CE at the best focal position, which is explained by the lack of efficient coupling between the laser and the plasma.
The role of DNA diffusion in solid phase polymerase chain reaction with gel-immobilized primers in planar and capillary microarray format
Authors: Drobyshev Alexei L. , Nasedkina Tatiana V. , Zakharova Natalia V.
Published: December 2009

The solid phase polymerase chain reaction (PCR) on a gel-based microarray system was studied under various durations of individual stages of the PCR cycle and spatial restriction of the reaction volume. Combining the experimental study with numerical modeling, we demonstrated that the diffusion of the PCR product in and out of a microarray element during the annealing and melting stages, respectively, is the main factor responsible for distinctive features of the studied type of PCR. The restriction of reaction volume leads to faster PCR signal growth. Particularly, the capillary array, whereby gel-based microarray elements are located on a glass bar inserted into capillary chamber, was found to be a suitable format for the development of the platform.
2009 Nobel Prize in Physics honors developments in information technology
Authors: Levi Barbara Goss
Published: December 2009

The new laureates helped lay the foundation for today’s flourishing fields of optical communication and digital photography.
Perception of ‘Best Likeness’ to Highly Familiar Faces of Self and Friend
Authors: Allen Hannah , Brady Nuala , Tredoux Colin
Published: November 2009

We investigated the idea that our memory for familiar faces involves an accurate representation of their unique spatial configuration and, further, whether this configuration may be caricatured in memory. In separate experimental blocks, thirty-five Irish participants were presented with a series of photographic images of their own face and of the face of a close friend, and were asked to choose the image which looked most like themselves or their friend. Both sets of images included an original full-face colour photograph, and photographic distortions ranging from a highly caricatured (+100%) to a highly anti-caricatured (–100%) version of the original, generated with reference to newly created average male and female Irish faces. Contrary to suggestions that we hold a slightly caricatured version of a familiar face in memory, the mean ‘best-likeness’ image, calculated across both self and friend trials, was an anti-caricature of −13.88% which was significantly different from 0 ( t69 = −5.34, p < 0.0001). The difference in the mean ‘best-likeness’ image chosen for self (–12.06%) and friend (–15.7%) was not significant ( t34 = 0.715, p = 0.48). These results are discussed with reference to our ability to discriminate facial shape, together with the possibility that we idealise the attractiveness of faces of those close to us.
The endogenous siRNA pathway is involved in heterochromatin formation in Drosophila
Authors: Fagegaltier Delphine , Bougé Anne-Laure , Berry Bassam , Poisot Émilie , Sismeiro Odile , Coppée Jean-Yves , Théodore Laurent , Voinnet Olivier , Antoniewski Christophe
Published: December 2009

A new class of small RNAs (endo-siRNAs) produced from endogenous double-stranded RNA (dsRNA) precursors was recently shown to mediate transposable element (TE) silencing in the
Drosophila
soma. These endo-siRNAs might play a role in heterochromatin formation, as has been shown in
S. pombe
for siRNAs derived from repetitive sequences in chromosome pericentromeres. To address this possibility, we used the viral suppressors of RNA silencing B2 and P19. These proteins normally counteract the RNAi host defense by blocking the biogenesis or activity of virus-derived siRNAs. We hypothesized that both proteins would similarly block endo-siRNA processing or function, thereby revealing the contribution of endo-siRNA to heterochromatin formation. Accordingly, P19 as well as a nuclear form of P19 expressed in
Drosophila
somatic cells were found to sequester TE-derived siRNAs whereas B2 predominantly bound their longer precursors. Strikingly, B2 or the nuclear form of P19, but not P19, suppressed silencing of heterochromatin gene markers in adult flies, and altered histone H3-K9 methylation as well as chromosomal distribution of histone methyl transferase Su(var)3–9 and Heterochromatin Protein 1 in larvae. Similar effects were observed in
dcr2
,
r2d2
, and
ago2
mutants. Our findings provide evidence that a nuclear pool of TE-derived endo-siRNAs is involved in heterochromatin formation in somatic tissues in
Drosophila
.
Structural and mechanistic basis behind the inhibitory interaction of PcTS on α-synuclein amyloid fibril formation
Authors: Lamberto Gonzalo R. , Binolfi Andrés , Orcellet María L. , Bertoncini Carlos W. , Zweckstetter Markus , Griesinger Christian , Fernández Claudio O.
Published: December 2009

