bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistics of two-particle dispersion in two-dimensional turbulence
Authors: Boffetta G. , Sokolov I. M.
Published: September 2002

We investigate Lagrangian relative dispersion in direct numerical simulation of two-dimensional inverse cascade turbulence. The analysis is performed by using both standard fixed time statistics and an exit time approach. The latter allows a more precise determination of the Richardson constant which is found to be g≃4 with a possible weak finite-size dependence. Our results show only small deviations with respect to the original Richardson’s description in terms of diffusion equation. These deviations are associated with the long-range correlated nature of the particles’ relative motion. The correlation, or persistence, parameter is measured by means of a Lagrangian “turning point” statistics.
Helium ion-implanted InGaAsP tunnel junction current blocking layers
Authors: Wang Hongsheng , Forrest Stephen R.
Published: September 2002

We experimentally investigate and model He+-implanted InGaAsP tunnel junctions used for lateral current confinement in vertical-cavity surface-emitting lasers (VCSELs). Prior to implantation, a 56-μm-diameter tunnel junction exhibits a peak-to-valley ratio of 2.2 and a differential resistance of 27 Ω at −2 V. After implantation at a dose of 3.3×1013 cm−2, the current under reverse bias reduces by a factor of >107. Placing tunnel junctions close to the laser active region does not degrade the gain in the quantum wells. With He+-implanted tunnel junctions, mirrorless test VCSEL structures up to 50 μm diameter have uniform current distribution across the entire light-emitting apertures.
Theory of electron stimulated desorption and dissociation of CO at transition metals
Authors: Corriol Cécile , Darling George R. , Holloway Stephen , Brenig Wilhelm , Andrianov Ivan , Klamroth Tillmann , Saalfrank Peter
Published: September 2002

Experiments, for the electron stimulated desorption of CO molecules from Ru(0001) [Wurm et al., Phys. Rev. Lett. 74, 2591 (1995)] are rationalized with the help of quantum wave packet methods using a bimodal two-state model. Besides a vibrational population inversion for the desorbing molecule we also find a small amount of dissociation, and an experimentally observed positive correlation between vibrational state and kinetic energy of the desorbing molecules. The role of vibrational excitation of the initial state is discussed. Classical trajectory calculations are found to be in good agreement with quantum dynamics thus allowing for a systematic exploration of the sensitivity of the results on details of the potential energy surfaces.
Epitaxial growth of an n-type ferromagnetic semiconductor CdCr2Se4 on GaAs(001) and GaP(001)
Authors: Park Y. D. , Hanbicki A. T. , Mattson J. E. , Jonker B. T.
Published: September 2002

We report the epitaxial growth of CdCr2Se4, an n-type ferromagnetic semiconductor, on both GaAs and GaP(001) substrates, and describe the structural, magnetic, and electronic properties. Magnetometry data confirm ferromagnetic order with a Curie temperature of 130 K, as in the bulk material. The magnetization exhibits hysteretic behavior with significant remanence, and an in-plane easy axis with a coercive field of ∼125 Oe. Temperature-dependent transport data show that the films are semiconducting in character and n type as grown, with room-temperature carrier concentrations of n∼1×1018 cm−3.
Metallic glass matrix composite with precipitated ductile reinforcement
Authors: Fan Cang , Ott R. T. , Hufnagel T. C.
Published: September 2002

We report a composite material consisting of precipitated micron-scale Ta-rich solid solution particles distributed in a bulk metallic glass matrix. The reinforcing ductile particles are precipitated during melting of the master alloy of glass-forming (Zr70Ni10Cu20)82Ta8Al10, by using previously prepared metastable Zr–Ta solid solution binary ingots. Upon cooling from the melt, the matrix undergoes a glass transition to produce an amorphous phase while the particles of precipitated Ta solid solution are distributed in the glass matrix. The resulting material not only shows high strength (∼2.1 GPa), but also has dramatically enhanced plastic strain to failure in uniaxial compression relative to single-phase bulk metallic glasses. The composite also displays limited tensile ductility.
Improved dc characteristics of AlGaN/GaN high-electron-mobility transistors on AlN/sapphire templates
Authors: Arulkumaran S. , Sakai M. , Egawa T. , Ishikawa H. , Jimbo T. , Shibata T. , Asai K. , Sumiya S. , Kuraoka Y. , Tanaka M. , Oda O.
Published: September 2002

High-electron-mobility transistors (HEMTs) have been demonstrated on both AlN/sapphire templates and sapphire substrates, and the dc characteristics of the fabricated devices were examined at room temperature. Better dc characteristics with high extrinsic transconductances and drain current densities were observed in the HEMTs grown on AlN/sapphire templates when compared with the HEMTs on sapphire substrates. Extrinsic transconductances of 214 and 137 mS/mm for Wg/Lg=15/2 μm HEMTs on AlN/sapphire templates and HEMTs on sapphire substrates were achieved, respectively. The enhancement of dc characteristics with small variations in threshold voltage (⩽130 mV) is due to the reduction of dislocation density (1.5×108 cm−2). The decrease of dark spot density has been confirmed in the GaN grown on AlN/sapphire templates using cathodoluminescence measurements. The advantage of using AlN/sapphire templates is that low dislocation density GaN layers at a high temperature can be grown without using low-temperature-grown GaN buffer layers.
Substitutional and interstitial carbon in wurtzite GaN
Authors: Wright A. F.
Published: September 2002

