bluuebunny/crossref_metadata_2025 huggingface dataset:
Comprehensive ab initio studies of nuclear magnetic resonance shielding and coupling constants in XH⋯O hydrogen-bonded complexes of simple organic molecules
Authors: Pecul Magdalena , Leszczynski Jerzy , Sadlej Joanna
Published: July 2002

The influence of hydrogen-bond formation on the scalar spin–spin coupling constants and NMR shielding constants in CH2O–H2O, C2H2–H2O, CH3OH–H2O, and (HCOOH)2 complexes has been calculated using correlated MO (MCSCF and MP2) methods. The hydrogen-bond transmitted spin–spin coupling constants are also reported. The iso- and anisotropic H1 shielding constants are found to be the most general parameters of the hydrogen bond. The changes in J1(XH) coupling in the proton donor also correlate with the XH⋯Y hydrogen bond strength. A similar correlation is found in the intermolecular J1h(HY) and J2h(XY) couplings. These couplings are substantial and dominated by the Fermi-contact term in contrast to the longer-range hydrogen-bond transmitted couplings where the noncontact terms prevail.
Rigorous estimates for a computer-assisted KAM theory
Authors: Celletti Alessandra , Chierchia Luigi
Published: July 2002

Nonautonomous Hamiltonian systems of one degree of freedom close to integrable ones are considered. Let ε be a positive parameter measuring the strength of the perturbation and denote by εc the critical value at which a given KAM (Kolmogorov–Arnold–Moser) torus breaks down. A computer-assisted method that allows one to give rigorous lower bounds for εc is presented. This method has been applied in Celletti–Falcolini–Porzio (to be published in Ann. Inst. H. Poincaré) to the Escande and Doveil pendulum yielding a bound which is within a factor 40.2 of the value indicated by numerical experiments.
Möller operators in classical relativistic two-particle scattering
Authors: De Bièvre S.
Published: July 2002

The scattering problem is formulated in a geometric language adapted to the description of classical relativistic two-body systems. Within the framework of manifestly covariant relativistic particle mechanics, precise, easily verifiable conditions with a clear physical interpretation are given on the interparticle interaction for the Möller operators to exist. The Möller operators are used to define the notion of an asymptotically free presymplectic structure on the evolution space and, using the results obtained, the existence and uniqueness of such a structure is discussed.
Mass, dual mass, and gravitational entropy
Authors: Magnon Anne
Published: July 2002

A mechanism (mathematical transformation) is considered by which a (Schwarzschild) black-hole singularity can be converted into a (Taub–NUT) (Newman–Unti–Tamburino) wire singularity, or equivalently topological modifications can be induced (e.g., transformation of S2×R2 topology into S3×R and conversely). A topological charge—an invariant of the transformation—emerges as a possible candidate for the description of gravitational entropy in the case of source-free solutions to Einstein’s equation with one Killing vector field. For a Schwarzschild black hole this invariant reduces to the area of the event horizon (or equivalently the Bolt charge) and it reduces to the square of the NUT charge (or equivalently the length of the closed timelike orbits) in the case of a Taub–NUT magnetic monopole. These considerations lead to the proposition that, under extreme conditions, gravitational clumping or entropy increase could be described by a modification in the characteristic classes of the space-time manifold due to the onset of nontrivial topological features. Further remarks are presented in view of the role of gravitational magnetic monopoles in quantum gravity, and of a possible relation between the notions of gravitational entropy and arrow of time.
Wake-free cylindrical and spherical waves in inhomogeneous media
Authors: Gottlieb H. P. W.
Published: July 2002

Radius-dependent wave-speed functions for two- and three-dimensional inhomogeneous media are discovered that yield explicit, general, radially symmetric solutions that are wake-free (tailess). For the corresponding finite regions, the eigenfrequency spectra for radially symmetric vibrations with Dirichlet boundary conditions are shown to be exactly harmonic.
Planar and axisymmetric walls in general relativity, a comparison
Authors: Lemos José P. S. , Ventura Ozemar S.
Published: July 2002

Through Birkhoff’s theorem it is shown that there are three planar walls with time-independent surface energy density, the Taub, Kasner, and Vilenkin walls. Under certain conditions Vilenkin’s planar wall is unique. In analogy with the planar case a family of axisymmetric walls are analyzed and compared.
A method for determining exact solutions to a class of nonlinear models based on introduction of differential constraints
Authors: Fusco D. , Manganaro N.
Published: July 2002

A nonhomogeneous system of partial differential equations of first order involving two dependent and two independent variables is considered. The introduction of suitable differential constraint equations permits to develop an approach to determining exact solutions to the model in point based on the use of the hodograph transformation. Within the present theoretical framework the governing systems of rate-type materials and flows in fluid-filled elastic tubes are investigated.
On competition between s- and d-symmetry types of the order parameter in high-temperature superconductors
Authors: Pashitskiı̆ È. A. , Pentegov V. I. , Semenov A. V.
Published: July 2002

