bluuebunny/crossref_metadata_2025 huggingface dataset:
Lichen Colonization and Recolonization of Two Recently Deglaciated Zones in the Maritime Antarctic
Authors: Valladares Fernando , Sancho Leopoldo G.
Published: January 2005

The stages of vegetation development close to two glacier fronts on two of the South Shetland Islands (Livingston and Robert) within the Maritime Antarctic were studied with special reference to saxicolous lichens. A lichenometric study of the crustose lichen Caloplaca sublobulata was carried out at both sites. On the moraine of Livingston Island, rock size played an important role in lichen development, explaining most of the differences observed in the diameter of C. sublobulata, the number of species, and the percentage of cover among the rocks studied. On Robert Island, the distance from the glacier front was associated with the lichen cover of the rocks but not with diameter of C. sublobulata This homogeneous distribution of C. sublobulata thallus size on the Robert Island study area points to a simultaneous recolonization of the whole zone by this lichen. The lichen development in the area studied on Robert Island seems to have been drastically affected by fluctuations in the persistence of snow cover following glacier front retreat. Tentative associations between ice retreat and colonization, on the o e hand, and changes in snow cover duration and the dynamic processes of extinction and recolonization, on the other, are suggested from comparison of the two zones.
Carbon Acquisition and Water Relations of Lichens in Polar Regions—Potentials and Limitations
Authors: Kappen L. , Sommerkorn M. , Schroeter B.
Published: January 2005

Primary production in lichens, as in all plants, is the result of gains and losses. Losses of carbon may be high under extreme environmental conditions. In the harsh climate of polar regions lichens use much carbon for maintenance and in their stress response. Thallus biomass is preserved through periods of darkness, extremely low temperatures and snow cover. On the other hand, lichens are photosynthetically active at temperatures below 0°C. They can make use of melted water from snow and ice and take up water vapour from snow. Physiological adaptations and environmental conditions under which growth is possible are discussed in terms of our recent results. A concept model describes the main parameters that influence carbon acquisition, growth and water relations of lichens in polar regions.
Climate Change and the Ecophysiological Response of Arctic Lichens
Authors: Nash Thomas H. , Olafsen Astrid G.
Published: January 2005

Under field conditions of optimal water hydration, lichen photosynthesis is primarily light-limited and nitrogen fixation is temperature-limited in both Peltigera canina and Stereocaulon tomentosum at Anaktuvuk Pass, Alaska. Thus, where duration of optimal hydration conditions remains unchanged from the present-day climate, the anticipated temperature increases in the Arctic may enhance nitrogen fixation in these lichens more than carbon gain. Because nitrogen frequently limits productivity in Arctic ecosystems, the results are potentially important to the many Arctic and subarctic ecosystems in which such lichens are abundant.
On the influence of a radial electric field on ripple-induced stochastic diffusion in tokamaks
Authors: ASSAS S. , ERIKSSON L.-G.
Published: January 2005

Strong plasma rotation is an important feature in many high-performance fusion plasmas, especially in plasmas with internal transport barriers (ITBs). An analysis of the impact of plasma rotation, or more specifically of a radial electric field, on the ripple-induced stochastic diffusion of energetic ions is presented. In particular, the influence of a radial electric field on the threshold for stochastic motion and on the diffusion path in real space is investigated.
Picturing the Black Box: On Blanks in Nineteenth Century Paintings and Photographs
Authors: Geimer Peter
Published: January 2005

Argument

In 1867 Edouard Manet painted the execution of the Mexican emperor Maximilian of Habsburg. Manet broke with the classical tradition of history painting, for he depicted the actual shooting itself instead of choosing moments before or after the execution. Thus, the painting refers to a moment that in real time would have been far too brief to be perceptible. Manet presented a portrait of living actors whose execution has already taken place. This depiction of the imperceptible invites comparison to contemporaneous photographs of extremely short periods of time: attempts to capture flying cannon balls (Thomas Kaife), to take flashlight portraits of patients that would undermine their bodies' reaction time (Albert Londe), to visualize the successive stages of a drop falling into water (Arthur Worthington). Like Manet these scientists referred to an “optical unconscious” (Walter Benjamin). A closer look at their work reveals that they dealt with a space of knowledge that went beyond the classical dichotomy between objectivity and imagination, scientific and artistic pictures.
László Moholy-Nagy's New Vision and the Aestheticization of Scientific Photography in Weimar Germany
Authors: Botar Oliver A. I.
Published: January 2005

Argument

I propose that both Moholy-Nagy's suggestions that products of applied, particularly scientific, photography be employed as exemplars for art photography, and his practice of integrating such applied photographs with art photographs in his publications and exhibitions, laid the groundwork for an aestheticization of scientific photography within the twentieth-century artistic avant-garde. This photographic “New Vision,” formulated in the 1920s, also effected a kind of “scientization” of art photography. Rather than Positivist mechanism, however, I argue that the science at play was “biocentrism,” the early twentieth-century worldview that can be described as Naturromantik updated by biologism. His key inspiration in this regard was one of the most important figures of biocentrism, the biologist and popular scientific writer Raoul Heinrich Francé, and his conception of Biotechnik [bionics], in which he proposed that all human technologies are based in natural technologies.

The biological, pure and simple, taken as the guide.

