bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystallographically oriented Fe nanocrystals formed in Fe-implanted TiO2
Authors: Zhou Shengqiang , Talut G. , Potzger K. , Shalimov A. , Grenzer J. , Skorupa W. , Helm M. , Fassbender J. , Čižmár E. , Zvyagin S. A. , Wosnitza J.
Published: April 2008

A comprehensive characterization of the structural and magnetic properties of Fe-implanted rutile TiO2(110) is presented. Fe and FeTiO3 (ilmenite) nanocrystals (NCs) are identified by synchrotron-radiation x-ray diffraction. The majority of Fe NCs are crystallographically oriented with respect to the matrix following the relation Fe(001)[010]∥TiO2(110)[11̱0]. Postannealing induced the out-diffusion of Fe and the growth of FeTiO3 at the cost of Fe NCs. Mössbauer spectroscopy and superconducting quantum interference device (SQUID) magnetometry reveal the corresponding evolution of magnetic properties, i.e., magnetization, and superparamagnetic blocking temperature. We unambiguously identify Fe NCs as the origin of the ferromagnetism. These Fe NCs possess a uniaxial in-plane magnetic anisotropy, such that the two Fe[100] axes are inequivalent.
High-spin alkali trimers on helium nanodroplets: Spectral separation and analysis
Authors: Nagl Johann , Auböck Gerald , Hauser Andreas W. , Allard Olivier , Callegari Carlo , Ernst Wolfgang E.
Published: April 2008

Electronic excitation spectra of homo- (K3,Rb3) and heteronuclear (K2Rb,KRb2) alkali trimers in the high-spin quartet state have been investigated in a broad spectral range (10600–17400cm−1). Ten new bands showing laser induced fluorescence (LIF) were measured. Due to the pickup statistics, overlapping spectra of all possible oligomers are present at once, complicating the unraveling and assignment of individual spectra. To circumvent the problem, two variations of beam depletion spectroscopy were employed in addition to the conventional analysis of the relation between signal and pickup pressure: A two-laser V-type double resonance scheme combining beam depletion with LIF, and a mass selective beam depletion scheme. In principle, these allow accurate separation of an arbitrary number of overlapping spectra. The benefits and drawbacks of each method are discussed. Assignment to electronic states is achieved by comparison with ab initio complete active space self-consistent field calculations of the excited electronic level structure of the molecules.
Temperature ratio dependence of ion temperature gradient and trapped electron mode instability thresholds
Authors: Casati Alessandro , Bourdelle C. , Garbet X. , Imbeaux F.
Published: April 2008

The impact of ion to electron temperature ratio Ti∕Te on the instability thresholds of ion temperature gradient (ITG) and trapped electron modes (TEMs) is both analytically and numerically addressed. An analytical kinetic approach is developed accounting for the ion (electron, respectively) magnetic drift resonance and for the electron (ion, respectively) nonresonant response. This leads to a significant temperature ratio dependence of both ITG and TEM instability thresholds. Moreover nonresonant effects are able to reverse the scaling of ITG and TEM thresholds with Ti∕Te beyond the flat density limit. Analytical predictions are successfully tested against linear gyrokinetic thresholds and growth rates using the code KINEZERO [C. Bourdelle et al., Nucl. Fusion 42, 892 (2002)].
Spectroscopy and efficient laser operation near 1.95μm of Tm3+ in disordered NaLu(WO4)2
Authors: Han Xiumei , Cano-Torres José M. , Rico Mauricio , Cascales Concepción , Zaldo Carlos , Mateos Xavier , Rivier Simon , Griebner Uwe , Petrov Valentin
Published: April 2008

Continuous wave and tunable laser operation at room temperature is demonstrated with single crystals of Tm-doped NaLu(WO4)2. Under Ti:sapphire laser pumping and tuning by an intracavity Lyot filter, the Tm3+ emission extended from 1799to2026nm. The maximum output power exceeded 0.5W and the maximum slope efficiency with respect to the absorbed power was above 58%. These good laser properties are related to the locally disordered tetragonal (I4¯) structure of the crystalline host. Using low temperature spectroscopy and energy level simulation methods, the energy levels and spectroscopic properties of Tm3+ have been shown to be compatible with the S4 site symmetry. This allowed the reliable determination of the emission cross sections related to the laser transitions.
Terahertz harmonic generation using a planar nanoscale unipolar diode at zero bias
Authors: Xu K. Y. , Lu X. F. , Song A. M. , Wang G.
Published: April 2008

The feasibility of harmonic generation in a planar nanoscale unipolar diode, called a self-switching device, is analyzed using a two-dimensional ensemble Monte Carlo method. A suitable channel width is chosen to enable a zero threshold voltage. The current-voltage and capacitance-voltage characteristics both demonstrate strong nonlinearity. It is shown that the noise of the device is below the shot noise level until about 1THz, after which two noise peaks arise. As such, a good signal-to-noise ratio can be achieved for up to six harmonics and beyond 1THz. The physical origins of the noise peaks are discussed.
Haloing, flocculation, and bridging in colloid-nanoparticle suspensions
Authors: Scheer Everett N. , Schweizer Kenneth S.
Published: April 2008

