bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarization imprint in ordered arrays of epitaxial ferroelectric nanostructures
Authors: Ma Wenhui , Hesse Dietrich
Published: April 2004

Sub 50-nm-high BaTiO3 epitaxial nanostructures were fabricated into well-ordered arrays on SrTiO3:Nb (001) single-crystal substrates using pulsed laser deposition through monolayer masks of monodisperse latex spheres (1 or 0.5 μm) and postdeposition annealing. Imprint was investigated in nanostructures of various dimensions using piezoresponse force microscopy. Piezoresponse domain imaging revealed a preferential downward prepolarization state for the as-prepared nanostructures, and consistently piezoresponse hysteresis loops exhibited negative polarization offsets. The offsets are suggested to be due to locked nonswitchable domains in an interface near region with a thickness of 7–14 nm.
Fatigue failure mechanisms of single-walled carbon nanotube ropes embedded in epoxy
Authors: Ren Y. , Fu Y. Q. , Liao K. , Li F. , Cheng H. M.
Published: April 2004

In this work, fatigue failure mechanisms of single-walled carbon nanotube (SWCNT) bundles embedded in epoxy matrix under repeated tensile load were studied. Observed damage and failure modes include: (1) splitting of SWCNT bundles, (2) kink formation and subsequent failure in SWCNTs, and (3) fracture of SWCNT bundles. Patterns of crack propagation under tension in SWCNTs were studied by molecular mechanics simulations, where defect-free SWCNTs and SWCNTs with two different modes of Stone–Wales defects were studied. It is demonstrated by the results of molecular mechanics simulation that the observed fracture surfaces of SWCNT can be reproduced reasonably well, suggesting possible fatigue failure mechanisms of SWCNT in the composite.
Temperature dependence and single-mode tuning behavior of second-harmonic generation in quantum cascade lasers
Authors: Gmachl C. , Owschimikow N. , Belyanin A. , Sergent A. M. , Sivco D. L. , Peabody M. L. , Cho A. Y. , Capasso F.
Published: April 2004

Second-harmonic generation (SHG) is reported in quantum cascade (QC) lasers with active regions that also support nonlinear cascades with large second order nonlinear susceptibility. SHG has been measured from 10 up to 250 K heat sink temperature, with about 1 μW of nonlinear power at 10 K and about 50 nW at 250 K. Single-mode and tunable SHG at 3.5 μm wavelength has been measured from single-mode QC distributed feedback lasers operating at the fundamental pump wavelength of 7.0 μm. Thermal tuning results in a tuning rate for the SHG emission of ∼0.2 nm/K for temperatures above ∼100 K.
Influence of a carrier supply layer on carrier density and drift mobility of AlGaN/GaN/SiC high-electron-mobility transistors
Authors: Marso M. , Bernát J. , Javorka P. , Kordoš P.
Published: April 2004

Sheet carrier concentration and low-field drift mobility of intentionally undoped and modulation-doped AlGaN/GaN heterostructures on SiC substrate were evaluated by capacitance-voltage and channel conductivity measurements. Sheet carrier concentration and average mobility at 0 V gate bias correspond to standard Hall results. Sheet carrier density increases from 6.8×1012 cm−2 for the undoped sample up to 1×1013 cm−2 for the device with the highest doping concentration, while the mobility decreases from 1800 to 1620 cm2/V s. The local mobility, on the other hand, depends only on the actual sheet carrier density and is not influenced by the doping concentration of the carrier supply layer. It reaches a maximum value of 2100 cm2/V s at a carrier density of 3×1012 cm−2.
Three-dimensional columnar optical nanostructures fabricated by using lithography-free templating approach
Authors: Ng Hou T. , Matthews K. , Chen Yi P. , Nguyen Pho , Li Jun , Han Jie , Meyyappan M.
Published: April 2004

Using a nanowire-based templating approach, lithography-free fabrication of three-dimensional circular optical nanostructures with a minimum physical size of 20 nm is demonstrated. Regular circular gratings with alternating transparent and opaque annular zones and having discrete nanometer length scale separations can be easily achieved. These columnar optical nanostructures could potentially be used for direct integration with other optical components.
Performance enhancement of InGaN light-emitting diodes by laser lift-off and transfer from sapphire to copper substrate
Authors: Tan Bee Sim , Yuan Shu , Kang Xue Jun
Published: April 2004

Laser lift-off technique was employed to carry out transfer of prefabricated InGaN multiple-quantum-well light-emitting diodes (LEDs) from sapphire onto Cu substrate. Silver epoxy was used as the bonding material. Characterization results showed tremendous device improvements in terms of maximum allowable current, light output power, and reliability from the use of conductive Cu substrate. LEDs on Cu could withstand a maximum current of 530 mA before breakdown while those on sapphire could only withstand 350 mA. At 40 mA, light output power of LEDs on sapphire and Cu was 0.74 and 0.95 mW, respectively. In addition, reliability test at constant current of 300 mA showed improvement in light output power for LEDs on Cu whereas LEDs on sapphire suffered deterioration with time.
Optical transition in discrete levels of Si quantum dots
Authors: Wu X. L. , Xue F. S.
Published: April 2004

Photoluminescence (PL) measurements have been carried out on Si quantum dots (QDs) with sizes of 2–4 nm embedded in Si oxide films. Conspicuous multiple PL peaks separated in energy by up to 104 meV are observed and attributed to optical transitions in discrete energy levels of Si QDs. A band-mixing model of direct and indirect gaps in a nanometer environment consisting of nc-Si core and SiOx surface layer has been proposed for calculation of electronic states. Good agreement is achieved between the observed PL peaks and calculated results. This work provides a path for realizing Si QD laser.
Dual insulated-gate field-effect transistors with cadmium sulfide active layer and a laminated polymer dielectric
Authors: Meth J. S. , Zane S. G. , Nunes G.
Published: April 2004