– Moholy-Nagy (1938, 198)
A ROLE FOR REGULATORY T CELLS IN MATERNO-FETAL TOLERANCE?
Authors: DUSSABLY LAURE M , SOMERSET DAVID A , KILBY MARK D , SANSOM DAVID M
Published: January 2005

Since the discovery that major histocompatibility complex (MHC) proteins provide a barrier to tissue transplantation between individuals it has been a paradox that, during pregnancy, the mother is able to retain a fetus that expresses MHC antigens from the father without rejection. The nature of this materno-fetal tolerance is not well understood and may involve multiple mechanisms. However since a major mechanism of histo-incompatible tissue rejection involves T lymphocytes, it seems plausible that controlling T cell responses is likely to be important. Recently, there has been considerable research activity focusing on the role of a sub-set of CD4+ T cells that express the interleukin-2 receptor chain CD25+. These CD4+CD25+ cells are termed regulatory T cells (Treg) based on their ability to prevent autoimmune tissue damage. In this review we discuss recent evidence that may link Treg to materno-fetal tolerance during pregnancy.
FETAL UROLOGY AND PRENATAL DIAGNOSIS: CURRENT THEMES
Authors: THOMAS DFM
Published: January 2005

Isolated case reports of the prenatal detection of urinary tract malformations first appeared in the literature in the late 1970s and by the mid 1980s prenatal diagnosis had already become a firmly established feature of paediatric urological practice throughout the developed world. The rapid development of prenatal diagnosis of urinary tract malformations can be attributed to the relatively high incidence of congenital anomalies within this system and the fact that the common anomalies are usually associated with dilatation or cystic pathology which is readily visualised by ultrasound. Although many aspects of prenatal diagnosis have been clarified by studies undertaken over the last twenty years some important questions have yet to be fully resolved – for example the long term natural history of asymptomatic uropathies such as pelvi ureteric obstruction and the benefit (if any) of prenatal diagnosis in reducing long term burden of chronic renal failure associated with posterior urethral valves.
Simultaneous Chandra X ray, Hubble Space Telescope ultraviolet, and Ulysses radio observations of Jupiter's aurora
Authors: Elsner R. F. , Lugaz N. , Waite J. H. , Cravens T. E. , Gladstone G. R. , Ford P. , Grodent D. , Bhardwaj A. , MacDowall R. J. , Desch M. D. , Majeed T.
Published: January 2005

Observations of Jupiter carried out by the Chandra Advanced CCD Imaging Spectrometer (ACIS‐S) instrument over 24–26 February 2003 show that the auroral X‐ray spectrum consists of line emission consistent with high‐charge states of precipitating ions, and not a continuum as might be expected from bremsstrahlung. The part of the spectrum due to oxygen peaks around 650 eV, which indicates a high fraction of fully stripped oxygen in the precipitating ion flux. A combination of the OVIII emission lines at 653 eV and 774 eV, as well as the OVII emission lines at 561 eV and 666 eV, are evident in the measure auroral spectrum. There is also line emission at lower energies in the spectral region extending from 250 to 350 eV, which could be from sulfur and/or carbon. The Jovian auroral X‐ray spectra are significantly different from the X‐ray spectra of comets. The charge state distribution of the oxygen ions implied by the measured auroral X‐ray spectra strongly suggests that independent of the source of the energetic ions, magnetospheric or solar wind, the ions have undergone additional acceleration. This spectral evidence for ion acceleration is also consistent with the relatively high intensities of the X rays compared with the available phase space density of the (unaccelerated) source populations of solar wind or magnetospheric ions at Jupiter, which are orders of magnitude too small to explain the observed emissions. The Chandra X‐ray observations were executed simultaneously with observations at ultraviolet wavelengths by the Hubble Space Telescope and at radio wavelengths by the Ulysses spacecraft. These additional data sets suggest that the source of the X rays is magnetospheric in origin and that the precipitating particles are accelerated by strong field‐aligned electric fields, which simultaneously create both the several‐MeV energetic ion population and the relativistic electrons observed in situ by Ulysses that are correlated with ∼40 min quasi‐periodic radio outbursts.
Spring precipitation variations over the western Himalaya, India, since A.D. 1731 as deduced from tree rings
Authors: Singh Jayendra , Yadav Ram R.
Published: January 2005

Spring precipitation, representative of regional‐scale features, was reconstructed since A.D. 1731 using 15 site ring width chronologies of Himalayan cedar (Cedrus deodara (Roxb. ex Lambert) G. Don), prepared from distantly located moisture‐stressed sites in the western Himalayan region. This is so far the strongest tree‐ring‐based precipitation reconstruction in terms of variance explained in the calibration model (A.D. 1897–1986) from the western Himalayan region. The twentieth century experienced the driest and wettest years in the whole reconstructed series. The 10‐ and 20‐year means also indicate extreme precipitation periods in the twentieth century. The increasing precipitation trend noticed in the reconstructed data of the late twentieth century closely matches with instrumental data.
Heat flow in proteins: Computation of thermal transport coefficients
Authors: Yu Xin , Leitner David M.
Published: January 2005