The identification of aggregation inhibitors and the investigation of their mechanism of action are fundamental in the quest to mitigate the pathological consequences of amyloid formation. Here, characterization of the structural and mechanistic basis for the antiamyloidogenic effect of phthalocyanine tetrasulfonate (PcTS) on α-synuclein (AS) allowed us to demonstrate that specific aromatic interactions are central for ligand-mediated inhibition of amyloid formation. We provide evidence indicating that the mechanism behind the antiamyloidogenic effect of PcTS is correlated with the trapping of prefibrillar AS species during the early stages of the assembly process. By using NMR spectroscopy, we have located the primary binding region for PcTS to a specific site in the N terminus of AS, involving the amino acid Tyr-39 as the anchoring residue. Moreover, the residue-specific structural characterization of the AS-PcTS complex provided the basis for the rational design of nonamyloidogenic species of AS, highlighting the role of aromatic interactions in driving AS amyloid assembly. A comparative analysis with other proteins involved in neurodegenerative disorders reveals that aromatic recognition interfaces might constitute a key structural element to target their aggregation pathways. These findings emphasize the use of aggregation inhibitors as molecular probes to assess structural and toxic mechanisms related to amyloid formation and the potential of small molecules as therapeutics for amyloid-related pathologies.
Calcium signaling cascade links dopamine D1–D2 receptor heteromer to striatal BDNF production and neuronal growth
Authors: Hasbi Ahmed , Fan Theresa , Alijaniaram Mohammad , Nguyen Tuan , Perreault Melissa L. , O'Dowd Brian F. , George Susan R.
Published: December 2009

Although the perturbation of either the dopaminergic system or brain-derived neurotrophic factor (BDNF) levels has been linked to important neurological and neuropsychiatric disorders, there is no known signaling pathway linking these two major players. We found that the exclusive stimulation of the dopamine D1–D2 receptor heteromer, which we identified in striatal neurons and adult rat brain by using confocal FRET, led to the activation of a signaling cascade that links dopamine signaling to BDNF production and neuronal growth through a cascade of four steps: (i) mobilization of intracellular calcium through Gq, phospholipase C, and inositol trisphosphate, (ii) rapid activation of cytosolic and nuclear calcium/calmodulin-dependent kinase IIα, (iii) increased BDNF expression, and (iv) accelerated morphological maturation and differentiation of striatal neurons, marked by increased microtubule-associated protein 2 production. These effects, although robust in striatal neurons from D5−/−mice, were absent in neurons from D1−/−mice. We also demonstrated that this signaling cascade was activated in adult rat brain, although with regional specificity, being largely limited to the nucleus accumbens. This dopaminergic pathway regulating neuronal growth and maturation through BDNF may have considerable significance in disorders such as drug addiction, schizophrenia, and depression.
Cardiovascular fitness is associated with cognition in young adulthood
Authors: Åberg Maria A. I. , Pedersen Nancy L. , Torén Kjell , Svartengren Magnus , Bäckstrand Björn , Johnsson Tommy , Cooper-Kuhn Christiana M. , Åberg N. David , Nilsson Michael , Kuhn H. Georg
Published: December 2009

During early adulthood, a phase in which the central nervous system displays considerable plasticity and in which important cognitive traits are shaped, the effects of exercise on cognition remain poorly understood. We performed a cohort study of all Swedish men born in 1950 through 1976 who were enlisted for military service at age 18 (
N
= 1,221,727). Of these, 268,496 were full-sibling pairs, 3,147 twin pairs, and 1,432 monozygotic twin pairs. Physical fitness and intelligence performance data were collected during conscription examinations and linked with other national databases for information on school achievement, socioeconomic status, and sibship. Relationships between cardiovascular fitness and intelligence at age 18 were evaluated by linear models in the total cohort and in subgroups of full-sibling pairs and twin pairs. Cardiovascular fitness, as measured by ergometer cycling, positively associated with intelligence after adjusting for relevant confounders (regression coefficient b = 0.172; 95% CI, 0.168–0.176). Similar results were obtained within monozygotic twin pairs. In contrast, muscle strength was not associated with cognitive performance. Cross-twin cross-trait analyses showed that the associations were primarily explained by individual specific, non-shared environmental influences (≥80%), whereas heritability explained <15% of covariation. Cardiovascular fitness changes between age 15 and 18 y predicted cognitive performance at 18 y. Cox proportional-hazards models showed that cardiovascular fitness at age 18 y predicted educational achievements later in life. These data substantiate that physical exercise could be an important instrument for public health initiatives to optimize educational achievements, cognitive performance, as well as disease prevention at the society level.
SCAP is required for timely and proper myelin membrane synthesis
Authors: Verheijen Mark H. G. , Camargo Nutabi , Verdier Valerie , Nadra Karim , de Preux Charles Anne-Sophie , Médard Jean-Jacques , Luoma Adrienne , Crowther Michelle , Inouye Hideyo , Shimano Hitoshi , Chen Su , Brouwers Jos F. , Helms J. Bernd , Feltri M. Laura , Wrabetz Lawrence , Kirschner Daniel , Chrast Roman , Smit August. B.
Published: December 2009