First-principles theoretical results are presented for substitutional and interstitial carbon in wurtzite GaN. Carbon is found to be a shallow acceptor when substituted for nitrogen (CN) and a shallow donor when substituted for gallium (CGa). Interstitial carbon (CI) is found to assume different configurations depending on the Fermi level: A site at the center of the c-axis channel is favored when the Fermi level is below 0.9 eV (relative to the valence band maximum) and a split-interstitial configuration is favored otherwise. Both configurations produce partly filled energy levels near the middle of the gap, and CI should therefore exhibit deep donor behavior in p-type GaN and deep acceptor behavior in n-type GaN. Formation energies for CN, CGa, and CI are similar, making it likely that CN acceptors will be compensated by other carbon species. CGa is predicted to be the primary compensating species when growth occurs under N-rich conditions while channel CI is predicted to be the primary compensating species under Ga-rich growth conditions. Self-compensation is predicted to be more significant under Ga-rich growth conditions than under N-rich conditions. Experimental evidence for self-compensation is discussed. Four carbon complexes are discussed. CN–VGa is found to be unstable when the Fermi level is above the middle of the gap due to the high stability of gallium vacancies (VGa). The CN–VGa complex was previously suggested as a source of the broad 2.2 eV luminescence peak often observed in n-type GaN. The present results indicate that this is unlikely. The CI–CN complex is capable of forming in carbon doped GaN grown under Ga-rich conditions if the mobility of the constituents is high enough. Experimental evidence for its existence is discussed.
Postsintering annealing induced extrinsic dielectric and piezoelectric responses in lead–zinc–niobate-based ferroelectric ceramics
Authors: Xia Feng , Yao Xi
Published: September 2002

The effect of postsintering annealing on the dielectric and piezoelectric responses in lead–zinc–niobate (PZN)-based ferroelectric ceramics was systematically studied in this work. Sintered PZN ceramics with the same composition were annealed at temperatures from 700 to 900 °C from 30 min to 24 h in various atmospheres. As a result, significant improvements in the dielectric and piezoelectric responses were observed in this system. After annealing, there was a huge increase of up to 130% in the dielectric constants, especially near the temperature of maximum dielectric constant. While the dielectric peaks became sharper and less diffuse, there was almost no change in frequency dispersion. Similar large improvements in the piezoelectric properties such as the coupling factor and piezoelectric constant were also observed. Optimized parameters, such as d33=642 pC/N, d31=−256 pC/N, kp=48% and εm=28 000, were achieved by annealing at 850 °C in an O2-rich atmosphere for 4–8 h. The observed improvement in the electrical properties can be attributed to an extrinsic contribution induced by domain wall motion. After annealing, the pinning effects caused by oxygen vacancies and internal stress on domain wall motion were largely reduced or eliminated, therefore a significantly reduced coercive field, an increased polarization level, and a greatly increased dielectric constant were observed in the PZN-based ferroelectric ceramics.
The interface screening model as origin of imprint in PbZrxTi1−xO3 thin films. I. Dopant, illumination, and bias dependence
Authors: Grossmann M. , Lohse O. , Bolten D. , Boettger U. , Schneller T. , Waser R.
Published: September 2002

Comprehensive imprint measurements on PbZrxTi1−xO3 (PZT) thin films were carried out. Different models, which were proposed in literature to explain imprint in ferroelectric thin films or a similar aging effect (internal bias) in ferroelectric bulk material, are reviewed. Discrepancies between the experimental results obtained on the PZT films in this work and the prediction of the literature models indicate that these models do not describe the dominant imprint mechanism in PZT thin films. Hence, in this work a model is proposed which suggests imprint to be caused by a strong electric field within a thin surface layer in which the ferroelectric polarization is smaller or even absent compared to the bulk of the film. With the proposed imprint model the influence of important experimental parameters like dopant, illumination, and bias dependence can be qualitatively explained.
Polarized luminescence from self-assembled, aligned, and cleaved supramolecules of highly ordered rodlike polymers
Authors: Knaapila M. , Ikkala O. , Torkkeli M. , Jokela K. , Serimaa R. , Dolbnya I. P. , Bras W. , ten Brinke G. , Horsburgh L. E. , Pålsson L.-O. , Monkman A. P.
Published: September 2002

A hierarchical self-assembly in comb-shaped supramolecules of conjugated rodlike polymers is reported. The supramolecules consist of poly(2,5-pyridinediyl), acid dopants, and hydrogen bonded alkyl side chains. A thermotropic smectic state with an exceptionally large coherence length is formed without additional solvent. This allows facile overall alignment resulting in high dichroism and polarized photoluminescence. Solid films are formed by cleaving side groups from the supramolecules which retain the optical anisotropy together with the high photoluminescence quantum yield of pristine polymer.
Enhanced antiferromagnetic exchange coupling in Fe/Si/Fe epitaxial trilayers with Fe0.5Si0.5 boundary layers
Authors: Gareev R. R. , Bürgler D. E. , Buchmeier M. , Schreiber R. , Grünberg P.
Published: September 2002