The rate of vibrational energy transfer and thermal transport coefficients are computed for two structurally distinct proteins, green fluorescent protein (GFP) and myoglobin. The computation of thermal transport coefficients exploits the scaling of the energy diffusion coefficient with the vibrational mode frequency of a protein. Near 300 K we find that vibrational energy transfer due to anharmonicity contributes substantially to thermal transport because of the localization of many thermally accessible normal modes. The thermal diffusivity for the β-barrel GFP is larger than that for myoglobin, particularly at low temperature due to a mean free path for vibrational energy propagation that is twice as large at low frequency. Vibrational energy transfer is also faster in GFP than in myoglobin for most vibrational modes.
The dipole moments of the excited states of FeC
Authors: Tzeli Demeter , Mavridis Aristides
Published: January 2005

With the purpose of comparing expectation dipole moment values 〈μ〉 with finite-field obtained dipole moments μFF, we recalculated by the finite-field method previously reported 〈μ〉 values of 38 excited states of FeC. In most of the cases μFF is significantly larger than 〈μ〉.
The absorption and excitation spectroscopy of matrix-isolated atomic manganese: Sites of isolation in the solid rare gases
Authors: Collier Martin A. , McCaffrey John G.
Published: January 2005

This study collects information from absorption and luminescence excitation spectra recorded for Mn atoms isolated in the solid rare gases Ar, Kr, and Xe and presents an analysis of the site occupancy, based on the polarizabilities of the rare gases and the observed spectral shifts. Two thermally stable sites of isolation exist for atomic Mn in solid Ar and Kr, while a single thermally stable site is present in Mn/Xe. Site occupancy assignments are based on the application of a polarizability model to the z 6P5/2←a 6S5/2; z 8P5/2←a 6S5/2, and y 6P5/2←a 6S5/2 electronic transitions of atomic Mn. From an analysis of the observed RG matrix-to-gas phase energy shifts for P←S type transitions, this model allows the association of certain site types occupied by metal atoms in the rare gas solids. The required condition being a linear dependence of the matrix shifts with rare gas polarizability for those metal atoms “trapped” in a particular site type. Application of the polarizability model in conjunction with trends observed in site dominance, established a connection between the blue sites in Ar and Kr and the single site in Xe. Use of the known Mg⋅RG ground state bond lengths facilitated an identification of the sites of Mn atom isolation assuming the transference of the known Mg⋅RG bond lengths to the Mn⋅RG systems. Substitutional site occupancy of atomic Mn is assigned to the blue sites in Ar and Kr and the single site in Xe, while tetra-vacancy site occupancy is assigned to the red sites in Ar and Kr. Consistent with these assignments, Mn atoms in solid Ar show a preference for trapping in tetra-vacancy sites whereas in solid Kr, single substitutional sites are preferred and in Xe, this is the only site observed.
Accurate hybrid stochastic simulation of a system of coupled chemical or biochemical reactions
Authors: Salis Howard , Kaznessis Yiannis
Published: January 2005

The dynamical solution of a well-mixed, nonlinear stochastic chemical kinetic system, described by the Master equation, may be exactly computed using the stochastic simulation algorithm. However, because the computational cost scales with the number of reaction occurrences, systems with one or more “fast” reactions become costly to simulate. This paper describes a hybrid stochastic method that partitions the system into subsets of fast and slow reactions, approximates the fast reactions as a continuous Markov process, using a chemical Langevin equation, and accurately describes the slow dynamics using the integral form of the “Next Reaction” variant of the stochastic simulation algorithm. The key innovation of this method is its mechanism of efficiently monitoring the occurrences of slow, discrete events while simultaneously simulating the dynamics of a continuous, stochastic or deterministic process. In addition, by introducing an approximation in which multiple slow reactions may occur within a time step of the numerical integration of the chemical Langevin equation, the hybrid stochastic method performs much faster with only a marginal decrease in accuracy. Multiple examples, including a biological pulse generator and a large-scale system benchmark, are simulated using the exact and proposed hybrid methods as well as, for comparison, a previous hybrid stochastic method. Probability distributions of the solutions are compared and the weak errors of the first two moments are computed. In general, these hybrid methods may be applied to the simulation of the dynamics of a system described by stochastic differential, ordinary differential, and Master equations.
Theoretical analysis of the microwave-drill near-field localized heating effect
Authors: Jerby E. , Aktushev O. , Dikhtyar V.
Published: January 2005

The microwave-drill principle [Jerby et al., Science 298, 587 (2002)] is based on a localized hot-spot effect induced by a near-field coaxial applicator. The microwave drill melts the nonmetallic material locally and penetrates mechanically into it to shape the hole. This paper presents a theoretical analysis of the thermal-runaway effect induced in front of the microwave drill. The model couples the Maxwell’s and heat equations including the material’s temperature-dependent properties. A finite-difference time-domain algorithm is applied in a two-time-scale numerical model. The simulation is demonstrated for mullite, and benchmarked in simplified cases. The results show a temperature rise of ∼103K∕s up to 1300K within a hot spot confined to a ∼4-mm width (∼0.1 wavelength). The input-port response to this near-field effect is modeled by equivalent time-varying lumped-circuit elements. Besides the physical insight, this theoretical study provides computational tools for design and analysis of microwave drills and for their real-time monitoring and adaptive impedance matching.
Thermally activated mechanisms of hydrogen abstraction by growth precursors during plasma deposition of silicon thin films
Authors: Bakos Tamas , Valipa Mayur S. , Maroudas Dimitrios
Published: January 2005