We report the fabrication of dual insulated gate thin-film transistors with chemical-bath deposited cadmium sulfide active layers. The cadmium sulfide was deposited from solution onto thermally oxidized silicon wafers to form the first semiconductor–dielectric interface. The terpolymer poly(tetrafluoroethylene-co-vinylidenefluoride-co-propylene) was laminated onto the semiconductor to create the second semiconductor–dielectric interface. This device geometry allows direct comparison of the behavior of the accumulated charge at these two very different interfaces. The mobility values for these devices are in the 0.1–1 cm2/Vs range, while the on/off ratios vary from 102 to 105. The ability to laminate a dielectric to a semiconductor enables new processing routes for large area transistor arrays.
Effect of quench on crystallinity and alignment of boron nanowires
Authors: Yun S. H. , Dibos A. , Wu J. Z. , Kim Do-Kyung
Published: April 2004

Fabrication of aligned single-crystalline boron nanowire (BNW) arrays has been a challenge so far. This motivated us to investigate the effect of quench on the crystallinity and alignment of BNW films made in a thermal vapor transfer process on Au-coated Si substrates. A remarkable improvement in both crystallinity and alignment has been observed in BNW films quenched from processing temperatures ⩾1100 °C. Without quench, the BNWs formed in the temperature range of 900–1100 °C are heavily tangled with either an amorphous or polycrystalline structure. If higher-temperature (⩾1100 °C) processing is followed by a quench, predominantly single-crystalline BNWs of a rhombohedral structure (a=10.94 Å and c=23.83 Å) and (031) orientation aligned along the normal of the substrates can be obtained. We speculate that the strain induced along the BNW at large temperature gradient is responsible for the observed quench effect.
Strain relaxation and its effect in exciton resonance energies of epitaxial ZnO layers grown on 6H-SiC substrates
Authors: Ashrafi A. B. M. Almamun , Binh Naguyen Thanh , Zhang Bao-ping , Segawa Yusaburo
Published: April 2004

Thickness-dependent strain relaxation and its role on exciton resonance energies of epitaxial ZnO layers grown on 6H-SiC substrates have been studied. The magnitudes of strain were determined experimentally by x-ray diffraction measurements. The strain ratios under biaxial stresses (Δc/c0)/(Δa/a0) of epitaxial ZnO layers grown on SiC and Al2O3 substrates were estimated to be 0.38 and 0.50, respectively. The strain-induced band shift δEA/δεzz for ZnO/SiC and ZnO/Al2O3 heterostructures was analyzed by photoluminescence with the values of 13.1 and 14.6 eV, respectively. These comparative strain-induced band shifts, as well as Poisson ratios, evidenced the role of lattice deformation kinetics induced by different lattice mismatches in the ZnO/SiC and ZnO/Al2O3 heterostructures.
Temperature-dependent characteristics of Pt Schottky contacts on n-type ZnO
Authors: Ip K. , Heo Y. W. , Baik K. H. , Norton D. P. , Pearton S. J. , Kim S. , LaRoche J. R. , Ren F.
Published: April 2004

The Schottky barrier height of Pt contacts on n-type (n∼1016 cm−3) thin film ZnO deposited by pulsed laser deposition was obtained from current–voltage measurements as a function of temperature. The resulting values ranged from 0.61±0.04 eV at 25 °C to 0.46±0.06 eV at 100 °C with saturation current densities of 1.5×10−4 A cm−2 (25 °C) to 6.0×10−2 A cm−2 (100 °C), respectively. The reverse current magnitude was larger than predicted by thermionic emission alone. The measured barrier height for Pt on ZnO is similar to the value reported for both Au and Ag rectifying contacts on this material.
Structure and electrical activity of rare-earth dopants in GaN
Authors: Filhol J.-S. , Jones R. , Shaw M. J. , Briddon P. R.
Published: April 2004

Density functional theory is used to investigate Eu, Er, and Tm rare earth (RE) impurities in GaN, paying particular attention to their structure, energetics, and electronic properties. The most stable site is when the RE is located at a Ga substitutional site but none of the defects possess deep energy levels. RE–VN close pairs are stable at temperatures greater than 1000 °C for 1 at. % RE doping and possess shallow levels possibly exciting intra-f luminescence in the visible. RE–VGa and RE-interstitial defects possess deep levels which exclude them as sources for visible RE-related luminescence.
Narrow photoluminescence linewidth (<17 meV) from highly uniform self-assembled InAs/GaAs quantum dots grown by low-pressure metalorganic chemical vapor deposition
Authors: Yang Tao , Tatebayashi Jun , Tsukamoto Shiro , Nishioka Masao , Arakawa Yasuhiko
Published: April 2004

We report highly uniform self-assembled InAs quantum dots (QDs) emitting at 1.3 μm, grown on GaAs substrates by low-pressure metalorganic chemical vapor deposition. By optimizing the InAs growth rate and capping the QDs with GaAs using triethylgallium as a gallium source, we have achieved a narrow photoluminescence (PL) inhomogeneous linewidth of 16.5 meV (at 7 K) from QDs with a density of 1.7×1010 cm−2. Furthermore, we show by temperature-dependent PL measurements that the QDs exhibit almost no dependence of linewidth on temperature due to their high uniformity.
Fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films
Authors: Chen Xidong , Sullivan J. P. , Friedmann T. A. , Gibson J. Murray
Published: April 2004

