bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of manganite films on silicon for uncooled bolometric applications
Authors: Choudhary R. J. , Ogale Anjali S. , Shinde S. R. , Hullavarad S. , Ogale S. B. , Venkatesan T. , Bathe R. N. , Patil S. I. , Kumar Ravi
Published: May 2004

Pulsed-laser-deposited polycrystalline/amorphous films of mixed-valent manganites [La0.7Ca0.3MnO3 (LCMO), La0.5Sr0.5MnO3 (LSMO), La0.5Ba0.5MnO3 (LBMO), and (La0.6Pr0.4)0.67Ca0.33MnO3 (LPCMO)] grown at low temperature (450 °C) on single crystal (001) silicon substrate are evaluated for uncooled bolometric applications. It is shown that the temperature coefficient of resistance (TCR) and electrical noise (Sv) depend on the chemical composition. The optimum performance is found for LCMO with TCR of ∼7% K−1 and spectral noise ∼8.9×10−13 V2/Hz. In LBMO and LSMO the noise is much lower, but so is the TCR (1.5%–2% K−1). In LPCMO the TCR is high (∼10% K−1) and the noise is as well.
Observation of nonequilibrium longitudinal optical phonons in InN and its implications
Authors: Liang W. , Tsen K. T. , Ferry D. K. , Lu Hai , Schaff William J.
Published: May 2004

Nonequilibrium longitudinal optical phonons in a high quality, single crystal wurtzite structure InN sample have been studied by picosecond Raman spectroscopy. Our experimental results demonstrate that the band gap of InN cannot be around 1.89 eV; but are consistent with a band gap of about 0.8 eV. In addition, they disprove the idea that 0.8 eV luminescence observed recently in InN is due to deep level radiative emission in InN.
Electroluminescence as internal light source for measurement of the photonic strength of random porous GaP
Authors: van Driel A. F. , Vanmaekelbergh D. , Kelly J. J.
Published: May 2004

During porous etching of GaP, electroluminescence ranging from the ultraviolet to the near-infrared is generated at the interface of the porous and the nonporous layer. This is used to measure the wavelength-dependent transmission of light through porous layers in a wide thickness range. Two types of porous structures, characterized by different pore sizes, were studied. The transmission of the emitted light gives valuable information about wavelength-dependent diffusion of light through porous GaP.
Low-loss GaInNAs saturable absorber mode locking a 1.3-μm solid-state laser
Authors: Liverini V. , Schön S. , Grange R. , Haiml M. , Zeller S. C. , Keller U.
Published: May 2004

We have demonstrated stable self-starting passive cw mode locking of a solid-state laser at about 1.3 μm using a GaInNAs semiconductor saturable absorber mirror (SESAM). GaInNAs SESAMs show negligible nonsaturable losses, low saturation fluences (11 μJ/cm2) and picosecond decay times which make them well-suited for self-starting and stable cw mode locking. Sub-10-ps pulses were produced with a Nd:YLF laser at 1314 nm. The incorporation of about 2% nitrogen into InGaAs redshifts the absorption edge above 1330 nm and reduces the strain in the saturable absorber grown on a GaAs/AlAs Bragg mirror. Final absorption edge adjustments have been made with thermal annealing which blueshifts the absorption edge.
Intense longitudinal electric fields generated from transverse electromagnetic waves
Authors: Miyaji Godai , Miyanaga Noriaki , Tsubakimoto Koji , Sueda Keiichi , Ohbayashi Ken
Published: May 2004

We present a simple method for generating an intense longitudinal electric field from transverse electromagnetic waves (laser pulses) with radially symmetric polarization, to which a liquid crystal device converts linear polarization with energy efficiency of ∼99%. The laser-generated longitudinal electric field was observed in two dimensions and distinguished from the transverse component using the optical Kerr shutter method. The measured amplitude was 1.1 GV/m at the focus of a pulsed Nd:YAG laser beam of 0.25-MW peak-power.
Interaction of vapor-deposited Ti and Au with molecular wires
Authors: Walker A. V. , Tighe T. B. , Stapleton J. , Haynie B. C. , Upilli S. , Allara D. L. , Winograd N.
Published: May 2004

We have investigated the interaction of vapor-deposited titanium and gold with a self-assembled monolayer (SAM) of 4-[4′-(phenylethynyl)-phenylethynyl]-benzenthiol, an unsubstituted oligo(phenylene-ethynylene), chemisorbed on a gold substrate, a typical SAM of interest for molecular electronics. Deposited titanium atoms are observed to react in a top-down fashion with the SAM molecules to form Ti–C bonds, destroying the monolayer structure. In contrast, deposited Au atoms undergo continuous penetration through the monolayer, even at high coverages, leaving the SAM “floating” on the Au substrate surface.
Self-assembled monolayer resist for atomic layer deposition of HfO2 and ZrO2 high-κ gate dielectrics
Authors: Chen Rong , Kim Hyoungsub , McIntyre Paul C. , Bent Stacey F.
Published: May 2004

A molecular layer resist for HfO2 and ZrO2 atomic layer deposition (ALD) was demonstrated by using self-assembled monolayers (SAMs). X-ray photoelectron spectroscopy and transmission electron microscopy indicated that silicon samples terminated with octadecyltrichlorosilane (ODTS) were deactivated with respect to subsequent ALD of metal oxide gate dielectrics, under conditions in which standard chemical oxide passivated substrates are highly effective templates for ALD film growth. A time-dependent investigation of SAM formation showed that the efficiency of deactivation depends strongly on the quality of the SAMs, with a high-quality, closely packed ODTS film crucial for achieving complete blocking of the HfO2 and ZrO2 ALD process.
Analysis of plasma treatment and vapor heat treatment for thin-film transistors by extracting trap densities at front and back interfaces
Authors: Kimura Mutsumi , Abe Daisuke , Inoue Satoshi , Shimoda Tatsuya , Tam Simon W.-B.
Published: May 2004