Myelination requires a massive increase in glial cell membrane synthesis. Here, we demonstrate that the acute phase of myelin lipid synthesis is regulated by sterol regulatory element-binding protein (SREBP) cleavage activation protein (SCAP), an activator of SREBPs. Deletion of SCAP in Schwann cells led to a loss of SREBP-mediated gene expression involving cholesterol and fatty acid synthesis. Schwann cell SCAP mutant mice show congenital hypomyelination and abnormal gait. Interestingly, aging SCAP mutant mice showed partial regain of function; they exhibited improved gait and produced small amounts of myelin indicating a slow SCAP-independent uptake of external lipids. Accordingly, extracellular lipoproteins partially rescued myelination by SCAP mutant Schwann cells. However, SCAP mutant myelin never reached normal thickness and had biophysical abnormalities concordant with abnormal lipid composition. These data demonstrate that SCAP-mediated regulation of glial lipogenesis is key to the proper synthesis of myelin membrane, and provide insight into abnormal Schwann cell function under conditions affecting lipid metabolism.
Prestimulus functional connectivity determines pain perception in humans
Authors: Ploner Markus , Lee Michael C. , Wiech Katja , Bingel Ulrike , Tracey Irene
Published: December 2009

Pain is a highly subjective experience that can be substantially influenced by differences in individual susceptibility as well as personality. How susceptibility to pain and personality translate to brain activity is largely unknown. Here, we report that the functional connectivity of two key brain areas before a sensory event reflects the susceptibility to a subsequent noxious stimulus being perceived as painful. Specifically, the prestimulus connectivity among brain areas related to the subjective perception of the body and to the modulation of pain (anterior insular cortex and brainstem, respectively) determines whether a noxious event is perceived as painful. Further, these effects of prestimulus connectivity on pain perception covary with pain-relevant personality traits. More anxious and pain-attentive individuals display weaker descending connectivity to pain modulatory brain areas. We conclude that variations in functional connectivity underlie personality-related differences in individual susceptibility to pain.
Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages
Authors: Jacques Pierre-André , Artero Vincent , Pécaut Jacques , Fontecave Marc
Published: December 2009

Hydrogen production through the reduction of water appears to be a convenient solution for the long-run storage of renewable energies. However, economically viable hydrogen production requests platinum-free catalysts, because this expensive and scarce (only 37 ppb in the Earth's crust) metal is not a sustainable resource [Gordon RB, Bertram M, Graedel TE (2006)
Proc Natl Acad Sci USA
103:1209–1214]. Here, we report on a new family of cobalt and nickel diimine-dioxime complexes as efficient and stable electrocatalysts for hydrogen evolution from acidic nonaqueous solutions with slightly lower overvoltages and much larger stabilities towards hydrolysis as compared to previously reported cobaloxime catalysts. A mechanistic study allowed us to determine that hydrogen evolution likely proceeds through a bimetallic homolytic pathway. The presence of a proton-exchanging site in the ligand, furthermore, provides an exquisite mechanism for tuning the electrocatalytic potential for hydrogen evolution of these compounds in response to variations of the acidity of the solution, a feature only reported for native hydrogenase enzymes so far.
Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites
Authors: Britt David , Furukawa Hiroyasu , Wang Bo , Glover T. Grant , Yaghi Omar M.
Published: December 2009