Epitaxial Fe/Fe0.5Si0.5/Si-wedge/Fe0.5Si0.5/Fe structures are prepared by thermal evaporation with Fe0.5Si0.5 boundary layers grown by coevaporation at 200 °C. Magnetic properties are examined with Brillouin light scattering and longitudinal magneto-optic Kerr effect hysteresis. The interlayer coupling is found to increase in excess of 8 mJ/m2 by introducing a boundary layer at the bottom interface. The coupling maximum shifts from 7 to 3 Å nominal Si thickness. This effect is related to reduced interdiffusion with the formation of an epitaxial, pinhole-free spacer at smaller thickness. Together with the strong increase of the coupling for decreasing spacer thickness, this results in an enhancement of the coupling.
A multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe
Authors: Ishibashi Taka-aki , Onishi Hiroshi
Published: September 2002

We present a multiplex infrared-visible sum-frequency (SF) spectrometer with wavelength tunability of the visible probe. The spectrometer is based on a two-channel optical parametric amplifier producing a broad-bandwidth femtosecond infrared pulse and a narrow-bandwidth picosecond visible pulse (470–800 and 401 nm). A SF spectrum over a 400 cm−1 spectral region was obtainable by a single measurement. SF spectra of an octadecanethiol monolayer on a gold substrate were measured with visible probes of different wavelengths from 401 to 633 nm. The spectral shape showed remarkable dependence on the visible probe wavelength. The dependence was ascribed to resonance to a surface electronic state of the substrate existing in the visible region.
High-efficiency solar cells based on Cu(InAl)Se2 thin films
Authors: Marsillac S. , Paulson P. D. , Haimbodi M. W. , Birkmire R. W. , Shafarman W. N.
Published: September 2002

A Cu(InAl)Se2 solar cell with 16.9% efficiency is demonstrated using a Cu(InAl)Se2 thin film deposited by four-source elemental evaporation and a device structure of glass/Mo/Cu(InAl)Se2/CdS/ZnO/indium tin oxide/(Ni/Algrid)/MgF2. A key to high efficiency is improved adhesion between the Cu(InAl)Se2 and the Mo back contact layer, provided by a 5-nm-thick Ga interlayer, which enabled the Cu(InAl)Se2 to be deposited at a 530 °C substrate temperature. Film and device properties are compared to Cu(InGa)Se2 with the same band gap of 1.16 eV. The solar cells have similar behavior, with performance limited by recombination through trap states in the space charge region in the Cu(InAl)Se2 or Cu(InGa)Se2 layer.
Achieving very-low-loss group velocity reduction without electromagnetically induced transparency
Authors: Deng L. , Hagley E. W. , Kozuma M. , Akamatsu D. , Payne M. G.
Published: September 2002

A Raman method is studied experimentally for potential optical-wave group velocity reduction in resonant medium. Specifically, we show that the proposed method can achieve significant group velocity reduction comparable to the conventional electromagnetically induced transparency (EIT) method that had been exclusively used to date. In addition, we show that this method has a significantly lower loss compared to the EIT method, and has rich dynamics such as probe pulse narrowing. Such features have applications in the domain of telecommunication technology.
Quantification of substitutional carbon loss from Si0.998C0.002 due to silicon self-interstitial injection during oxidation
Authors: Carroll M. S. , Sturm J. C.
Published: September 2002

The empirical reaction of substitutional carbon with silicon self-interstitials in Si0.998C0.002 layers pseudomorphically grown on Si (100) substrates has been quantified at 850 °C. During annealing of a sample with a thin Si0.998C0.002 layer capped with a thin crystalline silicon layer, in either oxygen or nitrogen ambient, carbon diffuses from the surface edge of the Si0.998C0.002 layer towards and out of the silicon surface. The extra number of carbon atoms lost during oxidation is found equal to the number of silicon interstitials injected by the oxidation process, strongly suggesting that each substitutional carbon reacts with a single self-interstitial to form a mobile interstitial carbon, whereby it diffuses to the surface. The mechanism appears the same in Si1−x−yGexCy and Si1−xCx films.
Low hydrogen content in trimethylsilane-based dielectric barriers deposited by inductively coupled plasma
Authors: Shieh Jia-Min , Tsai Kou-Chiang , Dai Bau-Tong
Published: September 2002

Dielectric barriers of trimethylsilane-based hydrogenated amorphous silicon carbide (a-SiCx:H) for damascene metalization were synthesized by inductively coupled plasma (ICP) chemical vapor deposition methods. The high ionization efficiency from ICP, resulting in a-SiCx:H films that contain only a little hydrogen, is explored as a major mechanism that yields such a film not only with a high breakdown field of 3.7 MV/cm and a low leakage current <6.0×10−10 A/cm2 (at 1.0 MV/cm), but also with a low stress (−5 MPa) and high hardness (11 GPa). Additionally, the dielectric constant of the a-SiCx:H films is maintained below 4.2.
AlN/GaN double-barrier resonant tunneling diodes grown by rf-plasma-assisted molecular-beam epitaxy
Authors: Kikuchi Akihiko , Bannai Ryo , Kishino Katsumi , Lee Chia-Ming , Chyi Jen-Inn
Published: September 2002

AlN/GaN double-barrier resonant tunneling diodes (DB–RTDs) were fabricated on (0001) Al2O3 substrates by molecular-beam epitaxy, using a rf-plasma nitrogen source. The AlN/GaN DB–RTDs were designed to have a 3-ML-thick GaN quantum well and 4-ML-thick AlN barrier layers sandwiched by Si-doped n-type GaN contact layers. The current–voltage characteristics of mesa diode samples showed clear negative differential resistance (NDR) at room temperature. The NDR was observed at 2.4 V with a peak current of 2.9 mA, which corresponds to 180 A/cm2. A peak-to-valley current ratio as high as 32 was obtained.
Long-range electron transfer and electronic transport through macromolecules
Authors: Zimbovskaya Natalya , Gumbs Godfrey
Published: September 2002