Integral equation theory with a hybrid closure approximation is employed to study the equilibrium structure of highly size asymmetric mixtures of spherical colloids and nanoparticles. Nonequilibrium contact aggregation and bridging gel formation is also qualitatively discussed. The effect of size asymmetry, nanoparticle volume fraction and charge, and the spatial range, strength, and functional form of colloid-nanoparticle and colloid-colloid attractions in determining the potential-of-mean force (PMF) between the large spheres is systematically explored. For hard, neutral particles with weak colloid-nanoparticle attraction qualitatively distinct forms of the PMF are predicted: (i) a contact depletion attraction, (ii) a repulsive form associated with thermodynamically stable “nanoparticle haloing,” and (iii) repulsive at contact but with a strong and tight bridging minimum. As the interfacial cohesion strengthens and becomes shorter range the PMF acquires a deep and tight bridging minimum. At sufficiently high nanoparticle volume fractions, a repulsive barrier then emerges which can provide kinetic stabilization. The charging of nanoparticles can greatly reduce the volume fractions where significant changes of the PMF occur. For direct and interfacial van der Waals attractions, the large qualitative consequences of changing the absolute magnitude of nanoparticle and colloid diameters at fixed size asymmetry ratio are also studied. The theoretical results are compared with recent experimental and simulation studies. Calculations of the real and Fourier space mixture structure at nonzero colloid volume fractions reveal complex spatial reorganization of the nanoparticles due to many body correlations.
Effects of surface treatments using O3 and NH3 on electrical properties and chemical structures of high-k HfO2 dielectric films on strained Si1−xGex∕Si substrates
Authors: Park Tae Joo , Kim Jeong Hwan , Jang Jae Hyuck , Na Kwang Duk , Hwang Cheol Seong , Won Jeong Yeon
Published: April 2008

This study examined the chemical interactions between an atomic-layer-deposited (ALD) HfO2 film and strained epitaxial Si1−xGex∕Si (x=0.1,0.2,0.3) substrates during film deposition and postdeposition annealing (PDA), as well as their influence on the electrical properties. The influences of various predeposition surface treatments under O3 (POT) and NH3 (PNT) were also investigated in order to improve the understanding of the chemical and structural changes in thin film systems. Particular focus was made on the migration behavior of Si and Ge in the interface between a HfO2 film and Si1−xGex substrate, which critically affects the capacitance equivalent thickness (CET). The interdiffusion of the substrate elements during ALD and PDA could be controlled by POT and PNT. PNT was effective in reducing the diffusion of both Si and Ge during the ALD and PDA, whereas POT reduced only Si diffusion. The surface treatments reduced the changes in the CET after PDA without any significant increase in the leakage current density. The migration of the substrate elements was traced by secondary ion mass spectroscopy, auger electron spectroscopy, high-resolution transmission microscopy and x-ray photoelectron spectroscopy.
Ultrasensitive proximity Josephson sensor with kinetic inductance readout
Authors: Giazotto Francesco , Heikkilä Tero T. , Pepe Giovanni Piero , Helistö Panu , Luukanen Arttu , Pekola Jukka P.
Published: April 2008

We propose a mesoscopic kinetic-inductance radiation detector based on a long superconductor-normal metal-superconductor Josephson junction. The operation of this proximity Josephson sensor relies on large kinetic inductance variations under irradiation due to the exponential temperature dependence of the critical current. Coupled with a dc superconducting quantum interference device readout, the PJS is able to provide a signal to noise (S/N) ratio up to ∼103 in the terahertz regime if operated as calorimeter, while electrical noise equivalent power as low as ∼7×10−20W∕Hz at 200mK can be achieved in the bolometer operation. The high performance together with the ease of fabrication make this structure attractive as an ultrasensitive cryogenic detector of terahertz electromagnetic radiation.
Concentrating colloids with electric field gradients. II. Phase transitions and crystal buckling of long-ranged repulsive charged spheres in an electric bottle
Authors: Leunissen Mirjam E. , van Blaaderen Alfons
Published: April 2008

We explored the usefulness of electric field gradients for the manipulation of the particle concentration in suspensions of charged colloids, which have long-ranged repulsive interactions. In particular, we studied the compression obtained by “negative” dielectrophoresis, which drives the particles to the regions of lowest field strength, thus preventing unwanted structural changes by induced dipole-dipole interactions. We used several sample cell layouts and suspension compositions, with a different range of the interparticle repulsions. For these systems, we obtained sufficient compression to observe a transition from the initial fluid phase to a random hexagonal close-packed crystal, as well as a body-centered cubic crystal. The heterogeneous dielectrophoretic crystallization mechanism involved an intriguing “pluglike” motion of the crystal, similar to what we have previously reported for hard-sphere suspensions. In this way, remarkably large single crystals were formed of several millimeters wide and a couple of centimeters long. Moreover, we found that these crystals could be compressed to such an extent that it led to an anisotropic deformation (“buckling”) and, upon subsequent relaxation, a reorientation of the lattice, while stacking errors disappeared. These striking differences with the compressed hard-sphere crystals that we studied before [M. E. Leunissen et al., J. Chem. Phys. 128, 164508 (2008).] are likely due to the smaller elastic moduli of the present lower-density soft-sphere crystals.
Resolution degradation caused by multispur effect in chemically amplified extreme ultraviolet resists
Authors: Kozawa Takahiro , Tagawa Seiichi , Shell Melissa
Published: April 2008

