Protein conformational fluctuations and free-energy transduction

Non-equilibrium fluctuations, whether imposed externally or driven by an energy-releasing chemical reaction, can cause a protein to cycle through several conformations. This cycling can drive a process thermodynamically uphill even though any one conformation considered independently catalyzes the process in the downhill direction. This is because the different conformations have different rate constants (energy barriers) between the states in the catalytic cycle. Even though each conformation individually obeys detailed balance, the flashing between different energy profiles gives rise to a ratchet effect. Further, by exploiting protein conformational dynamics, a single stochastic input can be converted into two phase-shifted internal parameters (e.g. a kinetic barrier height and a binding well energy). This allows the output process to be driven nearly adiabatically, explaining in part the very high efficiencies observed for some biological energy-transduction processes. The results apply equally to driving a biochemical reaction away from equilibrium by an enzyme, to formation of an osmotic gradient across a membrane by a molecular pump, or to motion and generation of force by a molecular motor. Received: 8 February 2002 / Accepted: 4 March 2002 / Published online: 22 April 2002     

Effect of phase shift in shape changing collision of solitons in coupled nonlinear Schrödinger equations

Soliton interactions in systems modelled by coupled nonlinear Schr?dinger (CNLS) equations and encountered in phenomena such as wave propagation in optical fibers and photorefractive media possess unusual features: shape changing intensity redistributions, amplitude dependent phase shifts and relative separation distances. We demonstrate these properties in the case of integrable 2-CNLS equations. As a simple example, we consider the stationary two-soliton solution which is equivalent to the so-called partially coherent soliton (PCS) solution discussed much in the recent literature. Received 1st October 2001 / Received in final form 4 February 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: lakshman@bdu.ernet.in     

Transfer reactions and sub-barrier fusion in 40Ca + 90, 96Zr

The two systems ⁴⁰Ca + ^90,96Zr have been studied by measuring nucleon transfer reactions at two energies near the Coulomb barrier, thus complementing the available sub-barrier fusion cross-sections. Angular distributions for various transfer channels have been determined. Significantly larger neutron transfer cross-sections are found for the target ⁹⁶Zr that exhibits the larger enhancement in the sub-barrier fusion cross-sections. All data have been analyzed with a new model for heavy-ion collisions that calculates simultaneously transfer cross-sections, fusion excitation functions and barrier distributions. The model gives a good account of both transfer and fusion data. Received: 2 May 2002 / Accepted: 4 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: montagnoli@pd.infn.it, Fax +39049 8277102, Tel. +39049 8277117. RID="b" ID="b"On leave from the China Institute for Atomic Energy, 102413 Beijing, China. Communicated by C. Signorini     

The case of RIB at intermediate energies: Isospin effects on nuclear dynamics

In the reaction dynamics of intermediate-energy radioactive beams we can probe highly asymmetric nuclear matter in compressed as well as dilute phases. In this report some predictions are presented, based on analytical results as well as on reaction simulations. We suggest a series of experiments with RIB aimed to shed light on isospin properties of nuclear interactions in the medium. Received: 1 May 2001 / Accepted: 4 December 2001     

Fabrication, morphology and structural characterization of ordered single-crystal Ag nanowires

Highly aligned Ag nanowires have been synthesized by dc electrodeposition within a hexagonal close-packed nanochannel anodic aluminum oxide template. The pore diameter varies from 20 nm to 50 nm depending on the anodization voltage and temperature for the two types of aqueous solutions, sulphuric and oxalic acids, respectively. The size and morphology of the Ag nanowire arrays were measured by scanning electron microscopy and transmission electron microscopy. The images indicate that the highly aligned Ag nanowires grow in the uniform nanochannels of the anodic alumina template and that the size of the nanowires depends on the size of the nanochannels. X-ray diffraction, selected area electron diffraction pattern and high-resolution transmission electron microscopy images show that the Ag nanowires are single-crystal. The temperature coefficient of resistivity (temperature range from 4.2 K to 300 K) of the Ag nanowire arrays decreases with decreasing diameter of the nanowires. Received: 5 November 2001 / Revised version: 12 March 2002 / Published online: 6 June 2002     

Σ- atoms and the ΣN interaction

It is shown that among four models of the Nijmegen baryon-baryon interaction only model F --which leads to a repulsive potential felt by the Σ hyperon inside the nucleus-- is consistent both with the analysis of Σ^- atoms and of the (K ^-,π) reactions. The Nijmegen models are used to determine the strong complex single-particle (s.p.) potential of Σ^-, and to calculate the strong-interaction shifts and widths of the lowest observed levels of Σ^- atoms. The results obtained with model F are in best agreement with the experimental data. Received: 15 January 2002 / Accepted: 14 March 2002     

