bluuebunny/crossref_metadata_2025 huggingface dataset:
A thermodynamic analysis of solvation in dipolar liquids
Authors: Matyushov Dmitry V. , Schmid Roland
Published: July 2002

The chemical potential of infinitely dilute dipole solvation is dissected into parts of cavity formation, dispersion forces, and dipolar interactions. It is this latter part whose treatment is improved here over previous models by applying the Padé approximation for the perturbation expansion. In this way allowance is made for local solvent structuring around the solute due to packing effects. The solvent is modelled by spherical hard molecules of spherical polarizability, centered dipole moment, and central dispersion potential. For realistic parameter values, the Padé result of dipole solvation falls between the predictions of the Onsager theory and the mean-spherical approximation (MSA), and approaches the continuum limit faster than the MSA. The latter is demonstrated to be just the low-density limit of the Padé form. Tested on experimental solvation free energies of nitromethane as the solute in select solvents, the Padé form is found to predict the polarity dependence of the chemical potential of dipole solvation far better than MSA and Onsager theories. We also set out to a qualitative analysis of hydration thermodynamics, in particular enthalpies and entropies. The calculated values of both components of the hydration free energy are substantially too negative compared to experiment, with an equality found between excess enthalpies and excess entropies times temperature. This is all the more meaningful as the excess entropy originates chiefly from cavity formation, and the excess enthalpy mainly from dispersion and dipolar forces. Both parts are thus derived from equations that are fully independent of one another. The excess enthalpy is identified with the solvent reorganization energy featuring the solvent–solvent interaction changes induced by the solute. This is the first time as it appears that, by applying the Padé approximation to the solvation problem, solvent reorganization energies for real systems can be extracted from experimental solvation data. The primary factor determining the solvent reorganization energy is found to be solute size. Since the solvent reorganization terms are locked into exact enthalpy–entropy compensation, in line with thermodynamic considerations, a simple solvent model as the present one is adequate to treat solvation free energies. Large negative entropies of hydration at constant pressure arise from the cavity formation term and are traced to two particular properties of water: small molecular size and low expansibility, with the latter being of greater impact. In fact, for all other solvents considered, the entropies of cavity formation at constant pressure are positive due to the high liquid expansibilities. These lead to overall small negative, or even positive, solvation entropies at constant pressure.
Size dependence of the photoabsorption spectra of Ar+n, n=4–25: A solvation effect on the Ar+3 chromophore
Authors: Ikegami Tsutomu , Iwata Suehiro
Published: July 2002

The photoabsorption spectra of argon cluster ions, Ar+n are calculated for n=4 to 25. The internal motion of the cluster is accounted for by the molecular dynamics method. The diatomics- in-molecules (DIM) potential energy surface is used for the calculation. There are basically two peaks in the spectra. At a low internal energy, the primary peak shifts from 510 to 550 nm at n≂10, and then shifts abruptly to 600 nm at n≂14. As the internal energy increases, the spectra become broad and the clear transition disappears. The spectral shift is explained by the solvation of the ion core in the cluster, with the rest of the constituent atoms acting as solvent atoms. The first red shift is due to the crossover of the energy levels between the ion core and the solvent shell. The second one takes place because the solvation energy is increased abruptly, which is explained in terms of the structural change in the solvation shell.
Degenerate four-wave mixing spectroscopy and spectral simulation of C2 in an atmospheric pressure oxy-acetylene flame
Authors: Kaminski C. F. , Hughes I. G. , Ewart P.
Published: July 2002

The d 3Πg↔a 3Πu Swan bands of C2 have been recorded with high resolution using DFWM in the nearly Doppler free, phase conjugate geometry. C2 was probed in a standard oxy-acetylene welding flame with excellent signal-to-noise ratio and spectral resolution. Theoretical spectra were simulated and fitted directly to the complex overlapping spectra. The good agreement obtained shows that DFWM holds promise to become a robust and reliable tool for flame thermometry. Current theories of DFWM are reviewed in context of the present work and advantages and disadvantages of the technique are discussed.
Dibromine monoxide, Br2O: The rotational spectrum and molecular properties
Authors: Müller Holger S. P. , Cohen Edward A.
Published: July 2002

The rotational spectra of Br792O, BrO7981Br, and Br812O in their ground vibrational states as well as Bro7981Br in its v2=1 state have been studied in selected regions between 90 and 523 GHz. Transitions involving a large range of quantum numbers, 6⩽J⩽123 and 0⩽Ka⩽12, have been observed permitting precise rotational and a large set of centrifugal distortion constants to be determined. All isotopic species as well as the excited state data were fit simultaneously. Ground-state effective and average structural parameters as well as an estimate of the equilibrium structure have been derived. The quartic distortion constants were used for a calculation of the harmonic force field. The complete quadrupole tensor has been determined. Its diagonalization reveals a largely covalent BrO bond with little π-bonding. The derived properties of Br2O are compared with those of related compounds such as Cl2O, HOBr, and HOCl.
On the transition coordinate for protein folding
Authors: Du Rose , Pande Vijay S. , Grosberg Alexander Yu. , Tanaka Toyoichi , Shakhnovich Eugene S.
Published: July 2002