A theory of electrical transport through molecular wires is used to estimate the electronic factor in the intramolecular electron transfer in porphyrin-nitrobenzene supermolecules, and to analyze its structure. The chosen molecules have complex donor and acceptor configurations, and relatively simple structure of the bridge, which enables us to concentrate our studies on the donor/acceptor coupling to the bridge. We present analytical and numerical results concerning the effect of donor/acceptor coupling to the bridge on the electron transfer process in molecules with complex donor/acceptor subsystems.
Effect on laser-ablation algorithms of reflection losses and nonnormal incidence on the anterior cornea
Authors: Jiménez J. R. , Anera R. G. , Jiménez del Barco L. , Hita E.
Published: September 2002

In this paper, we provide an analytic expression for an adjustment factor in ablation algorithms for photorefractive laser surgery which take into account reflection losses and the nonnormal incidence on the cornea. We evaluate the influence of this factor on certain ocular parameters, calculating alterations in the radius of anterior cornea and corneal asphericity when we use the algorithm given by Munnerlyn’s paraxial formula. Our data indicate that this adjustment factor should be considered in the ablation algorithms that are currently being proposed in customized corneal ablation and that need great accuracy for the correction of eye aberrations.
Highly efficient polymer light-emitting devices using a phosphorescent sensitizer
Authors: He Gufeng , Chang Shun-Chi , Chen Fang-Chung , Li Yongfang , Yang Yang
Published: September 2002

Highly efficient single-layer polymer light-emitting diodes that employ Nile Red as a fluorescent dye, with a green phosphorescent-sensitizer, bis(2-phenyl pyridinato-N,C2′) iridium (acetylacetonate) doped in a PVK/PBD host are demonstrated. The function of the phosphorescent sensitizer is to convert the triplet exciton into a singlet exciton during the energy transfer process. Therefore, ideally, all the excitons can be utilized. For comparison, devices with the same structure, but using a fluorescent sensitizer instead of a phosphorescent sensitizer, were fabricated. The efficiency of the phosphor-sensitized device is 6.4 cd/A, almost triple that of lumophor-sensitized or nonsensitized devices. This result indicates that not only singlet excitons but also triplet excitons are efficiently transferred from the host to the fluorescent dye when a phosphorescent material is used as a sensitizer.
Conductance quantization in magnetic nanowires electrodeposited in nanopores
Authors: Elhoussine F. , Mátéfi-Tempfli S. , Encinas A. , Piraux L.
Published: September 2002

Magnetic nanocontacts have been prepared by a templating method that involves the electrodeposition of Ni within the pores of track-etched polymer membranes. The nanocontacts are made at the extremity of a single Ni nanowire either inside or outside the pores. The method is simple, flexible, and controllable as the width of the constriction can be varied reversibly by controlling the potential between the electrodeposited nanowire and a ferromagnetic electrode. At room temperature, the electrical conductance shows quantization steps in units of e2/h, as expected for ferromagnetic metals without spin degeneracy. Our fabrication method enables future investigation of ballistic spin transport phenomena in electrodeposited magnetic nanocontacts.
Time-dependent plane wave packet formulation of quantum scattering with application to H+D2→HD+D
Authors: Althorpe Stuart C.
Published: September 2002

We outline a new time-dependent wave packet formulation of quantum scattering theory. The theory obtains the differential cross section directly from the time-evolution of a plane wave packet, bypassing the usual S-matrix formulas. We introduce the theory for potential scattering, generalize it to reactive scattering, then explain how the theory was used recently to interpret the H+D2→HD (v=3 j=0)+D reaction in collaboration with experiment [S. C. Althorpe, F. Fernández-Alonso, B. D. Bean, J. D. Ayers, A. E. Pomerantz, R. N. Zare, and E. Wrede, Nature (London) 416, 67 (2002)]. We also present new findings of quantum interference in the H+D2 reaction.
Devising a method to measure film adhesion through modeling
Authors: Hsueh C. H.
Published: September 2002

A method for measuring film adhesion is devised. It consists of a series of patterned film strips with different widths on a substrate, such that the driving force for each strip to debond increases as strip width increases when the system is subjected to residual and/or applied stresses. In this case, only those strips with sufficient driving force debond while the others remain attached to the substrate. Hence, qualitative comparison of the interface debond energy can be obtained by comparing the strip width below which debonding no longer occurs. By calculating the driving force for each strip to debond, the interface debond energy can be quantitatively determined.
Public Demand for Medicines, Price Regulation, and Government — Industry Relationships in Spain
Authors: Nonell Rosa , Borrell Joan-Ramon
Published: September 2002

