bluuebunny/crossref_metadata_2025 huggingface dataset:
Liner conformality in ionized magnetron sputter metal deposition processes
Authors: Hamaguchi S. , Rossnagel S. M.
Published: July 2002

The conformality of thin metal films (liners) formed on high-aspect-ratio trench structures in ionized magnetron sputter deposition processes is studied numerically and experimentally. The numerical simulator (SHADE) used to predict the surface topography is based on the shock-tracking method for surface evolution. The simulation results are in good agreement with experimentally observed thin-film topography. It is shown that combination of direct deposition and trench-bottom resputtering results in good conformality of step coverages and the amount of the resputtering needed for the good conformality is almost independent of trench aspect ratios.
Optimization of the refractory x-ray mask fabrication sequence
Authors: Cummings K. D. , Dauksher W. J. , Johnson W. A. , Laudon M. F. , Engelstad R.
Published: July 2002

This article discusses the effects different process flows have on the material and control requirements for refractory (subtractive) x-ray masks. Our investigation of x-ray masks shows that a film’s stress uniformity is more critical (for distortion) than the magnitude of the stress. In particular, we find for the wafer sequence the requirement on the membrane film’s stress uniformity is the most critical specification. For the membrane sequence, the requirement is for stress uniformity for films that are removed from the membrane (after pattern formation). We find that SiC reduces the distortions from the film stress gradient, however, the frame structure determines the distortions from a film’s mean stress. Finally, we conclude the best choice of an x-ray mask flow should be decided on our ability to modify wafer equipment and processes, to control the stress uniformity of the membrane film, to control the stress and uniformity of the resist, hardmask, and absorber films, and finally to handle the lithography required to create predistorted mask patterns.
Covalent immobilization of immunoglobulins G and Fab′ fragments on gold substrates for scanning force microscopy imaging in liquids
Authors: Droz Eric , Taborelli Mauro , Descouts Pierre , Wells Timothy N. C. , Werlen Raymond C.
Published: July 2002

Scanning force microscopy offers the possibility of observing protein molecules under liquid environment. The main difficulty in obtaining reproducible images is given by the low adhesion of the molecules to the substrate. Physisorbed molecules are displaced by the scanning tip or are resuspended in the medium. We have therefore performed a covalent immobilization of immunoglobulin G (IgG) or its monovalent Fab′ fragment on gold surfaces thanks to thiol groups. For this purpose, multiple thiol groups were chemically introduced into the IgG molecule by treatment with Traut’s reagent. As an alternative, for a Fab′ fragment, we prepared molecules with a single thiol group located close to the C terminus of the truncated heavy chain. Both immobilization techniques enable us to observe clearly discernable individual molecules in liquid media. The grafting of Fab′ fragments on gold surface open new opportunities to study protein interactions.
Nanoparticle impact micromachining
Authors: Gruber A. , Gspann J.
Published: July 2002

Nanoparticles of about 1000 molecules of CO2 accelerated to 20 km/s are used for surface micromachining. Isolated impacts are found to generate hillocks of about 0.5 nm in height and 20 nm in radius on silicon. The cluster material penetrates up to 20 nm into the target, suggesting the initial formation of transitory craters that relax by elastic rebound.
Interactive effects in reactive ion etching of W1−xGex
Authors: van der Drift E. , Dinh B. Q. , Verhoeven P. A. , Fakkeldij E. J. M. , Zuiddam M. R. , Zijlstra T.
Published: July 2002

Reactive ion etching characteristics of chemical vapor deposition-deposited W1−xGex alloys in Cl2, SF6, and SF6/O2/He plasmas were investigated. The interactive role of the germanium component in the overall etch process was unraveled. To this aim etch rates were studied as a function of the Ge content and at different temperatures. Etched profiles give useful additional information about the nature of the etch mechanism. The underlying surface reactions were more quantitatively identified by x-ray photoelectron spectroscopy on dry etched surfaces.
Lithography with a mask of block copolymer microstructures
Authors: Harrison Christopher , Park Miri , Chaikin Paul M. , Register Richard A. , Adamson Douglas H.
Published: July 2002

Dense, periodic arrays of holes and troughs have been fabricated in silicon, silicon nitride, and germanium. The holes are approximately 20 nanometers (nm) wide, 20 nm deep, spaced 40 nm apart, and uniformly patterned with 3×1012 holes on a three inch wafer. To access this length scale, self-assembling resists were synthesized to produce either a layer of hexagonally ordered polyisoprene (PI) spheres or parallel cylinders of polybutadiene (PB) in a polystyrene (PS) matrix. The PI spheres or PB cylinders were then degraded and removed with ozone to produce a PS mask for pattern transfer by fluorine-based reactive ion etching. A PS mask of spherical voids was used to fabricate a lattice of holes and a mask of cylindrical voids was used to produce parallel troughs. This technique accesses a length scale difficult to produce by conventional lithography and opens a route for the patterning of surfaces via self-assembly.
Development of a stainless steel tube resistant to corrosive Cl2 gas for use in semiconductor manufacturing
Authors: Ohmi T. , Yoshida M. , Matudaira Y. , Shirai Y. , Nakamura O. , Gozyuki M. , Hashimoto Y.
Published: July 2002