Hydrogen (H) plasma treatment, oxygen (O) plasma treatment and water (H2O)-vapor heat treatment for polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) have been analyzed by separately extracting trap density at a front silicon-oxide interface (DF) and trap density at a back interface (DB). It is found that the H plasma treatment is apt to generate DF and DB. The O plasma treatment reduces DF, while the H2O-vapor heat treatment reduces both DF and DB. Improvement of transistor characteristics of poly-Si TFTs depends on understanding these results.
Reduced threshold current of a quantum dot laser in a short period superlattice of indirect-band gap
Authors: Sun Gregory , Soref Richard A. , Khurgin Jacob B.
Published: May 2004

We propose the idea of making quantum dot lasers by embedding direct-band gap quantum dots in a short period superlattice whose band gap is indirect. This technique reduces the threshold current and its temperature dependence. We show that a higher characteristic-temperature T0 can be achieved in a quantum dot laser with indirect GaAs/AlAs superlattice barriers compared to that with direct GaAs barriers.
Formation chemistry of high-density nanocraters on the surface of sapphire substrates with an in situ etching and growth mechanism of device-quality GaN films on the etched substrates
Authors: Hao M. , Ishikawa H. , Egawa T.
Published: May 2004

An efficient method has been investigated to grow GaN films with sapphire substrates being treated in situ metalorganic chemical vapor deposition reactor for a special effect rather than simple thermal cleaning. First, a thin GaN layer is grown on the sapphire substrate. And then it is almost etched away by thermal decomposition. It is found that the decomposition of GaN induces the decomposition of sapphire resulting in the formation of high-density nanocraters on its surface. Finally the device-quality GaN film is regrown on the etched substrate with residual gallium droplets as nucleation sites. The chemistry of the etching process and the mechanism of the final GaN growth process have been discussed. The distinct feature of this method is the in situ formation of high-density nanocraters on the surface of the substrate. A rough interface between the substrate and GaN can improve the efficiency of the light-emitting diode built on it greatly.
Coulomb blockade in a silicon/silicon–germanium two-dimensional electron gas quantum dot
Authors: Klein L. J. , Slinker K. A. , Truitt J. L. , Goswami S. , Lewis K. L. M. , Coppersmith S. N. , van der Weide D. W. , Friesen Mark , Blick R. H. , Savage D. E. , Lagally M. G. , Tahan Charlie , Joynt Robert , Eriksson M. A. , Chu J. O. , Ott J. A. , Mooney P. M.
Published: May 2004

We report the fabrication and electrical characterization of a single electron transistor in a modulation doped silicon/silicon–germanium heterostructure. The quantum dot is fabricated by electron beam lithography and subsequent reactive ion etching. The dot potential and electron density are modified by laterally defined side gates in the plane of the dot. Low temperature measurements show Coulomb blockade with a single electron charging energy of 3.2 meV.
Femtosecond-laser-activated fluorescence from silver oxide nanoparticles
Authors: Gleitsmann T. , Stegemann B. , Bernhardt T. M.
Published: May 2004

A method for fast spatially resolved photoactivation of fluorescence from silver oxide nanoparticle films is presented. High activation rates are achieved through fast scanning of a focused high repetition rate 800 nm femtosecond laser with a peak power of 6.5 MW/cm2 over the sample. The activated areas of the thin films show fluorescence after subsequent excitation with an Ar+/Kr+ laser. The fluorescent properties were investigated by means of scanning laser microscopy at ambient conditions. Depending on excitation wavelengths (488 or 568 nm) strong fluorescence in the green or red domain was observed. The activation efficiencies were examined both as a function of exposure time and femtosecond laser power. Sensitivity, reliability and possible mechanisms for the photoactivation process are discussed.
A prototype microthermophotovoltaic power generator
Authors: Yang W. M. , Chou S. K. , Shu C. , Li Z. W. , Xue H.
Published: May 2004

A prototype microthermophotovoltaic (micro-TPV) power generator is described in this letter. The system is made of a SiC (silicon carbide) emitter, a simple nine-layer dielectric filter, and a GaSb (gallium antimony) photovoltaic cell array. In a microcombustor of 0.113 cm3 in volume, when the flow rate of hydrogen is 4.20 g/h, the micro-TPV system is able to deliver an electrical power output of 1.02 W, corresponding to an open-circuit electrical voltage of 2.28 V and a short-circuit current of 0.59 A. The prototype of the micro-TPV system will make it possible for us to substitute batteries with micropower generators in micromechanical devices in the near future.
Influence of misfit dislocations on the electrical properties of CdTe layers grown by molecular beam epitaxy on InSb
Authors: Khattak G. M. , Matthews G. W. , Scott C. G. , Yousaf M.
Published: May 2004

The electrical properties of n-CdTe films grown by molecular beam epitaxy on (001) InSb substrates have been studied with a view to examining the influence of dislocations arising from strain relaxation in these materials. It is shown that the observed nonuniformity in free carrier density as a function of depth through the films can be explained by electron trapping at dislocations assuming that the density of dislocations diminishes with distance from the substrate surface in proportion to the spatially distributed strain within the films. From a comparison of the experimentally deduced and theoretically derived values for the maximum dislocation density, it is concluded that CdTe layers grown using a stoichiometric Cd/Te flux are characterized by an interfacial layer, probably In2Te3, consistent with independent evidence from x-ray diffraction studies.
All-optical switching due to state filling in quantum dots
Authors: Prasanth R. , Haverkort J. E. M. , Deepthy A. , Bogaart E. W. , van der Tol J. J. G. M. , Patent E. A. , Zhao G. , Gong Q. , van Veldhoven P. J. , Nötzel R. , Wolter J. H.
Published: May 2004