To understand the kinetics of protein folding, we introduce the concept of a “transition coordinate” which is defined to be the coordinate along which the system progresses most slowly. As a practical implementation of this concept, we define the transmission coefficient for any conformation to be the probability for a chain with the given conformation to fold before it unfolds. Since the transmission coefficient can serve as the best possible measure of kinetic distance for a system, we present two methods by which we can determine how closely any parameter of the system approximates the transmission coefficient. As we determine that the transmission coefficient for a short-chain heteropolymer system is dominated by entropic factors, we have chosen to illustrate the methods mentioned by applying them to geometrical properties of the system such as the number of native contacts and the looplength distribution. We find that these coordinates are not good approximations of the transmission coefficient and therefore, cannot adequately describe the kinetics of protein folding.
A laser photolysis/time-resolved Fourier transform infrared emission study of OH(X 2Π,v) produced in the reaction of alkyl radicals with O(3P)
Authors: Lindner Jörg , Loomis Richard A. , Klaassen Jody J. , Leone Stephen R.
Published: July 2002

The emission spectra of vibrationally excited hydroxyl radical products formed in the reactions of alkyl radicals with O(3P) atoms are detected using a laser photolysis/time-resolved Fourier transform infrared spectroscopy technique. For the reaction between oxygen atoms and ethyl, the radicals are produced simultaneously by the 193 nm photolysis of the precursors SO2 and diethyl ketone, respectively. The observed initial OH(v) product vibrational state distribution for the C2H5+O(3P) reaction is 0.18±0.03, 0.23±0.04, 0.29±0.05, 0.23±0.07, and 0.07±0.04 for v=1 to 5, respectively. The population inversion is best explained by a direct abstraction mechanism for this radical–radical reaction. Vibrationally excited hydroxyl radicals are also observed in the O+ethyl, O+n-propyl, and O+i-propyl reactions when using alkyl iodides as precursors of the alkyl radicals, although quantitative detail is not obtained due to competing reaction processes.
Photodissociation spectroscopy of the Ca+–N2 complex
Authors: Pullins S. H. , Reddic J. E. , France M. R. , Duncan M. A.
Published: July 2002

The weakly bound complex Ca+–N2 is prepared in a pulsed nozzle/laser vaporization cluster source and studied with mass-selected photodissociation spectroscopy. The chromophore giving rise to the electronic transition is the 2P←2S atomic transition of Ca+. The appearance of spin–orbit doublets in the vibrationally resolved spectrum, as expected for a 2∏r←2∑+ transition, confirms that the complex is linear. The electronic transition in the complex lies to the red of the atomic resonance line indicating that the complex is more strongly bound in the excited state than in the ground state. The vibrationally resolved spectrum contains progressions in the Ca+–N2 stretching mode and in a combination of this stretch with the N–N stretch. Extrapolation of the Ca+–N2 stretch determines the excited state dissociation energy to be D0′=6500±500 cm−1, and an energetic cycle determines the ground state value to be D0″=1755±500 cm−1 (5.02 kcal/mol). The 2∏r(2,0,0)←2∑+(0,0,0) vibronic transition has been rotationally resolved yielding the bond lengths: rCaN=2.75 Å and rNN=1.15 Å for the 2∑+ ground state; rCaN=2.48 Å and rNN=1.17 Å for the 2∏ excited state.
Thermally induced morphological transition from lamella to gyroid in a binary blend of diblock copolymers
Authors: Sakurai Shinichi , Umeda Hideo , Furukawa Chizuko , Irie Hiroshi , Nomura Shunji , Hyun Lee Hee , Kon Kim Jin
Published: July 2002

We report an experimental result of the thermally induced morphological transition from lamellar to gyroid phase in a binary blend of polystyrene-block-polyisoprene (SI) diblock copolymers. Two SI copolymers employed in this study have almost the same molecular weights but different volume fractions of polystyrene (PS) block (φPS=0.26 and 0.65). The blend was prepared by these two SI diblock copolymers and the overall volume fraction of PS block in the blend, φPS¯, was 0.58. Time-resolved small-angle x-ray scattering (SAXS) experiments were conducted to reveal morphological structures as a function of temperature from 120 to 205 °C during the heating with a rate of 2 °C/min. It was observed from SAXS results that the lamellar phase (L) was found below 164 °C, the gyroid phase (G) above 188 °C, and the coexistence of L and G phases between these temperatures. The gyroid phase was confirmed by transmission electron micrograph. The transition from L to G phase is different from a thermally induced transition of kinetically locked morphology formed by the vitrification of the PS matrix, because it was also found by a separate SAXS experiment for the specimen annealed for long times in which the lamellar phase was stable up to 140 °C, which is well above the glass transition temperature of the PS block. A temperature sweep of SAXS experiment with a smaller heating rate was a useful method to roughly estimate the transition temperature. Moreover, it was found from this experiment that the position of the first-order peak does not change remarkably upon the transition. Although the thermoreversibility of the transition between L and G phases has not yet been confirmed, it does not necessarily mean that the lamellar morphology is not stable at lower temperatures.
Vacuum ultraviolet laser pulsed field ionization photoelectron studies of polyatomic species: Accurate ionization energies of CH3SH and CH3CH2SH
Authors: Cheung Y.-S. , Huang J.-C. , Ng C. Y.
Published: July 2002