In order to develop a corrosion-free halogen gas delivery system for use in semiconductor manufacturing, Cr2O3 passivation of ferritic and austenitic stainless steels has been investigated. It has been shown that a Cr2O3 layer can be formed on an electropolished (EP) surface of ferritic 26Cr–1Mo stainless steel by oxidation above 600 °C, but excellent corrosion resistance performance to Cl2 gas can be obtained only by oxidation above 700 °C. On the other hand, on the EP surface of austenitic SUS 316L, a corrosion-resistant Cr2O3 layer is not formed by oxidation even up to 800 °C.
Application of interferometric broadband imaging alignment on an experimental x-ray stepper
Authors: Moon Euclid E. , Lee Jawoong , Everett Patrick N. , Smith Henry I.
Published: July 2002

A series of experiments were performed with an interferometric-broadband imaging (IBBI) alignment system on an experimental x-ray lithography stepper. These experiments demonstrated sub-1 nm consistency of independent IBBI measurements and the ability to feedback lock the mask relative to a wafer to within a mean of 0.0 nm and a standard deviation of 1.4 nm. Comparisons of displacement measurements made with IBBI and closed-loop piezo drives confirmed scale consistency to within about 1.5%. In the absence of feedback control, spurious relative displacements were observed, due to temperature gradients and the nonrigid mechanics. This argues in favor of feedback control during exposure. The robustness of IBBI in allowing nanometer-level alignment measurements during x-ray exposure, with remotely located long-working-distance, low-cost microscopes should enable it to be used as a stand-alone alignment system or as an adjunct to global alignment.
Shot-noise and edge roughness effects in resists patterned at 10 nm exposure
Authors: Rau Nicholas , Stratton Fred , Fields Charles , Ogawa Taro , Neureuther Andrew , Kubena Randy , Willson Grant
Published: July 2002

The experimental shot-noise effects and line-edge roughness are reported for two positive and two negative tone chemically amplified resists (IBM Apex-E, Shipley UVIIHS, IBM ENR, and Shipley SAL-601, respectively) produced by high resolution (10 nm) focused ion-beam exposure. Scanning electron micrographs at the resolution limit for each resist (50–70 nm) showed that the positive resists became negative in tone and that edge roughness was reasonable. Shot-noise effects causing arrays of 10 nm posts to print or not to print at exposure events of 7, 14, and 28 average ions per post were observed in SAL-601 and agree with Poisson statistics. Single exposure events were not observed in any resist possibly owing to the fact that the working minimum exposure level at the resolution limit of the resist material required several overlapping events to print.
Mechanical properties, stress evolution and high-temperature thermal stability of nanolayered Mo–Si–N/SiC thin films
Authors: Torri P. , Hirvonen J.-P. , Kung H. , Lu Y.-C. , Nastasi M. , Gibson P. N.
Published: July 2002

A study of the microstructure, thermal stability, nanoindentation mechanical properties, and residual stress evolution of nanolayered Mo–Si–N/SiC thin films as a function of vacuum annealing time and temperature is reported. Multilayers of Mo–Si–N (MoSi2.2N2.5) and SiC were deposited by magnetron sputtering from planar MoSi2 and SiC targets onto single crystal silicon wafers. The relative amount of both components was varied (12.5–50 vol. % of SiC) while keeping the bilayer thickness constant (12 nm), or the bilayer thickness was varied (6–24 nm) with constant Mo–Si–N to SiC ratio (25 vol. % of SiC). Mechanical properties were measured by nanoindentation on as-deposited films and films annealed in vacuum at 500 and 900 °C. Microstructure and thermal stability were examined by cross-sectional transmission electron microscopy, glancing angle x-ray diffraction and nuclear resonance broadening. Stress evolution induced by thermal annealing was determined by measuring optically the change in curvature of coated silicon beams. In the as-deposited state, all films exhibited an amorphous microstructure. At 900 °C SiC still remained amorphous, but Mo–Si–N had developed a microstructure where nanocrystals of Mo5Si3 were embedded in an amorphous matrix. The interface between Mo–Si–N and SiC was indirectly shown to be stable at least up to 41 h annealing at 1075 °C in vacuum. The potential of Mo–Si–N as a barrier layer against intermixing between nanolayered MoSi2 and SiC at 900 °C has been demonstrated. Hardness, modulus and residual stress followed the volume fraction rule of mixture of both constituents of the nanolayered Mo–Si–N/SiC structure. Consequently, by optimizing the volume fraction of the constituents, zero residual stress on a silicon substrate is possible after annealing.
Analysis of Joule heating in multilevel interconnects
Authors: Shen Y.-L.
Published: July 2002

Metal Joule heating in interconnects is studied numerically. Representative models featuring multilevel interconnect structures (from single level to eight level) are used for the finite element analysis. Particular attention is devoted to the effects of current density, the number of conductor levels, and the materials used as the metallization and interlevel dielectrics. Transient heat conduction analyses are carried out to quantify the temperature rise. It is found that increasing the total number of metal levels and/or switching the dielectric from silicon oxide to polymer-based low-dielectric-constant materials can cause substantial temperature increases, which points out that interconnect Joule heating can become a reliability problem in future applications. The maximum temperature increases with the current density and the metal wiring density in an exponential manner. Incorporating low-k dielectrics based on an embedded insertion scheme can greatly relieve the heating problem. An example of reducing the temperature rise by adapting a cooling mechanism at the packaging level is also illustrated numerically. The effects of constant versus pulsed dc currents are discussed.
Si single-electron tunneling transistor with nanoscale floating dot stacked on a Coulomb island by self-aligned process
Authors: Nakajima Anri , Futatsugi Toshiro , Kosemura Kinjiro , Fukano Tetsu , Yokoyama Naoki
Published: July 2002