In this letter, we report fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films and the effect of annealing on this ordering. Annealed and unannealed diamond-like carbon films have almost identical short-range order. Our fluctuation microscopy results, however, indicate the presence of medium range order or clustering in the films on a lateral length scale that exceeds 1 nm. Within the clustered regions, the dominant local ordering appears to be diamond-like, and graphite-like ordering is not observed. Thermal annealing up to 600 °C leads to an increase in diamond-like clustering with no onset of graphite-like clustering. However, after high temperature annealing up to 1000 °C, graphite-like clustering becomes apparent as a result of the conversion of diamond-like carbon to graphite-like carbon. The results on the as-deposited films and films annealed up to 600 °C suggest that a spontaneous medium range ordering process occurs in diamond-like carbon films during and subsequent to film growth, and this may play an important role in stress relaxation.
InP/InGaAs heterojunction bipolar transistors with low-resistance contact on heavily doped InP emitter layer
Authors: Kim Moonjung , Kim Choul-Young , Kwon Young-Se
Published: April 2004

InP/InGaAs heterojunction bipolar transistors (HBTs) with low-resistance Ti/Pt/Au contact directly on heavily doped InP emitter layer have been demonstrated. A specific contact resistance of Ti/Pt/Au to n-type InP with doping concentration of 2×1019 cm−3 was investigated with the tunneling model theoretically and transmission line model (TLM) experimentally, revealing that it depends greatly on the doping level. TLM measurements exhibited a low specific contact resistance of 3.5×10−7 Ω cm2, which can be applicable to the fabrication of HBTs. InP/InGaAs HBTs with n+-InP emitter layer have been demonstrated with excellent dc characteristics, including a low offset voltage of 0.12 V, a knee voltage of 0.5 V, and a current gain of 28. These results verify that the heavily doped InP emitter layer allows a low-resistance contact.
Functional and evolutionary genomics of
Mycobacterium tuberculosis
: Insights from genomic deletions in 100 strains
Authors: Tsolaki Anthony G. , Hirsh Aaron E. , DeRiemer Kathryn , Enciso Jose Antonio , Wong Melissa Z. , Hannan Margaret , de la Salmoniere Yves-Olivier L. Goguet , Aman Kumiko , Kato-Maeda Midori , Small Peter M.
Published: April 2004

To better understand genome function and evolution in
Mycobacterium tuberculosis
, the genomes of 100 epidemiologically well characterized clinical isolates were interrogated by DNA microarrays and sequencing. We identified 68 different large-sequence polymorphisms (comprising 186,137 bp, or 4.2% of the genome) that are present in H37Rv, but absent from one or more clinical isolates. A total of 224 genes (5.5%), including genes in all major functional categories, were found to be partially or completely deleted. Deletions are not distributed randomly throughout the genome but instead tend to be aggregated. The distinct deletions in some aggregations appear in closely related isolates, suggesting a genomically disruptive process specific to an individual mycobacterial lineage. Other genomic aggregations include distinct deletions that appear in phylogenetically unrelated isolates, suggesting that a genomic region is vulnerable throughout the species. Although the deletions identified here are evidently inessential to the causation of disease (they are found in active clinical cases), their frequency spectrum suggests that most are weakly deleterious to the pathogen. For some deletions, short-term evolutionary pressure due to the host immune system or antibiotics may favor the elimination of genes, whereas longer-term physiological requirements maintain the genes in the population.
CO migration in native and mutant myoglobin: Atomistic simulations for the understanding of protein function
Authors: Nutt David R. , Meuwly Markus
Published: April 2004

Molecular dynamics simulations of the events after the photodissociation of CO in the myoglobin mutant L29F in which leucine is replaced by phenylalanine are reported. Using both classical and mixed quantum-classical molecular dynamics calculations, we observed the rapid motion of CO away from the distal heme pocket to other regions of the protein, in agreement with recent experimental results. The experimentally observed and calculated infrared spectra of CO after dissociation are also in good agreement. We compared the results with data from simulations of WT myoglobin. As the time resolution of experimental techniques is increased, theoretical methods and models can be validated at the atomic scale by direct comparison with experiment.
OH–Pd(0) interaction as a stabilizing factor in palladium-catalyzed allylic alkylations
Authors: Hallman Kristina , Frölander Anders , Wondimagegn Tebikie , Svensson Mats , Moberg Christina
Published: April 2004

In palladium-catalyzed alkylations of allylic acetates with malonate as nucleophile, catalysts with oxazoline ligands bearing hydroxymethyl substituents in 4-position have been shown by density functional theory computations to undergo a conformational change on nucleophilic attack, which is accompanied by reduction of Pd(II) to Pd(0). The conformations of the Pd(0) complexes were shown to be governed by the presence of a hydrogen bond with the metal center acting as a hydrogen bond acceptor. The conformational change, which is absent in catalysts with
O
-alkylated analogs, largely affects the enantioselectivity of the catalytic process. This process is a previously uninvestigated example of where this type of weak hydrogen bond has been shown to influence the stereochemistry of a chemical reaction.
Catalytic asymmetric addition of diphenylzinc to cyclic α,β-unsaturated ketones
Authors: Li Hongmei , García Celina , Walsh Patrick J.
Published: April 2004

We have examined the use of our bis(sulfonamide) diol ligand (1) in the asymmetric addition of phenyl groups to cyclic α,β-unsaturated ketones. Good to excellent enantioselectivities have been obtained with cyclic enones bearing alkyl substituents in the 2 position (71–97% enantiomeric excess). Furthermore, excellent enantioselectivities have been observed in the asymmetric phenylation of cyclic enones with 2-iodo and 2-bromo substituents. The results of this study broaden the scope of the asymmetric additions to ketones promoted by the titanium catalyst derived from ligand 1.
Cu(I)-catalyzed direct addition and asymmetric addition of terminal alkynes to imines
Authors: Wei Chunmei , Mague Joel T. , Li Chao-Jun
Published: April 2004