We report all-optical switching due to state filling in quantum dots (QDs) within a Mach–Zehnder interferometric switch (MZI). The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530–1570 nm probe beam is switched by optical excitation of one MZI arm. By exciting below the InGaAsP band gap, we prove that the refractive index nonlinearity is entirely due to the QDs. The switching efficiency is 5 rad/(μW absorbed power), corresponding to a 6 fJ switching energy. Probe wavelength insensitivity was obtained using a broad size distribution of QDs.
Thermophysical properties of a highly superheated and undercooled Ni–Si alloy melt
Authors: Wang H. P. , Cao C. D. , Wei B.
Published: May 2004

The surface tension of superheated and undercooled liquid Ni–5 wt % Si alloy was measured by an electromagnetic oscillating drop method over a wide temperature range from 1417 to 1994 K. The maximum undercooling of 206 K (0.13 TL) was achieved. The surface tension of liquid Ni–5 wt % Si alloy is 1.697 N m−1 at the liquidus temperature 1623 K, and its temperature coefficient is −3.97×10−4 N m−1 K−1. On the basis of the experimental data of surface tension, the other thermophysical properties such as the viscosity, the solute diffusion coefficient, and the density of liquid Ni–5 wt % Si alloy were also derived.
Terahertz quantum-well photodetector
Authors: Liu H. C. , Song C. Y. , SpringThorpe A. J. , Cao J. C.
Published: May 2004

The design and projected performance of quantum-well infrared photodetectors (QWIP) for the terahertz (1–10 THz) or the very-far-infrared region are presented together with our initial demonstration of a GaAs/AlGaAs QWIP working at photon energies below the optical phonons. We point out the problem with this initial device, discuss possible causes, and suggest areas of improvement.
Bistable nanoelectromechanical devices
Authors: Ziegler Kirk J. , Lyons Daniel M. , Holmes Justin D. , Erts Donats , Polyakov Boris , Olin Håkan , Svensson Krister , Olsson Eva
Published: May 2004

A combined transmission electron microscopy-scanning tunneling microscopy (TEM-STM) technique has been used to investigate the force interactions of silicon and germanium nanowires with gold electrodes. The I(V) data obtained typically show linear behavior between the gold electrode and silicon nanowires at all contact points, whereas the linearity of I(V) curves obtained for germanium nanowires were dependent on the point of contact. Bistable silicon and germanium nanowire-based nanoelectromechanical programmable read-only memory (NEMPROM) devices were demonstrated by TEM-STM. These nonvolatile NEMPROM devices have switching potentials as low as 1 V and are highly stable making them ideal candidates for low-leakage electronic devices.
Effects of the one-dimensional quantum barriers in pyramidal quantum dots
Authors: Michelini F. , Dupertuis M.-A. , Kapon E.
Published: May 2004

We investigated theoretically the effects of the one-dimensional barriers on the electronic states in a quantum dot grown on a nonplanar substrate. These peculiar barriers drastically modify the confined carrier wave functions and their energies. Connecting the quantum wire to the pyramidal quantum dot leads to an enhancement in the ground to first excited state separation energy and a simultaneous large reduction of the energy size sensitivity.
Persistent photocurrent and surface trapping in GaN Schottky ultraviolet detectors
Authors: Katz O. , Bahir G. , Salzman J.
Published: May 2004

GaN-based Schottky detectors were implemented and their photoresponse as a function of the incident power and time was measured. The measured photoresponse shows gain saturation and persistent photoconductivity behavior. These effects are shown here to be related to each other, arising from a nonideal semiconductor surface. A microscopic model of the gain mechanism to explain these observations is presented. Trap density at the semiconductor metal interface, characteristic lifetime, and carrier capture coefficient are extracted from our measurements.
Low-loss fiber accessible plasmon waveguide for planar energy guiding and sensing
Authors: Maier Stefan A. , Barclay Paul E. , Johnson Thomas J. , Friedman Michelle D. , Painter Oskar
Published: May 2004

A metal nanoparticle plasmon waveguide for electromagnetic energy transport utilizing dispersion engineering to increase lateral energy confinement via a two-dimensional pattern of Au dots on an optically thin Si membrane is described. Using finite-difference time-domain simulations and coupled-mode theory, we show that phase-matched evanescent excitation from conventional fiber tapers is possible with efficiencies >90% for realistic geometries. Energy loss in this waveguide is mainly due to material absorption, allowing for 1/e energy decay distances of about 320 μm for excitation at telecommunication frequencies. This concept can be extended to the visible regime and promises applications in optical energy guiding, optical sensing, and switching.
Formation of highly aligned ZnO tubes on sapphire (0001) substrates
Authors: Zhang B. P. , Binh N. T. , Wakatsuki K. , Segawa Y. , Yamada Y. , Usami N. , Kawasaki M. , Koinuma H.
Published: May 2004

ZnO tubes were epitaxially grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. The tubes grew along the substrate normal and were characterized by hexagon-shaped cross sections. All of the tubes possessed the same epitaxial relationships with respect to the substrate. Both reactor pressure and growth temperature were found to play an important role in the formation of ZnO tubes. Spiral column growth mode was found to be responsible for the formation of ZnO tubes.
Self-assembled growth and optical emission of silver-capped silicon nanowires
Authors: Qiu T. , Wu X. L. , Yang X. , Huang G. S. , Zhang Z. Y.
Published: May 2004