We studied experimentally and theoretically in detail a Si single-electron tunneling transistor which has a nanoscale floating dot gate stacked on a Coulomb island by a self-aligned process. At 4.2 K, this device exhibits drain current (Id) oscillations due to the Coulomb blockade effect besides the quantized threshold voltage (Vth) shifts with a hysteresis resulting from a single-electron tunneling between the channel and the floating dot gate. The periodicity of the Coulomb oscillation, the voltage separation (ΔVw) between the adjacent two voltages where the Vth shift occurs are consistent with those calculated from the geometrical consideration. The Coulomb oscillation disappeared at room temperature, however, the quantized Vth shifts and hysteresis curves, which are basic operations of single-electron memory, were observed up to room temperature. The fluctuation of electron number in the floating dot was theoretically analyzed at room temperature and we obtained consistent results with the experiments. The high on/off current ratio of the Coulomb oscillation combined with the quantized Vth shifts suggests that it might be possible to produce a memory with ultralow power consumption.
Plasma sources for electrons and ion beams
Authors: Leung Ka-Ngo
Published: July 2002

Plasma devices are commonly used for the production of ion beams. It has been demonstrated that the multicusp generator can produce very low energy ion beams for ion projection lithography applications. The multicusp source has also found important applications in focused ion beam systems. With its high and uniform plasma density, attempts have been made to extract high brightness electron beams from this type of plasma source, making it also useful for electron beam lithography applications.
Variable axis lens of mixed electrostatic and magnetic fields and its application in electron-beam lithography systems
Authors: Zhao Yan , Khursheed Anjam
Published: July 2002

Existing variable axis lens (VAL) concepts are based upon the power series expansion of the lens field, which theoretically limits them to situations where the focusing and in-lens deflection fields are of the same type, usually magnetic. In this article, a general VAL condition is derived from the paraxial trajectory equation that is applicable to any combined focusing and in-lens deflection system with mixed magnetic and electrostatic fields. Two basic configurations of mixed-field VALs are predicated. The feasibility of a magnetic VAL using in-lens electrostatic deflectors for e-beam lithography applications is analyzed. Deflection aberration simulations show that this mixed-field VAL is comparable to its pure-field counterpart.
All-photoplastic, soft cantilever cassette probe for scanning force microscopy
Authors: Genolet G. , Despont M. , Vettiger P. , Anselmetti D. , de Rooij N. F.
Published: July 2002

Multiple single-lever probes for scanning force microscopy arranged in a cassette design and made of a low-stiffness photoplastic material have been developed and successfully tested by imaging DNA molecules. The new concept consists of a column of a one-dimensional array of cantilevers with integrated tips, the first of which is used for imaging and the others are spares in case the first one becomes degraded. When this happens, the lever is mechanically removed from the chip so that the next spare becomes ready for scanning. The probes are fabricated of a photoplastic material, which allows simple batch fabrication and facilitates realization of specific mechanical properties for cantilevers—such as low stiffness—that are very difficult to achieve with classical silicon technology. The batch fabrication process, based on spin coating and subsequent near-ultraviolet exposure and development steps of the photoplastic SU-8, results in well-controlled and uniform mechanical properties of the probes within the same chip, as well as for different chips over a 4 in. wafer. Successful images of soft, condensed-matter samples were taken using these cassette probes, making consecutive readdressing of the same DNA molecule with two different cantilevers possible. Thus, a worn-out cantilever can be replaced by a new fresh one with only small positional adjustments and without any changes in the operating conditions. This ease of operation was provided by the cassette concept and the excellent uniformity of the mechanical cantilever characteristics.
Diapiric Flow at Subduction Zones: A Recipe for Rapid Transport
Authors: Hall Paul S. , Kincaid Chris
Published: July 2002

Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as ∼30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism. Results indicate that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid, yielding transport times commensurate with those indicated by uranium-thorium studies.
Runaway Growth of Planetary Embryos Facilitated by Massive Bodies in a Protoplanetary Disk
Authors: Kortenkamp Stephen J. , Wetherill George W. , Inaba Satoshi
Published: July 2002

About 30% of detected extrasolar planets exist in multiple-star systems. The standard model of planet formation cannot easily accommodate such systems and has difficulty explaining the odd orbital characteristics of most extrasolar giant planets. We demonstrate that the formation of terrestrial-size planets may be insulated from these problems, enabling much of the framework of the standard model to be salvaged for use in complex systems. A type of runaway growth is identified that allows planetary embryos to form by a combination of nebular gas drag and perturbations from massive companions—be they giant planets, brown dwarfs, or other stars.
Coordination of PIC Assembly and Chromatin Remodeling During Differentiation-Induced Gene Activation
Authors: Soutoglou Evi , Talianidis Iannis
Published: July 2002

We analyzed the ordered recruitment of factors to the human α
1
antitrypsin promoter around the initial activation of the gene during enterocyte differentiation. We found that a complete preinitiation complex, including phosphorylated RNA pol II, was assembled at the promoter long before transcriptional activation. The histone acetyltransferases CBP and P/CAF were recruited subsequently, but local histone hyperacetylation was delayed. After transient recruitment of the human Brahma homolog hBrm, remodeling of the neighboring nucleosome coincided with transcription initiation. The results suggest that, at this promoter, chromatin reconfiguration is a defining step of the initiation process, acting after the assembly of the Pol II machinery.
RNA Silencing: The Genome's Immune System
Authors: Plasterk Ronald H. A.
Published: July 2002