A Cu(I)-catalyzed direct addition of alkynes to imines was developed. The process is simple and provides a diverse range of propargylamines in high enantiomeric excess and good yield both in water and in toluene. The absolute configuration of such addition products has been determined by x-ray crystallography.
Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes: Catalyst and substrate studies leading to a mechanistic working model
Authors: Lautens Mark , Fagnou Keith
Published: April 2004

Catalyst and substrate studies have been performed on the rhodium-catalyzed asymmetric ring opening reaction. A working model is advanced that involves oxidative insertion with retention to form an organorhodium intermediate that then undergoes nucleophilic attack with inversion. Kinetic and competition experiments have uncovered evidence for a proton transfer step in the catalytic cycle that may activate both the allylrhodium intermediate and the nucleophile. We have also conducted experiments designed to understand which properties of the PPF-P
t
Bu
2
ligand contribute to the high reactivities and enantioselectivities.
Mapping knowledge domains: Characterizing PNAS
Authors: Boyack Kevin W.
Published: April 2004

A review of data mining and analysis techniques that can be used for the mapping of knowledge domains is given. Literature mapping techniques can be based on authors, documents, journals, words, and/or indicators. Most mapping questions are related to research assessment or to the structure and dynamics of disciplines or networks. Several mapping techniques are demonstrated on a data set comprising 20 years of papers published in PNAS. Data from a variety of sources are merged to provide unique indicators of the domain bounded by PNAS. By using funding source information and citation counts, it is shown that, on an aggregate basis, papers funded jointly by the U.S. Public Health Service (which includes the National Institutes of Health) and non-U.S. government sources outperform papers funded by other sources, including by the U.S. Public Health Service alone. Grant data from the National Institute on Aging show that, on average, papers from large grants are cited more than those from small grants, with performance increasing with grant amount. A map of the highest performing papers over the 20-year period was generated by using citation analysis. Changes and trends in the subjects of highest impact within the PNAS domain are described. Interactions between topics over the most recent 5-year period are also detailed.
Searching for intellectual turning points: Progressive knowledge domain visualization
Authors: Chen Chaomei
Published: April 2004

This article introduces a previously undescribed method progressively visualizing the evolution of a knowledge domain's cocitation network. The method first derives a sequence of cocitation networks from a series of equal-length time interval slices. These time-registered networks are merged and visualized in a panoramic view in such a way that intellectually significant articles can be identified based on their visually salient features. The method is applied to a cocitation study of the superstring field in theoretical physics. The study focuses on the search of articles that triggered two superstring revolutions. Visually salient nodes in the panoramic view are identified, and the nature of their intellectual contributions is validated by leading scientists in the field. The analysis has demonstrated that a search for intellectual turning points can be narrowed down to visually salient nodes in the visualized network. The method provides a promising way to simplify otherwise cognitively demanding tasks to a search for landmarks, pivots, and hubs.
Extracting knowledge from the World Wide Web
Authors: Henzinger Monika , Lawrence Steve
Published: April 2004

The World Wide Web provides a unprecedented opportunity to automatically analyze a large sample of interests and activity in the world. We discuss methods for extracting knowledge from the web by randomly sampling and analyzing hosts and pages, and by analyzing the link structure of the web and how links accumulate over time. A variety of interesting and valuable information can be extracted, such as the distribution of web pages over domains, the distribution of interest in different areas, communities related to different topics, the nature of competition in different categories of sites, and the degree of communication between different communities or countries.
Evolution of document networks
Authors: Menczer Filippo
Published: April 2004

How does a network of documents grow without centralized control? This question is becoming crucial as we try to explain the emergent scale-free topology of the World Wide Web and use link analysis to identify important information resources. Existing models of growing information networks have focused on the structure of links but neglected the content of nodes. Here I show that the current models fail to reproduce a critical characteristic of information networks, namely the distribution of textual similarity among linked documents. I propose a more realistic model that generates links by using both popularity and content. This model yields remarkably accurate predictions of both degree and similarity distributions in networks of web pages and scientific literature.
Crossmaps: Visualization of overlapping relationships in collections of journal papers
Authors: Morris Steven A. , Yen Gary G.
Published: April 2004

A crossmapping technique is introduced for visualizing multiple and overlapping relations among entity types in collections of journal articles. Groups of entities from two entity types are crossplotted to show correspondence of relations. For example, author collaboration groups are plotted on thexaxis against groups of papers (research fronts) on theyaxis. At the intersection of each pair of author group/research front pairs a circular symbol is plotted whose size is proportional to the number of times that authors in the group appear as authors in papers in the research front. Entity groups are found by agglomerative hierarchical clustering using conventional similarity measures. Crossmaps comprise a simple technique that is particularly suited to showing overlap in relations among entity groups. Particularly useful crossmaps are: research fronts against base reference clusters, research fronts against author collaboration groups, and research fronts against term co-occurrence clusters. When exploring the knowledge domain of a collection of journal papers, it is useful to have several crossmaps of different entity pairs, complemented by research front timelines and base reference cluster timelines.
Mapping topics and topic bursts in PNAS
Authors: Mane Ketan K. , Börner Katy
Published: April 2004