The vacuum ultraviolet (VUV) pulsed field ionization photoelectron (PFI-PE) spectra for CH3SH and CH3CH2SH have been obtained near their ionization thresholds. Using a semiempirical simulation scheme, we have obtained satisfactory fits to fine structures resolved in the VUV-PFI-PE spectra, yielding accurate ionization energies of 76 256.3±2.9 cm−1 (9.454 58±0.000 36 eV) and 74 948.7±2.9 cm−1 (9.292 46±0.000 36 eV) for CH3SH and CH3CH2SH, respectively.
High resolution spectroscopy and spectral simulation of C2 using degenerate four-wave mixing
Authors: Lloyd G. M. , Ewart P.
Published: July 2002

Degenerate four-wave mixing in the sub-Doppler phase conjugate geometry was used to record high resolution spectra of the d 3Πg–a 3Πu (0–0) Swan band of C2 produced in an oxy-acetylene flame. The line positions of isolated transitions were measured to an accuracy of ∼3×10−3 cm−1 and calibrated using a Fizeau interferometer system. The data obtained from these spectra was used to calculate rotational constants and lambda doubling parameters for the 3Π states from which the line positions for the whole band were calculated. Noticeable improvements between experimental and calculated line positions are seen when compared to previously published values. The effect of inaccuracies in line positions on the simulation of degenerate four-wave mixing spectra is discussed and some examples of the improvement in simulation using the newly calculated line positions are presented.
Effects of a fumed silica network on kinetics of phase separation in polymer blends
Authors: Chakrabarti Amitabha
Published: July 2002

We have carried out a systematic investigation of the kinetics of domain growth of a model binary mixture in contact with a fractal network similar to the fumed silica network used in recent experiments with polymer blends. This network has selective affinity for one component of the blend. We study the morphology of the growing domains and the wetting layer, the growth exponent, and dynamical scaling behavior for a critical composition of the mixture. We find that the characteristic size of domains grows as t1/3 for deep quenches, as long as the average domain size is small compared to the average “pore” radius of the unoccupied region. This suggests that the kinetics of domain growth at intermediate times is not appreciably perturbed by the presence of the network. For off-critical compositions, we study domain growth in two different situations where either the majority or the minority component of the blend wets the network. When the majority component wets the network, a network-induced nucleation is possible for sufficiently off-critical mixtures due to the reduction or elimination of the local nucleation barrier for the minority phase near the network. For this reason, minority droplets nucleate predominantly near the network before they appear in the bulk. When the minority phase wets the network, a slowdown in wetting layer growth is found to occur at late times due to a depletion of the system of its minority component.
Component segmental mobilities in an athermal polymer blend: Quasielastic incoherent neutron scattering versus simulation
Authors: Doxastakis M. , Kitsiou M. , Fytas G. , Theodorou D. N. , Hadjichristidis N. , Meier G. , Frick B.
Published: July 2002

The local dynamics of the miscible blend of cis-1,4 polyisoprene (PI) (70% by weight) with 1,2 polybutadiene (PVE) (30% by weight) is studied. Quasielastic incoherent neutron scattering (QENS) experiments have provided the dynamic structure factor for each component in the blend far above the glass transition temperature. Molecular dynamics simulations on the same system have given segmental relaxation functions in good agreement with the experiments. Both methods reveal differences in the mobilities of each component, even at high temperatures, emphasizing intramolecular factors. Remarkably, the segmental relaxation of the PVE component in the PI/PVE blend rich in PI resembles that of PI and not of the pure PVE.
Nonequilibrium phase diagram of a polydisperse system: A molecular dynamics study
Authors: Komatsugawa Hiroshi , Nosé Shuichi
Published: July 2002

We investigate the structural changes of a polydisperse system undergoing an oscillator shear flow by use of a nonequilibrium molecular dynamics simulation. The nonequilibrium phase diagrams are investigated in the space of the amplitude of the shear rate and the number density at several polydispersities. At small polydispersity, phase changes to two types of layered structures are observed and this phenomenon is similar to that in the monodisperse case. With increase of polydispersity, the system becomes disordered, and an amorphous phase is observed in the equilibrium state. Furthermore, we investigate the size dependence of structural changes in the polydisperse system. In consequence, we observe less of a dependence on the phase changes in our oscillatory shear case.
Millimeter-wave spectroscopy and coupled cluster calculations for NCCP
Authors: Bizzocchi Luca , Esposti Claudio Degli , Botschwina Peter
Published: July 2002

The rotational spectrum of the unstable NCCP molecule has been investigated in the millimeter-wave region. The ground-state spectra of the most abundant isotopomer and of the C13 and N15 containing species were studied, and lines in the v2, v3, v4, and v5 vibrationally excited states were detected for the normal isotopomer. Electric quadrupole and magnetic spin–rotation coupling constants of the nitrogen nucleus were also determined. The experimental work was assisted by high level coupled-cluster single double triple [CCSD(T)] calculations, performed using the cc-pVQZ basis, which provided accurate predictions for the αr vibration–rotation coupling constants and the ground-state rotational constants of the less abundant isotopic species. r0 and rs molecular structures of NCCP were derived directly from the experimental ground-state rotational constants of four different isotopomers, and an accurate equilibrium structure could be evaluated by combining theoretically computed vibration–rotation coupling constants with experimental ground-state rotational constants.
Nonsingular spiked harmonic oscillator
Authors: Aguilera-Navarro V. C. , Guardiola R.
Published: July 2002