Genomes are databases sensitive to invasion by viruses. In recent years, a defense mechanism has been discovered, which turns out to be conserved among eukaryotes. The system can be compared to the immune system in several ways: It has specificity against foreign elements and the ability to amplify and raise a massive response against an invading nucleic acid. The latter property is beginning to be understood at the molecular level.
Burst Conditions of Explosive Volcanic Eruptions Recorded on Microbarographs
Authors: Morrissey Meghan M. , Chouet Bernard A.
Published: July 2002

Explosive volcanic eruptions generate pressure disturbances in the atmosphere that propagate away either as acoustic or as shock waves, depending on the explosivity of the eruption. Both types of waves are recorded on microbarographs as 1- to 0.1-hertz N-shaped signals followed by a longer period coda. These waveforms can be used to estimate burst pressures and gas concentrations in explosive volcanic eruptions and provide estimates of eruption magnitudes.
Image Improvement from a Sodium-Layer Laser Guide Star Adaptive Optics System
Authors: Max C. E. , Olivier S. S. , Friedman H. W. , An J. , Avicola K. , Beeman B. V. , Bissinger H. D. , Brase J. M. , Erbert G. V. , Gavel D. T. , Kanz K. , Liu M. C. , Macintosh B. , Neeb K. P. , Patience J. , Waltjen K. E.
Published: July 2002

A sodium-layer laser guide star beacon with high-order adaptive optics at Lick Observatory (Mount Hamilton, California) produced a factor of 2.4 intensity increase and a factor of 2 decrease in full width at half maximum for an astronomical point source, compared with image motion compensation alone. The image full widths at half maximum were identical for laser and natural guide stars (0.3 arc second). The Strehl ratio with the laser guide star was 65 percent of that with a natural guide star. This technique should allow ground-based telescopes to attain the diffraction limit, by correcting for atmospheric distortions.
Migrating Planets
Authors: Murray N. , Hansen B. , Holman M. , Tremaine S.
Published: July 2002

A planet orbiting in a disk of planetesimals can experience an instability in which it migrates to smaller orbital radii. Resonant interactions between the planet and planetesimals remove angular momentum from the planetesimals, increasing their eccentricities. Subsequently, the planetesimals either collide with or are ejected by the planet, reducing the semimajor axis of the planet. If the surface density of the planetesimals exceeds a critical value, corresponding to ∼0.03 solar mass of gas inside the orbit of Jupiter, the planet will migrate inward a large distance. This instability may explain the presence of Jupiter-mass objects in small orbits around nearby stars.
Bimetallic System for Nitrogen Fixation: Ruthenium-Assisted Protonation of Coordinated N
2
on Tungsten with H
2
Authors: Nishibayashi Yoshiaki , Iwai Shotaro , Hidai Masanobu
Published: July 2002

Treatment of the tungsten dinitrogen complex
cis
-[W(N
2
)
2
(PMe
2
Ph)
4
] (Me = methyl, Ph = phenyl) with an equilibrium mixture of [RuCl(dppp)
2
]X and
trans
-[RuCl(η
2
-H
2
)(dppp)
2
]X [X = BF
4
, PF
6
, or OSO
2
CF
3
; dppp = 1,3-bis(diphenylphosphino)propane] under 1 atmosphere of dihydrogen at 55 degrees Celsius for 24 hours gave NH
3
in moderate yield. The same reaction in the presence of acetone produced acetone azine in high yield. None of these reactions proceeded in the absence of dihydrogen.
Identification of a Blue Photoluminescent Composite Material from a Combinatorial Library
Authors: Wang Jingsong , Yoo Young , Gao Chen , Takeuchi Ichiro , Sun Xiaodong , Chang Hauyee , Xiang X.-D. , Schultz Peter G.
Published: July 2002

A quaternary combinatorial masking strategy was used in conjunction with photolithography to generate compositionally diverse thin-film phosphor libraries containing 1024 different compositions on substrates 2.5 centimeters square. A parallel imaging system and scanning spectrophotometer were used to identify and characterize compositions in the library with interesting luminescent behavior. Optimal compositions were identified with the use of gradient libraries, in which the stoichiometry of a material was varied continuously. This process led to the identification of an efficient blue photoluminescent composite material, Gd
3
Ga
5
O
12
/SiO
2
. Experimental evidence suggests that luminescence in this material may arise from interfacial effects between SiO
2
and Gd
3
Ga
5
O
12
.
Off-Axis Crustal Thickness Across and Along the East Pacific Rise Within the MELT Area
Authors: Canales J. Pablo , Detrick Robert S. , Bazin Sara , Harding Alistair J. , Orcutt John A.
Published: July 2002

Wide-angle seismic data along the Mantle Electromagnetic and Tomography (MELT) arrays show that the thickness of 0.5- to 1.5-million-year-old crust of the Nazca Plate is not resolvably different from that of the Pacific Plate, despite an asymmetry in depth and gravity across this portion of the East Pacific Rise. Crustal thickness on similarly aged crust on the Nazca plate near a magmatically robust part of the East Pacific Rise at 17°15′S is slightly thinner (5.1 to 5.7 kilometers) than at the 15°55′S overlapping spreading center (5.8 to 6.3 kilometers). This small north-south off-axis crustal thickness difference may reflect along-axis temporal variations in magma supply, whereas the across-axis asymmetry in depth and gravity must be caused by density variations in the underlying mantle.
HIV-1 Regulatory/Accessory Genes: Keys to Unraveling Viral and Host Cell Biology
Authors: Emerman Michael , Malim Michael H.
Published: July 2002