Scientific research is highly dynamic. New areas of science continually evolve; others gain or lose importance, merge, or split. Due to the steady increase in the number of scientific publications, it is hard to keep an overview of the structure and dynamic development of one's own field of science, much less all scientific domains. However, knowledge of “hot” topics, emergent research frontiers, or change of focus in certain areas is a critical component of resource allocation decisions in research laboratories, governmental institutions, and corporations. This paper demonstrates the utilization of Kleinberg's burst detection algorithm, co-word occurrence analysis, and graph layout techniques to generate maps that support the identification of major research topics and trends. The approach was applied to analyze and map the complete set of papers published in PNAS in the years 1982-2001. Six domain experts examined and commented on the resulting maps in an attempt to reconstruct the evolution of major research areas covered by PNAS.
Mixed-membership models of scientific publications
Authors: Erosheva Elena , Fienberg Stephen , Lafferty John
Published: April 2004

PNAS is one of world's most cited multidisciplinary scientific journals. The PNAS official classification structure of subjects is reflected in topic labels submitted by the authors of articles, largely related to traditionally established disciplines. These include broad field classifications into physical sciences, biological sciences, social sciences, and further subtopic classifications within the fields. Focusing on biological sciences, we explore an internal soft-classification structure of articles based only on semantic decompositions of abstracts and bibliographies and compare it with the formal discipline classifications. Our model assumes that there is a fixed number of internal categories, each characterized by multinomial distributions over words (in abstracts) and references (in bibliographies). Soft classification for each article is based on proportions of the article's content coming from each category. We discuss the appropriateness of the model for the PNAS database as well as other features of the data relevant to soft classification.
Copper-catalyzed asymmetric conjugate reduction as a route to novel β-azaheterocyclic acid derivatives
Authors: Rainka Matthew P. , Aye Yimon , Buchwald Stephen L.
Published: April 2004

A chiral copper-hydride catalyst for the asymmetric conjugate reduction of α,β-unsaturated carbonyl compounds has been used for the reduction of substrates containing β-nitrogen substituents. A new set of reaction conditions has allowed for a variety of β-azaheterocyclic acid derivatives to be synthesized in excellent yields and with high degrees of enantioselectivity. In addition, the effect that the nature of the nitrogen substituent has on the rate of the conjugate reduction reaction has been explored.
Remarkably diastereoselective synthesis of a chiral biphenyl diphosphine ligand and its application in asymmetric hydrogenation
Authors: Qiu Liqin , Wu Jing , Chan Shusun , Au-Yeung Terry T.-L. , Ji Jian-Xin , Guo Rongwei , Pai Cheng-Chao , Zhou Zhongyuan , Li Xingshu , Fan Qing-Hua , Chan Albert S. C.
Published: April 2004

Essentially complete atropdiastereoselectivity was realized in the preparation of biaryl diphosphine dioxide by asymmetric intramolecular Ullmann coupling and oxidative coupling with central-to-axial chirality transfer. A bridged
C
2
-symmetric biphenyl phosphine ligand possessing additional chiral centers on the linking unit of the biphenyl groups was synthesized. No resolution step was required for the preparation of the enantiomerically pure chiral ligand. These findings offer a general and practical tool for the development of previously uninvestigated atropdiastereomeric biaryl phosphine ligands. The diphosphine ligand was found to be highly effective in the asymmetric hydrogenation of α- and β-ketoesters, 2-(6′-methoxy-2′-naphthyl)propenoic acid, β-(acylamino)acrylates, and enol acetates.
Air-stable, storable, and highly efficient chiral zirconium catalysts for enantioselective Mannich-type, aza Diels–Alder, aldol, and hetero Diels–Alder reactions
Authors: Kobayashi Shū , Ueno Masaharu , Saito Susumu , Mizuki Yumiko , Ishitani Haruro , Yamashita Yasuhiro
Published: April 2004

For the synthesis of optically active compounds, chiral catalysts have attracted much attention because large quantities of optically active molecules can be prepared from a small amount of a chiral source. However, many chiral catalysts are often unstable in air (oxygen) and/or in the presence of water. This is especially the case in chiral Lewis acid catalysis, because most Lewis acids are air- and moisture-sensitive. Therefore, many catalysts are prepared
in situ
in an appropriate solvent just before use, and they cannot be stored for extended periods. We have developed air-stable, storable, and highly efficient chiral zirconium Lewis acids. The catalysts promoted asymmetric Mannich-type, aza Diels–Alder, aldol, and hetero Diels–Alder reactions efficiently with high enantioselectivities. A key to stabilizing the catalysts is an appropriate combination of chiral zirconium Lewis acids with molecular sieves, and the zirconium–molecular sieves-combined catalysts can be stored for extended periods in air at room temperature without loss of activity. Moreover, it has been demonstrated that the catalysts can be recovered and reused.
Editing the stereochemical elements in an iridium catalyst for enantioselective allylic amination
Authors: Leitner Andreas , Shu Chutian , Hartwig John F.
Published: April 2004

Individual diastereomeric phosphoramidites and mixtures of diastereomeric phosphoramidites were evaluated in the iridium-catalyzed amination of allylic carbonates. The original process was conducted with a phosphoramidite ligand containing a resolved 2,2-dihydroxy-1,1-binaphthyl (BINOL) group and a diastereomerically and enantiomerically pure bis(phenethyl)amino group. Evaluation of the structure of the active catalyst and relative rates for reactions in the presence of catalysts containing diastereomeric ligands led to the identification of a phosphoramidite that provided the amination product with enantiomeric excess similar to the original, more structurally and stereochemically complex ligand and that contains a racemic BINOLate and an
N
-benzylphenethylamino group on phosphorus.
Light exposure stimulates formation of A2E oxiranes in a mouse model of Stargardt's macular degeneration
Authors: Radu Roxana A. , Mata Nathan L. , Bagla Aarti , Travis Gabriel H.
Published: April 2004

