bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and thermodynamic properties of molten alkali chlorides
Authors: Ballone P. , Pastore G. , Tosi M. P.
Published: February 2003

Self-consistent calculations of partial pair distribution functions and thermodynamic properties are presented for molten alkali chlorides in a nonpolarizable ion model. The theory starts from the hypernetted chain approximation and analyzes the role of bridge diagrams both for a two-component ionic plasma on a neutralizing background and for a binary ionic liquid of cations and anions. A simple account of excluded-volume effects suffices for a good description of the pair distribution functions in the two-component plasma, in analogy with earlier work on one-component fluids. The interplay of Coulomb attractions and repulsions in the molten salt requires, on the other hand, the inclusion of (i) excluded–volume effects for the various ion pairs as in a mixture of hard spheres with nonadditive radii and (ii) medium-range Coulomb effects reflected mainly in the like-ion correlations. All these effects are included approximately in an empirical evaluation of the bridge functions, with numerical results which compare very well with computer simulation data. A detailed discussion of the results against experimental structural data is then given in the case of molten sodium chloride.
The EUV photoexcitation of a H2S molecule
Authors: Ibuki Toshio
Published: February 2003

Hydrogen sulfide was excited by the EUV Ne i (16.85 eV) and He i (21.22 eV) resonance lines. By using optical emission spectroscopy, the H2S+ (Ã 2A1) radical cation and the H Balmer emission were detected. The Franck–Condon factors for the H2S+ (Ã 2A1, v′2 =v→X̃ 2B1, v=2=0) progressions were estimated. It has been suggested that the H2S+ (Ã 2A1) ion at the v′2=7 vibrational level would start predissociation into H2(1Σ+g) +S+ (4Su) via the repulsive 1 4A″ potential surface. The absolute fluorescence cross sections for the H2S+ (Ã→X̃) and the H Balmer series were deduced at the Ne i and He i resonance lines.
Predissociation processes in N2+2, O2+2, and NO2+ studied by ion kinetic energy spectroscopy
Authors: Curtis J. M. , Boyd R. K.
Published: February 2003

Experimental values of kinetic energy release T, for predissociation processes of diatomic di-cations N2+2, O2+2, and NO2+ have been obtained using techniques of ion-kinetic-energy spectroscopy on a double-focusing mass spectrometer. Both unimolecular and collision-induced processes were studied. As a result, the present data confirm earlier work but include additional transitions not reported previously. Values of T obtained by entirely different technqiues are shown to be consistent with the present work when differences in energy resolution and experimental time window are considered. Attempts to correlate all available information on these diatomic di-cations (Auger, double-charge-transfer and kinetic-energy loss spectra, as well as appearance energy measurements) are hampered by lack of high-level a priori calculations of the potential curves.
Response of liquid carbon disulfide to shock compression. II. Experimental design and measured Hugoniot information
Authors: Sheffield S. A.
Published: February 2003

Carbon disulfide (CS2) is known to encounter a region of high compressibility upon shock loading which is conjectured to accompany a reaction precipitated by the shock loading. One candidate explanation for this is that the CS2 reacts chemically with an accompanying negative volume change. An idealized double-shock experiment is proposed to probe the process using conventional shock measurables. Shock experiments based on this idea used a sapphire-faced projectile to impact a plastic cell containing the CS2, constructed with a sapphire back plate which served as an anvil to produce the second shock. Electromagnetic velocity gauges were used to monitor the particle velocity history at several Lagrangian positions in the liquid. The ‘‘unreacted’’ Hugoniot was deduced from measurements made as the initial shock traversed the CS2. Upon reflection of the initial shock from the sapphire anvil, an evolving two-wave structure was observed, a direct result of the increased compressibility. The Hugoniot cusp at the onset of the high compressibility region is estimated to occur at about 4.6 GPa, a value somewhat lower than previously reported. Based on an analysis of the states achieved in the two-wave structure, the reaction is inferred to be much more temperature than pressure dependent, adding strength to the hypothesis that a chemical reaction rather than a phase transition is occurring.
Crystal field analysis of Tm3+ and Yb3+ in YPO4 and LuPO4
Authors: Becker P. C. , Hayhurst T. , Shalimoff G. , Conway J. G. , Edelstein N. , Boatner L. A. , Abraham M. M.
Published: February 2003

The optical and near infrared absorption of dilute Tm3+ and Yb3+ impurities in YPO4 and LuPO4 single crystals have been measured at liquid helium and nitrogen temperatures. For Tm3+, the spectral region from 5 000 to 38 000 cm−1 was examined and Zeeman spectra were obtained in the visible region. The observed transitions were assigned and fit to a semiempirical Hamiltonian with adjustable parameters via a least-squares procedure. Satisfactory fits and good agreement between the calculated and measured g values were obtained. For Yb3+, there are more parameters than experimental levels, so ζ, B20, B40, and B44 were adjusted, while B60 and B64 were fixed at the values found for Tm3+. Energy levels and Zeeman splittings calculated with these parameters are in good agreement with the measured quantities.
The ν3 fundamental bands of HN+2, DN+2, and DCO+
Authors: Foster S. C. , McKellar A. R. W.
Published: February 2003