The authors' principal purpose is to study why the government in Spain has maintained price regulations on medicines during the last thirty-five years. The hypothesis is that price regulations were related to the regulatory style of the Spanish government in the 1960s and, since the last 1970s, have become a key instrument for preventing the pharmaceutical costs of the National Health System from rising. In order to test this hypothesis, two questions are considered. The first relates to how business interests and their main representative bodies have managed to influence the implementation of price regulations during the last thirty-five years. The second question concerns the extent to which changes in the prices of medicines have been effective in preventing increased costs of the Spanish National Health System drug bill between 1964 ad 1994. After analyzing the political economy of the price regulation of medicines and pharmaceutical cost-limiting policies during the last thirty-five years in Spain, and after estimating a function of the public demand for medicines between 1964 and 1994 in Spain, the authors extract the following two conclusions. First, price regulation has become a very useful instrument for preventing public pharmaceutical costs from rising since the late 1970s in Spain. Second, business interests have found it increasingly useful over the last thirty-five years to establish a stable framework in the market for medicines based on price regulation and the public provision of medicines.
Maximizing Memory Data Reuse for Lower Power Motion Estimation
Authors: Kim Bo-Sung , Cho Jun-Dong
Published: September 2002

This paper presents a new VLSI architecture of the Motion Estimation in MPEG‐2. Previously, a number of full search block matching algorithms (BMA) and architectures using systolic array have been proposed for motion estimation. However, the architectures have an inefficiently large number of external memory accesses. Recently, to reduce the number of accesses in one search block, a block matching method within a search area to reuse the search data is provided using systolic process arrays. To further reduce the data access and computation time during the block matching, we propose a new approach through the reuse of the previously‐search data in two dimensions. Our new architecture in this paper is an extension from our previous work such that we reuse the previously‐searches area not only between two consecutive columns but also between two consecutive rows, so as to entirely remove redundant memory accesses. Experimental results show that our architecture of increased area by 81% can reduce 98% of memory accesses. Total power reduction is 86% in power estimation by SPICE model.
Origin and geologic features of Taal Lake, Philippines
Authors: Ramos Emmanuel G.
Published: September 2002

Taal Lake was formed by a series of catastrophic volcanic eruptions and other geologic processes whose character slowly evolved as the large basinal depression and the lake took form. During the early stage of Taal's life, large scale magmatic activity was very common in this part of Luzon including the caldera-forming eruptions that created parts of Laguna de Bai and the smaller scale eruptions that formed the cones and volcanic peaks in the area. Around Taal, large eruptions were carving large bowl-shaped depressions on one part while depositing thick pyroclastic rocks on more distant sites. During the last stages of the large scale magmatic eruptions, a lake must have been already present, leading to the phreatic eruptions that created the crossbedded, base-surge pyroclastics found on the upper layers of the terrain around the lake. These phreatic eruptions created smaller circular depressions that later coalesced to form the present caldera. With much of the magma exhausted after periods of protracted eruptions, the scale of volcanism was reduced and mostly centered in the region where Volcano Island is now located. Combined with the hills near Balite and Bilibinwang, development of Volcano Island formed an east-west trending ridge that divided the lake in two. Tectonic uplift and erosion on the northern shores of the young lake and the presence of deeper craters on the south led to contrasts in the depths of the depressions. Smaller scale geologic processes marked the last stage in the development of the lake. Landslides modified the steep slopes, and large blocks of land were shifted vertically and horizontally along faults by slow tectonic movements. More sediments shed from the small region drained by the lake were being deposited and volcanism continued mostly on Volcano Island.

Numerous large circular craters on the shores of the lake suggest the explosive origin of the whole Taal Lake depression. The flat-bottomed lake is shallower by about 100 m on the north. Faults are found mostly on the northern slopes, where large-scale landslide blocks are also common. Landslide features are also found underwater particularly near the steep shores. Most of the historic volcanism had been centered on Taal Volcano Island, and these are often of the explosive phreatic type reflecting the influence of the lake on magmatic activity. The resources and the people residing near the lake are still under the threat of the same active geologic processes that created Taal Lake.
Water quality trends and trophic state assessment of Laguna de Bay, Philippines
Authors: Barril Carlito R. , Tumlos Elvira T.
Published: September 2002

Monitoring of key water quality parameters in Laguna de Bay was carried out from 1986 to 1996 to determine trends in water quality and the trophic state of the lake. Monthly sampling and field analyses were made at randomly selected sites of the lake. Three-year trend analysis of the annual mean values of some key water quality parameters showed the worsening condition of the lake. Ammonia-nitrogen, nitrate-nitrogen, ortho-phosphate, total phosphorus, chemical oxygen demand, turbidity, conductivity, and chlorides increased while alkalinity, clarity, and dissolved oxygen decreased. The lake is judged to be extremely trophic or dystrophic. Uses of the lake as a fishery resource and as a source of domestic water supply are briefly discussed.
Phosphoinositide 3-kinase regulates β2-adrenergic receptor endocytosis by AP-2 recruitment to the receptor/β-arrestin complex
Authors: Naga Prasad Sathyamangla V. , Laporte Stéphane A. , Chamberlain Dean , Caron Marc G. , Barak Larry , Rockman Howard A.
Published: September 2002

Internalization of β-adrenergic receptors (βARs) occurs by the sequential binding of β-arrestin, the clathrin adaptor AP-2, and clathrin. D-3 phosphoinositides, generated by the action of phosphoinositide 3-kinase (PI3K) may regulate the endocytic process; however, the precise molecular mechanism is unknown. Here we demonstrate that βARKinase1 directly interacts with the PIK domain of PI3K to form a cytosolic complex. Overexpression of the PIK domain displaces endogenous PI3K from βARK1 and prevents βARK1-mediated translocation of PI3K to activated β2ARs. Furthermore, disruption of the βARK1/PI3K interaction inhibits agonist-stimulated AP-2 adaptor protein recruitment to the β2AR and receptor endocytosis without affecting the internalization of other clathrin dependent processes such as internalization of the transferrin receptor. In contrast, AP-2 recruitment is enhanced in the presence of D-3 phospholipids, and receptor internalization is blocked in presence of the specific phosphatidylinositol-3,4,5-trisphosphate lipid phosphatase PTEN. These findings provide a molecular mechanism for the agonist-dependent recruitment of PI3K to βARs, and support a role for the localized generation of D-3 phosphoinositides in regulating the recruitment of the receptor/cargo to clathrin-coated pits.
The cell biology ofListeria monocytogenesinfection
Authors: Portnoy Daniel A. , Auerbuch Victoria , Glomski Ian J.
Published: September 2002