Human immunodeficiency virus type–1 (HIV-1) manipulates fundamental host cell processes in sophisticated ways to achieve optimum replicative efficiency. Recent studies have provided new details on the molecular interactions of HIV-1 with its host cell. For example, HIV-1 encodes a protein that regulates transcriptional elongation by interacting with a cellular cyclin-dependent kinase, another that activates the specific nuclear export of viral RNA, and several others that affect the intracellular trafficking of viral and host cell proteins. Detailed analysis of the interplay between these viral proteins and normal cellular activities has provided new insights into central questions of virology and host cell biology.
Prototype of a Heme Chaperone Essential for Cytochrome c Maturation
Authors: Schulz Henk , Hennecke Hauke , Thöny-Meyer Linda
Published: July 2002

Heme, the iron-containing cofactor essential for the activity of many enzymes, is incorporated into its target proteins by unknown mechanisms. Here, an
Escherichia coli
hemoprotein, CcmE, was shown to bind heme in the bacterial periplasm by way of a single covalent bond to a histidine. The heme was then released and delivered to apocytochrome c. Thus, CcmE can be viewed as a heme chaperone guiding heme to its appropriate biological partner and preventing illegitimate complex formation.
Abnormal Morphogenesis But Intact IKK Activation in Mice Lacking the IKKα Subunit of IκB Kinase
Authors: Hu Yinling , Baud Véronique , Delhase Mireille , Zhang Peilin , Deerinck Thomas , Ellisman Mark , Johnson Randall , Karin Michael
Published: July 2002

The oligomeric IκB kinase (IKK) is composed of three polypeptides: IKKα and IKKβ, the catalytic subunits, and IKKγ, a regulatory subunit. IKKα and IKKβ are similar in structure and thought to have similar function—phosphorylation of the IκB inhibitors in response to proinflammatory stimuli. Such phosphorylation leads to degradation of IκB and activation of nuclear factor κB transcription factors. The physiological function of these protein kinases was explored by analysis of IKKα-deficient mice. IKKα was not required for activation of IKK and degradation of IκB by proinflammatory stimuli. Instead, loss of IKKα interfered with multiple morphogenetic events, including limb and skeletal patterning and proliferation and differentiation of epidermal keratinocytes.
Activation of a Floral Homeotic Gene in
Arabidopsis
Authors: Busch Maximilian A. , Bomblies Kirsten , Weigel Detlef
Published: July 2002

The patterned expression of floral homeotic genes in
Arabidopsis
depends on the earlier action of meristem-identity genes such as
LEAFY
, which encodes a transcription factor that determines whether a meristem will generate flowers instead of leaves and shoots. The LEAFY protein, which is expressed throughout the flower, participates in the activation of homeotic genes, which are expressed in specific regions of the flower. Analysis of a LEAFY-responsive enhancer in the homeotic gene
AGAMOUS
indicates that direct interaction of LEAFY with this enhancer is required for its activity in plants. Thus, LEAFY is a direct upstream regulator of floral homeotic genes.
High Frequency of Cryptic Deleterious Mutations in
Caenorhabditis elegans
Authors: Davies Esther K. , Peters Andrew D. , Keightley Peter D.
Published: July 2002

Deleterious mutations with very small phenotypic effects could be important for several evolutionary phenomena, but the extent of their contribution has been unknown. Fitness effects of induced mutations in lines of
Caenorhabditis elegans
were measured using a system for which the number of deleterious point mutations in the DNA can be estimated. In fitness assays, only about 4 percent of the deleterious mutations fixed in each line were detectable. The remaining 96 percent, though cryptic, are significant for mutation load and, potentially, for the evolution of sex.
TCR-Mediated Internalization of Peptide-MHC Complexes Acquired by T Cells
Authors: Huang Jing-Feng , Yang Young , Sepulveda Homero , Shi Weixing , Hwang Inkyu , Peterson Per A. , Jackson Michael R. , Sprent Jonathan , Cai Zeling
Published: July 2002

Peptide–major histocompatibility complex protein complexes (pMHCs) on antigen-presenting cells (APCs) are central to T cell activation. Within minutes of peptide-specific T cells interacting with APCs, pMHCs on APCs formed clusters at the site of T cell contact. Thereafter, these clusters were acquired by T cells and internalized through T cell receptor–mediated endocytosis. During this process, T cells became sensitive to peptide-specific lysis by neighboring T cells (fratricide). This form of immunoregulation could explain the “exhaustion” of T cell responses that is induced by high viral loads and may serve to down-regulate immune responses.
The Genetic Program of Hematopoietic Stem Cells
Authors: Phillips Robert L. , Ernst Robin E. , Brunk Brian , Ivanova Natalia , Mahan Mark A. , Deanehan Julia K. , Moore Kateri A. , Overton G. Christian , Lemischka Ihor R.
Published: July 2002

Blood cell production originates from a rare population of multipotent, self-renewing stem cells. A genome-wide gene expression analysis was performed in order to define regulatory pathways in stem cells as well as their global genetic program. Subtracted complementary DNA libraries from highly purified murine fetal liver stem cells were analyzed with bioinformatic and array hybridization strategies. A large percentage of the several thousand gene products that have been characterized correspond to previously undescribed molecules with properties suggestive of regulatory functions. The complete data, available in a biological process–oriented database, represent the molecular phenotype of the hematopoietic stem cell.
Millennial-Scale Instability of the Antarctic Ice Sheet During the Last Glaciation
Authors: Kanfoush Sharon L. , Hodell David A. , Charles Christopher D. , Guilderson Thomas P. , Mortyn P. Graham , Ninnemann Ulysses S.
Published: July 2002