Structured silver-capped silicon nanowires were fabricated via electroless metal deposition on a silicon wafer in an ionic silver HF solution through selective chemical etching. Their formation mechanism was explained on the basis of self-assembled localized microscopic electrochemical cell model. The metal–semiconductor composite nanostructure was found to have a strong ultraviolet-emitting property with an emission peak position at 330 nm. The intensity of the peak nonmonotonously varies with annealing temperature and time, but its position remains unchanged. Emission and excitation spectral analyses suggest that the ultraviolet emission is closely related to the existence of silver vacancy defects in silver nanocaps formed during sample fabrication. A specific vacancy density is required to produce the strong ultraviolet emission.
Thermal stability of epitaxial SrRuO3 films as a function of oxygen pressure
Authors: Lee Ho Nyung , Christen Hans M. , Chisholm Matthew F. , Rouleau Christopher M. , Lowndes Douglas H.
Published: May 2004

The thermal stability of electrically conducting SrRuO3 thin films grown by pulsed-laser deposition on (001) SrTiO3 substrates has been investigated by atomic force microscopy and reflection high-energy electron diffraction (RHEED) under reducing conditions (25–800 °C in 10−7–10−2 Torr O2). The as-grown SrRuO3 epitaxial films exhibit atomically flat surfaces with single unit-cell steps, even after exposure to air at room temperature. The films remain stable at temperatures as high as 720 °C in moderate oxygen ambients (>1 mTorr), but higher temperature anneals at lower pressures result in the formation of islands and pits due to the decomposition of SrRuO3. Using in situ RHEED, a temperature and oxygen pressure stability map was determined, consistent with a thermally activated decomposition process having an activation energy of 88 kJ/mol. The results can be used to determine the proper conditions for growth of additional epitaxial oxide layers on high quality electrically conducting SrRuO3.
Hydrogen adsorption and desorption by potassium-doped superactivated carbon
Authors: Kojima Yoshitsugu , Suzuki Nobuaki
Published: May 2004

Potassium-doped superactivated carbon can adsorb 1.6 wt % of hydrogen at room temperature under 5 MPa. This value was greater than that of potassium-doped graphite (hydrogen adsorption capacity: 1.0 wt %). Kinetics of the hydrogen adsorption of the potassium-doped superactivated carbon was remarkably improved. The hydrogen stored in the potassium-doped superactivated carbon was mainly released at high temperatures (440–1370 K). The high hydrogen adsorption capacity and the improved kinetics of this system may be derived from the small-sized graphen and the high surface area.
Characterization of hafnium aluminate gate dielectrics deposited by liquid injection metalorganic chemical vapor deposition
Authors: Potter R. J. , Marshall P. A. , Chalker P. R. , Taylor S. , Jones A. C. , Noakes T. C. Q. , Bailey P.
Published: May 2004

Thin films of hafnium aluminate, with varying aluminum content, have been deposited by liquid injection metalorganic chemical vapor deposition using the metal alkoxide precursors hafnium methyl-methoxy-propanolate and aluminum iso-propoxide. X-ray diffraction analysis showed that the films were amorphous at aluminum concentrations above 7 at. %. Postdeposition annealing indicated that the oxide-transition temperature from amorphous to crystalline increased with aluminum content. Medium-energy ion scattering showed that up to 900 °C, internal oxidation of the silicon substrate had been inhibited. The capacitance–voltage characteristics of the films significantly improved following annealing in dry air.
Intramolecular Interactions in Carbenes Derived During the Pyrolysis of N-Alkenylisoxazolones
Authors: Cox Matthew , Dixon Michael , Lister Troy , Prager Rolf H.
Published: May 2004

Flash vacuum pyrolysis of ethyl 3-(3-methyl-5-oxo-2,5-dihydroisoxazole-2-yl)-3-phenylpropenoate yields 4-ethoxy-3-hydroxy-2-methyl-5-phenylpyridine in addition to the expected pyrrole. The structure assignment is based on two-dimensional NMR and computational evidence. The pyrolysis of the corresponding dimethyl amide gives a mixture of three 3-hydroxypyridines, in addition to the pyrrole. Evidence is presented for the formation of intermediate 1,4-oxazepines by a six-electron electrocyclic reaction of a carbene with ester or amide carbonyl groups.
Some Stereochemical Aspects of the Strecker Synthesis and the Bucherer - Bergs Reaction
Authors: Wermuth Urs D. , Jenkins Ian D. , Bott Raymond C. , Byriel Karl A. , Smith Graham
Published: May 2004

Both the Strecker and Bucherer–Bergs reactions convert the norbornane keto ester methyl bicyclo[2.2.1]hept-6-one-2-endo-carboxylate into the lactam 6-endo-aminobicyclo[2.2.1]heptane-2-endo-carboxylic acid-γ-lactam-6-exo-carboxylic acid. This lactam is unusually stable and cannot be hydrolyzed to the corresponding amino acid. The stereochemistry in the Strecker reaction, in which the amino group is endo, is contrary to that expected from literature precedent. The stereochemistry in the Bucherer–Bergs reaction, in which the amino group is also endo, has been confirmed by X-ray crystallographic analysis of the intermediate spirohydantoin (±)-bicyclo[2.2.1]heptane-2-endo-carboxylic acid-6-spiro-5′-hydantoin.
The trans-[py2AlCl4] - (‘py’ = pyridine) Anion, Structurally Characterized in Two Pyridinium Salts
Authors: Junk Peter C. , Raston Colin L. , Skelton Brian W. , White Allan H.
Published: May 2004

Salts of the trans-[py2AlCl4]– anion have been structurally characterized by ‘low’-temperature single crystal structure determinations of the adducts 2AlCl3 · 2HCl · 4py {=(pyH)2[(py)2AlCl4](AlCl4)}, and 2AlCl3 · 3HCl · 7py {=(pyH)3[(py)2AlCl4]2Cl}, extending the structurally characterized members of the array [(py)nAlCl(6–n)](n–3)± with ‘octahedral’ six-coordinate metal atom geometries, the members for n = 3 (mer) and 4 (trans) having been previously characterized.
Coordination Complexes of Tin(IV) Chloride with Unidentate Nitrogen Bases
Authors: Grigsby Warren A. , Morien Travis S. , Raston Colin L. , Skelton Brian W. , White Allan H.
Published: May 2004