Listeria monocytogenes has emerged as a remarkably tractable pathogen to dissect basic aspects of cell biology, intracellular pathogenesis, and innate and acquired immunity. In order to maintain its intracellular lifestyle, L. monocytogenes has evolved a number of mechanisms to exploit host processes to grow and spread cell to cell without damaging the host cell. The pore-forming protein listeriolysin O mediates escape from host vacuoles and utilizes multiple fail-safe mechanisms to avoid causing toxicity to infected cells. Once in the cytosol, the L. monocytogenes ActA protein recruits host cell Arp2/3 complexes and enabled/vasodilator-stimulated phosphoprotein family members to mediate efficient actin-based motility, thereby propelling the bacteria into neighboring cells. Alteration in any of these processes dramatically reduces the ability of the bacteria to establish a productive infection in vivo.
The WD repeat protein, Mdv1p, functions as a molecular adaptor by interacting with Dnm1p and Fis1p during mitochondrial fission
Authors: Tieu Quinton , Okreglak Voytek , Naylor Kari , Nunnari Jodi
Published: September 2002

Yeast mitochondrial fission is a multistep process during which the dynamin-related GTPase, Dnm1p, assembles into punctate structures that associate with the outer mitochondrial membrane and mediate mitochondrial division. Steps in the Dnm1p-dependent process of fission are regulated by the actions of the WD repeat protein, Mdv1p, and the mitochondrial outer membrane protein, Fis1p. Our previous studies suggested a model where Mdv1p functions to regulate fission at a post-Dnm1p assembly step and Fis1p functions at two distinct steps, at an early point, to regulate Dnm1p assembly, and later, together with Mdv1p, to facilitate Dnm1p-dependent mitochondrial fission. To test this model, we have examined the physical and functional relationship between Mdv1p and Fis1p and present genetic, biochemical, and two-hybrid data indicating that a Fis1p–Mdv1p complex is required to regulate mitochondrial fission. To further define the role of Mdv1p in fission, we examined the structural features of Mdv1p required for its interactions with Dnm1p and Fis1p. Data from two-hybrid analyses and GFP-tagged domains of Mdv1p indicate that it contains two functionally distinct domains that enable it to function as a molecular adaptor to regulate sequential interactions between Dnm1p and Fis1p and catalyze a rate-limiting step in mitochondrial fission.
Yersiniatype III secretion
Authors: Cornelis Guy R.
Published: September 2002

Pathogenic Yersinia spp (Yersinia pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica) have evolved an exquisite method for delivering powerful effectors into cells of the host immune system where they inhibit signaling cascades and block the cells' response to infection. Understanding the molecular mechanisms of this system has provided insight into the processes of phagocytosis and inflammation.
Employing graduates in SMEs: towards a research agenda
Authors: Holden Rick , Jameson Stephanie
Published: September 2002

In the context of a somewhat turbulent graduate labour market, attention is being focused on the employment of graduates in small‐ and medium‐sized enterprises (SMEs). This paper takes an initial “sounding” of our understanding about the transition of graduates into such organisations. While research data provides some insight into the barriers which work to discourage more SMEs from recruiting graduates, this understanding appears inadequate and insufficiently segmented to provide a detailed knowledge of the problems. A prevailing assumption is that graduates lack skills required by SME employers. Yet the limited research findings reveal ambiguity about the extent to which SMEs effectively deploy graduate labour. The article proposes an agenda that highlights the need for two types of research. First, a clearer picture of current trends in the SME graduate labour market. Second, a richer understanding of the real experience of graduates, and their managers, in relation to employment in an SME and the implications of such for both the supply and demand sides of the graduate labour market.
Zinc‐substituted Desulfovibrio gigas desulforedoxins: Resolving subunit degeneracy with nonsymmetric pseudocontact shifts
Authors: Goodfellow Brian J. , Nunes Sofia G. , Rusnak Frank , Moura Isabel , Ascenso Carla , Moura José J.G. , Volkman Brian F. , Markley John L.
Published: September 2002