Selective capture of CO
2
, which is essential for natural gas purification and CO
2
sequestration, has been reported in zeolites, porous membranes, and amine solutions. However, all such systems require substantial energy input for release of captured CO
2
, leading to low energy efficiency and high cost. A new class of materials named metal-organic frameworks (MOFs) has also been demonstrated to take up voluminous amounts of CO
2
. However, these studies have been largely limited to equilibrium uptake measurements, which are a poor predictor of separation ability, rather than the more industrially relevant kinetic (dynamic) capacity. Here, we report that a known MOF, Mg-MOF-74, with open magnesium sites, rivals competitive materials in CO
2
capture, with 8.9 wt. % dynamic capacity, and undergoes facile CO
2
release at significantly lower temperature, 80 °C. Mg-MOF-74 offers an excellent balance between dynamic capacity and regeneration. These results demonstrate the potential of MOFs with open metal sites as efficient CO
2
capture media.
Structural insights into tail-anchored protein binding and membrane insertion by Get3
Authors: Bozkurt Gunes , Stjepanovic Goran , Vilardi Fabio , Amlacher Stefan , Wild Klemens , Bange Gert , Favaloro Vincenzo , Rippe Karsten , Hurt Ed , Dobberstein Bernhard , Sinning Irmgard
Published: December 2009

Tail-anchored (TA) membrane proteins are involved in a variety of important cellular functions, including membrane fusion, protein translocation, and apoptosis. The ATPase Get3 (Asna1, TRC40) was identified recently as the endoplasmic reticulum targeting factor of TA proteins. Get3 consists of an ATPase and α-helical subdomain enriched in methionine and glycine residues. We present structural and biochemical analyses of Get3 alone as well as in complex with a TA protein, ribosome-associated membrane protein 4 (Ramp4). The ATPase domains form an extensive dimer interface that encloses 2 nucleotides in a head-to-head orientation and a zinc ion. Amide proton exchange mass spectrometry shows that the α-helical subdomain of Get3 displays considerable flexibility in solution and maps the TA protein-binding site to the α-helical subdomain. The non-hydrolyzable ATP analogue AMPPNP-Mg
2+
- and ADP-Mg
2+
-bound crystal structures representing the pre- and posthydrolysis states are both in a closed form. In the absence of a TA protein cargo, ATP hydrolysis does not seem to be possible. Comparison with the ADP·AlF
4
−
-bound structure representing the transition state (Mateja A, et al. (2009)
Nature
461:361–366) indicates how the presence of a TA protein is communicated to the ATP-binding site. In vitro membrane insertion studies show that recombinant Get3 inserts Ramp4 in a nucleotide- and receptor-dependent manner. Although ATP hydrolysis is not required for Ramp4 insertion per se, it seems to be required for efficient insertion. We postulate that ATP hydrolysis is needed to release Get3 from its receptor. Taken together, our results provide mechanistic insights into posttranslational targeting of TA membrane proteins by Get3.
A theory of microtubule catastrophes and their regulation
Authors: Brun Ludovic , Rupp Beat , Ward Jonathan J. , Nédélec François
Published: December 2009

Dynamic instability, in which abrupt transitions occur between growing and shrinking states, is an intrinsic property of microtubules that is regulated by both mechanics and specialized proteins. We discuss a model of dynamic instability based on the popular idea that growth is maintained by a cap at the tip of the fiber. The loss of this cap is thought to trigger the transition from growth to shrinkage, called a catastrophe. The model includes longitudinal interactions between the terminal tubulins of each protofilament and “gating rescues” between neighboring protofilaments. These interactions allow individual protofilaments to transiently shorten during a phase of overall microtubule growth. The model reproduces the reported dependency of the catastrophe rate on tubulin concentration, the time between tubulin dilution and catastrophe, and the induction of microtubule catastrophes by walking depolymerases. The model also reproduces the comet tail distribution that is characteristic of proteins that bind to the tips of growing microtubules.
Versatile ultrathin nanoporous silicon nitride membranes
Authors: Vlassiouk Ivan , Apel Pavel Y. , Dmitriev Sergey N. , Healy Ken , Siwy Zuzanna S.
Published: December 2009