It is shown that strong anisotropy of the quasi-two-dimensional electron spectrum in high-temperature superconductors and the competition between attraction and repulsion in the electron–electron interaction can lead either to anisotropic s-, or d-type of Cooper pairing depending on the system parameters. In the case when attraction prevails on the entire Fermi surface (e.g., due to electron–phonon interaction), the sxy- or s*-symmetry in the superconducting order parameter is advantageous from the energy point of view. At the same time, the dx2−y2-symmetry of the gap is observed in the case of repulsion in the entire Brillouin zone (which is typical, for example, of the electron–magnon interaction) as well as in the case of effective attraction on the regions of Fermi surface with the maximum density of states. In the latter case, the superconducting transition temperature Tc is always higher than in the former case, indicating the important role of additional attraction in the mechanism of high-temperature superconductivity.
Metamagnetism in perovskites RMnO3+x (R=Gd, Tb, Dy)
Authors: Troyanchuk O. Ya. , Kasper N. V. , Szymczak H. , Nabialek A.
Published: July 2002

A magnetic study of perovskites RMnO3+x (R = Nd, Eu, Gd, Tb, Dy) is reported. The compounds NdMnO2.99 and EuMnO2.99 are weak ferromagnets with TN=88 and 49 K respectively. It is shown that GdMnO3 undergoes a transition from antiferromagnetic to weak ferromagnetic state upon cooling. The antiferromagnetic ordering of the magnetic moments in the Gd sublattice is observed below 6 K. The magnetic moments of TbMnO3.01 and DyMnO3.01 are ordered below 7 K. The external field induces an antiferromagnet–ferromagnet transition in the rare-earth sublattice. The critical fields for GdMnO3,TbMnO3.01, and DyMnO3.01 are 5, 10, and 7 kOe, respectively.
The sound of a singularity
Authors: Caflisch Russel E. , Lund Fernando
Published: July 2002

The sound generated by the singularity of a rolled-up vortex sheet is computed. A strong pressure wave is generated, whose Fourier transform is a power of frequency, with the exponent a characteristic of the singularity.
Viscosity-induced mode splitting and potential energy criterion for mode stability
Authors: Ruzeng Zhu
Published: July 2002

This paper points out that viscosity can induce mode splitting in a uniform infinite cylinder of an incompressible fluid with self-gravitation, and that the potential energy criterion cannot be appropriate to all normal modes obtained, i.e., there will be stable modes with negative potential energy (Σ<0). Therefore the condition Σ>0 is not necessary, although sufficient, for the stability of a mode in an incompressible static fluid or magnetohydrodynamics (MHD) system, which is a correction of both Hare’s [Philos. Mag. 8, 1305 (1959)] and Chandrasekhar’s [Hydrodynamic and Hydromagnetic Stability (Oxford U.P., Oxford, 1961), p. 604] stability criterion for a mode. These results can also be extended to compressible systems with a polytropic exponent.
Forces acting on a low-velocity region within a turbulent boundary layer
Authors: Kistler Alan L. , Markis Theodore
Published: July 2002

One of the well-documented characteristics of the wall turbulent flows is the formation of low-velocity regions at the wall that are accelerated by the surrounding fluid. In this work, a low-velocity region was artificially created close to the wall, and its evolution in time was followed using hot-wire techniques. The acceleration and consequently the force on this region were calculated. An investigation of some of the possible force mechanisms that can act on the flow revealed that the dominant one is related to an ‘‘added mass’’ effect. It is possible that similar forces act on the low-velocity regions that exist in a natural turbulent flow.
Flow measurements behind attached ring-type turbulence promoters
Authors: Bullock K. J. , Lai J. C. S. , Walker T. B.
Published: July 2002

The flow field behind attached ring-type turbulence promoters of various sizes in a smooth pipe has been investigated by measuring the mean velocity and turbulence intensity profiles for downstream distances up to 100 promoter heights at a Reynolds number of 20 500 (based on pipe diameter and average velocity). Measurements have been limited to distances downstream of the flow reattachment region. The results indicate that for the smallest tested promoter with nondimensional height h+=12.5, the disturbance introduced does not have much influence in the core region and the flow behind the promoter reorganizes and recovers very quickly (within a streamwise distance equivalent to 20–30 promoter heights). On the other hand, for the largest promoter tested (h+=100), the flow in the core is significantly perturbed and the return of the flow to a self-preserving state has not been achieved, even at a downstream distance of 100 promoter heights. Furthermore,the mean velocity profile in the core region of the pipe with the largest promoter is much more uniform than the corresponding smooth pipe profile, indicating enhanced mixing. The longitudinal turbulence energy spectra in the sublayer, buffer layer, and logarithmic layer suggest that the sublayer structures are maintained and/or enhanced by large wallward motions originated in the core. These results are fairly consistent with the vortex model of Kobashi and Ichijo [Exp. Fluids 4, 49 (1986)].
Bubble formation in dense fluidized beds
Authors: Hernández Juan A. , Jiménez Javier
Published: July 2002

It has been known for a long time that fluidized beds of solid powder in gas may become unstable for certain values of relative and mass fraction, and that this behavior is important in determining the mixing properties of the bed. Several different mechanisms have been proposed for this instability, but no broad consensus is available on which are the dominant ones. That question is studied here by means of a linear stability analysis and a fully nonlinear spectral numerical simulation of a two-fluid model for the bed. Two instabilities are identified, both of which depend on the viscous interaction between phases. A first, one-dimensional, instability breaks the bed into alternating bands of high and lower concentrations, and is due to the interaction of the volumetric coupling between phases and the interphase friction. Another, secondary, two-dimensional instability deforms these slugs, and results in the formation of large bubbles that later coalesce into longitudinal streamers, which seem to be the final state of the bed. Both mechanisms are different from a third one that had been proposed by other investigators, and which depends on the interaction of volumetric coupling and buoyancy forces. This latter mechanism was also identified but found to be unimportant for these flows. Examples of the temporal evolution of slightly perturbed initial fields into the streamer state are given, and the relevant parameter ranges are identified. The similarities and differences with other two-phase flows with comparable volume fractions including bubbly flows, are explained. [Supported by Spain–U.S. Technological Cooperation Committee.]
Properties of ion temperature gradient drift instabilities in H-mode plasmas
Authors: Hahm T. S. , Tang W. M.
Published: July 2002