Desulfovibrio gigas desulforedoxin (Dx) consists of two identical peptides, each containing one [Fe‐4S] center per monomer. Variants with different iron and zinc metal compositions arise when desulforedoxin is produced recombinantly from Escherichia coli. The three forms of the protein, the two homodimers [Fe(III)/Fe(III)]Dx and [Zn(II)/Zn(II)]Dx, and the heterodimer [Fe(III)/Zn(II)]Dx, can be separated by ion exchange chromatography on the basis of their charge differences. Once separated, the desulforedoxins containing iron can be reduced with added dithionite. For NMR studies, different protein samples were prepared labeled with 15N or 15N + 13C. Spectral assignments were determined for [Fe(II)/Fe(II)]Dx and [Fe(II)/Zn(II)]Dx from 3D 15N TOCSY‐HSQC and NOESY‐HSQC data, and compared with those reported previously for [Zn(II)/Zn(II)]Dx. Assignments for the 13Cα shifts were obtained from an HNCA experiment. Comparison of 1H–15N HSQC spectra of [Zn(II)/Zn(II)]Dx, [Fe(II)/Fe(II)]Dx and [Fe(II)/Zn(II)]Dx revealed that the pseudocontact shifts in [Fe(II)/Zn(II)]Dx can be decomposed into inter‐ and intramonomer components, which, when summed, accurately predict the observed pseudocontact shifts observed for [Fe(II)/Fe(II)]Dx. The degree of linearity observed in the pseudocontact shifts for residues ≥8.5 Å from the metal center indicates that the replacement of Fe(II) by Zn(II) produces little or no change in the structure of Dx. The results suggest a general strategy for the analysis of NMR spectra of homo‐oligomeric proteins in which a paramagnetic center introduced into a single subunit is used to break the magnetic symmetry and make it possible to obtain distance constraints (both pseudocontact and NOE) between subunits.
How do two unrelated antibodies, HyHEL‐10 and F9.13.7, recognize the same epitope of hen egg‐white lysozyme?
Authors: Pons Jaume , Stratton Jennifer R. , Kirsch Jack F.
Published: September 2002

The anti‐hen egg‐white lysozyme (HEWL) antibodies HyHEL‐10 and F9.13.7 recognize a common epitope. The structures of the complexes differ, however, in the numbers of electrostatic and hydrogen‐bond interactions and in the distributions of contacts between the light and heavy chains. The equilibria and kinetics characterizing the F9.13.7 complex formation were evaluated for both wild‐type and mutant derivatives of HEWL to help to understand how the different contacts are effectively used in the complexes with the two antibodies. Three epitope hot spots, Y20, K96, and R73 (destabilization > 4 kcal/mole), were found by alanine scanning mutagenesis. The first two constitute two of the three hot spots in the HyHEL‐10 complex. The hot spots of the HyHEL‐10 paratope are centered on the HEWL epitope; whereas R73 (HEWL), the only important light‐chain‐contacting residue, is clearly separated from the other hot spots of the F9.13.7 complex. The larger number of epitope warm plus hot spots found in the F9.13.7 complex compared with that of HyHEL‐10 shows that the specificity of the former is greater even though the KD value is 20‐fold larger. Conservative mutations showed that the specificity enhancement is related to the greater number of functional polar and hydrogen bond interactions in the F9.13.7 complex. Alanine scanning mutagenesis would not have illuminated these distinctions. It is shown that the concept of antigen specificity, as defined by cross‐reactivity with natural variant antigens, is flawed by phylogenetic bias, and that specificity can only be defined by the use of unbiased epitopes, which are conveniently accessed by site‐directed mutagenesis.
PrfA protein of Bacillus species: Prediction and demonstration of endonuclease activity on DNA
Authors: Rigden Daniel J. , Setlow Peter , Setlow Barbara , Bagyan Irina , Stein Richard A. , Jedrzejas Mark J.
Published: September 2002

The prfA gene product of Gram‐positive bacteria is unusual in being implicated in several cellular processes; cell wall synthesis, chromosome segregation, and DNA recombination and repair. However, no homology of PrfA with other proteins has been evident. Here we report a structural relationship between PrfA and the restriction enzyme PvuII, and thereby produce models that predict that PrfA binds DNA. Indeed, wild‐type Bacillus stearothermophilus PrfA, but not a catalytic site mutant, nicked one strand of supercoiled plasmid templates leaving 5′‐phosphate and 3′‐hydroxyl termini. This activity, much lower on linear or relaxed circular double‐stranded DNA or on single‐stranded DNA, is consistent with a role for this protein in chromosome segregation, DNA recombination, or DNA repair.
A simple model for polyproline II structure in unfolded states of alanine‐based peptides
Authors: Pappu Rohit V. , Rose George D.
Published: September 2002

The striking similarity between observed circular dichroism spectra of nonprolyl homopolymers and that of regular left‐handed polyproline II (PII) helices prompted Tiffany and Krimm to propose in 1968 that unordered peptides and unfolded proteins are built of PII segments linked by sharp bends. A large body of experimental evidence, accumulated over the past three decades, provides compelling evidence in support of the original hypothesis of Tiffany and Krimm. Of particular interest are the recent experiments of Shi et al. who find significant PII structure in a short unfolded alanine‐based peptide. What is the physical basis for PII helices in peptide and protein unfolded states? The widely accepted view is that favorable chain‐solvent hydrogen bonds lead to a preference for dynamical fluctuations about noncooperative PII helices in water. Is this preference simply a consequence of hydrogen bonding or is it a manifestation of a more general trend for unfolded states which are appropriately viewed as chains in a good solvent? The prevalence of closely packed interiors in folded proteins suggests that under conditions that favor folding, water—which is a better solvent for itself than for any polypeptide chain—expels the chain from its midst, thereby maximizing chain packing. Implicit in this view is a complementary idea: under conditions that favor unfolding, chain‐solvent interactions are preferred and in a so‐called good solvent, chain packing density is minimized. In this work we show that minimization of chain packing density leads to preferred fluctuations for short polyalanyl chains around canonical, noncooperative PII‐like conformations. Minimization of chain packing is modeled using a purely repulsive soft‐core potential between polypeptide atoms. Details of chain‐solvent interactions are ignored. Remarkably, the simple model captures the essential physics behind the preference of short unfolded alanine‐based peptides for PII helices. Our results are based on a detailed analysis of the potential energy landscape which determines the system's structural and thermodynamic preferences. We use the inherent structure formalism of Stillinger and Weber, according to which the energy landscape is partitioned into basins of attraction around local minima. We find that the landscape for the experimentally studied seven‐residue alanine‐based peptide is dominated by fluctuations about two noncooperative structures: the left‐handed polyproline II helix and its symmetry mate.
A CH domain‐containing N terminus in NuMA?
Authors: Novatchkova Maria , Eisenhaber Frank
Published: September 2002