The ν3 (N–N or C–O stretch) fundamental bands of HN+2,DN+2, and DCO+ in the 5 μm region have been studied using a tunable infrared diode laser source. The absorption cell was a large liquid-nitrogen-cooled hollow cathode, and a discharge modulation scheme was used to enhance sensitivity and discriminate in favor of molecular ions. We observed 22 transitions for HN+2, 10 transitions for DN+2, and 15 transitions for DCO+. The analyses were straightforward because ground state rotational spectra are known for all three species. The resulting experimental ν3 band origins are 2257.8667(13), 2024.0141(13), and 1904.0640(20) cm−1, and the α3 (≡B″–B′) values are 329.7(12), 239.0(13), and 206.6(12) MHz for HN+2,DN+2, and DCO+, respectively.
Viscomagnetic effect: j-magnitude weighting for Ar–N2
Authors: Snider R. F.
Published: February 2003

A continuing question in the study of the viscomagnetic effect has been the dependence on j magnitude, of the angular momentum polarization. It has been generally accepted that neither the normalized nor the unrenormalized angular momentum quadrupole correctly interprets the experimental results. IOS calculations of the production and relaxation cross sections are performed keeping the full j-magnitude dependence. Predictions of the field dependence of the viscomagnetic effect are made and it is found that the j dependence of both the production cross sections and of the relaxation matrix influence the detailed field dependence of the viscomagnetic effect.
Monte Carlo Fourier path integral methods in chemical dynamics
Authors: Doll J. D.
Published: February 2003

Monte Carlo Fourier path integral methods are discussed, both with respect to their use for equilibrium statistical mechanics and to their use in chemical dynamics. It is argued that such techniques offer a practical, direct route to complex temperature density matrix elements necessary to implement recent quantum reactive flux formalisms. Applications to a one-dimensional test problem (the Eckart barrier) are discussed. A simple (classical) approximation scheme involving a temperature-dependent effective potential is also considered.
Test of variational transition state theory against accurate quantal results for a reaction with very large reaction-path curvature and a low barrier
Authors: Truhlar Donald G. , Garrett Bruce C. , Hipes Paul G. , Kuppermann Aron
Published: February 2003

We present three sets of calculations for the thermal rate constants of the collinear reaction I+HI→IH+I: accurate quantum mechanics, conventional transition state theory (TST), and variational transition state theory (VTST). This reaction differs from previous test cases in that it has very large reaction-path curvature but hardly any tunneling. TST overestimates the accurate results by factors of 2×1010, 2×104, 57, and 19 at 40, 100, 300, and 1000 K, respectively. At these same four temperatures the ratios of the VTST results to the accurate quantal ones are 0.3, 0.8, 1.1, and 1.4, respectively. We conclude that the variational transition states are meaningful, even though they are computed from a reaction-path Hamiltonian with large curvature, which is the most questionable case.
I*(5 2P1/2) quenching kinetics
Authors: Burrows Michael D.
Published: February 2003

Rate constants for the quenching of excited I(5 2P1/2) atoms by I2, O2, ICl, and Cl2 have been measured using time resolved I(2P1/2→2P3/2) 1.315 μm fluorescence from iodine atoms produced by the UV laser induced photodissociation of either CF3I or i-C3F7I. Using the pulsed photodissociation of CF3I at 248 nm as an I(5 2P1/2) source the measured rates (cm3/molecule s) were (3.0±0.1)×10−11, (2.1±0.1)×10−11, (3.3±0.4)×10−11, and (2.0±0.1)×10−14 for I2, O2, ICl, and Cl2, respectively. Using the pulsed photodissociation of i-C3F7I at 308 nm as an I(5 2P1/2) source, similar rates were measured for I* quenching by I2 and ICl, but the I(5 2P1/2)+Cl2 quenching rate increased strongly with increasing amounts of Cl2 photodissociation. For approximately 1.8% Cl2 dissociation at 308 nm the measured rate constant for I(5 2P1/2)+Cl2 quenching was (4.3±0.1)×10−13 cm3/molecule s. This increase in the I*+Cl2 rate is attributed to the two body quenching reaction I*+Cl→I+Cl with a rate constant on the order of 1.5×10−11 cm3/molecule s.
Photochemical site conversion: Octatetraene in n-octane
Authors: Ackerman John R. , Forman Stuart A. , Katz Larry , Kohler Bryan E. , Wu Konrad T.
Published: February 2003

Irradiation of n-alkane solutions of all-trans octatetraene at temperatures below 10 K produces a number of photoproducts. This paper reports spectroscopic and thermodynamic properties of one of these which has been identified to be the all-trans isomer in a second, less stable crystallographic site than that produced by slowly cooling through a succession of equilibrium states. This identification follows from the excited state lifetime and high resolution fluorescence and one- and two-photon excitation spectra for this photoproduct in n-octane which show that: (1) states of this photoproduct are only slightly shifted with respect to those of the all-trans precursor; (2) the fluorescence lifetime is nearly the same as that measured previously for the all-trans precursor; (3) the photoproduct is only stable at temperatures below 20 K, at higher temperatures it reverts to the all-trans isomer in the minimum energy site; and (4) the photoproduct is centrosymmetric. In the ground state the conversion of this photoproduct into the minimum energy site in n-octane proceeds over a barrier of 0.59+/−0.07 kcal mol−1. The barrier for this process in n-hexane is 0.68+/−0.16 kcal mol−1.
Comment on ‘‘Extensions of the molecular dynamics simulation method. II. Isothermal systems’’
Authors: Evans Denis J. , Morriss Gary P.
Published: February 2003

Haile and Gupta have given derivations of isothermal equations of motion for a system of N particles. The first derivation, based on generalized potentials, gives equations of motion which are derivable from a Hamiltonian and hence have four rather than three constants of the motion. These equations of motion have no obvious relevance to equilibrium statistical mechanics. Derivations which lead to isothermal equations of motion which have direct utility to statistical mechanics have been given by Evans, Hoover, and Nosé.
Excited states via the AGP polarization propagator. I. Application to Li2
Authors: Sangfelt E. , Kurtz H. A. , Elander N. , Goscinski O.
Published: February 2003