Single- and multiple-nanopore membranes are both highly interesting for biosensing and separation processes, as well as their ability to mimic biological membranes. The density of pores, their shape, and their surface chemistry are the key factors that determine membrane transport and separation capabilities. Here, we report silicon nitride (SiN) membranes with fully controlled porosity, pore geometry, and pore surface chemistry. An ultrathin freestanding SiN platform is described with conical or double-conical nanopores of diameters as small as several nanometers, prepared by the track-etching technique. This technique allows the membrane porosity to be tuned from one to billions of pores per square centimeter. We demonstrate the separation capabilities of these membranes by discrimination of dye and protein molecules based on their charge and size. This separation process is based on an electrostatic mechanism and operates in physiological electrolyte conditions. As we have also shown, the separation capabilities can be tuned by chemically modifying the pore walls. Compared with typical membranes with cylindrical pores, the conical and double-conical pores reported here allow for higher fluxes, a critical advantage in separation applications. In addition, the conical pore shape results in a shorter effective length, which gives advantages for single biomolecule detection applications such as nanopore-based DNA analysis.
Long-term economic growth stimulus of human capital preservation in the elderly
Authors: Manton Kenneth G. , Gu Xi-Liang , Ullian Arthur , Tolley H. Dennis , Headen Alvin E. , Lowrimore Gene
Published: December 2009

Health care is a crucial factor in US economic growth, because growing health care costs have made US corporations less competitive than their counterparts in countries where central governments assume most of those costs. In this paper we illustrate a second, possibly more powerful, effect of health care expenditures on the long term pace of US economic growth, i.e., that such investments in aging populations helps preserve human capital to later ages. In addition, as current investment in health care improves health and functional status, the future demand for health care as well as future health care costs will be constrained. These are crucial factors in countries experiencing rapid population aging. US labor force projections do not directly represent the effects of health care investment on the health of the future labor force, and federal health cost projections do not reflect the trajectory of health changes. Health dynamic projections suggest the effects of health care investment are large and growth stimulating. Projections done for the time period used by the Congressional Budget Office in budget mark-ups (2010–2020) are presented in the
supporting information
.
Synergistic effect of HIF-1α gene therapy and HIF-1-activated bone marrow-derived angiogenic cells in a mouse model of limb ischemia
Authors: Rey Sergio , Lee KangAe , Wang C. Joanne , Gupta Kshitiz , Chen Shaoping , McMillan Alexandra , Bhise Nupura , Levchenko Andre , Semenza Gregg L.
Published: December 2009

Ischemia induces the production of angiogenic cytokines and the homing of bone-marrow-derived angiogenic cells (BMDACs), but these adaptive responses become impaired with aging because of reduced expression of hypoxia-inducible factor (HIF)-1α. In this study, we analyzed the effect of augmenting HIF-1α levels in ischemic limb by intramuscular injection of AdCA5, an adenovirus encoding a constitutively active form of HIF-1α, and intravenous administration of BMDACs that were cultured in the presence of the prolyl-4-hydroxylase inhibitor dimethyloxalylglycine (DMOG) to induce HIF-1 expression. The combined therapy increased perfusion, motor function, and limb salvage in old mice subjected to femoral artery ligation. Homing of BMDACs to the ischemic limb was dramatically enhanced by intramuscular AdCA5 administration. DMOG treatment of BMDACs increased cell surface expression of β
2
integrins, which mediated increased adherence of BMDACs to endothelial cells. The effect of DMOG was abolished by coadministration of the HIF-1 inhibitor digoxin or by preincubation with a β
2
integrin-blocking antibody. Transduction of BMDACs with lentivirus LvCA5 induced effects similar to DMOG treatment. Thus, HIF-1α gene therapy increases homing of BMDACs to ischemic muscle, whereas HIF-1 induction in BMDACs enhances their adhesion to vascular endothelium, leading to synergistic effects of combined therapy on tissue perfusion.
Geographic Risk Management: A Spatial Study of Mentally Disordered Offenders Discharged from Forensic Psychiatric Care
Authors: Melnychuk RachelM. , Verdun-Jones SimonN. , Brink Johann
Published: December 2009

We investigated the impact of neighborhood and community factors on the reintegration of forensic patients leaving custodial care in British Columbia, Canada. Using geographic information systems (GIS) techniques, the residential locations of a sample of forensic patients were tracked over time and mapped in relation to each other. The frequency of a patient's return to hospital was monitored and the reasons for these returns were recorded after each unsuccessful community placement. The analysis of the findings suggested that patients who were released to certain, socially disorganized neighborhoods returned to inpatient care at a higher frequency. These neighborhoods exhibited many destabilizing features that may have significant influence on the long-term success or failure of discharge patients, such as low income, high unemployment, poor educational achievement, and concentrated rental accommodation. Further research is needed in order to explore not only the influence of neighbourhood destabilizers on the length of community placements for forensic mental health patients, but also the underlying rationale for locating patient services in socially disorganized areas.