Records of ice-rafted detritus (IRD) concentration in deep-sea cores from the southeast Atlantic Ocean reveal millennial-scale pulses of IRD delivery between 20,000 and 74,000 years ago. Prominent IRD layers correlate across the Polar Frontal Zone, suggesting episodes of Antarctic Ice Sheet instability. Carbon isotopes (δ
13
C) of benthic foraminifers, a proxy of deepwater circulation, reveal that South Atlantic IRD events coincided with strong increases in North Atlantic Deep Water (NADW) production and inferred warming (interstadials) in the high-latitude North Atlantic. Sea level rise or increased NADW production associated with strong interstadials may have resulted in destabilization of grounded ice shelves and possible surging in the Weddell Sea region of West Antarctica.
Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa
Authors: Kivelson Margaret G. , Khurana Krishan K. , Russell Christopher T. , Volwerk Martin , Walker Raymond J. , Zimmer Christophe
Published: July 2002

On 3 January 2000, the Galileo spacecraft passed close to Europa when it was located far south of Jupiter's magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward Jupiter. This pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of Europa's internal field. The Galileo magnetometer measured changes in the magnetic field predicted if a current-carrying outer shell, such as a planet-scale liquid ocean, is present beneath the icy surface. The evidence that Europa's field varies temporally strengthens the argument that a liquid ocean exists beneath the present-day surface.
Function of an Axonal Chemoattractant Modulated by Metalloprotease Activity
Authors: Galko Michael J. , Tessier-Lavigne Marc
Published: July 2002

The axonal chemoattractant netrin-1 guides spinal commissural axons by activating its receptor DCC (Deleted in Colorectal Cancer). We have found that chemical inhibitors of metalloproteases potentiate netrin-mediated axon outgrowth in vitro. We have also found that DCC is a substrate for metalloprotease-dependent ectodomain shedding, and that the inhibitors block proteolytic processing of DCC and cause an increase in DCC protein levels on axons within spinal cord explants. Thus, potentiation of netrin activity by inhibitors may result from stabilization of DCC on the axons, and proteolytic activity may regulate axon migration by controlling the number of functional extracellular axon guidance receptors.
Deep Thoughts on the Forbidden
Authors: Douglas Mary
Published: July 2002

The Forest of Taboos
Morality, Hunting, and Identity Among the Huaulu of the Moluccas. Valerio Valeri. University of Wisconsin Press, Madison, 2000. 533 pp. $69.95. ISBN 0-299-16210-9. Paper, $29.95. ISBN 0-299-16214-1.

In this beautifully written, densely argued, and difficult book, Valeri interweaves straightforward ethnographic reporting with an ambitious treatise on taboo. Douglas believes it will become a classic text in anthropology and that it offers a major contribution to theories of the mind and to the philosophy of religion.
A New Astrophysical Setting for Chondrule Formation
Authors: Krot Alexander N. , Meibom Anders , Russell Sara S. , O'D. Alexander Conel M. , Jeffries Timothy E. , Keil Klaus
Published: July 2002

Chondrules in the metal-rich meteorites Hammadah al Hamra 237 and QUE 94411 have recorded highly energetic thermal events that resulted in complete vaporization of a dusty region of the solar nebula (dust/gas ratio of about 10 to 50 times solar). These chondrules formed under oxidizing conditions before condensation of iron-nickel metal, at temperatures greater than or equal to 1500 K, and were isolated from the cooling gas before condensation of moderately volatile elements such as manganese, sodium, potassium, and sulfur. This astrophysical environment is fundamentally different from conventional models for chondrule formation by localized, brief, repetitive heating events that resulted in incomplete melting of solid precursors initially residing at ambient temperatures below approximately 650 K.
Purification and Biochemical Characterization of the VIM-1 Metallo-β-Lactamase
Authors: Franceschini Nicola , Caravelli Berardo , Docquier Jean-Denis , Galleni Moreno , Frère Jean-Marie , Amicosante Gianfranco , Rossolini Gian Maria
Published: July 2002

VIM-1 is a new group 3 metallo-β-lactamase recently detected in carbapenem-resistant nosocomial isolates of
Pseudomonas aeruginosa
from the Mediterranean area. In this work, VIM-1 was purified from an
Escherichia coli
strain carrying the cloned
bla
VIM-1
gene by means of an anion-exchange chromatography step followed by a gel permeation chromatography step. The purified enzyme exhibited a molecular mass of 26 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and an acidic pI of 5.1 in analytical isoelectric focusing. Amino-terminal sequencing showed that mature VIM-1 results from the removal of a 26-amino-acid signal peptide from the precursor. VIM-1 hydrolyzes a broad array of β-lactam compounds, including penicillins, narrow- to expanded-spectrum cephalosporins, carbapenems, and mechanism-based serine-β-lactamase inactivators. Only monobactams escape hydrolysis. The highest catalytic constant/

K
m

ratios (>10
6
M
−1
· s
−1
) were observed with carbenicillin, azlocillin, some cephalosporins (cephaloridine, cephalothin, cefuroxime, cefepime, and cefpirome), imipenem, and biapenem. Kinetic parameters showed remarkable variability with different β-lactams and also within the various penam, cephem, and carbapenem compounds, resulting in no clear preference of the enzyme for any of these β-lactam subfamilies. Significant differences were observed with some substrates between the kinetic parameters of VIM-1 and those of other metallo-β-lactamases. Inactivation assays carried out with various chelating agents (EDTA, 1,10-
o
-phenanthroline, and pyridine-2,6-dicarboxylic acid) indicated that formation of a ternary enzyme-metal-chelator complex precedes metal removal from the zinc center of the protein and revealed notable differences in the inactivation parameters of VIM-1 with different agents.
Impact of Flavophospholipol and Vancomycin on Conjugational Transfer of Vancomycin Resistance Plasmids
Authors: Riedl Sabine , Ohlsen Knut , Werner Guido , Witte Wolfgang , Hacker Jörg
Published: July 2002