Extreme ultraviolet (EUV) (92.5 eV) radiation is expected as a next-generation exposure source. When EUV photons enter resists, they are absorbed by resist molecules and photoelectrons are emitted. Photoelectrons with the initial energy of approximately 80 eV further induce ionization and generate ion pairs. Because the inelastic mean free path of these electrons is less than 1 nm, ion pairs are so narrowly distributed that they interact with each other through Coulomb forces. In this study, the details in the interaction among ion pairs were investigated by using a Monte Carlo simulation. It was found that the electric interaction between ion pairs significantly degrade the initial acid distribution. The dependence of resolution degradation and multispur effect on acid generator concentration was clarified. The acid generator loading of >10 wt % is effective for the suppression of resolution degradation.
Exciton dynamics in current-injected single quantum dot at 1.55μm
Authors: Miyazawa Toshiyuki , Nakaoka Toshihiro , Usuki Tatsuya , Arakawa Yasuhiko , Takemoto Kazuya , Hirose Shinnichi , Okumura Shigekazu , Takatsu Motomu , Yokoyama Naoki
Published: April 2008

We investigate the exciton dynamics in a current-injected single InAs quantum dot (QD) which emits 1.55μm photons. Photon antibunching behavior is observed using a single electroluminescence line of a single QD. The radiative lifetime of this line determined by time-resolved measurement is 1.59ns. The single exciton recombination time agrees with the lifetime calculated with an eight-band kp model. We examine a high drive rate operation of the device by changing the delay time between two electrical pulses. These results demonstrate that our device has the potential to achieve telecommunication band subgigahertz single-photon emission with electrical pulses.
Quantitative force mapping of an optical vortex trap
Authors: Zhao Yiqiong , Milne Graham , Edgar J. Scott , Jeffries Gavin D. M. , McGloin David , Chiu Daniel T.
Published: April 2008

This paper describes the quantitative force mapping of micron-sized particles held in an optical vortex trap. We present a simple and efficient model, which accounts for the diffraction of the strongly localized optical field of the tightly focused laser beam, the spherical aberration introduced by the dielectric glass-to-water interface, employs the multidipole approximation for force calculations, and is able to reproduce, with quantitative agreement, the experimentally measured force map.
Dirac–Sobolev inequalities and estimates for the zero modes of massless Dirac operators
Authors: Balinsky A. , Evans W. D. , Saito Y.
Published: April 2008

The paper analyzes the decay of any zero modes that might exist for a massless Dirac operator H≔α⋅(1∕i)∇+Q, where Q is 4×4 matrix valued and of order O(∣x∣−1) at infinity. The approach is based on inversion with respect to the unit sphere in R3 and establishing embedding theorems for Dirac–Sobolev spaces of spinors f which are such that f and Hf lie in (Lp(R3))4, 1⩽p<∞.
Tunneling characteristics across nanoscale metal ferric junction lines into doped Si
Authors: Wang Keqiang , Stehlik Jiri , Wang Jian-Qing
Published: April 2008

Tunneling properties were studied on nanofabricated metal ferric tunnel structures on phosphorus doped silicon by measuring I-V characteristics and differential conductance versus bias over a wide temperature range. These properties were found to have weak temperature dependences up to 250K, unambiguously showing tunneling characteristics from metal ferric nanolines into Si via AlOx insulating barrier. Direct correlation was observed between thermal activation of deep levels in bulk Si and metal-semiconductor tunneling, revealing feasible mechanisms influencing the interfacial transport.
Reaction of barium oxide threshold voltage tuning layers with SiO2 and HfO2∕SiO2 gate dielectrics
Authors: Copel M.
Published: April 2008

We have investigated the reactions of ultrathin BaO capping layers with SiO2 and HfO2∕SiO2 dielectrics using medium energy ion scattering. BaO readily forms a silicate at high temperatures, intermixing with SiO2. Unlike other silicate-forming systems, Ba diffuses throughout the volume of available SiO2, creating a dilute metal oxide. However, when deposited on a HfO2∕SiO2 layer, a Ba silicate layer nucleates at the HfO2∕SiO2 interface, leaving an SiO2-like buffer layer. The reaction with SiO2 is markedly different from other silicate-forming metal oxides, where nucleation of distinct phases is observed.
Effect of pressure on the properties of phosphorus-doped p-type ZnO thin films grown by radio frequency-magnetron sputtering
Authors: Hwang Dae-Kue , Oh Min-Suk , Choi Yong-Seok , Park Seong-Ju
Published: April 2008

We report the effect of pressure on the surface morphology, electrical and optical properties of phosphorus (P)-doped p-type ZnO grown by radio frequency magnetron sputtering. The nanorod structures of P-doped ZnO films became dense and flat with decreasing pressure. The Hall effect measurement of the films grown at a pressure of 5–20mTorr showed an n-type conductivity after rapid thermal annealing. However, the films grown at a low pressure of 1mTorr showed a p-type conductivity with a hole concentration of 4.71×1818∕cm3. This result showed that the pressure of rf-magnetron sputtering plays a critical role in the growth of P-doped p-type ZnO.
High-k gate stack on germanium substrate with fluorine incorporation
Authors: Xie Ruilong , Yu Mingbin , Lai Mei Ying , Chan Lap , Zhu Chunxiang
Published: April 2008

In this letter, a postgate CF4-plasma treatment is proposed and demonstrated on germanium (Ge) metal-oxide-semiconductor capacitors and the effects of fluorine (F) incorporation have been studied on both high-k∕Ge gate stacks without any surface passivation and with Si surface passivation. Our results show that F is effectively introduced into the gate stack by CF4 treatment and segregates near high-k∕Ge interface. Electrical characteristics such as frequency dispersion, interface state density (Dit), and gate leakage are improved after F incorporation. Interface quality of high-k∕Ge gate stack is further improved by combining Si surface passivation and postgate CF4 treatment, with its Dit as low as 4.85×1011cm−2eV−1.
Electrical control of spin coherence in ZnO
Authors: Ghosh S. , Steuerman D. W. , Maertz B. , Ohtani K. , Xu Huaizhe , Ohno H. , Awschalom D. D.
Published: April 2008