A single crystal X-ray study shows the product of reaction between quinuclidine, ‘qn’, and tin(IV) chloride in toluene solution to be, unusually, a five-coordinate neutral complex [qnSnCl4]. Sn—N is 2.282(5) Å in an axial site of a trigonal bipyramidal array, trans to an Sn—Cl bond 2.367(2) with Sn—Cl (equatorial) 2.323(2)–2.333(2) Å. With 1,4-pyrazine, ‘pz’, as adumbrated by earlier spectroscopic work, a 1 : 2 (centro-symmetric) adduct, trans-[pz2SnCl4] is obtained (rather than a pyrazine bridged polymer) [Sn—N 2.246(1), Sn—Cl 2.3883(4), 2.3980(4) Å].
The Reaction of Highly Hindered Silicon Halides with Nucleophiles
Authors: Galligan Janine K. , Happer D. Alan R. , Wilson Gareth R. S.
Published: May 2004

(Me3Si)3CSi(CD3)2I, (Me3Si)3CSi(CD3)2Br, and (Me3Si)3CSi(CD3)2H solvolyze in refluxing acetic acid in the presence of HgX2 to give, in addition to (Me3Si)3CSi(CD3)2X and (Me3Si)3CSi(CD3)2OAc, isomers with structures (Me3Si)2C(SiMe2X)Si(CD3)2Me and (Me3Si)2C(SiMe2OAc)Si(CD3)2Me. The first two also react if AgX is used in place of HgX2, but (Me3Si)3CSi(CD3)2H is inert. In refluxing trifluoroacetic acid (Me3Si)3CSi(CD3)2I and (Me3Si)3CSi(CD3)2H solvolyze even in the absence of a catalyst to give similar products. The products distributions are believed to arise from nucleophilic attack on a methyl-bridged cationic intermediate.
Self-Assembly of a Charge-Neutral Molecular Square
Authors: Ali Mohammad Akbar , Bernhardt Paul V. , Kiem Chong Lee , Mirza Aminul Huq
Published: May 2004

Complexation of cadmium(II) by the ditopic (bis-tridentate) thiocarbazone ligand 1,5-bis(6-methyl-2-pyridyl- methylene)thiocarbonohydrazide, H2L1, results in the self-assembly of a charge-neutral 2 × 2 molecular grid, [Cd4(L1)4], comprising four metals and four ligands in an interlocked cyclic array. The solid-state structure of this tetramer has been established by X-ray crystallography and in solution by 1H NMR spectroscopy. The presence of lower molecular weight oligomers was identified by both NMR and ESI-MS.
Terpyridine - Ruthenium Complexes as Building Blocks for New Metallo-Supramolecular Architectures
Authors: Hofmeier Harald , Andres Philip R. , Hoogenboom Richard , Herdtweck Eberhardt , Schubert Ulrich S.
Published: May 2004

Supramolecular architectures are of great interest in modern materials research. The directed synthesis of asym-metric 2,2′:6′,2′′-terpyridine ruthenium(II) complexes is an important tool towards such systems. In this contribution, we report the synthesis of asymmetric terpyridine ruthenium(II) complexes as models for supramolecular architectures and polymers. Terpyridines, bearing different functional groups in the 4′-position, were complexed with unfunctionalized terpyridine ligands using Ru(III)/Ru(II) chemistry. The resulting compounds were characterized by UV-vis, one- and two-dimensional NMR spectroscopy as well as MALDI-TOF mass spectrometry. In addition, X-ray structure analysis was performed for one selected example.
Responses of Xanthoria parietina thalli to environmentally relevant concentrations of hexavalent chromium
Authors: Toppi Luigi Sanità di , Musetti Rita , Marabottini Rosita , Corradi Maria Grazia , Vattuone Zulema , Favali Maria Augusta , Badiani Maurizio
Published: May 2004

Thalli of the lichen Xanthoria parietina (L.) Th. Fr. were soaked for either 24 or 48 h in a buffered medium in the presence of environmentally relevant concentrations (4.8 and 9.6 μM) of hexavalent chromium [Cr(VI)]. Treatment effects on the antioxidant status, differential distribution and fate of Cr(VI) among the mycobiont and the photobiont cells, and potential damage to cell ultrastructure in the two bionts, were evaluated. The adopted conditions of low Cr(VI) stress caused: (i) an increase in the level of ascorbic acid and a decrease in that of reduced glutathione, as well as a moderate increase in guaiacol peroxidase activity, only observed after treatment with 9.6 μM Cr(VI); (ii) no changes in malondialdehyde content; (iii) a remarkable Cr accumulation in the mycobiont cytosol and compartmentalisation in the mycobiont vacuoles;(iv) a modest apoplastic Cr immobilisation by the outer part of the cell walls, of both the mycobiont and the photobiont. The response of X.�parietina to low concentrations of Cr(VI) appears to be a complex phenomenon, which might reflect maintenance of cellular homeostatic equilibria, rather than specific response pathways.
Neutrality, niches, and dispersal in a temperate forest understory
Authors: Gilbert Benjamin , Lechowicz Martin J.
Published: May 2004