Hydrogen abstraction by growth precursors is the dominant process responsible for reducing the hydrogen content of amorphous silicon thin films grown from SiH4 discharges at low temperatures. Besides direct (Eley-Rideal) abstraction, gas-phase radicals may first adsorb on the growth surface and abstract hydrogen in a subsequent process, giving rise to thermally activated precursor-mediated (PM) and Langmuir-Hinshelwood (LH) abstraction mechanisms. Using results of first-principles density functional theory (DFT) calculations on the interaction of SiH3 radicals with the hydrogen-terminated Si(001)-(2×1) surface, we show that precursor-mediated abstraction mechanisms can be described by a chemisorbed SiH3 radical hopping between overcoordinated surface Si atoms while being weakly bonded to the surface before encountering a favorable site for hydrogen abstraction. The calculated energy barrier of 0.39 eV for the PM abstraction reaction is commensurate with the calculated barrier of 0.43–0.47 eV for diffusion of SiH3 on the hydrogen-terminated Si(001)-(2×1) surface, which allows the radical to sample the entire surface for hydrogen atoms to abstract. In addition, using the same type of DFT analysis we have found that LH reaction pathways involve bond breaking between the silicon atoms of the chemisorbed SiH3 radical and the film prior to hydrogen abstraction. The LH reaction pathways exhibit energy barriers of 0.76 eV or higher, confining the abstraction only to nearest-neighbor hydrogens. Furthermore, we have found that LH processes compete with radical desorption from the hydrogen-terminated Si(001)-(2×1) surface and may be suppressed by the dissociation of chemisorbed SiH3 radicals into lower surface hydrides. Analysis of molecular-dynamics simulations of the growth process of plasma deposited silicon films have revealed that qualitatively similar pathways for thermally activated hydrogen abstraction also occur commonly on the amorphous silicon growth surface.
Silicon interstitial injection during dry oxidation of SiGe∕Si layers
Authors: Napolitani E. , Di Marino M. , De Salvador D. , Carnera A. , Spadafora M. , Mirabella S. , Terrasi A. , Scalese S.
Published: January 2005

The injection of Si self-interstitial atoms during dry oxidation at 815°C of very shallow SiGe layers grown on Si (001) by molecular-beam epitaxy (MBE) has been investigated. We first quantified the oxidation enhanced diffusion (OED) of two boron deltas buried into the Si underlying the oxidized SiGe layers. Then, by simulating the interstitial diffusion in the MBE material with a code developed on purpose, we estimated the interstitial supersaturation (S) at the SiGe∕Si interface. We found that S (a) is lower than that observed in pure Si, (b) is Ge-concentration dependent, and (c) has a very fast transient behavior. After such a short transient, the OED is completely suppressed, and the suppression lasts for long annealing times even after the complete oxidation of the SiGe layer. The above results have been related to the mechanism of oxidation of SiGe in which the Ge piles up at the SiO2∕SiGe interface by producing a thin and defect-free layer with a very high concentration of Ge.
Resonant magnetic tunnel junction at 0°K: I-V characteristics and magnetoresistance
Authors: Dartora C. A. , Cabrera G. G.
Published: January 2005

In this paper we analyze the main transport properties of a simple resonant magnetic tunnel junction (FM-IS-METAL-IS-FM structure) taking into account both elastic and magnon-assisted tunneling processes at low voltages and temperatures near 0°K. We show the possibility of magnetoresistance inversion as a consequence of inelastic processes and spin-dependent transmission coefficients. Resonant tunneling can also explain the effect of scattering by impurities located inside an insulating barrier.
Threshold voltage shift of heteronanocrystal floating gate flash memory
Authors: Zhu Yan , Zhao Dengtao , Li Ruigang , Liu Jianlin
Published: January 2005

Simulations of threshold voltage shift of a p-channel Ge/Si heteronanocrystal floating gate memory device were carried out using both a numerical two-dimensional Poisson–Boltzmann method and an equivalent circuit model. The results show that the presence of a Ge dot on top of a Si dot significantly prolongs the retention time of the device, indicated by the time decay behavior of the threshold voltage shift. Both methods lead to consistent results that an increase in the thickness of either the Si dot or Ge dot will result in a reduction of the threshold voltage shift. Additionally, the threshold voltage shift increases significantly as the heteronanocrystal density increases. Nevertheless, only a weak dependence of threshold voltage shift on the tunneling oxide thickness was found.
Spatial and chemical interface asymmetry in Fe/MgO/Fe(001) heterostructures
Authors: Palomares F. J. , Munuera C. , Boubeta C. Martínez , Cebollada A.
Published: January 2005

The chemical nature of the different interfaces and possible segregation effects are studied in fully epitaxial Fe/MgO/Fe (001) oriented heterostructures fabricated by combined sputtering plus laser ablation deposition techniques. Auger electron spectroscopy depth profiling experiments showed that the interfaces of the MgO spacer with the Fe layers are different, the inner MgO/Fe being spatially broader and with an FeO interlayer, while that of the outer Fe/MgO is spatially narrower with no evidence of FeO formation. This spatial and chemical asymmetry is interpreted in terms of the different deposition procedures, which affect the formation of both interfaces. No Fe segregation in the MgO layers or MgO segregation in the Fe films is observed.
Thermal quenching of cathodoluminescence from ZnS:Ag,Cl powder phosphors
Authors: Abrams B. L. , Williams L. , Bang J.-S. , Holloway P. H.
Published: January 2005