Electric field enhanced electron spin coherence is characterized using time-resolved Faraday rotation spectroscopy in n-type ZnO epilayers grown by molecular beam epitaxy. An in-plane dc electric field E almost doubles the transverse spin lifetime at 20K without affecting the effective g factor. This effect persists until high temperatures, but decreases with increasing carrier concentration. Comparisons of the variations in the spin lifetime, the carrier recombination lifetime, and photoluminescence lifetimes indicate that the applied E enhances the radiative recombination rate. All observed effects are independent of crystal directionality and are performed at low magnetic fields (B<0.2T).
Understanding nanoscale damage at a crack tip of multilayered metallic composites
Authors: Zhu X. F. , Li Y. P. , Zhang G. P. , Tan J. , Liu Y.
Published: April 2008

Through quantitative focused ion beam cross-sectional characterization technique, we directly observed shear displacement in a range from several nanometers to a few tens of nanometers occurring in the plastic deformation zone ahead of a crack tip in nanolayered Cu∕Ta composite subjected to tensile load. As a result, shear-mode fracture of the Cu∕Ta laminate composite was eventually caused. A theoretical analysis of the interface barrier strength of nanolayered metallic composites here and reported in literature finds a critical individual layer thickness below which the nature of fracture of the nanolayered composites tends to be shearing failure.
Magnetostructural transformation in Ni–Mn–In–Co ribbons
Authors: Liu J. , Scheerbaum N. , Hinz D. , Gutfleisch O.
Published: April 2008

A series of Ni50−xMn37In13Cox (x=0–9) ribbons was synthesized using the melt-spinning technique. Different ordered structures (5M, 7M, B2, and L21) were formed depending on composition. Both martensitic transformation temperature and entropy change increase with increasing Co for 0⩽x⩽3, while they decrease with further increasing to 3<x⩽9. Increasing Co increases the Curie temperature of the austenite but decreases that of the martensite. An optimized composition range of 4.5⩽x⩽5.5 is obtained where a magnetic-field-induced reverse transformation from nonmagnetic martensite to ferromagnetic austenite is realized.
Cross-sectional scanning photoelectron microscopy and spectroscopy of wurtzite InN∕GaN heterojunction: Measurement of “intrinsic” band lineup
Authors: Wu Chung-Lin , Lee Hong-Mao , Kuo Cheng-Tai , Chen Chia-Hao , Gwo Shangjr
Published: April 2008

A method for studying heterojunction band lineups on the submicrometer scale is demonstrated by using synchrotron-radiation photoelectron microscopy and spectroscopy. In particular, an in situ sample cleavage technique is adopted here to reveal the cross-sectional, nonpolar a-plane face of InN∕GaN heterojunction grown on Si(111) along the polar −c axis with fully relaxed lattice structure, eliminating the polarization effects associated with the interface charge/dipole normal to the cleaved surface. The “intrinsic” valence band offset at the cleaved InN∕GaN heterojunction has been determined to be 0.78eV. Additionally, using known material parameters, the values of InN∕GaN conduction band offset and InN electron affinity are also estimated.
Molecular-scale structural distortion near vacancies in pentacene
Authors: Seo S. , Grabow L. C. , Mavrikakis M. , Hamers R. J. , Thompson N. J. , Evans P. G.
Published: April 2008

Molecular vacancies form in both of the crystallographic basis sites of thin pentacene crystals. Features in scanning tunneling microscopy images of these crystals correspond to the exposed terminal atoms of molecules. The (001) and (001¯) surfaces of pentacene are distinguishable, which allows for the identification of the absolute orientation of crystals and for the unambiguous assignment of the position of molecules relative to each vacancy. For vacancies in each molecular basis site of the pentacene (001) surface, the image feature associated with one molecular nearest neighbor is displaced by significantly more than other molecules.
Static optical sorting in a laser interference field
Authors: Jákl Petr , Čižmár Tomáš , Šerý Mojmír , Zemánek Pavel
Published: April 2008

We present a unique technique for optical sorting of heterogeneous suspensions of microparticles, which does not require the flow of the immersion medium. The method employs the size-dependent response of suspended dielectric particles to the optical field of three intersecting beams that form a fringelike interference pattern. We experimentally demonstrate sorting of a polydisperse suspension of polystyrene beads of diameters 1, 2, and 5.2μm and living yeast cells.
Two-photon excited luminescence spectral distribution observation in wide-gap semiconductor crystals
Authors: Noor A. S. M. , Miyakawa A. , Kawata Y. , Torizawa M.
Published: April 2008

The use of photoluminescence excited with two-photon process for characterizing the defect and impurity level in wide-gap semiconductor is discussed in this paper. Defects of polycrystalline zinc selenide (ZnSe) is observed deep inside the crystal. Two types of defects can be detected based on the spectral luminescence image. One type of defect can be seen in the entire spectrum images. Meanwhile, other types of defects can only be observed at higher energy of the spectrum, from 460to465nm. This study represents works of identifying crystals defect in wide gap materials by two-photon luminescence technique.
Ultrafast metal-insulator varistors based on tunable Al2O3 tunnel junctions
Authors: Weimer Michael A. , Hakim Luis F. , King David M. , Liang Xinhua , Weimer Alan W. , George Steven M. , Li Peng , Groner Markus D.
Published: April 2008