A perturbative study of a class of nonsingular spiked harmonic oscillators defined by the Hamiltonian H=−d2/dr 2+r 2+λ/r α in the domain [0,∞] is carried out, in the two extremes of a weak coupling and a strong coupling regimes. A path has been found to connect both expansions for α near 2.
On the Green-functions technique and phase velocity approximation of axially symmetric fields in stratified media
Authors: Ahmad Falih , Razzaghi Mohsen
Published: July 2002

In this paper a numerical method is developed to solve a one-dimensional inverse scattering problem associated with a pair of two coupled partial differential equations. These equations are for the Green functions that can be used for the optimization of electromagnetic energy within selected regions of a medium in which the propagation is taking place. The method is based upon constructing the Nth degree interpolation polynomial to approximate the Green functions and the phase velocity function using Legendre–Gauss–Lobatto collocation points. An example is given to demonstrate the accuracy of the developed method.
Inhomogeneous Kasner-type cylindrically symmetric models in Kaluza–Klein space–time
Authors: Patel L. K. , Dadhich Naresh
Published: July 2002

We consider a cylindrically symmetric metric separable in space and time variables having Kasnerian time dependence in the Kaluza–Klein space–time. We obtain perfect fluid models that admit the dimensional reduction yielding the usual four-dimensional space–time as the universe expands. The models are inhomogeneous and they have the Kasnerian vacuum solution as their matter-free limit. The five-dimensional Kasnerian vacuum solution goes over on dimensional reduction to the radiation Friedman–Robertson–Walker flat model.
Non-covariance of the generalized holonomies: Examples
Authors: Schilling Troy A.
Published: July 2002

A key aspect of a recent proposal for a generalized loop representation of quantum Yang–Mills theory and gravity is considered. Such a representation of the quantum theory has been expected to arise via consideration of a particular algebra of observables – given by the traces of the holonomies of generalized loops. We notice, however, a technical subtlety, which prevents us from reaching the conclusion that the generalized holonomies are covariant with respect to small gauge transformations. Further analysis is given which shows that they are not covariant with respect to small gauge transformations; their traces are not observables of the gauge theory. This result indicates what may be a serious complication to the use of generalized loops in physics, but does not affect the ordinary loop representation.
Debye potentials for Maxwell and Dirac fields from a generalization of the Killing–Yano equation
Authors: Benn I. M. , Charlton Philip , Kress Jonathan
Published: July 2002

By using conformal Killing–Yano tensors, and their generalizations, we obtain scalar potentials for both the source-free Maxwell and massless Dirac equations. For each of these equations we construct, from conformal Killing–Yano tensors, symmetry operators that map any solution to another.
Hydrodynamic dispersion and pore geometry in consolidated rock
Authors: Gist G. A. , Thompson A. H. , Katz A. J. , Higgins R. L.
Published: July 2002

Experimental measurements of the dispersion of tracer particles in flow through natural porous media are compared with a percolation model. The experiments show that tracer dispersion is a sensitive function of the width of the pore-size distribution as measured by mercury capillary pressure. Measurements of capillary pressure (or electrical conductivity) are used to estimate the geometric correlation length of the dominant flow path in the rock. Percolation theory is used to derive a power-law relationship between the correlation length and the ratio of the dispersivity to the average grain size. The experimental value of the power-law exponent is in agreement with the theoretical prediction. Measurements on samples containing a residual saturation of wetting epoxy show no significant change in dispersion behavior. This result mediates against dispersion models requiring trapping in dead-end pores. Tracer concentration profiles exhibit anomalous long-time tails in two cases. In carbonate rocks, we associate long-time tails with macroscopic permeability heterogeneities. In sandstones, long-time tails occur in samples with a very narrow pore-size distribution. These samples may have permeability heterogeneities as a result of defects in the packing density. In the limit of low flow velocity, the long-time tail disappears, suggesting a convective mechanism associated with flow heterogeneities at a millimeter-or-larger scale.
On the Yakhot–Orszag renormalization group method for deriving turbulence statistics and models
Authors: Smith L. M. , Reynolds W. C.
Published: July 2002

The purpose of this paper is to provide an independent, comprehensive, critical review of the ‘‘renormalization group’’ theory of turbulence developed by Yakhot and Orszag [J. Sci. Comput. 1, 3 (1986)]. The first part of the paper is a confirmation of their basic theory for the Navier–Stokes equations, followed by a discussion of approximations in the scale removal procedure. The main part of the paper examines the YO derivations of the velocity-derivative skewness and the transport equation for the energy dissipation rate ℰ. An algebraic error in the derivation of the skewness is corrected. Several problems are identified in the derivation of the ℰ equation which suggest that the derivation should be reformulated. The present paper has become the basis for such a reformulation, to be published separately by Yakhot and Smith.
Rayleigh–Taylor instability of a plasma–vacuum boundary in the limit of a large Larmor radius
Authors: Velikovich A. L.
Published: July 2002