The influence of vancomycin and flavophospholipol (FPL) on the transfer rate of conjugative plasmids harboring the vancomycin resistance operon
vanA
was determined in several clinical and animal isolates of
Enterococcus faecium
. FPL significantly inhibited the frequency of transfer of conjugative VanA plasmids up to 70-fold. Vancomycin had no significant effect on the transfer rate of VanA plasmids.
Broad-Spectrum Antiviral Activity of the IMP Dehydrogenase Inhibitor VX-497: a Comparison with Ribavirin and Demonstration of Antiviral Additivity with Alpha Interferon
Authors: Markland W. , McQuaid T. J. , Jain J. , Kwong A. D.
Published: July 2002

The enzyme IMP dehydrogenase (IMPDH) catalyzes an essential step in the de novo biosynthesis of guanine nucleotides, namely, the conversion of IMP to XMP. The major event occurring in cells exposed to competitive IMPDH inhibitors such as ribavirin or uncompetitive inhibitors such as mycophenolic acid (MPA) is a depletion of the intracellular GTP and dGTP pools. Ribavirin is approved as an inhaled antiviral agent for treatment of respiratory syncytial virus (RSV) infection and orally, in combination with alpha interferon (IFN-α), for the treatment of chronic hepatitis C virus (HCV) infection. VX-497 is a potent, reversible uncompetitive IMPDH inhibitor which is structurally unrelated to other known IMPDH inhibitors. Studies were performed to compare VX-497 and ribavirin in terms of their cytotoxicities and their efficacies against a variety of viruses. They included DNA viruses (hepatitis B virus [HBV], human cytomegalovirus [HCMV], and herpes simplex virus type 1 [HSV-1]) and RNA viruses (respiratory syncytial virus [RSV], parainfluenza-3 virus, bovine viral diarrhea virus, Venezuelan equine encephalomyelitis virus [VEEV], dengue virus, yellow fever virus, coxsackie B3 virus, encephalomyocarditis virus [EMCV], and influenza A virus). VX-497 was 17- to 186-fold more potent than ribavirin against HBV, HCMV, RSV, HSV-1, parainfluenza-3 virus, EMCV, and VEEV infections in cultured cells. The therapeutic index of VX-497 was significantly better than that of ribavirin for HBV and HCMV (14- and 39-fold, respectively). Finally, the antiviral effect of VX-497 in combination with IFN-α was compared to that of ribavirin with IFN-α in the EMCV replication system. Both VX-497 and ribavirin demonstrated additivity when coapplied with IFN-α, with VX-497 again being the more potent in this combination. These data are supportive of the hypothesis that VX-497, like ribavirin, is a broad-spectrum antiviral agent.
In Vitro Assessment of Antifungal Therapeutic Potential of Salivary Histatin-5, Two Variants of Histatin-5, and Salivary Mucin (MUC7) Domain 1
Authors: Situ Hongsa , Bobek Libuse A.
Published: July 2002

Human salivary histatin-5 (Hsn-5) is a 24-residue peptide that possesses potent antifungal activity in vitro. The
MUC7
gene encodes human salivary low-molecular-weight mucin (MG2). The candidacidal activity of MUC7 domain 1 (MUC7 D1, the N-terminal 51 amino acid residues of MUC7) in vitro has also been demonstrated. In this study, we have investigated the antifungal therapeutic potential of Hsn-5, its two variants, R12I/K17N and R12I/H21L, and MUC7 D1. First, these peptides were tested for activities against different clinically important fungi. We found them to possess broad-spectrum antifungal activities; specifically, most exhibited excellent in vitro activity against eight clinically important fungal strains tested, including
Candida albicans
and
Candida glabrata
and their azole-resistant counterparts and
Cryptococcus neoformans
and its amphotericin B-resistant counterpart. These findings also suggest that the mechanism of action of both Hsn-5 and MUC7 D1 for these fungi is different from that of amphotericin B or azole antifungal agents. Second, we examined the stability of these peptides in whole human saliva and human serum. In saliva, the Hsn-5 variants R12I/K17N and R12I/H21L and MUC7 D1 degraded at a lower rate than Hsn-5. In human serum, MUC7 D1 was also more stable than Hsn-5; both peptides were more stable in serum than in saliva. Third, we examined the cytotoxicity of these peptides using human erythrocytes and two human cell lines (KB and HSG). No (or very low) hemolytic activity was observed with any of the four peptides, even at the highest protein concentration tested (200 μM), while amphotericin B caused 100% hemolysis at only 12.5 μM. The toxic effects of Hsn-5 and MUC7 D1 toward KB and HSG cells were also much lower than that of amphotericin B as measured by trypan blue exclusion. Together, these findings indicate that the investigated peptides possess high antifungal therapeutic potential, in particular for the treatment of drug-resistant fungal strains associated with immunocompromised (particularly human immunodeficiency virus-infected) patients. The same peptides could also be used as components of artificial saliva for patients with salivary dysfunction.
Role of Human Immunodeficiency Virus (HIV) Type 1 Envelope in the Anti-HIV Activity of the Betulinic Acid Derivative IC9564
Authors: Holz-Smith Sonia L. , Sun I-Chen , Jin Lei , Matthews Thomas J. , Lee Kuo-Hsiung , Chen Chin Ho
Published: July 2002