Ultrafast metal-insulator varistors have been fabricated using atomic layer deposition. A high-density matrix of micron-sized spherical Ni particles conformally coated with ∼7.5–22nm Al2O3 films exhibited transient response times (∼0.3ns), capacitances (∼45pF), leakage currents (∼33pA), and nonlinearities (α∼380) which were all markedly improved over conventional metal oxide varistors. These characteristics result from the Fowler–Nordheim tunneling of electrons through uniform Al2O3 tunnel junctions separating adjacent particles within the matrix.
Hole confinement in quantum islands in Ga(AsSb)∕GaAs∕(AlGa)As heterostructures
Authors: Horst S. , Chatterjee S. , Hantke K. , Klar P. J. , Nemeth I. , Stolz W. , Volz K. , Bückers C. , Thränhardt A. , Koch S. W. , Rühle W. , Johnson S. R. , Wang J.-B. , Zhang Y.-H.
Published: April 2008

Formation of self-organized Ga(AsSb) quantum islands during growth is shown to occur in a series of Ga(AsSb)∕GaAs∕(AlGa)As heterostructures, resulting in an in-plane hole confinement of several hundred meV. The shape of the in-plane confinement potential is nearly parabolic and, thus, yields almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum islands are 100nm in diameter and exhibit an in-plane variation of the Sb concentration of more than 30%. Up to seven bound hole states are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation density, lattice temperature, and excitation photon energy and reveal fast carrier capture into, and relaxation within, the quantum islands. The advantages of such structures as active laser material are discussed.
On the dc and noise properties of the gate current in epitaxial Ge p-channel metal oxide semiconductor field effect transistors with TiN∕TaN∕HfO2∕SiO2 gate stack
Authors: Maji Debabrata , Crupi Felice , Giusi Gino , Pace Calogero , Simoen Eddy , Claeys Cor , Rao V. Ramgopal
Published: April 2008

In this paper, we report the dc and noise properties of the gate current in epitaxial Ge p-channel metal oxide field effect transistors (pMOSFETs) with a Si passivated surface. The gate stack consists of HfO2∕SiO2 dielectric with TiN∕TaN metal gate. The observed temperature dependence of the gate current indicates that the dominant charge transport mechanism through the gate dielectric consists of Poole–Frenkel conduction. Gate current 1∕f noise is more than two orders higher in the case of Ge pMOSFETs when compared to reference Si pMOSFETs. Ge outdiffusion into the gate oxide is the suspected cause for the enhanced Poole–Frenkel conduction and the high gate current 1∕f noise in Ge pMOSFETs.
Synthesis, Characterization, and Antibacterial and Cytotoxic Study of Metal Complexes with Schiff Base Ligands
Authors: Shi Lei , Fang Rui-Qin , Xue Jia-Yu , Xiao Zhu-Ping , Tan Shu-Hua , Zhu Hai-Liang
Published: April 2008

The synthesis of 16 metal complexes from four Schiff bases prepared from 5-chloro-2-hydroxybenzaldehyde and primary amines has been described. The synthesized Schiff base ligands and their complexes were characterized by elemental analyses, spectroscopic (UV, IR, 1H and 13C NMR, electrospray ionization-mass spectrometry) methods, and magnetic and conductance measurements. Furthermore, complexes 1a, 1b, 3d, 4a, and 4d were characterized by X-ray diffraction analysis. After the structural characterization, all the compounds were tested in vitro for their antibacterial (Bacillus subtilis, Escherichia coli, Pseudomonas fluorescens, and Staphylococcus aureus) activities. The cytotoxic activities of the synthesized compounds were evaluated in vitro against human chronic myeloid leukaemia cells (K562) and a human nasopharyngeal epidermoid tumour cell line. The results indicated that most of the complexes showed good cytotoxic activity against human cancer cell lines but weak cytotoxic activity against a human normal cell line (L02). Among the compounds tested, the cobalt complexes 1a, 2a, 3a, and 4a showed the most favourable antibacterial and cytotoxic activities.
Porphyrazines: Designer Macrocycles by Peripheral Substituent Change
Authors: Fuchter Matthew J. , Zhong Chang , Zong Hong , Hoffman Brian M. , Barrett Anthony G. M.
Published: April 2008

It is rare that such a diverse array of applications can be realized from a single basic molecular unit, however, such is the power of the tetrapyrrolic macrocycle motif. Indeed, their potential in areas such as chemical dyes, optical sensors, optoelectronics, and biomedical agents is a function of their rich electronic and optical properties. While the naturally occurring porphyrins and the synthetic phthalocyanines have been extensively studied, the related tetraazaporphyrins or porphyrazines remain comparatively underdeveloped. Since porphyrazines maintain a unique position in this family: analogous derivatives are virtually inaccessible for the porphyrins, and direct fusion of heteroatomic substituents onto the porphyrazine β-positions results in a more pronounced effect compared with the substitution of an equivalent group onto the benzenoid rings of the phthalocyanine; a driving force exists to further explore the synthesis and applications of these novel macrocycles. This review will provide a historical overview of the synthetic strategies towards functionalized porphyrazines and describe new strategies towards the preparation and applications of heteroatom-appended porphyrazines, particularly in the context of their multimetallic complexes, catalysis, surface chemistry, and as biomedical agents.
Synthesis of Covalently-Linked Phthalocyanine–Phthalocyanine and Porphyrin–Phthalocyanine Dimers
Authors: Lyubimtsev Alexey , Iqbal Zafar , Hanack Michael
Published: April 2008