Experimental results from tokamaks such as DIII-D [Phys. Fluids 31, 3738 (1988)] and JET [Proceedings of the 15th European Conference on Controlled Fusion and Plasma Heating (European Physical Society, Budapest, 1988), Vol. 12B, Part 1, p. 2239] have indicated that the electron density profile in H-mode (‘‘high confinement’’) discharges can be nearly flat over most of the plasma and in some cases, even inverted (outwardly peaked). These conditions have very interesting implications for pictures of anomalous thermal transport, based on the presence of ion temperature gradient (ηi ) drift instabilities. The present paper includes (i) a new derivation of the ion temperature gradient threshold for weak density grandient ηi modes when ion transit resonances are taken into account; and (ii) the first derivation of threshold conditions for the onset of ηi modes when ηi is negative, along with the properties of these negative ηi instabilities when the thresholds are exceeded. Possible consequences for confinement in H-mode plasmas are discussed.
Observation of fast stochastic ion heating by drift waves
Authors: McChesney J. M. , Bellan P. M. , Stern R. A.
Published: July 2002

Anomalously fast ion heating has been observed in the Caltech Encore tokamak [Phys. Rev. Lett. 59, 1436 (1987)], with the use of laser-induced fluorescence. This heating was found to be independent of electron temperature, but was well correlated with the presence of large-amplitude drift-Alfvén waves. Evidence is presented that suggests that the heating is stochastic and occurs when the ion displacement due to polarization drift becomes comparable to the perpendicular wavelength, i.e., when k⊥ (mik⊥ φ0/qB2)∼1. Stochastic heating may also be the cause of the anomalously high ion temperatures observed in reversed-field pinches.
Theory of ion-temperature-gradient-driven turbulence in tokamaks
Authors: Lee G. S. , Diamond P. H.
Published: July 2002

An analytic theory of ion-temperature-gradient-driven turbulence in tokamaks is presented. Energy-conserving, renormalized spectrum equations are derived and solved in order to obtain the spectra of stationary ion-temperature-gradient-driven turbulence. Corrections to mixing-length estimates are calculated explicitly. The resulting anomalous ion thermal diffusivity χi=0.4[(π/2)ln(1+ηi)]2[(1+ηi)/τ]2 ρ2pcs/Ls is derived and is found to be consistent with experimentally deduced thermal diffusivities. The associated electron thermal and particle diffusivity, and particle and heat-pinch velocities are also calculated. The effect of impurity gradients on saturated ion-temperature-gradient-driven turbulence is discussed and a related explanation of density profile steepening during Z-mode operation is proposed.
Nonlinear gyrokinetic equations for tokamak microturbulence
Authors: Hahm T. S.
Published: July 2002

A nonlinear electrostatic gyrokinetic Vlasov equation as well as a Poisson equation have been derived in a form suitable for particle simulation studies of tokamak microturbulence and associated anomalous transport. This work differs from the existing nonlinear gyrokinetic theories in toroidal geometry, since the present equations conserve energy while retaining the crucial linear and nonlinear polarization physics. In the derivation, the action-variational Lie perturbation method is utilized in order to preserve the Hamiltonian structure of the original Vlasov–Poisson system. Emphasis is placed on the dominant physics of the collective fluctuations in toroidal geometry, rather than on details of particle orbits.
The solar-wind driven magnetosphere–ionosphere as a complex dynamical system
Authors: Horton W. , Smith J. P. , Weigel R. , Crabtree C. , Doxas I. , Goode B. , Cary J.
Published: July 2002

The solar-wind driven magnetosphere–ionosphere system is a classic example of a complex dynamical system (CDS). The defining properties of a CDS are (1) sensitivity to initial conditions; (2) multiple space-time scales; (3) bifurcation sequences with hysteresis in transitions between attractors; and (4) noncompositionality. Noncompositionality means that the behavior of the system as a whole is different from the dynamics of its subcomponents taken with passive or no couplings. In particular the dynamics of the geomagnetic tail plasma depends on its coupling to the dissipative ionospheric plasma and on the nature of the solar-wind driving electric field over a suitably long (many hours) previous time interval. These complex dynamical system features are shown here in detail using the known WINDMI model for the solar-wind driven magnetosphere–ionosphere (MI) system. Numerous features in the bifurcation sequence are identified with known substorm and storm characteristics.
Reduction of fall times in Ga0.47In0.53As photoconductive receivers through back gating
Authors: Chen C. Y. , Cho A. Y. , Olsson N. A. , Garbinski P. A.
Published: July 2002

We have observed the reduction of fall times in Ga0.47In0.53As photoconductive receivers by applying a reverse back gate voltage. A reduction of fall times from 7 to ≊1 ns was obtained with a back gate voltage as low as 1.1 V. The increase in response speeds reduced the bit error rates at 420 Mb/s from 3×10−6 to 10−8 at 1.55 μm, and should improve the receiver sensitivity by ≊1 dB. More importantly, the increase in bandwidths may allow the detector to operate without equalizers for certain bit rates. This is the first demonstration of a back gated photoconductive receiver operating in the 1.1–1.65-μm range.
Reactions of XeF2 chemisorbed on Si(111) 7×7
Authors: Roop B. , Joyce S. , Schultz Jocelyn C. , Shinn N. D. , Steinfeld J. I.
Published: July 2002