Thermal quenching of cathodoluminescence (CL) was studied by incrementally increasing the temperature of a ZnS:Ag,Cl phosphor without exposure to a continuous electron beam and measuring the decreased CL intensity. A characteristic thermal quenching temperature of 150 °C with an activation energy (Ea) of 0.87 eV was measured. In addition to the reduced CL intensity, the spectra shifted to longer wavelengths and changed shape at elevated temperature due to band gap narrowing at high temperatures and to copper contamination from the heater stage. The CL spectral distribution and intensity were 100% recoverable upon cooling back to room temperature when the electron beam exposure was <1C∕cm2.
Photocurrent in epitaxial GaN
Authors: Salis M. , Anedda A. , Quarati F. , Blue A. J. , Cunningham W.
Published: January 2005

A simple kinetic model concerning photocurrent in epitaxially grown GaN is presented. Utilizing a minimal set of rate equations and kinetic parameters, it is shown that in the presence of hole centers with small probabilities of electron-hole recombinations, the time dependence of photocurrent is ruled by competition between capture of conduction band electrons by deep electron traps and electron-hole recombinations. If the probability of electron capture exceeds that of recombination, the decay of current after excitation is turned off shows the usual persistent photocurrent trend. If, on the contrary, the probability of recombination is larger than that of electron capture, a slow photocurrent quenching, past a maximum, can be observed. In some circumstances, after excitation is turned off, the current drops below the steady dark current, at which point the negative persistent photoconductivity effect comes into play.
Transport efficiency of vacuum arc plasma in a curved magnetic filter
Authors: Hu Yawei , Li Liuhe , Xu Ming , Liu Youming , Cai Xun , Brown Ian G. , Chu Paul K.
Published: January 2005

We describe two methods for increasing the transmission efficiency of vacuum arc plasma through curved magnetic filters. In the first method the substrate is connected to the anode or biased to a negative voltage. In the second method a metal grid is placed between the substrate and the exit of the magnetic filter, and biased to a positive voltage whereas the substrate is biased negatively. The ion saturation current and electron saturation current of the plasma between the filter exit and the substrate were measured using a current collector plate and a Langmuir probe, respectively, and the ion density estimated. For the experimental conditions of the work described here, the measured ion flux (ion saturation current) near the duct exit was increased by up to about 80% (from 140 to 250 mA), and the measured ion density was increased by up to about 40% (from 3.7×1011 to 5.2×1011cm−3). These results can be explained by the ambipolar influence of enhanced electron flow on the accompanying plasma ion component, leading also to enhanced ion flow.
Spin-polarized reflection in a two-dimensional electron system
Authors: Chen Hong , Heremans J. J. , Peters J. A. , Govorov A. O. , Goel N. , Chung S. J. , Santos M. B.
Published: January 2005

We present a method to create spin-polarized beams of ballistic electrons in a two-dimensional electron system in the presence of spin–orbit interaction. Scattering of a spin-unpolarized injected beam from a lithographic barrier leads to the creation of two fully spin-polarized side beams, in addition to an unpolarized specularly reflected beam. Experimental magnetotransport data on InSb∕InAlSb heterostructures demonstrate the spin-polarized reflection in a mesoscopic geometry.
Laser Doppler velocimetry based on the photoacoustic effect in a CO2 laser
Authors: Choi Jong-woon , Yu Moon-jong , Kopica Mirek , Woo Sam-yong , Choi Yong-Seok
Published: January 2005

We report a simple laser Doppler velocimeter in which the photoacoustic effect was used to measure the rotation wheel speed. A Doppler signal, caused by mixing a returning wave with an originally existing wave inside the CO2 laser cavity, was detected using a microphone in the laser tube. Frequency of the microphone output was in proportion to the rotation speed of a wheel and is dependent on the cosine of the angle between the direction of the laser beam and tangent of wheel velocity. A Doppler-shifted frequency as high as 34kHz was detected using this method. A frequency response of a few megahertz is expected from the laser Doppler velocimeter based on the photoacoustic effect in a CO2 laser by using a wider bandwidth microphone.
Dressed particle simulation of dusty plasmas
Authors: Jenko F. , Joyce G. , Thomas H. M.
Published: January 2005

A dressed particle approach to numerical simulation of dusty plasmas is presented. Since the dust grains in the laboratory are usually immersed in a streaming plasma, their interaction potential has a complex spatial structure which can be computed within the framework of linear response theory. For typical experimental parameters the two-body force is sufficiently short-ranged to make molecular dynamics techniques feasible. As an application of this approach, the formation of two-dimensional clusters is investigated and compared with experimental results.
A mechanical microscope: High-speed atomic force microscopy
Authors: Humphris A. D. L. , Miles M. J. , Hobbs J. K.
Published: January 2005