The synthesis of the dimeric homonuclear phthalocyanines (Pc) 4a, 4b, 5a and 5b and the dimeric heteronuclear Pc 4c and 5c is reported. The reactions are carried out by coupling the monomeric Pc 1a and 1b each containing a phenolic OH group in one of its substituents with the bromoalkyl-substituted Pc 2a,b or 3a,b respectively. From the spectral data of 4a–c and 5a–c, it can be concluded that in dichloromethane and toluene as solvents, these binuclear Pc are equilibrating between cofacial and open conformations with the open form predominating. Changing the solvent polarity, e.g. by adding methanol, the equilibrium is shifted towards a cofacial conformation. Treatment of the Pc 2a,b and 3a,b with 5-(4′-oxyphenyl)-10,15,20-triphenylporphin (6) in a basic solution gives the porphyrins (P)-Pc dimers 7a,b and 8a,b respectively. In dichloromethane, the open conformation is preferred for the P-Pc dimers.
Flavin Core as Electron Acceptor Component in a Zinc(II)-Phthalocyanine-Based Dyad
Authors: Gouloumis Andreas , Rahman G. M. Aminur , Abel Julia , de la Torre Gema , Vázquez Purificación , Echegoyen Luis , Guldi Dirk M. , Torres Tomas
Published: April 2008

A zinc(ii)-phthalocyanine-flavin dyad has been synthesized by Heck-type cross-coupling between a flavin that bears a p-iodophenyl group and a phthalocyanine functionalized with a vinyl moiety. Electrochemical experiments reveal that no significant interaction occurs at the ground state between the two electroactive subunits. However, the occurrence of a photoinduced electron transfer in this donor–acceptor conjugate is observed in transient absorption experiments. Charge-separation (i.e., 4.0 × 1011 s–1) and charge-recombination dynamics in benzonitrile (2.2 × 1010 s–1) reveal a remarkable stabilization of the radical ion pair in this solvent.
Drosophila
germ-line modulation of insulin signaling and lifespan
Authors: Flatt Thomas , Min Kyung-Jin , D'Alterio Cecilia , Villa-Cuesta Eugenia , Cumbers John , Lehmann Ruth , Jones D. Leanne , Tatar Marc
Published: April 2008

Ablation of germ-line precursor cells in
Caenorhabditis elegans
extends lifespan by activating DAF-16, a forkhead transcription factor (FOXO) repressed by insulin/insulin-like growth factor (IGF) signaling (IIS). Signals from the gonad might thus regulate whole-organism aging by modulating IIS. To date, the details of this systemic regulation of aging by the reproductive system are not understood, and it is unknown whether such effects are evolutionarily conserved. Here we report that eliminating germ cells (GCs) in
Drosophila melanogaster
increases lifespan and modulates insulin signaling. Long-lived germ-line-less flies show increased production of
Drosophila insulin-like peptides
(
dilps
) and hypoglycemia but simultaneously exhibit several characteristics of IIS impedance, as indicated by up-regulation of the
Drosophila
FOXO (dFOXO) target genes
4E-BP
and
l
(
2
)
efl
and the insulin/IGF-binding protein
IMP-L2
. These results suggest that signals from the gonad regulate lifespan and modulate insulin sensitivity in the fly and that the gonadal regulation of aging is evolutionarily conserved.
RETRACTED: The antibody zalutumumab inhibits epidermal growth factor receptor signaling by limiting intra- and intermolecular flexibility
Authors: Lammerts van Bueren Jeroen J. , Bleeker Wim K. , Brännström Annika , von Euler Anne , Jansson Magnus , Peipp Matthias , Schneider-Merck Tanja , Valerius Thomas , van de Winkel Jan G. J. , Parren Paul W. H. I.
Published: April 2008

The epidermal growth factor receptor (EGFR) activates cellular pathways controlling cell proliferation, differentiation, migration, and survival. It thus represents a valid therapeutic target for treating solid cancers. Here, we used an electron microscopy-based technique (Protein Tomography) to study the structural rearrangement accompanying activation and inhibition of native, individual, EGFR molecules. Reconstructed tomograms (3D density maps) showed a level of detail that allowed individual domains to be discerned. Monomeric, resting EGFR ectodomains demonstrated large flexibility, and a number of distinct conformations were observed. In contrast, ligand-activated EGFR complexes were detected only as receptor dimers with ring-like conformations. Zalutumumab, a therapeutic inhibitory EGFR antibody directed against domain III, locked EGFR molecules into a very compact, inactive conformation. Biochemical analyses showed bivalent binding of zalutumumab to provide potent inhibition of EGFR signaling. The structure of EGFR–zalutumumab complexes on the cell surface visualized by Protein Tomography indicates that the cross-linking spatially separates the EGFR molecules' intracellular kinase domains to an extent that appears incompatible with the induction of signaling. These insights into the mechanisms of action of receptor inhibition may also apply to other cell-surface tyrosine kinase receptors of the ErbB family.
Compression of auditory space during rapid head turns
Authors: Leung Johahn , Alais David , Carlile Simon
Published: April 2008

Studies of spatial perception during visual saccades have demonstrated compressions of visual space around the saccade target. Here we psychophysically investigated perception of auditory space during rapid head turns, focusing on the “perisaccadic” interval. Using separate perceptual and behavioral response measures we show that spatial compression also occurs for rapid head movements, with the auditory spatial representation compressing by up to 50%. Similar to observations in the visual system, this occurred only when spatial locations were measured by using a perceptual response; it was absent for the behavioral measure involving a nose-pointing task. These findings parallel those observed in vision during saccades and suggest that a common neural mechanism may subserve these distortions of space in each modality.
Global transcriptomic analysis ofCyanothece51142 reveals robust diurnal oscillation of central metabolic processes
Authors: Stöckel Jana , Welsh Eric A. , Liberton Michelle , Kunnvakkam Rangesh , Aurora Rajeev , Pakrasi Himadri B.
Published: April 2008