The Rayleigh–Taylor instability of a plasma–vacuum boundary is studied with the aid of an ideal one-fluid Hall model corresponding to the limit of a large ion Larmor radius. The global Rayleigh–Taylor instability mode, whose growth rate is Γ(k)=(gk)1/2, still exists in this case, but is not necessarily the dominating one. The exact upper bound for the growth rates of all the other instability modes is found in the low-beta limit. For large wave numbers the growth rate of the fastest instability mode is estimated as Γmax(k)=gk/Ωi.
The thermal nature of the triple point of a Mach reflection
Authors: Ben-Dor G. , Takayama K. , Needham C. E.
Published: July 2002

An experimental investigation followed by some theoretical considerations suggests that the triple point of a three-shock confluence behaves as a ‘‘hot spot.’’ Numerical calculations using the hull code, which was developed by the U. S. Air Force Weapons Laboratory (National Technical Information Service Document No. ADB 014070/LP) and modified by S-Cubed [Shock Waves and Shock Tubes (Stanford University Press, Stanford, CA 1986), pp. 407–413] have provided supporting evidence for hot spot behavior near the triple point. Furthermore, the numerical calculations also show a spike in the internal energy, the pressure, and the density near the triple point.
Experimental and theoretical evaluation of the k−11 spectral law
Authors: Turan Ö. , Azad R. S. , Kassab S. Z.
Published: July 2002

In this paper the k−11 spectral law is examined using the experimental data from fully developed pipe, boundary layer, and diffuser flows (here, k1 denotes a one-dimensional longitudinal wavenumber). The type of pressure gradient present in a wall-bounded flow has been found to play a controlling role on the presence and extent of the k−11 spectral law. In fully developed pipe and boundary layer flows, under favorable and zero pressure gradients, respectively, the occurrence of the k−11 spectral law shows similar characteristics. On the other hand, the presence of adverse pressure gradient has been observed to modify these characteristics.
Measurements of a longitudinal vortex generated by a rectangular jet in a turbulent boundary layer
Authors: Zhang Xin , Collins Michael W.
Published: July 2002

Measurements were made on the mean flowfield created through the interaction between a two-dimensional flat plate turbulent boundary layer on a flat plate and an inclined jet. The jet was generated by a nozzle of rectangular exit and was pitched and skewed to the oncoming flow. A total of three pitch angles and two jet velocity ratios were tested. The measurements were performed with a three-component laser Doppler anemometer system in a low speed wind tunnel. The dominant feature of the flow physics is a single longitudinal vortex produced as a result of jet/boundary layer interaction, with additional induced secondary features in the near-wall area in the spanwise direction. A salient feature of the flow is its maximum vorticity position which is located underneath the center of the vortex. Within the measured flow parameter range, the vortex development differs also from that of a round jet, differences in the velocity distribution are observed. The study provides contributions to flow physics.
The long-time motion of vortex sheets with surface tension
Authors: Hou T. Y. , Lowengrub J. S. , Shelley M. J.
Published: July 2002

We study numerically the simplest model of two incompressible, immiscible fluids shearing past one another. The fluids are two-dimensional, inviscid, irrotational, density matched, and separated by a sharp interface under a surface tension. The nonlinear growth and evolution of this interface is governed by only the competing effects of the Kelvin–Helmholtz instability and the dispersion due to surface tension. We have developed new and highly accurate numerical methods designed to treat the difficulties associated with the presence of surface tension. This allows us to accurately simulate the evolution of the interface over much longer times than has been done previously. A surprisingly rich variety of behavior is found. For small Weber numbers, where there are no unstable length-scales, the flow is dispersively dominated and oscillatory behavior is observed. For intermediate Weber numbers, where there are only a few unstable length-scales, the interface forms elongating and interpenetrating fingers of fluid. At larger Weber numbers, where there are many unstable scales, the interface rolls-up into a “Kelvin-Helmholtz” spiral with its late evolution terminated by the collision of the interface with itself, forming at that instant bubbles of fluid at the core of the spiral. Using locally refined grids, this singular event (a “topological” or “pinching” singularity) is studied carefully. Our computations suggest at least a partial conformance to a local self-similar scaling. For fixed initial data, the pinching singularity times decrease as the surface tension is reduced, apparently towards the singularity time associated with the zero surface tension problem, as studied by Moore and others. Simulations from more complicated, multi-modal initial data show the evolution as a combination of these fingers, spirals, and pinches.
A class of exact multipolar vortices
Authors: Crowdy Darren
Published: July 2002

A class of exact solutions to the steady Euler equations representing finite area patches of nonuniform vorticity is presented. It is demonstrated that the solutions constitute a special class of steady multipolar vortical structures and have many qualitative similarities with the multipolar equilibria observed in two-dimensional flows at high Reynolds numbers. The results provide insights into the mathematical structure of the two-dimensional Euler equation that, it is argued, underlies the occurrence of such multipolar coherent structures in real physical flows. Moreover, the new solutions possess the interesting feature of being completely “invisible” in that their presence cannot be detected anywhere outside the support of the vorticity.
Multidimensional behavior of an electrostatic ion wave in a magnetized plasma
Authors: Nishinari Katsuhiro , Abe Kanji , Satsuma Junkichi
Published: July 2002

Three-dimensional nonlinear evolution equations of an electrostatic ion wave in the short wave region in a magnetized plasma are derived by means of the reductive perturbation method. It is shown that, in some cases, the evolution equations reduce to the Davey–Stewartson 1 equations, which are known to admit solutions with localized structure in higher dimension. It is also shown that there is a possibility of collapse of localized structure in the case of wave propagation parallel to a magnetic field.
Room-temperature photoluminescence times in a GaAs/AlxGa1−xAs molecular beam epitaxy multiple quantum well structure
Authors: Fouquet J. E. , Siegman A. E.
Published: July 2002