The stable overlayer composition resulting from dissociative chemisorption of XeF2 on the Si(111)7×7 surface has been studied by x-ray photoelectron spectroscopy and thermal desorption spectrometry. Evidence is found for the existence of fluorine covalently bonded in the bulk; no evidence for unreacted interstitial fluorine is found.
Ion-assisted etching of silicon by SF6
Authors: Oostra D. J. , Haring A. , de Vries A. E. , Sanders F. H. M. , Miyake K.
Published: July 2002

Sputtering of silicon by 3-keV Ar+ ions in the presence of a molecular SF6 beam has been investigated by mass spectrometry and time-of-flight measurements. At temperatures below 100 K chemical reactions are induced between the silicon and an adsorbed layer of SF6 under ion bombardment leading to newly formed molecular products. The main products formed are SiFx′ (x=0–4) and a small amount of SiSFy compounds (y=0–2). The kinetic energies of these molecular species are for the major part in the 0.1-eV region which excludes evaporation at substrate temperature to be a dominant mechanism for erosion. An effective sputtering yield for silicon of approximately 15 is obtained.
Identification of the major residual donor in unintentionally doped InP grown by molecular beam epitaxy
Authors: Martin T. , Stanley C. R. , Iliadis A. , Whitehouse C. R. , Sykes D. E.
Published: July 2002

The properties of InP grown unintentionally doped by molecular beam epitaxy are generally limited by residual free-carrier concentrations (ND−NA)∼1016 cm−3. By combining two depth profiling techniques, secondary ion mass spectrometry and electrochemical capacitance-voltage measurements, the single donor impurity responsible for these high free-electron concentrations has been identified. Conclusive proof is presented which resolves the source of the impurity.
Picosecond degenerate four-wave mixing in soluble polydiacetylenes
Authors: Dennis W. M. , Blau W. , Bradley D. J.
Published: July 2002

The observation of optical phase conjugation utilizing the intrinsic Kerr-like nonlinear susceptibilities of two soluble polydiacetylenes is reported. Phase conjugate reflectivities of up to 40% were observed on a timescale of ≲100 ps and a value of χ(3)xyyx =7×10−12 esu evaluated.
New optical gyroscope based on the ring passive phase conjugator
Authors: Fischer Baruch , Sternklar Shmuel
Published: July 2002

A new optical rotation sensor is described. It is a ring passive phase conjugator in which the ring may consist of a multimode fiber. A nonreciprocal phase shift in the ‘‘passive’’ like fiber ring activates a grating movement and subsequent frequency detuning of the beams in a photorefractive four-wave mixer. This device has the advantages of natural reciprocal behavior of phase conjugate beams (essential for rotation sensing) and has several adjustable controlling parameters. It reveals a new class of interferometry in which changes in the ring’s optical phases, the beam’s intensities and losses, and the mixing crystal’s efficiency and electric field modulate a frequency detuning of the oscillating beams.
Effect of nonlinear gain reduction on semiconductor laser wavelength chirping
Authors: Koch T. L. , Linke R. A.
Published: July 2002

A simple wavelength chirping formula is presented which includes small nonlinearities in the net gain such as spectral hole burning. The two dominant terms are a laser-structure-independent derivative or ‘‘transient’’ chirp and a structure-dependent ‘‘adiabatic’’ chirp, each with distinctly different lightwave system consequences. The two chirp contributions are indirectly related through their mutual association with relaxation oscillations. Time-resolved spectral measurements on a number of different laser structures support the results.
Magneto-optic recording materials with direct overwrite capability
Authors: Shieh Han-Ping D. , Kryder Mark H.
Published: July 2002

The possibility of achieving direct overwrite capability in magneto-optic thin films is demonstrated. By using films having a compensation temperature tens of degrees higher than the ambient temperature, the demagnetizing field can effectively be used as a bias field for thermomagnetic writing in which writing and erasing of domains are accomplished in the absence of a magnetic field. A read-before-write scheme is proposed to facilitate the write/erase laser trigger signal to perform overwriting in real time.
Enhanced oxygen diffusion in silicon at thermal donor formation temperature
Authors: Lee S.-Tong , Fellinger Paul
Published: July 2002

From the outdiffusion profiles of oxygen in Czochralski silicon at 500 °C measured with secondary ion mass spectrometry (SIMS), oxygen diffusivities of 2.5–4.0×10−14 cm2 s−1 are deduced, which are enhanced nearly four orders of magnitude relative to the normal diffusivity. Diffusion of implanted oxygen-18 in float-zone silicon at 400–525 °C yields SIMS profiles consisting of an exponential decay of oxygen concentration. The exponential profiles of oxygen-18 above background (1014 cm−3) as deep as 4–16 μm also show direct evidence of enhanced long-range oxygen diffusion. The results of enhanced oxygen diffusion can be explained with a fast-diffusing species, which is in dynamical equilibrium with the interstitial oxygen.
Heterojunction bipolar transistor using pseudomorphic GaInAs for the base
Authors: Enquist P. M. , Ramberg L. R. , Najjar F. E. , Schaff W. J. , Eastman L. F.
Published: July 2002