Cyanobacteria are photosynthetic organisms and are the only prokaryotes known to have a circadian lifestyle. Unicellular diazotrophic cyanobacteria such asCyanothecesp. ATCC 51142 produce oxygen and can also fix atmospheric nitrogen, a process exquisitely sensitive to oxygen. To accommodate such antagonistic processes, the intracellular environment ofCyanotheceoscillates between aerobic and anaerobic conditions during a day–night cycle. This is accomplished by temporal separation of the two processes: photosynthesis during the day and nitrogen fixation at night. Although previous studies have examined periodic changes in transcript levels for a limited number of genes inCyanotheceand other unicellular diazotrophic cyanobacteria, a comprehensive study of transcriptional activity in a nitrogen-fixing cyanobacterium is necessary to understand the impact of the temporal separation of photosynthesis and nitrogen fixation on global gene regulation and cellular metabolism. We have examined the expression patterns of nearly 5,000 genes inCyanothece51142 during two consecutive diurnal periods. Our analysis showed that ≈30% of these genes exhibited robust oscillating expression profiles. Interestingly, this set included genes for almost all central metabolic processes inCyanothece51142. A transcriptional network of all genes with significantly oscillating transcript levels revealed that the majority of genes encoding enzymes in numerous individual biochemical pathways, such as glycolysis, oxidative pentose phosphate pathway, and glycogen metabolism, were coregulated and maximally expressed at distinct phases during the diurnal cycle. These studies provide a comprehensive picture of how a physiologically relevant diurnal light–dark cycle influences the metabolism in a photosynthetic bacterium.
Calculation of protein–ligand binding free energy by using a polarizable potential
Authors: Jiao Dian , Golubkov Pavel A. , Darden Thomas A. , Ren Pengyu
Published: April 2008

The binding of charged ligands benzamidine and diazamidine to trypsin was investigated by using a polarizable potential energy function and explicit-water molecular dynamics simulations. The binding free energies were computed from the difference between the free energies of decoupling the ligand from water and protein environments. Both the absolute and the relative free energies from the perturbation simulations agree with experimental measurements to within 0.5 kcal·mol
−1
. Comparison of free-energy components sampled from different thermodynamic paths indicates that electrostatics is the main driving force behind benzamidine recognition of trypsin. The contribution of electronic polarization to binding appears to be crucial. By computing the free-energy contribution caused by the polarization between the ligand and its surroundings, we found that polarization has the opposite effect in dissimilar environments. Although polarization favors ligand solvation in water, it weakens the protein–ligand attraction by screening the electrostatic interaction between trypsin and benzamidine. We also examined the relative binding free energies of a benzamidine analog diazamidine to trypsin. The changes in free energy on benzamidine-diazamidine substitution were tens of kilocalories in both water and trypsin environments; however, the change in the total binding free energy is <2 kcal·mol
−1
because of cancellation, consistent with the experimental results. Overall, our results suggest that the use of a polarizable force field, given adequate sampling, is capable of achieving chemical accuracy in molecular simulations of protein–ligand recognition.
A polyketide macrolactone synthase from the filamentous fungus
Gibberella zeae
Authors: Zhou Hui , Zhan Jixun , Watanabe Kenji , Xie Xinkai , Tang Yi
Published: April 2008

Resorcylic acid lactones represent a unique class of fungal polyketides and display a wide range of biological activities, such as nanomolar inhibitors of Hsp90 and MAP kinase. The biosynthesis of these compounds is proposed to involve two fungal polyketide synthases (PKS) that function collaboratively to yield a 14-membered macrolactone with a resorcylate core. We report here the reconstitution of
Gibberella zeae
PKS13, which is the nonreducing PKS associated with zearalenone biosynthesis. Using a small molecule mimic of the natural hexaketide starter unit, we reconstituted the entire repertoire of PKS13 activities
in vitro
, including starter-unit selection, iterative condensation, regioselective C2–C7 cyclization, and macrolactone formation. PKS13 synthesized both natural 14-membered and previously uncharacterized 16-membered resorcylic acid lactones, indicating relaxed control in both iterative elongation and macrocyclization. PKS13 exhibited broad starter-unit specificities toward fatty acyl-CoAs ranging in sizes between C6 and C16 and displayed the highest activity toward decanoyl-CoA. PKS13 was shown to be active in
Escherichia coli
and synthesized numerous alkyl pyrones and alkyl resorcylic esters without exogenously supplied precursors. We demonstrated that PKS13 can interact with
E. coli
fatty acid biosynthetic machinery and can be primed with fatty-acyl ACPp at low-micromolar concentrations. PKS13 synthesized new polyketides in
E. coli
when the culture was supplemented with synthetic precursors, showcasing its utility in precursor-directed biosynthesis. PKS13 is therefore a highly versatile polyketide macrolactone synthase that is useful in the engineered biosynthesis of polyketides, including resorcylic acid lactones that are not found in nature.
Critical role of a K
+
channel in
Plasmodium berghei
transmission revealed by targeted gene disruption
Authors: Ellekvist Peter , Maciel Jorge , Mlambo Godfree , Ricke Christina H. , Colding Hanne , Klaerke Dan A. , Kumar Nirbhay
Published: April 2008