Time-resolved photoluminescence measurements at room temperature of the n=1 heavy hole transition in a GaAs/AlxGa1−xAs multiple quantum well structure reveal a single-exponential decay with τ≊1 ns over a wide range of excitation densities. Time-integrated photoluminescence increases as the square of excitation energy density. These data indicate that the observed decay rate is due to nonradiative recombination. Free carriers, not excitons, govern radiative recombination in this sample.
Time and wavelength resolved nonlinear optical spectroscopy of a polydiacetylene in the solid state using picosecond dye laser pulses
Authors: Carter G. M. , Thakur M. K. , Chen Y. J. , Hryniewicz J. V.
Published: July 2002

Degenerate four-wave mixing of tunable dye laser pulses in good optical quality single crystal platelets of poly bis(p-toluene sulfonate) of 2, 4-hexadiyne-1, 6 diol (PTS) polydiacetylene was used to measure the spectral and temporal dependence of the material’s third order nonlinear optical susceptibility χ(3). For the wavelength range measured ‖χ(3)‖ varied from 9×10−9 esu at 6515 Å to 5×10−10 esu at 7015 Å. The material’s response time was determined to be less than the optical pulse width (≂6 ps) over this wavelength range.
Surface plasmon polariton enhanced light emission from Schottky diodes
Authors: Köck A. , Beinstingl W. , Berthold K. , Gornik E.
Published: July 2002

We have investigated the light emission from forward- and reverse-biased sinusoidally structured Ag/n-GaAs Schottky diodes. Sinusoidally structured Schottky junctions show increased light emission because of the radiative decay of excited surface plasmon polaritons, resulting in drastically enhanced quantum efficiency. A model explaining excitation and emission of surface plasmon polaritons is presented.
Combinatorial computational method gives new picomolar ligands for a known enzyme
Authors: Grzybowski Bartosz A. , Ishchenko Alexey V. , Kim Chu-Young , Topalov George , Chapman Robert , Christianson David W. , Whitesides George M. , Shakhnovich Eugene I.
Published: July 2002

Combinatorial small molecule growth algorithm was used to design inhibitors for human carbonic anhydrase II. Two enantiomeric candidate molecules were predicted to bind with high potency (with R isomer binding stronger than S), but in two distinct conformations. The experiments verified that computational predictions concerning the binding affinities and the binding modes were correct for both isomers. The designed R isomer is the best-known inhibitor (
K
d
∼ 30 pM) of human carbonic anhydrase II.
Identification of a functional role for lipid asymmetry in biological membranes: Phosphatidylserine-skeletal protein interactions modulate membrane stability
Authors: Manno Sumie , Takakuwa Yuichi , Mohandas Narla
Published: July 2002

Asymmetric distribution of phospholipids is ubiquitous in the plasma membranes of many eukaryotic cells. The majority of the aminophospholipids are located in the inner leaflet whereas the cholinephospholipids are localized predominantly in the outer leaflet. Several functional roles for asymmetric phospholipid distribution in plasma membranes have been suggested. Disruption of lipid asymmetry creates a procoagulant surface on platelets and serves as a trigger for macrophage recognition of apoptotic cells. Furthermore, the dynamic process of phospholipid translocation regulates important cellular events such as membrane budding and endocytosis. In the present study, we used the red cell membrane as the model system to explore the contribution of phospholipid asymmetry to the maintenance of membrane mechanical properties. We prepared two different types of membranes in terms of their phospholipid distribution, one in which phospholipids were scrambled and the other in which the asymmetric distribution of phospholipids was maintained and quantitated their mechanical properties. We documented that maintenance of asymmetric distribution of phospholipids resulted in improved membrane mechanical stability. The greater difficulty in extracting the spectrin–actin complex at low-ionic strength from the membranes with asymmetric phospholipid distribution further suggested the involvement of interactions between aminophospholipids in the inner leaflet and skeletal proteins in modulating mechanical stability of the red cell membrane. These findings have enabled us to document a functional role of lipid asymmetry in regulating membrane material properties.
Rat strain-specific actions of 17β-estradiol in the mammary gland: Correlation between estrogen-induced lobuloalveolar hyperplasia and susceptibility to estrogen-induced mammary cancers
Authors: Harvell Djuana M. E. , Strecker Tracy E. , Tochacek Martin , Xie Benjamin , Pennington Karen L. , McComb Rodney D. , Roy Shyamal K. , Shull James D.
Published: July 2002