An atomic force microscope capable of obtaining images in less than 20ms is presented. By utilizing a microresonator as a scan stage, and through the implementation of a passive mechanical feedback loop with a bandwidth of more than 2MHz, a 1000-fold increase in image acquisition rate relative to a conventional atomic force microscope is obtained. This has allowed images of soft crystalline and molten polymer surfaces to be collected in 14.3ms, with a tip velocity of 22.4cms−1 while maintaining nanometer resolution.
Pseudo-half-metallicity in the double perovskite Sr2CrReO6 from density-functional calculations
Authors: Vaitheeswaran G. , Kanchana V. , Delin A.
Published: January 2005

The electronic structure of the spintronic material Sr2CrReO6 is studied by means of full-potential linear muffin-tin orbital method. Scalar relativistic calculations predict Sr2CrReO6 to be half-metallic with a magnetic moment of 1μB. When spin–orbit coupling is included, the half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital moment appears, resulting in a significant increase of the total magnetic moment to 1.28μB. This moment is significantly larger than the experimental moment of 0.9μB. A possible explanation of this discrepancy is that the anti-site disorder in Sr2CrReO6 is significantly larger than hitherto assumed.
Angle-dispersive neutron diffraction under high pressure to 10GPa
Authors: Klotz S. , Strässle Th. , Rousse G. , Hamel G. , Pomjakushin V.
Published: January 2005

We present a method which allows high-quality powder neutron diffraction patterns to be obtained under pressure by angle-dispersive diffraction to at least 10GPa. This technique uses a new type of Paris–Edinburgh press in conjunction with sintered boron nitride anvils. As an example, we show NiO diffraction patterns obtained under purely hydrostatic pressures up to 10GPa. These data were collected within a few hours, and are free from any contaminating signal from the pressure cell. High-resolution nuclear and magnetic structural information can be readily extracted by Rietveld refinements, without additional data correction. This technique will allow powder neutron diffraction at elevated pressures to become a standard tool on continuous neutron facilities.
Probing counterion modulated repulsion and attraction between nucleic acid duplexes in solution
Authors: Bai Yu , Das Rhiju , Millett Ian S. , Herschlag Daniel , Doniach Sebastian
Published: January 2005

Understanding biological and physical processes involving nucleic acids, such as the binding of proteins to DNA and RNA, DNA condensation, and RNA folding, requires an understanding of the ion atmosphere that surrounds nucleic acids. We have used a simple model DNA system to determine how the ion atmosphere modulates interactions between duplexes in the absence of specific metal ion-binding sites and other complicated interactions. In particular, we have tested whether the Coulomb repulsion between nucleic acids can be reversed by counterions to give a net attraction, as has been proposed recently for the rapid collapse observed early in RNA folding. The conformation of two DNA duplexes tethered by a flexible neutral linker was determined in the presence of a series of cations by small angle x-ray scattering. The small angle x-ray scattering profiles of two control molecules with distinct shapes (a continuous duplex and a mimic of the compact DNA) were in good agreement with predictions, establishing the applicability of this approach. Under low-salt conditions (20 mM Na
+
), the tethered duplexes are extended because of a Coulombic repulsion estimated to be 2–5 kT/bp. Addition of high concentrations of Na
+
(1.2 M), Mg
2+
(0.6 M), and spermidine
3+
(75 mM) resulted in electrostatic relaxation to a random state. These results indicate that a counterion-induced attractive force between nucleic acid duplexes is not significant under physiological conditions. An upper limit on the magnitude of the attractive potential under all tested ionic conditions is estimated.
A role for galanin in antidepressant actions with a focus on the dorsal raphe nucleus
Authors: Lu Xiaoying , Barr Alasdair M. , Kinney Jefferson W. , Sanna Pietro , Conti Bruno , Behrens M. Margarita , Bartfai Tamas
Published: January 2005

Selective serotonin reuptake inhibitors, such as fluoxetine (FLX), are the most commonly used drugs in the treatment of major depression. However, there is a limited understanding of their molecular mechanism of action. Although the acute effect of selective serotonin reuptake inhibitors in elevating synaptic serotonin concentrations is well known, the clinical amelioration of depressive symptoms requires 14-21 days of treatment, suggesting that numerous other rearrangements of function in the CNS must take place. In the present study, we demonstrated that 14 days of FLX treatment up-regulated galanin mRNA levels by 100% and GalR2-binding sites by 50%, in the rat dorsal raphe nucleus, where galanin coexists with serotonin. Furthermore, a galanin receptor antagonist, M40, attenuated the antidepressant-like effect of FLX in the forced swim test, a rodent preclinical screen commonly used to evaluate antidepressant-like efficacy. Direct activation of galanin receptors by a galanin receptor agonist, galnon, was found to produce an antidepressant-like effect in the same task. Two other antidepressant treatments also affected the galaninergic system in the monoaminergic nuclei: Electroconvulsive shock elevated galanin mRNA levels in dorsal raphe nucleus, whereas sleep deprivation increased galanin mRNA levels in the locus coeruleus, further underlining the connection between activation of the galaninergic system and antidepressant action of various clinically proven treatments.
Visual speech speeds up the neural processing of auditory speech
Authors: van Wassenhove Virginie , Grant Ken W. , Poeppel David
Published: January 2005