We report on replacing the GaAs base in the AlGaAs/GaAs heterojunction bipolar transistor with pseudomorphic GaInAs. Base regions consisting of uniform Ga0.95In0.05As and graded Ga1−yInyAs (y=0.0–0.05) are compared to base regions consisting of GaAs. The highest dc common emitter current gain is obtained with the graded Ga1−yInyAs base.
Interferometric detection of ultrasound at rough surfaces using optical phase conjugation
Authors: Paul M. , Betz B. , Arnold W.
Published: July 2002

Optical detection of ultrasonic signals of 7.5 MHz center frequency has been carried out by heterodyne interferometry at the rough surface of an aluminum sample. The distorted wave front was reconstructed by optical phase conjugation leading to an increase of the light-gathering power of the interferometer.
Scanning tunneling microscope as a micromechanical tool
Authors: McCord M. A. , Pease R. F. W.
Published: July 2002

The need to fashion materials on a submicron scale is now well recognized. In a scanning tunneling microscope we have been able to achieve nanometer scale control of the depth of penetration of the probe into a thin insulating film, and by laterally traversing the probe we have been able to machine away submicron-wide, 20-nm-thick strips of the insulating film without damage to the substrate or probe. This could represent a new approach to ultrafine machining. However, the detailed mechanism of how the tunneling current through the film can be used to control the machining depth is still unclear.
Selectively δ-doped AlxGa1−xAs/GaAs heterostructures with high two-dimensional electron-gas concentrations n2DEG≥1.5×1012 cm−2 for field-effect transistors
Authors: Schubert E. F. , Cunningham J. E. , Tsang W. T. , Timp G. L.
Published: July 2002

The δ-doping concept is applied to selectively doped heterostructures in the AlxGa1−xAs/GaAs material system. High two-dimensional electron-gas concentrations ≥1.5×1012 cm−2 are obtained at T=300 K in such selectively δ-doped heterostructures due to (i) size quantization in the AlxGa1−xAs and (ii) localization of donor impurities within one atomic monolayer. Shubnikov–de Haas measurements yield n2DEG =1.1×1012 cm−2 at 300 mK and at a spacer thickness of 25 Å. Selectively δ-doped heterostructure transistors (SΔDHT’s) are fabricated and have excellent characteristics due to the enhanced electron-gas concentrations achieved. A very high transconductance of gm ≂360 mS/mm at a gate length of 1.2 μm is obtained in depletion-mode SΔDHT’s at T=300 K.
Growth and characterization of molecular beam epitaxial GaAs layers on porous silicon
Authors: Lin T. L. , Sadwick L. , Wang K. L. , Kao Y. C. , Hull R. , Nieh C. W. , Jamieson D. N. , Liu J. K.
Published: July 2002

GaAs layers have been grown on porous silicon (PS) substrates with good crystallinity by molecular beam epitaxy. In spite of the surface irregularity of PS substrates, no surface morphology deterioration was observed on epitaxial GaAs overlayers. A 10% Rutherford backscattering spectroscopy minimum channeling yield for GaAs-on-PS layers as compared to 16% for GaAs-on-Si layers grown under the same condition indicates a possible improvement of crystallinity when GaAs is grown on PS. Transmission electron microscopy reveals that the dominant defects in the GaAs-on-PS layers are microtwins and stacking faults, which originate from the GaAs/PS interface. GaAs is found to penetrate into the PS layers. n-type GaAs/p-type PS heterojunction diodes were fabricated with good rectifying characteristics.
Current-voltage characteristics of In0.53Ga0.47As/In0.52Al0.48As resonant tunneling barrier structures grown by molecular beam epitaxy
Authors: Sugiyama Yoshihiro , Inata Tsuguo , Muto Shunichi , Nakata Yoshiaki , Hiyamizu Satoshi
Published: July 2002

Well width dependence and temperature dependence of negative differential resistance characteristics of InGaAs/InAlAs resonant tunneling barrier diodes were investigated. Peak current density was almost independent of temperature, while the valley current density increased with increasing temperature above 100 K. At room temperature, a peak-to-valley current ratio of 5.5 with a peak current density of 4.8×104 A/cm2 was obtained by reducing the quantum well width to a narrow 32.2 Å. This is the largest peak-to-valley current ratio ever reported for resonant tunneling barrier diodes at room temperature.
Light modulation of Rubisco in
Arabidopsis
requires a capacity for redox regulation of the larger Rubisco activase isoform
Authors: Zhang Ning , Kallis Russell P. , Ewy Robert G. , Portis Archie R.
Published: July 2002

The light activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)
in vivo
requires the presence of Rubisco activase, a nuclear-encoded chloroplast protein that consists of two isoforms arising from alternative splicing in most plants. We examined the function of each isoform by characterizing Rubisco activation in transgenic
Arabidopsis
plants that express only one or both isoforms, as compared with the wild type. In plants expressing only the shorter isoform, Rubisco activity was as high as in the wild type under saturating light, but the activity was not down-regulated at intensities limiting for photosynthesis. In contrast, in plants expressing only the longer isoform, Rubisco activity was down-regulated at limiting light, but the activity was slightly lower and increased much more slowly at saturating light intensities as compared with the wild type. Light regulation of Rubisco similar to that in the wild-type plants was observed in the progeny of a genetic cross of these two transformants in which both isoforms were again present. When the capacity to redox regulate the activity of the larger activase isoform was eliminated by replacement of the critical cysteine residues in the carboxyl-terminal extension unique to this isoform, Rubisco activity in saturating light was similar to the wild type, but the ability of the larger isoform to down-regulate Rubisco activity at limiting light intensities in transgenic plants was almost abolished. These results indicate that the light modulation of Rubisco under limiting light is mainly due to the ability to regulate the activity of Rubisco activase by redox changes in the stroma.
Forming classes by stimulus frequency: Behavior and theory
Authors: Rosenthal Orna , Fusi Stefano , Hochstein Shaul
Published: July 2002