An approximation to the polarization propagator using an optimized antisymmetrized geminal power (AGP) wave function as a reference state is discussed. As an application, potential energy curves and spectroscopic constants for some of the low-lying electronic states in Li2 are given. Transition moments and radiative lifetimes for the experimentally observed B 1Πu–X 1Σ+g and A 1Σ+u–X 1Σ+g transitions are also presented. Excellent agreement with experiment and theoretical calculations of multiconfigurational self-consistent field (MCSCF) type is obtained. off
Local density pseudopotential calculations for molecules: O2 and Mo2O2S2(S2)2−2
Authors: Bernholc J. , Holzwarth N. A. W.
Published: February 2003

We report a development of a self-consistent, local density pseudopotential method for calculation of the electronic properties of large molecules. Angular momentum dependent, first-principles ionic pseudopotentials are used to describe the valence-core interactions. Average correlation effects are included in the local density sense. The wave functions, the charge density and the exchange-correlation potential are expanded in atom-centered Gaussian orbitals of s, p, and d symmetry. These atomic basis sets account for nonspherical (non-muffin tin) effects and are easily transferable to different geometries and other environments. The total energy of the system is accurately evaluated. The method is tested on the O2 molecule and found to give the orbital energies, the equilibrium distance and the vibrational frequency in good agreement with previous all-electron calculations. As the first application to a large system, we have investigated the electronic structure of the Mo2O2S2(S2)2−2 anion. In this anion, the molybdenum atoms are fivefold coordinated: to one oxygen atom, two bridging sulfur atoms, and a molecular S2. From the energy level diagram, the oxygen atoms are found to form approximately triple bonds to the molybdenum atoms, the Mo–S and Mo–S2 bonds being successively weaker. The molybdenum atoms form a single Mo–Mo bond and are formally in the Mo(V) oxidation state. The highest occupied molecular orbitals are, however, the S2 π* orbitals, oriented perpendicular to the Mo-S2 planes and located almost 1 eV above the Mo–Mo bond energy. The lowest unoccupied molecular orbitals are the relatively closely spaced π* antibonding Mo–O, Mo–Mo, and Mo–S2 orbitals.
A two-isotope higher-order RKR-type inversion procedure
Authors: Schwartz Carey , Le Roy Robert J.
Published: February 2003

The Rydberg–Klein–Rees (RKR) method is a valuable everyday tool for determining diatom potential energy curves. However, the RKR method is based upon the first-order WKB approximation, and while in most cases the potential curves it yields are adequate, there exist situations in which their deficiencies are unacceptable. This work addresses the above problem by presenting and testing a new higher-order, RKR-like inversion procedure which is exact within the third-order WKB approximation. The new method relies upon the existence of data for two isotopes. The turning point expressions obtained in the new method have exactly the same structure as the ordinary RKR expressions, and the procedure requires no iteration. Tests for two well-defined model problems show that the present method can yield potentials which are more accurate than those obtained from first-order RKR calculations, independent of whether or not the latter includes the widely used Kaiser correction.
Electron correlation in the Coulomb hole model. Comparison of methods
Authors: San-Fabián Emilio , Moscardó Federico
Published: February 2003

Methods, based both on the density functional and on the correlation factor, are analyzed by applying them to the correlation energy calculation of two- and four-electron closed shell atoms; so that only the short range electron correlation is considered. The method based on the correlation factor seems to be more promising than those of the density functional. However, in the present state of development, there are some serious deficiencies that limit their application to atomic and molecular calculations.
A
b i
n
i
t
i
o comparison of H bonds and Li bonds. Complexes of LiF, LiCl, HF, and HCl with NH3
Authors: Latajka Z. , Scheiner Steve
Published: February 2003

A b initio calculations are carried out on the complexes H3N–LiF, H3N–LiCl and their analogs H3N–HF and H3N–HCl as well as the isolated subunits. Double-zeta basis sets, augmented by two sets of polarization functions, are used in conjunction with second-order Moller–Plesset perturbation theory (MP2) for evaluation of electron correlation effects. The Li bonds are found to be substantially stronger than their H-bonding counterparts, due in large measure to the greater dipole moments of the LiX subunits. Correlation has a large effect on the geometry and energetics of both H-bonded complexes, reducing the intermolecular separation and contributing between 20% and 40% to the total complexation energy. In contrast, the SCF and MP2 results for the Li bonds are nearly identical. The small net effect of correlation in these complexes is ascribed to cancellation between incorporation of dispersion and reduction of the electrostatic component. Another distinction between the two types of bonds arises from consideration of the stretch of the LiX and HX bonds resulting from complex formation. Whereas correlation increases the magnitude of this bond elongation in the H bonds, an opposite trend is noted in the Li bonds.
Theory of the phase-change behavior of hydrogen in metals containing metallic impurities: The hydrogen–niobium–molybdenum system
Authors: Shirley Arthur I. , Hall Carol K. , Sahni Paramdeep S. , King Nancy J. P.
Published: February 2003