Recessive Stargardt's macular degeneration is a blinding disease of children caused by mutations in the
ABCA4
(
ABCR
) gene. Mice with a knockout mutation in
abcr
accumulate toxic lipofuscin pigments in ocular tissues, similar to affected humans. The major fluorophore of lipofuscin is the
bis
-retinoid,
N
-retinylidene-
N
-retinylethanolamine (A2E). In the current study, we sought to define the effect of increasing light on A2E accumulation. We crossed the
abcr
–/–
mutation onto an albino background. The retinoid profiles in albino mice indicated higher retinal illuminance than in pigmented mice exposed to similar ambient light. Unexpectedly, A2E levels were not higher in the albino mice. Also, A2E levels in
abcr
–/–
mice reared under cyclic light at 30, 120, or 1,700 lux were similar. Thus, increased retinal illuminance was not correlated with higher A2E. A2E has been shown to undergo light-dependent oxidation to yield a series of A2E epoxides or oxiranes. These oxiranes react with DNA
in vitro
, suggesting a potential mechanism for A2E cytotoxicity. We analyzed ocular tissues from
abcr
–/–
mice for A2E oxiranes by mass spectrometry. Unlike A2E, the oxiranes were more abundant in albino vs. pigmented
abcr
–/–
mice, and in
abcr
–/–
mice exposed to increasing ambient light. These observations suggest that both the biosynthesis of A2E and its conversion to oxiranes are accelerated by light. Finally, we showed that the formation of A2E oxiranes is strongly suppressed by treating the
abcr
–/–
mice with Accutane (isotretinoin), an inhibitor of rhodopsin regeneration.
Membrane channel structure of
Helicobacter pylori
vacuolating toxin: Role of multiple GXXXG motifs in cylindrical channels
Authors: Kim Sanguk , Chamberlain Aaron K. , Bowie James U.
Published: April 2004

Helicobacter pylori
is a human pathogen responsible for severe gastric diseases such as peptic ulcers, gastric adenocarcinoma, and gastric lymphoma. Vacuolating toxin (VacA) is crucial in facilitating the colonization of the gastric lining by inducing cell apoptosis and immune suppression. VacA inserts into membranes and forms a hexameric, anion-selective pore. Here we present a structural model of the VacA pore that strongly resembles the structure of an unrelated anion-selective channel, MscS. In our model, Gly residues in GXXXG motifs pack against small Ala or Val side chains to generate the pore. Our model suggests that the same design of two anion-selective channels was achieved by two different evolutionary paths and provides insight into the mechanism of VacA function.
A Review of Pityriasis Rubra Pilaris and Rheumatologic Associations
Authors: Chan Happy , Liu Fu Tong , Naguwa Stanley
Published: April 2004

Pityriasis rubra pilaris (PRP) is a rare group of hyperkeratotic,
papulosquamous disease that can be acquired or inherited. There
have been reported cases of rheumatologic associations, mainly arthritis
and dermatomyositis. In this review article, we will explore the clinical
presentation and classification, rheumatologic associations and treatment
modalities of PRP. In addition, we will also report a case of PRP with seronegative arthritis.
Suppression of Leukotriene B4 Generation by Ex-vivo Neutrophils Isolated from Asthma Patients on Dietary Supplementation with Gammalinolenic Acid‐containing Borage Oil: Possible Implication in Asthma
Authors: Ziboh Vincent A. , Naguwa Stanley , Vang Kao , Wineinger Julie , Morrissey Brian M. , McIntyre J. A. , Watnik Mitchell , Gershwin M. Eric
Published: April 2004

Dietary gammalinolenic acid (GLA), a potent inhibitor of 5‐lipoxygenase (5‐LOX) and suppressor of
leukotriene B4 (LTB4), can attenuate the clinical course of rheumatoid arthritics, with negligible side
effects. Since Zileuton, also an inhibitor of 5‐LOX, attenuates asthma but with an undesirable side
effect, we investigated whether dietary GLA would suppress biosynthesis of PMN‐LTB4 isolated from
asthma patients and attenuate asthma. Twenty‐four mild‐moderate asthma patients (16–75 years) were
randomized to receive either 2.0 g daily GLA (borage oil) or corn oil (placebo) for 12 months. Blood
drawn at 3 months intervals was used to prepare sera for fatty acid analysis, PMNs for determining
phospholipid fatty acids and for LTB4 generation. Patients were monitored by daily asthma scores,
pulmonary function, and exhaled NO. Ingestion of daily GLA (i) increased DGLA (GLA metabolite) in
PMN‐phospholipids; (ii) increased generation of PMN‐15‐HETrE (5‐LOX metabolite of DGLA).
Increased PMN‐DGLA/15‐HETrE paralleled the decreased PMN generation of proinflammatory LTB4.
However, the suppression of PMN‐LTB4 did not reveal statistically significant suppression of the
asthma scores evaluated. Nonetheless, the study demonstrated dietary fatty acid modulation of
endogenous inflammatory mediators without side effects and thus warrant further explorations into the
roles of GLA at higher doses, leukotrienes and asthma.
Identification of Germinal Center B Cells in Blood from HIV‐infected Drug‐naive Individuals in Central Africa
Authors: Béniguel Lydie , Bégaud Evelyne , Cognasse Fabrice , Gabrié Philippe , Mbolidi Christophe D. , Sabido Odile , Marovich Mary A. , deFontaine Christiane , Frésard Anne , Lucht Frédéric , Genin Christian , Garraud Olivier
Published: April 2004