Visual classification is the way we relate to different images in
our environment as if they were the same, while relating differently to
other collections of stimuli (e.g., human vs. animal faces). It is
still not clear, however, how the brain forms such classes, especially
when introduced with new or changing environments. To isolate a
perception-based mechanism underlying class representation, we studied
unsupervised classification of an incoming stream of simple images.
Classification patterns were clearly affected by stimulus frequency
distribution, although subjects were unaware of this distribution.
There was a common bias to locate class centers near the most frequent
stimuli and their boundaries near the least frequent stimuli. Responses
were also faster for more frequent stimuli. Using a minimal,
biologically based neural-network model, we demonstrate that a simple,
self-organizing representation mechanism based on overlapping tuning
curves and slow Hebbian learning suffices to ensure classification.
Combined behavioral and theoretical results predict large tuning
overlap, implicating posterior infero-temporal cortex as a possible
site of classification.
Crystal cataracts: Human genetic cataract caused by protein crystallization
Authors: Pande Ajay , Pande Jayanti , Asherie Neer , Lomakin Aleksey , Ogun Olutayo , King Jonathan , Benedek George B.
Published: July 2002

Several human genetic cataracts have been linked recently to point
mutations in the γD crystallin gene. Here we provide a molecular
basis for lens opacity in two genetic cataracts and suggest that the
opacity occurs because of the spontaneous crystallization of the mutant
proteins. Such crystallization of endogenous proteins
leading to pathology is an unusual event. Measurements of the
solubility curves of crystals of the Arg-58 to His and Arg-36 to Ser
mutants of γD crystallin show that the mutations dramatically lower
the solubility of the protein. Furthermore, the crystal nucleation rate
of the mutants is enhanced considerably relative to that of the
wild-type protein. It should be noted that, although there is a marked
difference in phase behavior, there is no significant difference in
protein conformation among the three proteins.
A maize cytokinin gene encoding an
O
-glucosyltransferase specific to
cis
-zeatin
Authors: Martin Ruth C. , Mok Machteld C. , Habben Jeffrey E. , Mok David W. S.
Published: July 2002

Zeatin is a naturally occurring cytokinin. Biosynthesis and
metabolism studies of zeatin have been directed mostly at the
trans
isomer, although
cis
-zeatin and its
riboside occur as major components in some plant species. It is not
known whether parallel regulatory pathways exist for the two isomers.
Based on the sequence of the gene
ZOG1
encoding a
trans
-zeatin
O
-glucosyltransferase from
Phaseolus
(EC
2.4.1.203
), a
cis
-zeatin-specific
O
-glucosyltransferase
was isolated from maize. This gene,
cisZOG1
, contains an
ORF of 1,401 nucleotides encoding a protein of 51.1 kDa with 41%
identity to the
Phaseolus
ZOG1 protein. Unexpectedly,
the maize enzyme recognizes as substrates
cis
-zeatin and
UDP-glucose but not
cis
-ribosylzeatin,
trans
-zeatin, or
trans
-ribosylzeatin.
This finding indicates the existence of cis-specific regulatory
elements in plants and suggests that
cis
-zeatin and
derivatives may be more important in cytokinin homeostasis than
currently recognized.
Regulation of the homeodomain CCAAT displacement/
cut
protein function by histone acetyltransferases p300/CREB-binding protein (CBP)-associated factor and CBP
Authors: Li SiDe , Aufiero Barbara , Schiltz R. Louis , Walsh Martin J.
Published: July 2002

The CCAAT displacement protein/
cut
homologue (CDP/
cut
) is a divergent homeodomain protein that is highly conserved through evolution and has properties of a potent transcriptional repressor. CDP/
cut
contains three conserved
cut
-repeat domains and a conserved homeobox, each involved in directing binding specificity to unique nucleotide sequence elements. Furthermore, CDP/
cut
may play a role as a structural component of chromatin through its direct interaction with nucleosomal DNA and association with nuclear matrix attachment regions. CDP/
cut
is cell-cycle regulated through interactions with Rb, p107, specific kinases and phosphatases directing the transcriptional activity of CDP/
cut
on such genes encoding p21
WAF1,CIP1
,
c-myc
, thymidine kinase, and histones. Our previous studies indicate that CDP/
cut
is associated with histone deacetylase activity and is associated with a corepressor complex through interactions with histone deacetylases. Here, we report the interaction of CDP/
cut
with CBP and p300/CREB-binding protein-associated factor (PCAF) along with the modification of CDP/
cut
by the histone acetyltransferase PCAF. Acetylation of CDP/
cut
by PCAF is directed at conserved lysine residues near the homeodomain region and regulates CDP/
cut
function. These observations are consistent with the ability of CDP/
cut
to regulate genes as a transcriptional repressor, suggesting acetylation as a mechanism that regulates CDP/
cut
function.
Deciphering the mechanism for the assembly of aromatic polyketides by a bacterial polyketide synthase.
Authors: Shen B , Hutchinson C R
Published: July 2002