A theory of the phase-change behavior of hydrogen in metals containing substitutional impurities is described. A lattice-gas model is developed which includes both direct electronic and indirect elastic interactions between hydrogen atoms. The interactions between hydrogen atoms and the randomly distributed impurity atoms are treated as a random field in analogy with the random field Ising model. The model is applied to the hydrogen–niobium–molybdenum system. Monte Carlo simulation is used to calculate the free energy and incoherent phase diagrams. The calculation predicts that even small amounts (5%–15%) of molybdenum cause the disappearance of the α–α′ coexistence region found in Nb–H. In addition the α–β transition, which is first order in Nb–H, becomes second order above a certain temperature (the tricritical point). These results are in qualitative agreement with experiment.
Molecular theory of the volume change accompanying contact-complex formation reactions in solution
Authors: Yoshimura Y. , Nakahara M.
Published: February 2003

A theoretical expression of the volume change accompanying the contact-complex formation in the hard-sphere fluid at infinite dilution has been derived by using the radial distribution function and the equation of state. It is revealed that the volume change is composed of the two additive terms related to (i) the molecular packing and (ii) the thermal motion of molecules. In order to comprehend the volume change at the molecular level, the numerical calculation of the volume change based on the theory developed has been performed as a function of the solvent density and the size ratio of the solute to the solvent molecules. The volume changes obtained at liquid densities are dominated by the molecular packing term, and their values in solvents of the common size are in the range of −5 to −10 cm3 mol−1 in reasonable agreement with the experimental results for the weak EDA complexes in solution.
Energies of the phases of ice at zero temperature and pressure
Authors: Whalley E.
Published: February 2003

The energies of the high-pressure phases of ice at zero temperature and pressure, relative to the energy of ice I, have been evaluated from the equilibrium pressures of the various pairs of phases extrapolated to zero temperature. The values should be useful for evaluating interaction potentials between water molecules.
A
b i
n
i
t
i
o description of large amplitude motions in solid N2. III. Libron–phonon coupling
Authors: Briels W. J. , Jansen A. P. J. , van der Avoird Ad
Published: February 2003

A new lattice dynamics scheme is proposed for handling librons, anharmonic translational vibrations, and translational-rotational coupling in molecular crystals. This scheme is an extension of earlier libron models which describe large amplitude librations or hindered rotations. The formalism is based on expanding the intermolecular potential in the molecular displacement coordinates, including cubic and quartic terms, while retaining the exact orientational dependence. Closed expressions are obtained via spherical tensor methods. After constructing separate mean field states for the molecular rotations and translations, using bases of tesseral harmonics and 3D harmonic oscillator functions, respectively, the intermolecular correlations are taken into account and simultaneously the translational-rotational coupling, by solving the equations of motion for the crystal according to the time-dependent Hartree or random-phase approximation. Application of the formalism to the ordered α and γ phases of solid nitrogen, using an ab initio potential, gave very satisfactory results.
Orbiting resonance model for recombination of physisorbed atoms
Authors: Schwartz Carey , Le Roy Robert J.
Published: February 2003

The orbiting resonance model for atomic recombination, originally developed for applications to gas phase kinetics, is adapted for describing the recombination of atoms physisorbed on a surface. The model assumes that a population of atoms is initially physisorbed on a surface on which the atoms are free to move about in two dimensions. Atom–atom collisions give rise to long-lived orbiting pairs, which may in turn lose internal energy and become truly bound. The surface lowers the dimensionality of the problem, plays the role of the third body whose participation preserves energy and momentum conservation, and its corrugation provides the perturbation driving the second (inelastic) step of the mechanism. The present work involves the application of this model to the recombination of H or D atoms on the (111) and (100) surfaces of a Xe crystal, to obtain overall second-order rate constants at T=4 and 10 K.
Anharmonic potential functions as derived from Raman intensities: Methane
Authors: Bermejo D. , Montero S.
Published: February 2003

The cubic force field of methane has been investigated on the basis of the analysis of Raman scattering cross sections of binary overtones and combination bands. Seven absolute cross sections, ten symmetry cubic force constants, and the second derivative of the mean CH bond polarizability are the original results of present work. Some of these cubic force constants are determined with high accuracy. Most Ĥ22 rovibrational constants calculated by means of present anharmonic force field are in good agreement with the experimental values reported by various authors.
Semiclassical multistate curve-crossing models: Reduction of the transition amplitudes to diabatic and adiabatic phase integrals
Authors: Atabek O. , Lefebvre R. , Jacon M.
Published: February 2003

A systematic procedure for deriving semiclassical transition amplitudes in multistate curve-crossing situations is used to prove that, apart from a phase factor, this amplitude can be expressed entirely with the action angles defining the diabatic and adiabatic levels. This formulation is useful in intermediate coupling regimes where coincidences between diabatic and adiabatic levels are responsible for the structure in cross sections. The way to use this transition amplitude when the initial or final (or both) channels are closed is indicated. A numerical example concerning resonance Raman scattering is worked out.
Electron thermalization in nanosecond pulse-ionized dry and humid air
Authors: Warman John M. , Zhou-lei Mei , van Lith Dick
Published: February 2003

The process of electron thermalization in dry and humid air has been studied by monitoring the relaxation of the microwave conductivity following pulsed ionization. The energy exchange rate coefficient Ku for dry air is found to decrease from 8×10−11 cm3 s−1 immediately following the pulse to approximately 1.3×10−11 cm3 s−1 at close to thermal energies. Small amounts of water vapor result in a marked increase in the rate of electron thermalization. The thermalization time τth =t(Te =1.1 T) at 294 K is given by τth ≂(1.3×105+2.2×108 FH2O)−1 s Torr with FH2O the fractional water content. The electron–ion recombination rate constant is found to increase from 3.1×10−7 cm3 s−1 at 2 Torr to 1.9×10−6 cm3 s−1 at 50 Torr for dry air and is approximately 1×10−6 cm3 s−1 for 1.1% H2O at 2 Torr total pressure.
ESR studies of the formyl radical in a CO matrix: Magnetophotoselective photolysis and thermally activated rotations
Authors: Adrian Frank J. , Bohandy J. , Kim B. F.
Published: February 2003