The genetically related ACI and Copenhagen (COP) rat strains
display diametrically opposed susceptibilities to mammary cancer
development when treated chronically with 17β-estradiol (E2). Here,
we compare the actions of E2 on cell proliferation and lobuloalveolar
development in the mammary glands of female ACI and COP rats. After 12
wk of E2 treatment, the mammary glands of ACI rats exhibited a
significantly greater proliferative response to E2, compared with COP
rats, as evidenced by quantification of S phase fraction and
development of lobuloalveolar hyperplasia. Focal regions of atypical
epithelial hyperplasia were observed in ACI, but not COP, rats. These
strain differences were not because of differences in circulating E2,
progesterone or, prolactin. Two-thirds of the induced mammary cancers
in ACI rats exhibited aneuploidy. The E2-induced mammary cancers
regressed when hormone treatment was discontinued, indicating that they
were estrogen-dependent. Progesterone receptor was expressed by the
great majority of epithelial cells within the E2-induced atypical
hyperplastic foci and the mammary carcinomas, suggesting a link between
these lesions. These data demonstrate a correlation between E2 action
in the induction of mammary cell proliferation and atypical epithelial
hyperplasia and genetically conferred susceptibility to E2-induced
mammary cancers.
Single-neuron correlates of subjective vision in the human medial temporal lobe
Authors: Kreiman Gabriel , Fried Itzhak , Koch Christof
Published: July 2002

Visual information from the environment is transformed into perceptual sensations through several stages of neuronal processing. Flash suppression constitutes a striking example in which the same retinal input can give rise to two different conscious visual percepts. We directly recorded the responses of individual neurons during flash suppression in the human amygdala, entorhinal cortex, hippocampus, and parahippocampal gyrus, allowing us to explore the neuronal responses in untrained subjects at a high spatial and temporal resolution in the medial temporal lobe. Subjects were patients with pharmacologically intractable epilepsy implanted with depth electrodes to localize the seizure onset focus. We observed that the activity of two thirds of all visually selective neurons followed the perceptual alternations rather than the retinal input. None of the selective neurons responded to a perceptually suppressed stimulus. Therefore, the activity of most individual neurons in the medial temporal lobe of naive human subjects directly correlates with the phenomenal visual experience.
The
Saccharomyces cerevisiae
Fin1 protein forms cell cycle-specific filaments between spindle pole bodies
Authors: van Hemert Martijn J. , Lamers Gerda E. M. , Klein Dionne C. G. , Oosterkamp Tjerk H. , Steensma H. Yde , van Heusden G. Paul H.
Published: July 2002

The
FIN1
gene from the yeast
Saccharomyces cerevisiae
encodes a basic protein with putative coiled-coil regions. Here we show that in large-budded cells a green fluorescent protein-Fin1 fusion protein is visible as a filament between the two spindle pole bodies. In resting cells the protein is undetectable, and in small-budded cells it is localized in the nucleus. During late mitosis it localizes on the spindle pole bodies. Filaments of cyano fluorescent protein-tagged Fin1 colocalize with filaments of green fluorescent protein-tagged Tub1 only in large-budded cells. By electron and atomic force microscopy we showed that purified recombinant Fin1p self-assembles into filaments with a diameter of ≈10 nm. Our results indicate that the Fin1 protein forms a cell cycle-specific filament, additional to the microtubules, between the spindle pole bodies of dividing yeast cells.
Refined molecular hinge between allosteric and catalytic domain determines allosteric regulation and stability of fungal chorismate mutase
Authors: Helmstaedt Kerstin , Heinrich Gabriele , Lipscomb William N. , Braus Gerhard H.
Published: July 2002

The yeast chorismate mutase is regulated by tyrosine as feedback inhibitor and tryptophan as crosspathway activator. The monomer consists of a catalytic and a regulatory domain covalently linked by the loop L220s (212–226), which functions as a molecular hinge. Two monomers form the active dimeric enzyme stabilized by hydrophobic interactions in the vicinity of loop L220s. The role of loop L220s and its environment for enzyme regulation, dimerization, and stability was analyzed. Substitution of yeast loop L220s in place of the homologous loop from the corresponding and similarly regulated
Aspergillus
enzyme (and the reverse substitution) changed tyrosine inhibition to activation. Yeast loop L220s substituted into the
Aspergillus
enzyme resulted in a tryptophan-inhibitable enzyme. Monomeric yeast chorismate mutases could be generated by substituting two hydrophobic residues in and near the hinge region. The resulting Thr-212→Asp–Phe-28→Asp enzyme was as stable as wild type, but lost allosteric regulation and showed reduced catalytic activity. These results underline the crucial role of this molecular hinge for inhibition, activation, quaternary structure, and stability of yeast chorismate mutase.
Differential fluorescence labeling of cysteinyl clusters uncovers high tissue levels of thionein
Authors: Yang Yi , Maret Wolfgang , Vallee Bert L.
Published: July 2002

The isolation of thionein (T) from tissues has not been reported
heretofore. T contains 20 cysteinyl residues that react with
7-fluorobenz-2-oxa-1,3-diazole-4-sulfonamide to form fluorescent
adducts. In metallothionein (MT) the cysteinyl residues, which
are bound to zinc, do not react. However, they do react in the presence
of a chelating agent such as EDTA. The resultant difference in chemical
reactivity provides a means to measure T in the absence of EDTA, (MT +
T) in its presence, and, of course, MT by difference. The
7-fluorobenz-2-oxa-1,3-diazole-4-sulfonamide derivative of T can be
isolated from tissue homogenates by HPLC and quantified
fluorimetrically with a detection limit in the femtomolar range and a
linear response over 3 orders of magnitude. Analysis of liver, kidney,
and brain of rats reveals almost as much T as MT. Moreover, in contrast
to earlier views, MT in tissue extracts appears to be less stable than
T. The existence of T in tissues under normal physiological conditions
has important implications for its function both in zinc metabolism and
the redox balance of the cell.
From flies' eyes to our ears: Mutations in a human class III myosin cause progressive nonsyndromic hearing loss DFNB30
Authors: Walsh Tom , Walsh Vanessa , Vreugde Sarah , Hertzano Ronna , Shahin Hashem , Haika Smadar , Lee Ming K. , Kanaan Moien , King Mary-Claire , Avraham Karen B.
Published: July 2002