Synchronous presentation of stimuli to the auditory and visual systems can modify the formation of a percept in either modality. For example, perception of auditory speech is improved when the speaker's facial articulatory movements are visible. Neural convergence onto multisensory sites exhibiting supra-additivity has been proposed as the principal mechanism for integration. Recent findings, however, have suggested that putative sensory-specific cortices are responsive to inputs presented through a different modality. Consequently, when and where audiovisual representations emerge remain unsettled. In combined psychophysical and electroencephalography experiments we show that visual speech speeds up the cortical processing of auditory signals early (within 100 ms of signal onset). The auditory–visual interaction is reflected as an articulator-specific temporal facilitation (as well as a nonspecific amplitude reduction). The latency facilitation systematically depends on the degree to which the visual signal predicts possible auditory targets. The observed auditory–visual data support the view that there exist abstract internal representations that constrain the analysis of subsequent speech inputs. This is evidence for the existence of an “analysis-by-synthesis” mechanism in auditory–visual speech perception.
ATP flux through creatine kinase in the normal, stressed, and failing human heart
Authors: Weiss Robert G. , Gerstenblith Gary , Bottomley Paul A.
Published: January 2005

The heart consumes more energy per gram than any other organ, and the creatine kinase (CK) reaction serves as its prime energy reserve. Because chemical energy is required to fuel systolic and diastolic function, the question of whether the failing heart is “energy starved” has been debated for decades. Despite the central role of the CK reaction in cardiac energy metabolism, direct measures of CK flux in the beating human heart were not previously possible. Using an image-guided molecular assessment of endogenous ATP turnover, we directly measured ATP flux through CK in normal, stressed, and failing human hearts. We show that cardiac CK flux in healthy humans is faster than that estimated through oxidative phosphorylation and that CK flux does not increase during a doubling of the heart rate-blood pressure product by dobutamine. Furthermore, cardiac ATP flux through CK is reduced by 50% in mild-to-moderate human heart failure (1.6 ± 0.6 vs. 3.2 ± 0.9 μmol/g of wet weight per sec,
P
< 0.0005). We conclude that magnetic resonance strategies can now directly assess human myocardial CK energy flux. The deficit in ATP supplied by CK in the failing heart is cardiac-specific and potentially of sufficient magnitude, even in the absence of a significant reduction in ATP stores, to contribute to the pathophysiology of human heart failure. These findings support the pursuit of new therapies that reduce energy demand and/or augment energy transfer in heart failure and indicate that cardiac magnetic resonance can be used to assess their effectiveness.
Genetic deletion of the Nogo receptor does not reduce neurite inhibition
in vitro
or promote corticospinal tract regeneration
in vivo
Authors: Zheng Binhai , Atwal Jasvinder , Ho Carole , Case Lauren , He Xiao-lin , Garcia K. Christopher , Steward Oswald , Tessier-Lavigne Marc
Published: January 2005

Axon regeneration failure in the adult mammalian CNS is attributed in part to the inhibitory nature of CNS myelin. Three myelin-associated, structurally distinct proteins, Nogo, myelin-associated glycoprotein, and oligodendrocyte myelin glycoprotein, have been implicated in this inhibition. Neuronal Nogo receptor (NgR) binds to each of the three inhibitors and has been proposed to mediate their inhibitory signals by complexing with a signal-transducing coreceptor, the neurotrophin receptor p75
NTR
. To assess the contribution of NgR to mediating myelin inhibitory signals and regeneration failure
in vivo
, we generated and characterized NgR-deficient mice.
Nogo
transcripts are up-regulated in
NgR
mutants, indicating that NgR regulates
Nogo in vivo
. However, neurite outgrowth from
NgR
-deficient postnatal dorsal root ganglion or cerebellar granule neurons is inhibited by myelin and by a Nogo-66 substrate to the same extent as is from wild-type neurons, whereas

p75
NTR

-deficient neurons are less inhibited. The NgR ligand-binding domain promotes neurite outgrowth on Nogo-66, regardless of the genotype of the neurons, indicating that the NgR ligand-binding domain can act independent of NgR. Thus, NgR is not essential for mediating inhibitory signals from CNS myelin, at least in the neurons tested, whereas p75
NTR
plays a central role in this response. Neither
NgR
-nor

p75
NTR

-deficient mice showed enhanced regeneration of corticospinal tract axons in comparison with wild-type controls after spinal dorsal hemisection. Our results thus fail to support a central role for NgR in axonal growth inhibition
in vitro
or in corticospinal tract regeneration block
in vivo
.
A coreceptor interaction between the CD28 and TNF receptor family members B and T lymphocyte attenuator and herpesvirus entry mediator
Authors: Gonzalez Lino C. , Loyet Kelly M. , Calemine-Fenaux Jill , Chauhan Vandana , Wranik Bernd , Ouyang Wenjun , Eaton Dan L.
Published: January 2005