Photolysis of the formyl radical (HCO) in a CO matrix at 13 K by yellow light (λ>500 nm) polarized with its electric vector parallel to an external magnetic field (HDC) is orientationally selective, as shown by changes in the powder ESR spectrum of the radical. The depleted orientation in the cyclic photolysis [HCO→hν H+CO; H+ (a different CO)→(randomly oriented HCO)] is (HCO plane) ⊥ HDC, showing that the largest component of the optical transition moment is perpendicular to the HCO plane, as predicted by theory for a nonrotating HCO. Around 25 K part of the photoinduced orientational anisotropy is lost due to incipient rotation about an axis close to the CO bond, but the remaining orientational anisotropy persists to higher temperatures (∼40 K), where it decays only slowly. Around 35 K this rotation becomes fast enough to yield a pseudoaxially symmetric powder ESR spectrum with g∥=1.9973,g⊥=2.0021, and A∥=A⊥=383.0 MHz. The rotation axis is closest to but significantly different from the minimum inertial axis.
Studies of reactions of importance in the stratosphere. V. Rate constants for the reactions O+NO2→NO+O2 and O+ClO→Cl+O2 at 298 K
Authors: Ongstad Andrew P. , Birks John W.
Published: February 2003

Rate constants for the reactions O+ClO→Cl+O2 (2) and O+NO2→NO+O2 (8) were measured at 298 K and 2.3 Torr total pressure using the discharge flow technique. Reaction rates were measured under pseudo-first-order conditions with either ClO or NO2 in large excess and the decay of oxygen atoms monitored by adding nitric oxide at the end of the flow tube and detecting the O+NO+M chemiluminescence. The O+ClO rate constant was measured using three different reactions of chlorine atoms to generate the ClO radical: Cl+O3→ClO+O2, (1) Cl+ClO2→2ClO, (6) Cl+Cl2O→Cl2+ClO. (7) As an internal check on the stoichiometries of the ClO source reactions and of the purities of the O3, ClO2, and Cl2O reagents, following each measurement of k2 an excess of nitric oxide was used to substitute NO2 for ClO via the reaction ClO+NO→Cl+NO2 (9) and the value of k8 measured. The second order rate constant for the O+ClO reaction was found to be independent of the ClO source used and is given by k2=(3.5±0.6)×10−11 cm3 molecule−1 s−1. The rate constant for the O+NO2 reaction, obtained by adding NO2 directly to the flow tube, is given by k8=(1.00±0.10)×10−11 cm3 molecule−1 s−1. The errors quoted for k2 and k8 include estimates of possible systematic errors.
Vibrational energy disposal in reactive and inelastic collisions of H(D)+HCl(DCl) at 1–3 eV
Authors: Wight Charles A. , Magnotta Frank , Leone Stephen R.
Published: February 2003

Vibrational energy disposal due to reactive exchange and unreactive translational-to-vibrational excitation in hyperthermal collisions of H+HCl and deuterated analogs is investigated by the excimer laser photolysis/infrared fluorescence technique. In the H+DCl system the excitation of DCl vibration increases from an energy efficiency of 4% to 9% as the initial H atom energy is increased from 1.4 to 3.1 eV. Only about 2% of the available energy appears as HCl vibration via H atom exchange, independent of the initial energy. The exchange reaction barrier is determined to be less than 0.95 eV (22 kcal/mol). At 2.3 eV collision energy the vibrational distribution of the HCl exchange product is relatively hot, with about 86% of the vibrational energy in molecules with v≥2. In the H+HCl system, where only the sum of inelastic and reactive collisions can be observed, the HCl vibrational distribution is markedly cooler, with only 60% of the vibrational energy in molecules with v≥2. The overall vibrational excitation efficiency of H+HCl is also a sensitive function of energy, increasing from 3% to 18% over the range 0.95 to 3.1 eV. This is consistent with an interpretation that the inelastic T–V excitation pathway is predominant. No information is obtained on the H atom abstraction channel.
Insights into glutamate transport regulation in human astrocytes: Cloning of the promoter for excitatory amino acid transporter 2 (EAAT2)
Authors: Su Zao-zhong , Leszczyniecka Magdalena , Kang Dong-chul , Sarkar Devanand , Chao Wei , Volsky David J. , Fisher Paul B.
Published: February 2003

Glutamate transport is central to neurotransmitter functions in the brain. Impaired glutamate transport induces neurotoxicity associated with numerous pathological processes, including stroke/ischemia, temporal lobe epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, Huntington's disease, HIV-1-associated dementia, and growth of malignant gliomas. Excitatory amino acid transporter-2 (EAAT2) is a major glutamate transporter in the brain expressed primarily in astrocytes. We presently describe the cloning and characterization of the human EAAT2 promoter, demonstrating elevated expression in astrocytes. Regulators of EAAT2 transport, both positive and negative, alter EAAT2 transcription, promoter activity, mRNA, and protein. These findings imply that transcriptional processes can regulate EAAT2 expression. Moreover, they raise the intriguing possibility that the EAAT2 promoter may be useful for targeting gene expression in the brain and for identifying molecules capable of modulating glutamate transport that could potentially inhibit, ameliorate, or prevent various neurodegenerative diseases.
Cells lying on a bed of microneedles: An approach to isolate mechanical force
Authors: Tan John L. , Tien Joe , Pirone Dana M. , Gray Darren S. , Bhadriraju Kiran , Chen Christopher S.
Published: February 2003