Modeling of Cu gettering in p- and n-type silicon and in poly-silicon

Based on experimental findings we set up calculations of numerical modeling of gettering efficiencies for Cu in various silicon wafers. Gettering efficiencies for Cu were measured by applying a reproducible spin-on contamination in the 10¹² atoms/cm² range, followed by a thermal treatment to redistribute the metallic impurity. Subsequently, the wafers were analyzed by a novel wet chemical layer-by-layer etching technique in combination with inductively coupled plasma mass spectrometry. We investigated p/p+ and n/n+ epitaxial wafers with different doping levels and different substrate-doping species. We have also investigated gettering efficiencies of phosphorus-diffused p- and n-type wafers. Heavilyboron doped silicon exhibited a gettering efficiency of ∼100%, while gettering by n+ silicon occurred for doping levels >3×10¹⁹ atoms/cm³ only. In another set of experiments we investigated the dependence of the gettering efficiency of p-type wafers with poly-silicon back sides for different cooling rates and Cu spiking levels. A strong dependence on both parameters was found. Cu gettering in p/p+ epitaxial wafers was modeled by calculating the increased solubility of Cu in p+ silicon compared to non-doped silicon taking into account the Fermi-level effect, which stabilizes donors in p+ silicon, and the pairing reaction between Cu and B. Calculated gettering efficiencies were in very good agreement with experimental results. Gettering in n+ silicon was similarly modeled in terms of pairing reactions and the Fermi-level effect. But, for n-type silicon, many experimental uncertainties existed; thus, we applied our expressions to solubility data of Hall and Racette to obtain the unknown parameters. The empirical calculations were in good agreement even with results on n/n+ wafers. For phosphorus-diffused wafers we had to consider an excess vacancy concentration of 1.2–5.5 times the equilibrium concentration to explain the experimental findings by the model. Gettering by poly-silicon back sides was simulated by solving the time-dependent diffusion equation with boundary conditions that take into account different surface reaction rates of silicon point defects. Using this advanced model, the experimentally measured gettering efficiencies were reproduced within the uncertainty of the measurement. Received: 3 September 2001 / Accepted: 4 September 2001 / Published online: 20 December 2001     

Decay of the neutron-rich isotope 113Ru to 113Rh

The decay of neutron-rich isotope ¹¹³Ru obtained as on-line mass separated product of proton-induced fission has been investigated by γγ coincidence and spectrum multiscaling measurements. Decay schemes for both low- and high-spin isomers of ¹¹³Ru have been constructed. The level scheme of ¹¹³Rh is considerably extended. Systematics of the lowest-lying rhodium levels is smooth. The picture of shape coexistence established for neutron-rich Rh isotopes near-neutron midshell is confirmed with the observation of a K = 1/2 deformed band, with its 3/2⁺ state at 600 keV being the lowest-lying level and of probable 7/2⁺ and 5/2⁺ band members. A large fraction of β feeding is found to populate high-lying levels in ¹¹³Rh. The GT strength in ¹¹³Ru^m decay is significantly larger than for the decay of ¹¹³Ru_g and of lighter rhodium isotopes. Received: 18 January 2001 / Accepted: 14 January 2002     

Heavy-element chemistry --Status and perspectives

In the past ten years, nuclear chemists have made considerable progress in developing fast on-line separation techniques, which allowed to chemically characterize the first four transactinide elements Rf (rutherfordium, Z = 104), Db (dubnium, Z = 105), Sg (seaborgium, Z = 106), and recently also Bh (bohrium, Z = 107). In all cases the isolated nuclides were unambiguously identified by observing genetically linked decay chains. Nuclides with production cross-sections of less than 100 pb and half-lives as short as a few seconds have been chemically isolated. Thus, chemists have discovered or significantly contributed to the characterization of the nuclear-decay properties of a number of transactinide nuclei. New techniques with greatly improved overall efficiencies should allow chemists to extend their studies to even heavier elements such as Hs (hassium, Z = 108) and to the recently discovered superheavy elements with Z = 112 and 114, which can be produced only with picobarn cross-sections. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: andreas.tuerler@psi.ch     

Restructuring of force networks

The compaction of granular packings or soils is a collective process which for higher densities becomes increasingly slower reaching glassy behaviour. We present a study of this problem from various points of view, in particular we will represent the evolving force network that percolates through the system by an inverse fiber rupture model. Received 15 March 2002 and Received in final form 29 July 2002