A fundamental goal of ecology is to understand what controls the distribution and abundance of species. Both environmental niches and trade-offs among species in dispersal and competitive ability have traditionally been cited as determinants of plant community composition. More recently, neutral models have shown that communities of species with identical life-history characteristics and no adaptation to environmental niches can form spatial distribution patterns similar to those found in nature, so long as the species have a limited dispersal distance. If there is a strong correlation between geographic distance and change in environmental conditions, however, such spatial patterns can arise through either neutral or niche-based processes. To test these competing theories, we developed a sampling design that decoupled distance and environment in the understory plant communities of an old-growth, temperate forest. We found strong evidence of niche-structuring but almost no support for neutral predictions. Dispersal limitation acted in conjunction with environmental gradients to determine species' distributions, and both functional and phylogenetic constraints appear to contribute to the niche differentiation that structures community assembly. Our results indicate that testing a neutral hypothesis without accounting for environmental gradients will at best cause unexplained variation in plant distributions and may well provide misleading support for neutrality because of a correlation between geographic distance and environment.
Inhibition of primary human T cell proliferation byHelicobacter pylorivacuolating toxin (VacA) is independent of VacA effects on IL-2 secretion
Authors: Sundrud Mark S. , Torres Victor J. , Unutmaz Derya , Cover Timothy L.
Published: May 2004

Recent evidence indicates that the secretedHelicobacter pylorivacuolating toxin (VacA) inhibits the activation of T cells. VacA blocks IL-2 secretion in transformed T cell lines by suppressing the activation of nuclear factor of activated T cells (NFAT). In this study, we investigated the effects of VacA on primary human CD4+T cells. VacA inhibited the proliferation of primary human T cells activated through the T cell receptor (TCR) and CD28. VacA-treated Jurkat T cells secreted markedly diminished levels of IL-2 compared with untreated cells, whereas VacA-treated primary human T cells continued to secrete high levels of IL-2. Further experiments indicated that the VacA-induced inhibition of primary human T cell proliferation was not attributable to VacA effects on NFAT activation or IL-2 secretion. We show here that VacA suppresses IL-2-induced cell-cycle progression and proliferation of primary human T cells without affecting IL-2-dependent survival. Through the analysis of a panel of mutant VacA proteins, we demonstrate that VacA-mediated inhibition of T cell proliferation requires an intact N-terminal hydrophobic region necessary for the formation of anion-selective membrane channels. Remarkably, we demonstrate that one of these mutant VacA proteins [VacA-Δ(6–27)] abrogates the immunosuppressive actions of wild-type VacA in a dominant-negative fashion. We suggest that VacA may inhibit the clonal expansion of T cells that have already been activated byH. pyloriantigens, thereby allowingH. pylorito evade the adaptive immune response and establish chronic infection.
Reduction in DNA-binding affinity of Cys
2
His
2
zinc finger proteins by linker phosphorylation
Authors: Jantz Derek , Berg Jeremy M.
Published: May 2004

Cys
2
His
2
zinc finger proteins make up the largest class of transcription factors encoded in the genomes of higher eukaryotes. Recent studies of the Ikaros transcription factor demonstrated that this zinc finger protein undergoes cell cycle-dependent changes in association with DNA that seem to be due to phosphorylation of Thr or Ser residues in the linker regions connecting adjacent zinc finger domains. The high degree of conservation of this linker sequence within the Cys
2
His
2
superfamily suggested a common mechanism for the cell cycle-dependent modulation of DNA-binding affinity throughout this large class of transcription factors. The effects of linker phosphorylation on DNA-binding affinity were investigated through a direct comparison of the DNA-binding properties of four synthetic zinc finger proteins produced by native chemical ligation. The four proteins, comprising three zinc finger domains joined by two consensus
Thr
-Gly-Glu-Lys-Pro linkers, correspond to all four possible combinations of linker Thr phosphorylation states. Fluorescence-based DNA-binding studies of a specific DNA-binding site revealed that phosphorylation of a single linker reduced binding affinity ≈40-fold, whereas phosphorylation of both linkers reduced binding affinity 130-fold. These results with purified components demonstrate that linker phosphorylation does, indeed, produce a significant reduction in DNA-binding affinity and support a model wherein a single cell cycle-dependent Ser/Thr kinase could simultaneously inactivate a large number of zinc finger transcription factors.
An invariant aspartic acid in the DNA glycosylase domain of
DEMETER
is necessary for transcriptional activation of the imprinted
MEDEA
gene
Authors: Choi Yeonhee , Harada John J. , Goldberg Robert B. , Fischer Robert L.
Published: May 2004

Helix-hairpin-helix DNA glycosylases are typically small proteins that initiate repair of DNA by excising damaged or mispaired bases. An invariant aspartic acid in the active site is involved in catalyzing the excision reaction. Replacement of this critical residue with an asparagine severely reduces catalytic activity but preserves enzyme stability and structure. The
Arabidopsis DEMETER
(
DME
) gene encodes a large 1,729-aa polypeptide with a 200-aa DNA glycosylase domain.
DME
is expressed primarily in the central cell of the female gametophyte. DME activates maternal allele expression of the imprinted
MEDEA
(
MEA
) gene in the central cell and is required for seed viability. We mutated the invariant aspartic acid at position 1304 in DME to asparagine (D1304N) to determine whether the catalytic activity of the DNA glycosylase domain is required for DME function
in vivo
. Transgenes expressing wild-type DME in the central cell rescue seed abortion caused by a mutation in the endogenous
DME
gene and activate maternal
MEA:GFP
transcription. However, transgenes expressing the D1304N mutant DME do not rescue seed abortion or activate maternal
MEA:GFP
transcription. Whereas ectopic expression of the wild-type DME polypeptide in pollen is sufficient to activate ectopic paternal
MEA
and
MEA:GUS
expression, equivalent expression of the D1304N mutant DME in pollen failed to do so. These results show that the conserved aspartic acid residue is necessary for DME to function
in vivo
and suggest that an active DNA glycosylase domain, normally associated with DNA repair, promotes gene transcription that is essential for gene imprinting.
Role for glyoxalase I in Alzheimer's disease
Authors: Chen Feng , Wollmer M. Axel , Hoerndli Frederic , Münch Gerald , Kuhla Björn , Rogaev Evgeny I. , Tsolaki Magdalini , Papassotiropoulos Andreas , Götz Jürgen
Published: May 2004