We describe an approach to manipulate and measure mechanical interactions between cells and their underlying substrates by using microfabricated arrays of elastomeric, microneedle-like posts. By controlling the geometry of the posts, we varied the compliance of the substrate while holding other surface properties constant. Cells attached to, spread across, and deflected multiple posts. The deflections of the posts occurred independently of neighboring posts and, therefore, directly reported the subcellular distribution of traction forces. We report two classes of force-supporting adhesions that exhibit distinct force–size relationships. Force increased with size of adhesions for adhesions larger than 1 μm
2
, whereas no such correlation existed for smaller adhesions. By controlling cell adhesion on these micromechanical sensors, we showed that cell morphology regulates the magnitude of traction force generated by cells. Cells that were prevented from spreading and flattening against the substrate did not contract in response to stimulation by serum or lysophosphatidic acid, whereas spread cells did. Contractility in the unspread cells was rescued by expression of constitutively active RhoA. Together, these findings demonstrate a coordination of biochemical and mechanical signals to regulate cell adhesion and mechanics, and they introduce the use of arrays of mechanically isolated sensors to manipulate and measure the mechanical interactions of cells.
kep1
interacts genetically with
dredd
/
Caspase-8
, and
kep1
mutants alter the balance of
dredd
isoforms
Authors: Di Fruscio Marco , Styhler Sylvia , Wikholm Eva , Boulanger Marie-Chloé , Lasko Paul , Richard Stéphane
Published: February 2003

The
Drosophila kep1
gene encodes an RNA binding protein related to the murine QUAKING apoptotic inducer. We have previously shown that
kep1
can induce apoptosis when transfected into different cell lines. To better define the role of Kep1 in apoptosis, we generated
kep1
null flies. These flies were viable, but females displayed reduced fertility, with approximately half of the eggs laid from
kep1
− homozygotes failing to hatch. In addition, loss of
kep1
suppressed
GMR-rpr
-mediated apoptosis in the
Drosophila
eye, and
kep1
mutant flies had increased susceptibility to
Escherichia coli
infection. We found that Kep1 bound
dredd
RNA
in vitro
, and that extracts prepared from
kep1
mutant ovaries had markedly reduced proteolytic cleavage activity toward the caspase-8 target substrate IETD-7-amino-4-trifluoromethyl coumarin. We observed increased levels of the β isoform of
dredd
mRNA in
kep1
mutants as compared with wild-type. Taken together, our results suggest that Kep1 regulates apoptosis by influencing the processing of
dredd
RNA.
Structure and allosteric regulation of the αXβ2 integrin I domain
Authors: Vorup-Jensen Thomas , Ostermeier Christian , Shimaoka Motomu , Hommel Ulrich , Springer Timothy A.
Published: February 2003

The integrin αXβ2 (CD11c/CD18, p150,95) binds ligands through the I domain of the αX subunit. Ligands include the complement factor fragment iC3b, a key component in the innate immune defense, which, together with the expression of αXβ2 on dendritic cells and on other leukocytes, suggests a role in the immune response. We now report the structure of the αX I domain resolved at 1.65 Å by x-ray crystallography. To analyze structural requirements for ligand binding we made a mutation in the αX I domain C-terminal helix, which increased the affinity for iC3b ≈200-fold to 2.4 μM compared with the wild-type domain affinity of ≈400 μM. Gel permeation chromatography supported a conformational change between the wild-type and mutated domains. Conservation of allosteric regulation in the αX I domain points to the functional importance of this phenomenon.
Chemosensory regulation of developmental gene expression in
Myxococcus
xanthus
Authors: Kirby John R. , Zusman David R.
Published: February 2003

The δ-proteobacterium
Myxococcus xanthus
coordinates its motility during aggregation and fruiting body formation. While searching for chemotaxis genes in
M. xanthus
, we identified a third chemotaxis-like gene cluster, the
che3
cluster, encoding homologs to two methyl-accepting chemotaxis proteins (MCPs), a CheW, a hybrid CheA, a CheB, a CheR, but no CheY. Mutations in
mcp3A
,
mcp3B
, and
cheA3
did not show obvious defects in motility or chemotaxis but did affect the timing of entry into development. Mutations in these genes caused early aggregation of starving cells, even at low cell densities. Furthermore, these mutants showed pronounced overexpression of the developmentally regulated Tn
5lac
fusions Ω4403, Ω4411, and Ω4521 as well as overexpression of
mbhA
and
tps
, markers for peripheral rods and aggregating cells, respectively. Divergently transcribed from the
che3
promoter region is another gene,
crdA
(
c
hemosensory
r
egulator of
d
evelopment), predicted to encode a transcriptional activator of σ
54
-dependent promoters. To test the hypothesis that CrdA functions as the cognate response regulator for the histidine kinase CheA3, CrdA and CheA3 were assayed and found to interact strongly in the yeast two-hybrid system. Mutant analysis showed that
crdA
cells were delayed in development (12–24 h) and delayed in MbhA production relative to the wild type. An
mcp3BcrdA
double mutant displayed the
crdA
phenotype, indicating that
crdA
is epistatic to
mcp3B
. We conclude that the Che3 chemotaxis-like system functions to control developmental gene expression by regulating a σ
54
transcriptional activator, CrdA.
Social exploitation of vitellogenin
Authors: Amdam Gro V. , Norberg Kari , Hagen Arne , Omholt Stig W.
Published: February 2003