To better understand the pathophysiology of B cell populations—the precursors of antibody secreting
cells—during chronic human immunodeficiency virus (HIV) infection, we examined the phenotype of
circulating B cells in newly diagnosed Africans. We found that all African individuals displayed low
levels of naive B cells and of memory‐type CD27+ B cells, and high levels of differentiated B cells. On
the other hand, HIV‐infected African patients had a population of germinal center B cells (i.e. CD20+,
sIgM-, sIgD+, CD77+, CD138±), which are generally restricted to lymph nodes and do not circulate
unless the lymph node architecture is altered. The first observations could be linked to the tropical
environment whereas the presence of germinal center B cells may be attributable to chronic exposure to
HIV as it is not observed in HIV‐negative African controls and HAART treated HIV‐infected
Europeans. It may impact the management of HIV infection in countries with limited access to HIV
drugs and urges consideration for implementation of therapeutic vaccines.
Development of T Lymphocytes in the Nasal‐associated Lymphoid Tissue (NALT) from Growing Wistar Rats
Authors: Sosa Gustavo A. , Roux María E.
Published: April 2004

The aim of the present report was to study the development of
several T‐lymphocyte subsets in the nasal‐associated lymphoid
tissue (NALT) of growing Wistar rats. CD5+ and CD4+ lymphocytes
gradually increased with age. A predominance of CD8α+ over CD4+ T
cells was found from 7 to 45 days but from 45 to 60 days of age
T helper cells outnumbered the cytotoxic subpopulation. The majority
of CD8+ T lymphocytes expressed the heterodimeric isoform. The
most relevant findings by immunohistochemistry are: (1) the
predominance of TCRγδ+ and CD8α+ cells at 7 days postpartum over
all the other T‐cell subpopulations; and (2) that TCRγβ+ outnumbered
TCRαβ+ T cells from 7 to 45 days postpartum whereas αβ T cells
predominated in 45‐ and 60‐day‐old rats. Besides, cytometric studies
have shown that the percentages of TCRγ+, CD8+, as well as the
population coexpressing both phenotypes (TCRγδ+CD8α+), were
significantly higher in rats at 7 days postpartum when compared to
60 day‐old rats. In the present study, the finding of a high number
of γδ+ and CD8+ T cells early in NALT development may indicate the
importance of these subpopulations in the protection of the nasal
mucosa in suckling and weaning Wistar rats.
Neurohypophysial Receptor Gene Expression by Thymic T Cell Subsets and Thymic T Cell Lymphoma Cell Lines
Authors: Hansenne I. , Rasier G. , Charlet-renard Ch. , Defresne M.-P. , Greimers R. , Breton C. , Legros J.-J. , Geenen V. , Martens H.
Published: April 2004

Abstract
Neurohypophysial oxytocin (OT) and vasopressin (VP) genes are
transcribed in thymic epithelium, while immature T lymphocytes express
functional neurohypophysial receptors. Neurohypophysial receptors belong
to the G protein‐linked seven‐transmembrane receptor superfamily and are encoded
by four distinct genes, OTR, V1R, V2R
and V3R. The objective of this study was to
identify the nature of neurohypophysial receptor in thymic T cell subsets purified by
immunomagnetic selection, as well as in murine thymic lymphoma cell lines RL12‐NP
and BW5147. OTR is transcribed in all thymic T cell subsets and T cell lines, while
V3R transcription is restricted to CD4+ CD8+ and CD8+ thymic cells. Neither V1R nor
V2R transcripts are detected in any kind of T cells. The OTR protein was identified by
immunocytochemistry on thymocytes freshly isolated from C57BL/6 mice. In murine fetal
thymic organ cultures, a specific OTR antagonist does not modify the percentage of T
cell subsets, but increases late T cell apoptosis further evidencing the involvement of
OT/OTR signaling in the control of T cell proliferation and survival. According to these
data, OTR and V3R are differentially expressed during T cell ontogeny. Moreover, the
restriction of OTR transcription to T cell lines derived from thymic lymphomas may be
important in the context of T cell leukemia pathogenesis and treatment.
Autoantibodies Profile in the Sera of Patients with Sjogren]s Syndrome: The ANA Evaluation—A Homogeneous, Multiplexed System
Authors: Gilburd Boris , Abu-Shakra Mahmoud , Shoenfeld Yehuda , Giordano Andrea , Bocci Elena Bartoloni , delle Monache Francesco , Gerli Roberto
Published: April 2004

Background: Flow‐based, multiplex bead arrays (MBA)
have been
developed for a variety of applications including the detection of antibodies
to extractable nuclear antigens (ENA). It offers a rapid and sensitive
method to assess multiple analyses in a single tube/well.

Purpose: To evaluate the Athena Multi‐Lyte ANA Test
System utilizes
Luminex Corporation′s MBA technology for the detection of antinuclear
antibodies (ANA) and
ENA antibodies in the sera of patients with Sjogren′s syndrome (SS).

Methods: MBA assay was used to detect ANA and
ENA antibodies in the
sera of 37 patients with SS and 96 sera from healthy subjects.

Results: All patients were women. Their mean age was 48.7 years
and the mean disease duration was 7.27 years. ANA was found in 3 (3%)
sera of healthy subjects by the AtheNA system and in 2 (2%) sera by the
ELISA kit. A 99% concordance between the 2 assays was found. A 94.6%
concordance between
the 2 assays was found by testing the sera of patients with SS for ANA.