P301L mutant tau transgenic mice develop neurofibrillary tangles, a histopathologic hallmark of Alzheimer's disease and frontotemporal dementia (FTDP-17). To identify differentially expressed genes and to gain insight into pathogenic mechanisms, we performed a stringent analysis of the microarray dataset obtained with RNA from whole brains of P301L mutant mice and identified a single up-regulated gene, glyoxalase I. This enzyme plays a critical role in the detoxification of dicarbonyl compounds and thereby reduces the formation of advanced glycation end products.
In situ
hybridization analysis revealed expression of glyoxalase I in all brain areas analyzed, both in transgenic and control mice. However, levels of glyoxalase I protein were significantly elevated in P301L brains, as shown by Western blot analysis and immunohistochemistry. Moreover, a glyoxalase I-specific antiserum revealed many intensely stained flame-shaped neurons in Alzheimer's disease brain compared with brains from nondemented controls. In addition, we examined a single nucleotide polymorphism predicting a nonconservative amino acid substitution at position 111 (E111A) in ethnically independent populations. We identified significant and consistent deviations from Hardy–Weinberg equilibrium, which points to the presence of selection forces. The E111A single nucleotide polymorphism was not associated with the risk for Alzheimer's disease in the overall population. Together, our data demonstrate the potential of transcriptomics applied to animal models of human diseases. They suggest a previously unidentified role for glyoxalase I in neurodegenerative disease.
Enlarged meristems and delayed growth in
plp
mutants result from lack of CaaX prenyltransferases
Authors: Running Mark P. , Lavy Meirav , Sternberg Hasana , Galichet Arnaud , Gruissem Wilhelm , Hake Sarah , Ori Naomi , Yalovsky Shaul
Published: May 2004

Meristems require a myriad of intercellular signaling pathways for coordination of cell division within and between functional zones and clonal cell layers. This control of cell division ensures a constant availability of stem cells throughout the life span of the meristem while limiting overproliferation of meristematic cells and maintaining the meristem structure. We have undertaken a genetic screen to identify additional components of meristem signaling pathways. We identified
pluripetala
(
plp
) mutants based on their dramatically larger meristems and increased floral organ number.
PLURIPETALA
encodes the α-subunit shared between protein farnesyltransferase and protein geranylgeranyltransferase-I.
plp
mutants also have altered abscisic acid responses and overall much slower growth rate.
plp
is epistatic to mutations in the β-subunit of farnesyltransferase and shows a synergistic interaction with
clavata3
mutants.
plp
mutants lead to insights into the mechanism of meristem homeostasis and provide a unique
in vivo
system for studying the functional role of prenylation in eukaryotes.
SecB is a bona fide generalized chaperone in
Escherichia coli
Authors: Ullers Ronald S. , Luirink Joen , Harms Nellie , Schwager Françoise , Georgopoulos Costa , Genevaux Pierre
Published: May 2004

It is known that the DnaK and Trigger Factor (TF) chaperones cooperate in the folding of newly synthesized cytosolic proteins in
Escherichia coli
. We recently showed that despite a very narrow temperature range of growth and high levels of aggregated cytosolic proteins,
E. coli
can tolerate deletion of both chaperones, suggesting that other chaperones might be involved in this process. Here, we show that the secretion-dedicated chaperone SecB efficiently suppresses both the temperature sensitivity and the aggregation-prone phenotypes of a strain lacking both TF and DnaK. SecB suppression is independent of a productive interaction with the SecA subunit of the translocon. Furthermore,
in vitro
cross-linking experiments demonstrate that SecB can interact both co- and posttranslationally with short nascent chains of both secretory and cytosolic proteins. Finally, we show that such cotranslational substrate recognition by SecB is greatly suppressed in the presence of ribosome-bound TF, but not by DnaK. Taken together, our data demonstrate that SecB acts as a bona fide generalized chaperone.
HIV-1-infected monocyte-derived dendritic cells do not undergo maturation but can elicit IL-10 production and T cell regulation
Authors: Granelli-Piperno Angela , Golebiowska Angelika , Trumpfheller Christine , Siegal Frederick P. , Steinman Ralph M.
Published: May 2004

Dendritic cells (DCs) undergo maturation during virus infection and thereby become potent stimulators of cell-mediated immunity. HIV-1 replicates in immature DCs, but we now find that infection is not accompanied by many components of maturation in either infected cells or uninfected bystanders. The infected cultures do not develop potent stimulating activity for the mixed leukocyte reaction (MLR), and the DCs producing HIV-1 gag p24 do not express CD83 and DC–lysosome-associated membrane protein maturation markers. If different maturation stimuli are applied to DCs infected with HIV-1, the infected cells selectively fail to mature. When DCs from HIV-1-infected patients are infected and cultured with autologous T cells, IL-10 was produced in 6 of 10 patients. These DC–T cell cocultures could suppress another immune response, the MLR. The regulation was partially IL-10-dependent and correlated in extent with the level of IL-10 produced. Suppressor cells only developed from infected patients, rather than healthy controls, and the DCs had to be exposed to live virus rather than HIV-1 gag peptides or protein. These results indicate that HIV-1-infected DCs have two previously unrecognized means to evade immune responses: maturation can be blocked reducing the efficacy of antigen presentation from infected cells, and T cell-dependent suppression can be induced.
Natural Killer T Cells Infiltrate Neuroblastomas Expressing the Chemokine CCL2
Authors: Metelitsa Leonid S. , Wu Hong-Wei , Wang Hong , Yang Yujun , Warsi Zamir , Asgharzadeh Shahab , Groshen Susan , Wilson S. Brian , Seeger Robert C.
Published: May 2004