Vitellogenin is a female-specific glucolipoprotein yolk precursor produced by all oviparous animals. Vitellogenin expression is under hormonal control, and the protein is generally synthesized directly before yolk deposition. In the honeybee (
Apis mellifera
), vitellogenin is not only synthesized by the reproductive queen, but also by the functionally sterile workers. In summer, the worker population consists of a hive bee group performing a multitude of tasks including nursing inside the nest, and a forager group specialized in collecting nectar, pollen, water, and propolis. Vitellogenin is synthesized in large quantities by hive bees. When hive bees develop into foragers, their juvenile hormone titers increase, and this causes cessation of their vitellogenin production. This inverse relationship between vitellogenin synthesis and juvenile hormone is opposite to the norm in insects, and the underlying proximate processes and life-history reasons are still not understood. Here we document an alternative use of vitellogenin by showing that it is a source for the proteinaceous royal jelly that is produced by the hive bees. Hive bees use the jelly to feed larvae, queen, workers, and drones. This finding suggests that the evolution of a brood-rearing worker class and a specialized forager class in an advanced eusocial insect society has been directed by an alternative utilization of yolk protein.
Differential effects of prostaglandin derived from ω-6 and ω-3 polyunsaturated fatty acids on COX-2 expression and IL-6 secretion
Authors: Bagga Dilprit , Wang Ling , Farias-Eisner Robin , Glaspy John A. , Reddy Srinivasa T.
Published: February 2003

Omega-6 (ω-6) polyunsaturated fatty acids (PUFA), abundant in the Western diet, are precursors for a number of key mediators of inflammation including the 2-series of prostaglandins (PG). PGE
2
, a cyclooxygenase (COX) metabolite of arachidonic acid, a ω-6 PUFA, is a potent mediator of inflammation and cell proliferation. Dietary supplements rich in ω-3 PUFA reduce the concentrations of 2-series PG and increase the synthesis of 3-series PG (e.g., PGE
3
), which are believed to be less inflammatory. However, studies on cellular consequences of increases in 3-series PG in comparison to 2-series PG have not been reported. In this study, we compared the effects of PGE
2
and PGE
3
on (
i
) cell proliferation in NIH 3T3 fibroblasts, (
ii
) expression and transcriptional regulation of the COX-2 gene in NIH 3T3 fibroblasts, and (
iii
) the production of an inflammatory cytokine, IL-6, in RAW 264.7 macrophages. PGE
3
, unlike PGE
2
, is not mitogenic to NIH 3T3 fibroblasts. PGE
2
and PGE
3
both induce COX-2 mRNA via similar signaling mechanisms; however, compared with PGE
2
, PGE
3
is significantly less efficient in inducing COX-2 gene expression. Furthermore, although both PGE
2
and PGE
3
induce IL-6 synthesis in RAW 264.7 macrophages, PGE
3
is substantially less efficient compared with PGE
2
. We further show that increasing the ω-3 content of membrane phospholipid results in a decrease in mitogen-induced PGE
2
synthesis. Taken together, our data suggest that successful replacement of ω-6 PUFA with ω-3 PUFA in cell membranes can result in a decreased cellular response to mitogenic and inflammatory stimuli.
Isolation and characterization of Staufen-containing ribonucleoprotein particles from rat brain
Authors: Mallardo Massimo , Deitinghoff Anke , Müller Juliane , Goetze Bernhard , Macchi Paolo , Peters Christopher , Kiebler Michael A.
Published: February 2003

Localized mRNAs are thought to be transported in defined particles to their final destination. These particles represent large protein complexes that may be involved in recognizing, transporting, and anchoring localized messages. Few components of these ribonucleoparticles, however, have been identified yet. We chose the strategy to biochemically enrich native RNA–protein complexes involved in RNA transport to identify the associated RNAs and proteins. Because Staufen proteins were implicated in intracellular RNA transport, we chose mammalian Staufen proteins as markers for the purification of RNA transport particles. Here, we present evidence that Staufen proteins exist in two different complexes: (
i
) distinct large, ribosome- and endoplasmic reticulum-containing granules preferentially found in the membrane pellets during differential centrifugation and (
ii
) smaller particles in the S100 from rat brain homogenates. On gel filtration of the S100, we identified soluble 670-kDa Staufen1-containing and 440-kDa Staufen2-containing particles. They do not cofractionate with ribosomes and endoplasmic reticulum but rather coenrich with kinesin heavy chain. Furthermore, the fractions containing the Staufen1 particles show a 15-fold enrichment of mRNAs compared with control fractions. Most importantly, these fractions are highly enriched in BC1, and, to a lesser extent, in the α-subunit of the Ca
2+
/calmodulin-dependent kinase II, two dendritically localized RNAs. Finally, both RNAs colocalize with Staufen1–hemagglutinin in particles in dendrites of transfected hippocampal neurons. We therefore propose that these Staufen1-containing particles may represent RNA transport intermediates that are in transit to their final destination within neurons.
Flexible lid to the p53-binding domain of human Mdm2: Implications for p53 regulation
Authors: McCoy Mark A. , Gesell Jennifer J. , Senior Mary M. , Wyss Daniel F.
Published: February 2003