Nuclear mitotic apparatus protein (NuMA) is an essential vertebrate component in organizing microtubule ends at spindle poles. The NuMA‐dynactin/dynein motor multiprotein complex not only explains the transport of NuMA along spindle fibers but also is linked to the process of microtubule focusing. The interaction sites of NuMA to dynein/dynactin have not been mapped. In the yet functionally uncharacterized N terminus of NuMA, we predict a calponin‐homology (CH) domain, a motif with binding activity for actin‐like molecules. We substantiate the primary sequence analysis‐based prediction with secondary structure and fold recognition analysis, and we propose the N‐terminal CH domain of NuMA as a likely interaction site for actin‐related protein 1 (Arp1) protein of the dynactin/dynein complex.
Adaptation of Alphaviruses to Heparan Sulfate: Interaction of Sindbis and Semliki Forest Viruses with Liposomes Containing Lipid-Conjugated Heparin
Authors: Smit Jolanda M. , Waarts Barry-Lee , Kimata Koji , Klimstra William B. , Bittman Robert , Wilschut Jan
Published: September 2002

Passage of Sindbis virus (SIN) in BHK-21 cells has been shown to select for virus mutants with high affinity for the glycosaminoglycan heparan sulfate (HS). Three loci in the viral spike protein E2 (E2:1, E2:70, and E2:114) have been identified that mutate during adaptation and independently confer on the virus the ability to bind to cell surface HS (W. B. Klimstra, K. D. Ryman, and R. E. Johnston, J. Virol. 72:7357-7366, 1998). In this study, we used HS-adapted SIN mutants to evaluate a new model system involving target liposomes containing lipid-conjugated heparin (HepPE) as an HS receptor analog for the virus. HS-adapted SIN, but not nonadapted wild-type SIN TR339, interacted efficiently with HepPE-containing liposomes at neutral pH. Binding was competitively inhibited by soluble heparin. Despite the efficient binding of HS-adapted SIN to HepPE-containing liposomes at neutral pH, there was no fusion under these conditions. Fusion did occur, however, at low pH, consistent with cellular entry of the virus via acidic endosomes. At low pH, wild-type or HS-adapted SIN underwent fusion with liposomes with or without HepPE with similar kinetics, suggesting that interaction with the HS receptor analog at neutral pH has little influence on subsequent fusion of SIN at low pH. Finally, Semliki Forest virus (SFV), passaged frequently on BHK-21 cells, also interacted efficiently with HepPE-containing liposomes, indicating that SFV, like other alphaviruses, readily adapts to cell surface HS. In conclusion, the liposomal model system presented in this paper may serve as a novel tool for the study of receptor interactions and membrane fusion properties of HS-interacting enveloped viruses.
Magnitude and Frequency of Cytotoxic T-Lymphocyte Responses: Identification of Immunodominant Regions of Human Immunodeficiency Virus Type 1 Subtype C
Authors: Novitsky V. , Cao H. , Rybak N. , Gilbert P. , McLane M. F. , Gaolekwe S. , Peter T. , Thior I. , Ndung'u T. , Marlink R. , Lee T. H. , Essex M.
Published: September 2002

A systematic analysis of immune responses on a population level is critical for a human immunodeficiency virus type 1 (HIV-1) vaccine design. Our studies in Botswana on (i) molecular analysis of the HIV-1 subtype C (HIV-1C) epidemic, (ii) frequencies of major histocompatibility complex class I HLA types, and (iii) cytotoxic T-lymphocyte (CTL) responses in the course of natural infection allowed us to address HIV-1C-specific immune responses on a population level. We analyzed the magnitude and frequency of the gamma interferon ELISPOT-based CTL responses and translated them into normalized cumulative CTL responses. The introduction of population-based cumulative CTL responses reflected both (i) essentials of the predominant virus circulating locally in Botswana and (ii) specificities of the genetic background of the Botswana population, and it allowed the identification of immunodominant regions across the entire HIV-1C. The most robust and vigorous immune responses were found within the HIV-1C proteins Gag p24, Vpr, Tat, and Nef. In addition, moderately strong responses were scattered across Gag p24, Pol reverse transcriptase and integrase, Vif, Tat, Env gp120 and gp41, and Nef. Assuming that at least some of the immune responses are protective, these identified immunodominant regions could be utilized in designing an HIV vaccine candidate for the population of southern Africa. Targeting multiple immunodominant regions should improve the overall vaccine immunogenicity in the local population and minimize viral escape from immune recognition. Furthermore, the analysis of HIV-1C-specific immune responses on a population level represents a comprehensive systematic approach in HIV vaccine design and should be considered for other HIV-1 subtypes and/or different geographic areas.