The betulinic acid derivative IC9564 is a potent anti-human immunodeficiency virus (anti-HIV) compound that can inhibit both HIV primary isolates and laboratory-adapted strains. However, this compound did not affect the replication of simian immunodeficiency virus and respiratory syncytial virus. Results from a syncytium formation assay indicated that IC9564 blocked HIV type 1 (HIV-1) envelope-mediated membrane fusion. Analysis of a chimeric virus derived from exchanging envelope regions between IC9564-sensitive and IC9564-resistant viruses indicated that regions within gp120 and the N-terminal 25 amino acids (fusion domain) of gp41 are key determinants for the drug sensitivity. By developing a drug-resistant mutant from the NL4-3 virus, two mutations were found within the gp120 region and one was found within the gp41 region. The mutations are G237R and R252K in gp120 and R533A in the fusion domain of gp41. The mutations were reintroduced into the NL4-3 envelope and analyzed for their role in IC9564 resistance. Both of the gp120 mutations contributed to the drug sensitivity. On the contrary, the gp41 mutation (R533A) did not appear to affect the IC9564 sensitivity. These results suggest that HIV-1 gp120 plays a key role in the anti-HIV-1 activity of IC9564.
In Vitro Susceptibility of
Coxiella burnetii
to Linezolid in Comparison with Its Susceptibilities to Quinolones, Doxycycline, and Clarithromycin
Authors: Gikas Achilleas , Spyridaki Ioanna , Scoulica Efstathia , Psaroulaki Anna , Tselentis Yannis
Published: July 2002

The in vitro susceptibility to linezolid shown by nine Greek isolates of
Coxiella burnetii
derived from patients with acute Q fever was investigated. MICs of linezolid were compared with those of pefloxacin, ciprofloxacin, ofloxacin, trovafloxacin, doxycycline, and clarithromycin using the shell vial assay. MICs of linezolid and clarithromycin ranged from 2 to 4 μg/ml; those of doxycycline, trovafloxacin, and ofloxacin ranged from 1 to 2 μg/ml; those of pefloxacin ranged from 1 to 4 μg/ml; and those of ciprofloxacin ranged from 4 to 8 μg/ml. Linezolid was effective in controlling intracellular parasites in cultures of Vero cells infected by
C
.
burnetii
. No bactericidal activity by linezolid was obtained against
C
.
burnetii
at 8 μg/ml.
Interactions of Posaconazole and Flucytosine againstCryptococcus neoformans
Authors: Barchiesi Francesco , Schimizzi Anna Maria , Najvar Laura K. , Bocanegra Rosie , Caselli Francesca , Di Cesare Simona , Giannini Daniele , Di Francesco Luigi Falconi , Giacometti Andrea , Carle Flavia , Scalise Giorgio , Graybill John R.
Published: July 2002

A checkerboard methodology, based on standardized methods proposed by the National Committee for Clinical Laboratory Standards for broth microdilution antifungal susceptibility testing, was applied to study the in vitro interactions of flucytosine (FC) and posaconazole (SCH 56592) (FC-SCH) against 15 isolates ofCryptococcus neoformans. Synergy, defined as a fractional inhibitory concentration (FIC) index of <0.50, was observed for 33% of the isolates tested. When synergy was not achieved, there was still a decrease in the MIC of one or both drugs when they were used in combination. Antagonism, defined as a FIC of >4.0, was not observed. The in vitro efficacy of combined therapy was confirmed by quantitative determination of the CFU ofC. neoformans486, an isolate against which the FC-SCH association yielded a synergistic interaction. To investigate the potential beneficial effects of this combination therapy in vivo, we established two experimental murine models of cryptococcosis by intracranial or intravenous injection of cells ofC. neoformans486. At 1 day postinfection, the mice were randomized into different treatment groups. One group each received each drug alone, and one group received the drugs in combination. While combination therapy was not found to be significantly more effective than each single drug in terms of survival, tissue burden experiments confirmed the potentiation of antifungal activity with the combination. Our study demonstrates that SCH and FC combined are significantly more active than either drug alone againstC. neoformansin vitro as well in vivo. These findings suggest that this therapeutic approach could be useful in the treatment of cryptococcal infections.
Pharmacokinetic-Pharmacodynamic Modeling of the Electroencephalogram Effect of Imipenem in Healthy Rats
Authors: Dupuis Antoine , Couet William , Paquereau Joël , Debarre Stanley , Portron Agnès , Jamois Candice , Bouquet Serge
Published: July 2002

A pharmacokinetic-pharmacodynamic (PK-PD) modeling approach was developed to investigate the epileptogenic activity of imipenem in rats. Initially, animals received an intravenous infusion of imipenem at a rate of 2.65 mg min
−1
for 30 min. Blood samples were collected for drug assay, and an electroencephalogram (EEG) was recorded during infusion and postinfusion. A dramatic delay was observed between concentrations of imipenem in serum and the EEG effect; this effect was accompanied by tremors and partial seizures. Indirect-effect models failed to describe these data, which were successfully fitted using an effect compartment model. The relationship between effect and concentration at the effect site was best described by a spline function. The elimination rate constant from the effect compartment was severalfold lower than that from the central compartment. The robustness of the model was then confirmed after administering the imipenem dose over 60 and 90 min. In conclusion, the successful PK-PD modeling of the imipenem EEG effect in rats constitutes a major improvement for better prediction of the epileptogenic risk associated with this antibiotic.