The stabilization of p53 against Mdm2-mediated degradation is an important event in DNA damage response. Initial models of p53 stabilization focused on posttranslational modification of p53 that would disrupt the p53–Mdm2 interaction. The N-terminal regions of both p53 and Mdm2 are modified
in vivo
in response to cellular stress, suggesting that modifications to Mdm2 also may affect the p53–Mdm2 interaction. Our NMR studies of apo-Mdm2 have found that, in addition to Mdm2 residues 25–109 that form the well ordered p53-binding domain that was observed in the p52–Mdm2 complex, Mdm2 residues 16–24 form a lid that closes over the p53-binding site. The Mdm2 lid, which is strictly conserved in mammals, may help to stabilize apo-Mdm2. It also competes weakly with peptidic and nonpeptidic antagonists. Modifications to the Mdm2 lid may disrupt p53–Mdm2 binding leading to p53 stabilization. Mdm2 and Mdm4 possess nearly identical p53-binding domains but different lids suggesting that lid modifications may select for p53 binding.
Glycosaminoglycan binding and oligomerization are essential for the
in vivo
activity of certain chemokines
Authors: Proudfoot Amanda E. I. , Handel Tracy M. , Johnson Zoë , Lau Elaine K. , LiWang Patricia , Clark-Lewis Ian , Borlat Frédéric , Wells Timothy N. C. , Kosco-Vilbois Marie H.
Published: February 2003

During organogenesis, immunosurveillance, and inflammation, chemokines selectively recruit leukocytes by activating seven-transmembrane-spanning receptors. It has been suggested that an important component of this process is the formation of a haptotactic gradient by immobilization of chemokines on cell surface glycosaminoglycans (GAGs). However, this hypothesis has not been experimentally demonstrated
in vivo
. In the present study we investigated the effect of mutations in the GAG binding sites of three chemokines, monocyte chemoattractant protein-1/CC chemokine ligand (CCL)2, macrophage-inflammatory protein-1β/CCL4, and RANTES/CCL5, on their ability to recruit cells
in vivo
. These mutant chemokines retain chemotactic activity
in vitro
, but they are unable to recruit cells when administered intraperitoneally. Additionally, monomeric variants, although fully active
in vitro
, are devoid of activity
in vivo
. These data demonstrate that both GAG binding and the ability to form higher-order oligomers are essential for the activity of particular chemokines
in vivo
, although they are not required for receptor activation
in vitro
. Thus, quaternary structure of chemokines and their interaction with GAGs may significantly contribute to the localization of leukocytes beyond migration patterns defined by chemokine receptor interactions.
In vivo
activation of a mutant μ-opioid receptor by antagonist: Future direction for opiate pain treatment paradigm that lacks undesirable side effects
Authors: Law Ping-Yee , Yang Jesse W. , Guo Xiaohong , Loh Horace H.
Published: February 2003

Based on the
in vitro
ability of opioid antagonists to activate a μ-opioid receptor mutant, S196A, we reasoned that when expressed in the appropriate sites
in vivo
, this mutant receptor could be used to elicit the analgesic effects of the opioids without the accompanying side effects, such as tolerance and dependence. To test this hypothesis, we introduced the S196A mutation into the mouse μ-opioid receptor by a knock-in strategy to test the ability of the opioid antagonist to produce analgesic effects. In these homozygous mice, we observed increased intrinsic efficacy of opioid analgesics with two antinociceptive tests: hot water tail-withdrawal and acetic acid-induced writhing tests. Opioid antagonists, such as naloxone and naltrexone, elicited antinociceptive effects similar to that of partial agonists. Most importantly, chronic treatment of the homozygous mice with naltrexone did not produce the expected tolerance response, whereas less physical dependence was observed than with chronic morphine treatment. Such
in vivo
properties suggest the possibility of using the S196A mutant of the μ-opioid receptor and opioid antagonists to minimize the spectrum of unwarranted side effects in pain management when opiate analgesics are used.
Facultative bacterial symbionts in aphids confer resistance to parasitic wasps
Authors: Oliver Kerry M. , Russell Jacob A. , Moran Nancy A. , Hunter Martha S.
Published: February 2003

Symbiotic relationships between animals and microorganisms are common in nature, yet the factors controlling the abundance and distributions of symbionts are mostly unknown. Aphids have an obligate association with the bacterium
Buchnera aphidicola
(the primary symbiont) that has been shown to contribute directly to aphid fitness. In addition, aphids sometimes harbor other vertically transmitted bacteria (secondary symbionts), for which few benefits of infection have been previously documented. We carried out experiments to determine the consequences of these facultative symbioses in
Acyrthosiphon pisum
(the pea aphid) for vulnerability of the aphid host to a hymenopteran parasitoid,
Aphidius ervi
, a major natural enemy in field populations. Our results show that, in a controlled genetic background, infection confers resistance to parasitoid attack by causing high mortality of developing parasitoid larvae. Compared with uninfected controls, experimentally infected aphids were as likely to be attacked by ovipositing parasitoids but less likely to support parasitoid development. This strong interaction between a symbiotic bacterium and a host natural enemy provides a mechanism for the persistence and spread of symbiotic bacteria.
DNA duplex–quadruplex exchange as the basis for a nanomolecular machine
Authors: Alberti Patrizia , Mergny Jean-Louis
Published: February 2003

There is currently great interest in the design of nanodevices that are capable of performing linear or rotary movements. Protein molecular machines are abundant in biology but it has recently been proposed that nucleic acids could also act as nanomolecular machines in model systems. Several types of movements have been described with DNA machines: rotation and “scissors-like” opening and closing. Here we show a nanomachine that is capable of an extension–contraction movement. The simple and robust device described here is composed of a single 21-base oligonucleotide and relies on a duplex–quadruplex equilibrium that may be fueled by the sequential addition of DNA single strands, generating a DNA duplex as a by-product. The interconversion between two well defined topological states induces a 5-nm two-stroke, linear motor-type movement, which is detected by fluorescence resonance energy transfer spectroscopy.