Immune cell cosignaling receptors are important modulators of immune cell function. For T cells, cosignaling receptors supply necessary secondary signals supporting activation or attenuation after engagement of antigen-presenting cells. The primary cosignaling receptors belong to either the Ig (CD28-like) or TNF receptor (TNFR) superfamilies. The CD28 family is comprised of coinhibitory and costimulatory receptors. The three coinhibitory receptors are cytotoxic T lymphocyte antigen 4, programmed death-1, and B and T lymphocyte attenuator (BTLA). Although cytotoxic T lymphocyte antigen 4 and programmed death-1 interact with B7-Ig family counter receptors, the ligand for BTLA is less clear. From a protein–protein interaction screen, we identified the TNFR family member herpesvirus entry mediator (HVEM) as a counter receptor for BTLA. Here we show that HVEM binds BTLA with high affinity and can form a ternary complex with its known ligands homologous to lymphotoxin, showing inducible expression, and competing with herpes simplex virus glycoprotein D for HVEM, a receptor expressed by T lymphocytes (LIGHT) or lymphotoxin α and BTLA. In addition, binding of HVEM to BTLA attenuates T cell activation, identifying HVEM/BTLA as a coinhibitory receptor pair. This study is a demonstration of a direct interaction between the primary T cell cosignaling receptors of the CD28 and TNFR families.
Expression of cellulose synthase-like (
Csl
) genes in insect cells reveals that
CslA
family members encode mannan synthases
Authors: Liepman Aaron H. , Wilkerson Curtis G. , Keegstra Kenneth
Published: January 2005

Glucuronoarabinoxylan, xyloglucan, and galactomannan are noncellulosic polysaccharides found in plant cell walls. All consist of β-linked glycan backbones substituted with sugar side chains. Although considerable progress has been made in characterizing the structure of these polysaccharides, little is known about the biosynthetic enzymes that produce them. Cellulose synthase-like (
Csl
) genes are hypothesized to encode Golgi-localized β-glycan synthases that polymerize the backbones of noncellulosic polysaccharides. To investigate this hypothesis, we used heterologous expression in
Drosophila
Schneider 2 (S2) cells to systematically analyze the functions of the gene products of a group of
Csl
genes from
Arabidopsis
and rice (
Oryza sativa
L.), including members from five
Csl
gene families (
CslA, CslC, CslD, CslE
, and
CslH
). Our analyses indicate that several members of the
CslA
gene family encode β-mannan synthases. Recombinant CslA proteins produce β-linked mannan polymers when supplied GDP-mannose. The same proteins can produce β-linked glucomannan heteropolymers when supplied both GDP-mannose and GDP-glucose. One CslA protein also produced β-linked glucan polymers when supplied GDP-glucose alone. Heterologous expression studies of additional candidate glycan synthases in insect cells or other systems may help identify other noncellulosic polysaccharide biosynthetic enzymes.
Methylator-induced, Mismatch Repair-dependent G2Arrest Is Activated through Chk1 and Chk2
Authors: Adamson Aaron W. , Beardsley Dillon I. , Kim Wan-Ju , Gao Yajuan , Baskaran R. , Brown Kevin D.
Published: January 2005

SN1 DNA methylating agents such as the nitrosourea N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) elicit a G2/M checkpoint response via a mismatch repair (MMR) system-dependent mechanism; however, the exact nature of the mechanism governing MNNG-induced G2/M arrest and how MMR mechanistically participates in this process are unknown. Here, we show that MNNG exposure results in activation of the cell cycle checkpoint kinases ATM, Chk1, and Chk2, each of which has been implicated in the triggering of the G2/M checkpoint response. We document that MNNG induces a robust, dose-dependent G2arrest in MMR and ATM-proficient cells, whereas this response is abrogated in MMR-deficient cells and attenuated in ATM-deficient cells treated with moderate doses of MNNG. Pharmacological and RNA interference approaches indicated that Chk1 and Chk2 are both required components for normal MNNG-induced G2arrest. MNNG-induced nuclear exclusion of the cell cycle regulatory phosphatase Cdc25C occurred in an MMR-dependent manner and was compromised in cells lacking ATM. Finally, both Chk1 and Chk2 interact with the MMR protein MSH2, and this interaction is enhanced after MNNG exposure, supporting the notion that the MMR system functions as a molecular scaffold at the sites of DNA damage that facilitates activation of these kinases.
cAMP-mediated Inhibition of DNA Replication and S Phase Progression: Involvement of Rb, p21Cip1, and PCNA
Authors: Naderi Soheil , Wang Jean Y.J. , Chen Tung-Ti , Gutzkow Kristine B. , Blomhoff Heidi K.
Published: January 2005

cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1proteins, since both processes were only marginally affected by increased levels of cAMP in Rb-/-and p21Cip1-/-3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase–specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase–specific antitumor agents.
Involvement of Sir2/4 in Silencing of DNA Breakage and Recombination on Mouse YACs during Yeast Meiosis
Authors: Klieger Yair , Yizhar Ofer , Zenvirth Drora , Shtepel-Milman Neta , Snoek Margriet , Simchen Giora
Published: January 2005

Yeast artificial chromosomes (YACs) that contain human DNA backbone undergo DNA double-strand breaks (DSBs) and recombination during yeast meiosis at rates similar to the yeast native chromosomes. Surprisingly, YACs containing DNA covering a recombination hot spot in the mouse major histocompatibility complex class III region do not show meiotic DSBs and undergo meiotic recombination at reduced levels. Moreover, segregation of these YACs during meiosis is seriously compromised. In meiotic yeast cells carrying the mutations sir2 or sir4, but not sir3, these YACs show DSBs, suggesting that a unique chromatin structure of the YACs, involving Sir2 and Sir4, protects the YACs from the meiotic recombination machinery. We speculate that the paucity of DSBs and recombination events on these YACs during yeast meiosis may reflect the refractory nature of the corresponding region in the mouse genome.