Aromatic polyketides are assembled by a type 11 (iterative) polyketide synthase (PKS) in bacteria. Understanding the enzymology of such enzymes should provide the information needed for the synthesis of novel polyketides through the genetic engineering of PKSs. Using a previously described cell-free system [B.S. & C.R.H. (1993) Science 262, 1535-1540], we studied a PKS enzyme whose substrate is not directly available and purified the TcmN polyketide cyclase from Streptomyces glaucescens. TcmN is a bifunctional protein that catalyzes the regiospecific cyclization of the Tcm PKS-bound linear decaketide to Tcm F2 and the 0-methylation of Tcm D3 to Tcm B3. In the absence of TcmN, the decaketide formed by the minimal PKS consisting of the TcmJKLM proteins undergoes spontaneous cyclization to form some Tcm F2 as well as SEK15 and many other aberrant shunt products. Addition of purified TcmN to a mixture of the other Tcm PKS components both restores and enhances Tcm F2 production. Interestingly, Tcm F2 but none of the aberrant products was bound tightly to the PKS. The results described support the notion that the polyketide cyclase, not the minimal PKS, dictates the regiospecificity for the cyclization of the linear polyketide intermediate. Furthermore, because the addition of TcmN to the TcmJKLM proteins results in a significant increase of the total yield of decaketide, interactions among the individual components of the Tcm PKS complex must give rise to the optimal PKS activity.
Genetic identification of a gene involved in constitutive, high-affinity nitrate transport in higher plants.
Authors: Wang R , Crawford N M
Published: July 2002

Two mutations have been found in a gene (NRT2) of Arabidopsis thaliana that specifically impair constitutive, high-affinity nitrate uptake. These mutants were selected for resistance to 0.1 mM chlorate in the absence of nitrate. Progency from one of the backcrossed mutants showed no constitutive uptake of nitrate below 0.5 mM at pH 7.0 in liquid culture (that is, within 30 min of initial exposure to nitrate). All other uptake activities measured (high-affinity phosphate and sulfate uptake, inducible high-affinity nitrate uptake, and constitutive low-affinity nitrate uptake) were present or nearly normal in the backcrossed mutant. Electrophysiological analysis of individual root cells showed that the nrt2 mutant showed little response to 0.25 mM of nitrate, whereas NRT2 wild-type cells showed an initial depolarization followed by recovery. At 10 mM of nitrate both the mutant and wild-type cells displayed similar, strong electrical responses. These results indicate that NRT2 is a critical and perhaps necessary gene for constitutive, high-affinity nitrate uptake in Arabidopsis, but not for inducible, high-affinity nor constitutive, low-affinity nitrate uptake. Thus, these systems are genetically distinct.
P2Y1 purinergic receptors in sensory neurons: contribution to touch-induced impulse generation.
Authors: Nakamura F , Strittmatter S M
Published: July 2002

Somatic sensation requires the conversion of physical stimuli into the depolarization of distal nerve endings. A single cRNA derived from sensory neurons renders Xenopus laevis oocytes mechanosensitive and is found to encode a P2Y1 purinergic receptor. P2Y1 mRNA is concentrated in large-fiber dorsal root ganglion neurons. In contrast, P2X3 mRNA is localized to small-fiber sensory neurons and produces less mechanosensitivity in oocytes. The frequency of touch-induced action potentials from frog sensory nerve fibers is increased by the presence of P2 receptor agonists at the peripheral nerve ending and is decreased by the presence of P2 antagonists. P2X-selective agents do not have these effects. The release of ATP into the extracellular space and the activation of peripheral P2Y1 receptors appear to participate in the generation of sensory action potentials by light touch.
Molecular markers reveal cryptic sex in the human pathogen Coccidioides immitis.
Authors: Burt A , Carter D A , Koenig G L , White T J , Taylor J W
Published: July 2002

Coccidioides immitis, cause of a recent epidemic of "Valley fever" in California, is typical of many eukaryotic microbes in that mating and meiosis have yet to be reported, but it is not clear whether sex is truly absent or just cryptic. To find out, we have undertaken a population genetic study using PCR amplification, screening for single-strand conformation polymorphisms, and direct DNA sequencing to find molecular markers with nucleotide-level resolution. Both population genetic and phylogenetic analyses indicate that C. immitis is almost completely recombining. To our knowledge, this study is the first to find molecular evidence for recombination in a fungus for which no sexual stage has yet been described. These results motivate a directed search for mating and meiosis and illustrate the utility of single-strand conformation polymorphism and sequencing with arbitrary primer pairs in molecular population genetics.
Human immunodeficiency virus type 1 viral background plays a major role in development of resistance to protease inhibitors.
Authors: Rose R E , Gong Y F , Greytok J A , Bechtold C M , Terry B J , Robinson B S , Alam M , Colonno R J , Lin P F
Published: July 2002