By the AtheNA system, none of the sera of 37
patients with SS had autoantibodies reacting with Sm, Jo‐1, dsDNA
or histones. Anti‐RNP antibody was found in 5.4% of the sera and 2.7%
of the sera reacted with Scl‐70 and histones. Anti‐SS/A and anti‐SS/B were
identified in 84 and 76% of the sera, respectively.

Conclusion: The AtheNa Multi‐Lyte ANA Test System offers
a sensitive and specific result for the detection of ANA and
ENA antibodies in the sera of patients with SS.
Identification of the Molecular Mechanism by which TLR Ligation and IFN‐γ Synergize to Induce Mig
Authors: Powell Jonathan D. , Boodoo Sada , Horton Maureen R.
Published: April 2004

Monokine Induced by Interferon‐ (MIG), a CXC chemokine, is a potent
inducer of T‐cell chemotaxis and activation and has been implicated in
the host response to viral infections and tumor immunity as well as in
the pathogenesis of autoimmunity and transplant rejection. Although it is known
that the Toll‐Like Receptor‐4 (TLR‐4) ligand LPS synergizes with IFN‐γ to
induce MIG expression in macrophages, the molecular mechanisms responsible for
the synergy have yet to be elucidated. We determined that the marked synergy
between LPS and IFN‐γ on MIG mRNA expression in mouse macrophages is
a result of LPS‐induced NF‐κB and IFN‐γ‐induced STAT. The synergy was not
dependent on new protein synthesis, was independent of TNF‐α, and occurred
at the level of gene transcription. We identified 2 NF‐κB sites located at ‐154
and ‐129 of the MIG promoter proximal to the ‐responsive element that mediated
this effect. Finally, we demonstrated that other TLR ligands (zymosan, double
stranded RNA and CpG) synergized with IFN‐γ to induce MIG in an NF‐κB
dependent fashion. These data emphasize the ability of bacterial and viral
products to activate/modify immune responses and promote
adaptive T cell immunity through the NF‐κB pathway.
Vertebrate development requires ARVCF and p120 catenins and their interplay with RhoA and Rac
Authors: Fang Xiang , Ji Hong , Kim Si-Wan , Park Jae-Il , Vaught Travis G. , Anastasiadis Panos Z. , Ciesiolka Malgorzata , McCrea Pierre D.
Published: April 2004

Using an animal model system and depletion-rescue strategies, we have addressed the requirement and functions of armadillo repeat gene deleted in velo-cardio-facial syndrome (ARVCF) and p120 catenins in early vertebrate embryogenesis. We find that xARVCF and Xp120 are essential to development given that depletion of either results in disrupted gastrulation and axial elongation, which are specific phenotypes based on self-rescue analysis and further criteria. Exogenous xARVCF or Xp120 cross-rescued depletion of the other, and each depletion was additionally rescued with (carefully titrated) dominant-negative RhoA or dominant-active Rac. Although xARVCF or Xp120 depletion did not appear to reduce the adhesive function of C-cadherin in standard cell reaggregation and additional assays, C-cadherin levels were somewhat reduced after xARVCF or Xp120 depletion, and rescue analysis using partial or full-length C-cadherin constructs suggested contributory effects on altered adhesion and signaling functions. This work indicates the required functions of both p120 and ARVCF in vertebrate embryogenesis and their shared functional interplay with RhoA, Rac, and cadherin in a developmental context.
Cytoplasmic foci are sites of mRNA decay in human cells
Authors: Cougot Nicolas , Babajko Sylvie , Séraphin Bertrand
Published: April 2004

Understanding gene expression control requires defining the molecular and cellular basis of mRNA turnover. We have previously shown that the human decapping factors hDcp2 and hDcp1a are concentrated in specific cytoplasmic structures. Here, we show that hCcr4, hDcp1b, hLsm, and rck/p54 proteins related to 5′–3′ mRNA decay also localize to these structures, whereas DcpS, which is involved in cap nucleotide catabolism, is nuclear. Functional analysis using fluorescence resonance energy transfer revealed that hDcp1a and hDcp2 interact in vivo in these structures that were shown to differ from the previously described stress granules. Our data indicate that these new structures are dynamic, as they disappear when mRNA breakdown is abolished by treatment with inhibitors. Accumulation of poly(A)+ RNA in these structures, after RNAi-mediated inactivation of the Xrn1 exonuclease, demonstrates that they represent active mRNA decay sites. The occurrence of 5′–3′ mRNA decay in specific subcellular locations in human cells suggests that the cytoplasm of eukaryotic cells may be more organized than previously anticipated.
Ca2+cyt negatively regulates the initiation of oocyte maturation
Authors: Sun Lu , Machaca Khaled
Published: April 2004

Ca2+ is a ubiquitous intracellular messenger that is important for cell cycle progression. Genetic and biochemical evidence support a role for Ca2+ in mitosis. In contrast, there has been a long-standing debate as to whether Ca2+ signals are required for oocyte meiosis. Here, we show that cytoplasmic Ca2+ (Ca2+cyt) plays a dual role during Xenopus oocyte maturation. Ca2+ signals are dispensable for meiosis entry (germinal vesicle breakdown and chromosome condensation), but are required for the completion of meiosis I. Interestingly, in the absence of Ca2+cyt signals oocytes enter meiosis more rapidly due to faster activation of the MAPK-maturation promoting factor (MPF) kinase cascade. This Ca2+-dependent negative regulation of the cell cycle machinery (MAPK-MPF cascade) is due to Ca2+cyt acting downstream of protein kinase A but upstream of Mos (a MAPK kinase kinase). Therefore, high Ca2+cyt delays meiosis entry by negatively regulating the initiation of the MAPK-MPF cascade. These results show that Ca2+ modulates both the cell cycle machinery and nuclear maturation during meiosis.