Normal vision in
Drosophila
requires NINAC, a class III myosin. Class III myosins are hybrid motor-signaling molecules, with an N-terminal kinase domain, highly conserved head and neck domains, and a class III-specific tail domain. In
Drosophila
rhabdomeres, NINAC interacts with actin filaments and with a PDZ scaffolding protein to organize the phototransduction machinery into a signaling complex. Recessive null mutations in
Drosophila
NINAC delay termination of the photoreceptor response and lead to progressive retinal degeneration. Here, we show that normal hearing in humans requires myosin IIIA, the human homolog of NINAC. In an extended Israeli family, nonsyndromic progressive hearing loss is caused by three different recessive, loss-of-function mutations in myosin IIIA. Of 18 affected relatives in Family N, 7 are homozygous and 11 are compound heterozygous for pairs of mutant alleles. Expression of mammalian myosin IIIA is highly restricted, with the strongest expression in retina and cochlea. The involvement of homologous class III myosins in both
Drosophila
vision and human hearing is an evolutionary link between these sensory systems.
Firing rates of hippocampal neurons are preserved during subsequent sleep episodes and modified by novel awake experience
Authors: Hirase Hajime , Leinekugel Xavier , Czurkó András , Csicsvari Jozsef , Buzsáki György
Published: July 2002

What determines the firing rate of cortical neurons in the absence of external sensory input or motor behavior, such as during sleep? Here we report that, in a familiar environment, the discharge frequency of simultaneously recorded individual CA1 pyramidal neurons and the coactivation of cell pairs remain highly correlated across sleep–wake–sleep sequences. However, both measures were affected when new sets of neurons were activated in a novel environment. Nevertheless, the grand mean firing rate of the whole pyramidal cell population remained constant across behavioral states and testing conditions. The findings suggest that long-term firing patterns of single cells can be modified by experience. We hypothesize that increased firing rates of recently used neurons are associated with a concomitant decrease in the discharge activity of the remaining population, leaving the mean excitability of the hippocampal network unaltered.
Functional activity and role of cation-efflux family members in Ni hyperaccumulation in
Thlaspi goesingense
Authors: Persans Michael W. , Nieman Ken , Salt David E.
Published: July 2002

The ability of
Thlaspi goesingense
to
hyperaccumulate Ni seems to be governed in part by enhanced
accumulation of Ni within leaf vacuoles. We have characterized genes
from
T. goesingense
encoding putative vacuolar metal ion
transport proteins, termed
m
etal
t
olerance
p
roteins (TgMTPs). These
proteins contain all of the features of cation-efflux family members,
and evidence indicates they are derived from a single genomic sequence
(
TgMTP1
) that gives rise to an unspliced
(
TgMTP1t1
) and a spliced (
TgMTP1t2
)
transcript. Heterologous expression of these transcripts in yeast
lacking the TgMTP1 orthologues COT1 and ZRC1 complements the metal
sensitivity of these yeast strains, suggesting that TgMTP1s are able to
transport metal ions into the yeast vacuole in a manner similar to COT1
and ZRC1. The unspliced and spliced TgMTP1 variants differ within a
histidine-rich putative metal-binding domain, and these sequence
differences are reflected as alterations in the metal specificities of
these metal ion transporters. When expressed in yeast,
TgMTP1t1
confers the highest level of tolerance to Cd,
Co, and Zn, whereas
TgMTP1t2
confers the highest
tolerance to Ni.
TgMTP1
transcripts are highly expressed
in
T. goesingense
compared with orthologues in the
nonaccumulators
Arabidopsis thaliana
,
Thlaspi
arvense
, and
Brassica juncea
. We propose that
the high-level expression of
TgMTP1
in
T.
goesingense
accounts for the enhanced ability of this
hyperaccumulator to accumulate metal ions within shoot vacuoles.
A new recommended dietary allowance of vitamin C for healthy young women
Authors: Levine Mark , Wang Yaohui , Padayatty Sebastian J. , Morrow Jason
Published: July 2002

The recently released Recommended Dietary Allowance of vitamin C
for women, 75 mg daily, was based on data for men. We now report
results of a depletion–repletion study with healthy young women
hospitalized for 186 +/− 28 days, using vitamin C doses of 30–2,500
mg daily. The relationship between dose and steady-state plasma
concentration was sigmoidal. Only doses above 100 mg were beyond the
linear portion of the curve. Plasma and circulating cells saturated at
400 mg daily, with urinary elimination of higher doses. Biomarkers of
endogenous oxidant stress, plasma and urine
F
2
-isoprostanes, and urine levels of a major metabolite of
F
2
-isoprostanes were unchanged by vitamin C at all doses,
suggesting this vitamin does not alter endogenous lipid
peroxidation in healthy young women. By using Food and Nutrition Board
guidelines, the data indicate that the Recommended Dietary Allowance
for young women should be increased to 90 mg daily.