The observed in vitro and in vivo benefit of combination treatment with anti-human immunodeficiency virus (HIV) agents prompted us to examine the potential of resistance development when two protease inhibitors are used concurrently. Recombinant HIV-1 (NL4-3) proteases containing combined resistance mutations associated with BMS-186318 and A-77003 (or saquinavir) were either inactive or had impaired enzyme activity. Subsequent construction of HIV-1 (NL4-3) proviral clones containing the same mutations yielded viruses that were severely impaired in growth or nonviable, confirming that combination therapy may be advantageous. However, passage of BMS-186318-resistant HIV-1 (RF) in the presence of either saquinavir or SC52151, which represented sequential drug treatment, produced viable viruses resistant to both BMS-186318 and the second compound. The predominant breakthrough virus contained the G48V/A71T/V82A protease mutations. The clone-purified RF (G48V/A71T/V82A) virus, unlike the corresponding defective NL4-3 triple mutant, grew well and displayed cross-resistance to four distinct protease inhibitors. Chimeric virus and in vitro mutagenesis studies indicated that the RF-specific protease sequence, specifically the Ile at residue 10, enabled the NL4-3 strain with the triple mutant to grow. Our results clearly indicate that viral genetic background will play a key role in determining whether cross-resistance variants will arise.
In vitro synthesis of oncogenic human papillomaviruses requires episomal genomes for differentiation-dependent late expression.
Authors: Frattini M G , Lim H B , Laimins L A
Published: July 2002

Human papillomavirus (HPV) types 16, 18, 31, and 51 are the etiologic agents of many anogenital cancers including those of the cervix. These "high risk" HPVs specifically target genital squamous epithelia, and their lytic life cycle is closely linked to epithelial differentiation. We have developed a genetic assay for HPV functions during pathogenesis using recircularized cloned HPV 31 genomes that were transfected together with a drug resistance marker into monolayer cultures of normal human foreskin keratinocytes, the natural host cell. After drug selection, cell lines were isolated that stably maintained HPV 31 DNA as episomes and underwent terminal differentiation when grown in organotypic raft cultures. In differentiated rafts, the expression of late viral genes, amplification of viral DNA, and production of viral particles were detected in suprabasal cells. This demonstrated the ability to synthesize HPV 31 virions from transfected DNA templates and allowed an examination of HPV functions during the vegetative viral life cycle. We then used this system to investigate whether an episomal genome was required for the induction of late viral gene expression. When an HPV 31 genome (31E1*) containing a missense mutation in the E1 open reading frame was transfected into normal human keratinocytes, the mutant viral sequences were found to integrate into the host cell chromosomal DNA with both early and late regions intact. While high levels of early viral gene transcription were observed, no late gene expression was detected in rafts of cell lines containing the mutant viral genome despite evidence of terminal differentiation. Therefore, the induction of late viral gene expression required that the viral genomes be maintained as extrachromosomal elements, and terminal differentiation alone was not sufficient. These studies provide the basis for a detailed examination of HPV functions during viral pathogenesis.
Human and
Saccharomyces cerevisiae
dolichol phosphate mannose synthases represent two classes of the enzyme, but both function in
Schizosaccharomyces pombe
Authors: Colussi Paul A. , Taron Christopher H. , Mack Jamey C. , Orlean Peter
Published: July 2002

Dolichol phosphate mannose (Dol-
P
-Man), formed upon transfer of Man from GDPMan to Dol-
P
, is a mannosyl donor in pathways leading to
N
-glycosylation, glycosyl phosphatidylinositol membrane anchoring, and
O
-mannosylation of protein. Dol-
P
-Man synthase is an essential protein in
Saccharomyces cerevisiae
. We have cloned cDNAs encoding human and
Schizosaccharomyces pombe
proteins that resemble
S. cerevisiae
Dol-
P
-Man synthase. Disruption of the gene for the
S. pombe
Dol-
P
-Man synthase homolog,
dpm1
+
, is lethal. The known Dol-
P
-Man synthase sequences can be divided into two classes. One contains the
S. cerevisiae
,
Ustilago maydis
, and
Trypanosoma brucei
enzymes, which have a COOH-terminal hydrophobic domain, and the other contains the human,
S. pombe
, and
Caenorhabditis
synthases, which lack a hydrophobic COOH-terminal domain. The two classes of synthase are functionally equivalent, because
S. cerevisiae DPM1
and its human counterpart both complement the lethal null mutation in
S. pombe dpm1
+
. The findings that Dol-
P
-Man synthase is essential in yeast and that the
Ustilago
and
Trypanosoma
synthases are in a different class from the human enzyme raise the possibility that Dol-
P
-Man synthase could be exploited as a target for inhibitors of pathogenic eukaryotic microbes.
Expression of an ortholog of replication protein A1 (RPA1) is induced by gibberellin in deepwater rice
Authors: van der Knaap Esther , Jagoueix Sandrine , Kende Hans
Published: July 2002

Internodes of deepwater rice are induced to grow rapidly when plants become submerged. This adaptation enables deepwater rice to keep part of its foliage above the rising flood waters during the monsoon season and to avoid drowning. This growth response is, ultimately, elicited by the plant hormone gibberellin (GA). The primary target tissue for GA action is the intercalary meristem of the internode. Using differential display of mRNA, we have isolated a number of genes whose expression in the intercalary meristem is regulated by GA. The product of one of these genes was identified as an ortholog of replication protein A1 (RPA1). RPA is a heterotrimeric protein involved in DNA replication, recombination, and repair and also in regulation of transcription. A chimeric construct, in which the single-stranded DNA-binding domain of rice
RPA1
was spliced into the corresponding region of yeast
RPA1
, was able to complement a yeast
rpa1
mutant. The transcript level of rice
RPA1
is high in tissues containing dividing cells.
RPA1
mRNA levels increase rapidly in the intercalary meristem during submergence and treatment with GA before the increase in the level of histone H3 mRNA, a marker for DNA replication.