CD1d-restricted Vα24-Jα18–invariant natural killer T cells (iNKTs) are potentially important in tumor immunity. However, little is known about their localization to tumors. We analyzed 98 untreated primary neuroblastomas from patients with metastatic disease (stage 4) for tumor-infiltrating iNKTs using TaqMan® reverse transcription polymerase chain reaction and immunofluorescent microscopy. 52 tumors (53%) contained iNKTs, and oligonucleotide microarray analysis of the iNKT+ and iNKT− tumors revealed that the former expressed higher levels of CCL2/MCP-1, CXCL12/SDF-1, CCL5/RANTES, and CCL21/SLC. Eight tested neuroblastoma cell lines secreted a range of CCL2 (0–21.6 ng/ml), little CXCL12 (≤0.1 ng/ml), and no detectable CCL5 or CCL21. CCR2, the receptor for CCL2, was more frequently expressed by iNKT compared with natural killer and T cells from blood (P < 0.001). Supernatants of neuroblastoma cell lines that produced CCL2 induced in vitro migration of iNKTs from blood of patients and normal adults; this was abrogated by an anti-CCL2 monoclonal antibody. CCL2 expression by tumors was found to inversely correlate with MYCN proto-oncogene amplification and expression (r = 0.5, P < 0.001), and MYCN-high/CCL2-low expression accurately predicted the absence of iNKTs (P < 0.001). In summary, iNKTs migrate toward neuroblastoma cells in a CCL2-dependent manner, preferentially infiltrating MYCN nonamplified tumors that express CCL2.
2B4 Acts As a Non–Major Histocompatibility Complex Binding Inhibitory Receptor on Mouse Natural Killer Cells
Authors: Lee Kyung-Mi , McNerney Megan E. , Stepp Susan E. , Mathew Porunelloor A. , Schatzle John D. , Bennett Michael , Kumar Vinay
Published: May 2004

Natural killer (NK) cells are critical in the immune response to tumor cells, virally infected cells, and bone marrow allografts. 2B4 (CD244) is expressed on all NK cells and the ligand for 2B4, CD48, is expressed on hematopoietic cells. Cross-linking 2B4 on NK cells with anti-2B4 monoclonal antibody leads to NK cell activation in vitro. Therefore, 2B4 is considered to be an activating receptor. Surprisingly, we have found, using antibody-blocking and 2B4-deficient NK cells, that NK lysis of CD48+ tumor and allogeneic targets is inhibited by 2B4 ligation. Interferon γ production by NK cells is also inhibited. Using a peritoneal tumor clearance assay, it was found that 2B4−/− mice have increased clearance of CD48+ tumor cells in vivo. Retroviral transduction of 2B4 was sufficient to restore inhibition in 2B4−/− primary NK cells. It was found that although mature NK cells express SH2D1A, in vitro–derived NK cells do not. However, both populations are inhibited by 2B4 ligation. This indicates that 2B4 inhibitory signaling occurs regardless of the presence of SH2D1A. These findings reveal a novel role for 2B4 as a non–major histocompatibility complex binding negative regulator of NK cells.
A Skin-selective Homing Mechanism for Human Immune Surveillance T Cells
Authors: Schaerli Patrick , Ebert Lisa , Willimann Katharina , Blaser Andrea , Roos Regula Stuber , Loetscher Pius , Moser Bernhard
Published: May 2004

Effective immune surveillance is essential for maintaining protection and homeostasis of peripheral tissues. However, mechanisms controlling memory T cell migration to peripheral tissues such as the skin are poorly understood. Here, we show that the majority of human T cells in healthy skin express the chemokine receptor CCR8 and respond to its selective ligand I-309/CCL1. These CCR8+ T cells are absent in small intestine and colon tissue, and are extremely rare in peripheral blood, suggesting healthy skin as their physiological target site. Cutaneous CCR8+ T cells are preactivated and secrete proinflammatory cytokines such as tumor necrosis factor–α and interferon-γ, but lack markers of cytolytic T cells. Secretion of interleukin (IL)-4, IL-10, and transforming growth factor–β was low to undetectable, arguing against a strict association of CCR8 expression with either T helper cell 2 or regulatory T cell subsets. Potential precursors of skin surveillance T cells in peripheral blood may correspond to the minor subset of CCR8+CD25− T cells. Importantly, CCL1 is constitutively expressed at strategic cutaneous locations, including dermal microvessels and epidermal antigen-presenting cells. For the first time, these findings define a chemokine system for homeostatic T cell traffic in normal human skin.
SLAM Family Receptors Regulate Immunity with and without SAP-related Adaptors
Authors: Veillette André
Published: May 2004

Two papers describing mice deficient in signaling lymphocyte activation molecule and 2B4 represent the first accounts of immune phenotypes in animals lacking members of the SLAM family of receptors. The findings provide definitive evidence of the importance of SLAM-related receptors in the regulation of T cell, macrophage, and natural killer cell functions.
A Human Telomerase-associated Nuclease
Authors: Oulton Rena , Harrington Lea
Published: May 2004

Ciliate and yeast telomerase possess a nucleolytic activity capable of removing DNA from the 3′ end of a single-stranded oligonucleotide substrate. The nuclease activity is thought to assist in enzyme proofreading and/or processivity. Herein, we report a previously uncharacterized human telomerase-associated nuclease activity that shares several properties with ciliate and yeast telomerases. Partially purified human telomerase, either from cell extracts or recombinantly produced, demonstrated an ability to remove 3′ nontelomeric nucleotides from a substrate containing 5′ telomeric DNA, followed by extension of the newly exposed telomeric sequence. This cleavage/extension activity was apparent at more than one position within the telomeric DNA and was influenced by sequences 5′ to the telomeric/nontelomeric boundary and by substitution with a methylphosphonate moiety at the telomeric/nontelomeric DNA boundary. Our data suggest that human telomerase is associated with an evolutionarily conserved nucleolytic activity and support a model in which telomerase-substrate interactions can occur distal from the 3′ primer end.