Regulated K
+
transport across the plasma membrane is of vital importance for the survival of most cells. Two K
+
channels have been identified in the
Plasmodium falciparum
genome; however, their functional significance during parasite life cycle in the vertebrate host and during transmission through the mosquito vector remains unknown. We hypothesize that these two K
+
channels mediate the transport of K
+
in the parasites, and thus are important for parasite survival. To test this hypothesis, we identified the orthologue of one of the
P. falciparum
K
+
channels, PfKch1, in the rodent malaria parasite
P. berghei
(PbKch1) and examined the biological role by performing a targeted disruption of the gene encoding PbKch1. The deduced amino acid sequence of the six transmembrane domains of PfKch1 and PbKch1 share 82% identity, and in particular the pore regions are completely identical. The PbKch1-null parasites were viable despite a marked reduction in the uptake of the K
+
congener
86
Rb
+
, and mice infected with PbKch1-null parasites survived slightly longer than mice infected with WT parasites. However, the most striking feature of the phenotype was the virtually complete inhibition of the development of PbKch1-null parasites in
Anopheles stephensi
mosquitoes. In conclusion, these studies demonstrate that PbKch1 contributes to the transport of K
+
in
P. berghei
parasites and supports the growth of the parasites, in particular the development of oocysts in the mosquito midgut. K
+
channels therefore may constitute a potential antimalarial drug target.
Remodeling of cellular cytoskeleton by the acid sphingomyelinase/ceramide pathway
Authors: Zeidan Youssef H. , Jenkins Russell W. , Hannun Yusuf A.
Published: April 2008

The chemotherapeutic agent cisplatin is widely used in treatment of solid tumors. In breast cancer cells, cisplatin produces early and marked changes in cell morphology and the actin cytoskeleton. These changes manifest as loss of lamellipodia/filopodia and appearance of membrane ruffles. Furthermore, cisplatin induces dephosphorylation of the actin-binding protein ezrin, and its relocation from membrane protrusions to the cytosol. Because cisplatin activates acid sphingomyelinase (ASMase), we investigate here the role of the ASMase/ceramide (Cer) pathway in mediating these morphological changes. We find that cisplatin induces a transient elevation in ASMase activity and its redistribution to the plasma membrane. This translocation is blocked upon overexpression of a dominant-negative (DN) ASMaseS508A mutant and by a DN PKCδ. Importantly; knockdown of ASMase protects MCF-7 cells from cisplatin-induced cytoskeletal changes including ezrin dephosphorylation. Reciprocally, exogenous delivery of D-e-C16-Cer, but not dihydro-C16-Cer, recapitulates the morphotropic effects of cisplatin. Collectively, these results highlight a novel tumor suppressor property for Cer and a function for ASMase in cisplatin-induced cytoskeletal remodeling.
eIF4GI links nutrient sensing by mTOR to cell proliferation and inhibition of autophagy
Authors: Ramírez-Valle Francisco , Braunstein Steve , Zavadil Jiri , Formenti Silvia C. , Schneider Robert J.
Published: April 2008

Translation initiation factors have complex functions in cells that are not yet understood. We show that depletion of initiation factor eIF4GI only modestly reduces overall protein synthesis in cells, but phenocopies nutrient starvation or inhibition of protein kinase mTOR, a key nutrient sensor. eIF4GI depletion impairs cell proliferation, bioenergetics, and mitochondrial activity, thereby promoting autophagy. Translation of mRNAs involved in cell growth, proliferation, and bioenergetics were selectively inhibited by reduction of eIF4GI, as was the mRNA encoding Skp2 that inhibits p27, whereas catabolic pathway factors were increased. Depletion or overexpression of other eIF4G family members did not recapitulate these results. The majority of mRNAs that were translationally impaired with eIF4GI depletion were excluded from polyribosomes due to the presence of multiple upstream open reading frames and low mRNA abundance. These results suggest that the high levels of eIF4GI observed in many breast cancers might act to specifically increase proliferation, prevent autophagy, and release tumor cells from control by nutrient sensing.
Transmission of HIV-1 Gag immune escape mutations is associated with reduced viral load in linked recipients
Authors: Goepfert Paul A. , Lumm Wendy , Farmer Paul , Matthews Philippa , Prendergast Andrew , Carlson Jonathan M. , Derdeyn Cynthia A. , Tang Jianming , Kaslow Richard A. , Bansal Anju , Yusim Karina , Heckerman David , Mulenga Joseph , Allen Susan , Goulder Philip J.R. , Hunter Eric
Published: April 2008

In a study of 114 epidemiologically linked Zambian transmission pairs, we evaluated the impact of human leukocyte antigen class I (HLA-I)–associated amino acid polymorphisms, presumed to reflect cytotoxic T lymphocyte (CTL) escape in Gag and Nef of the virus transmitted from the chronically infected donor, on the plasma viral load (VL) in matched recipients 6 mo after infection. CTL escape mutations in Gag and Nef were seen in the donors, which were subsequently transmitted to recipients, largely unchanged soon after infection. We observed a significant correlation between the number of Gag escape mutations targeted by specific HLA-B allele–restricted CTLs and reduced VLs in the recipients. This negative correlation was most evident in newly infected individuals, whose HLA alleles were unable to effectively target Gag and select for CTL escape mutations in this gene. Nef mutations in the donor had no impact on VL in the recipient. Thus, broad Gag-specific CTL responses capable of driving virus escape in the donor may be of clinical benefit to both the donor and recipient. In addition to their direct implications for HIV-1 vaccine design, these data suggest that CTL-induced viral polymorphisms and their associated in vivo viral fitness costs could have a significant impact on HIV-1 pathogenesis.