Descripción: Dielectronic recombination data for nitrogen-like ions forming oxygen-like ions has been calculated as part of the assembly of a level-resolved dielectronic recombination database necessary for the modelling of dynamic finite-density plasmas (Badnell et al. 2003). Dielectronic recombination rate coefficients for a selection of ions from O+ to Xe47+ are presented and the results discussed.

Descripción: A programme is outlined for the assembly of a comprehensive dielectronic recombination database within the generalized collisional-radiative (GCR) framework. It is valid for modelling ions of elements in dynamic finite-density plasmas such as occur in transient astrophysical plasmas such as solar flares and in the divertors and high transport regions of magnetic fusion devices. The resolution and precision of the data are tuned to spectral analysis and so are sufficient for prediction of the dielectronic recombination contributions to individual spectral line emissivities. The fundamental data are structured according to the format prescriptions of the Atomic Data and Analysis Structure (ADAS) and the production of relevant GCR derived data for application is described and implemented following ADAS. The requirements on the dielectronic recombination database are reviewed and the new data are placed in context and evaluated with respect to older and more approximate treatments. Illustrative results validate the new high-resolution zero-density dielectronic recombination data in comparison with measurements made in heavy-ion storage rings utilizing an electron cooler. We also exemplify the role of the dielectronic data on GCR coefficient behaviour for some representative light and medium weight elements.

Descripción: The conventional random phase approximation (RPA) of the polarization propagator theory and a computational method based on continuous transformation of origin for the current density (CTOCD) induced within the electron cloud by an external homogeneous, static magnetic field has been employed to calculate atomic contributions to magnetic susceptibilities. The diamagnetic part of the magnetic susceptibility is written in terms of the polarization propagator. Since the paramagnetic term may also be obtained from the propagator it is thus possible to compute both contributions at the same level of approximation. The evaluated average susceptibility is independent of the origin of the vector potential, but depends on the origin of the reference frame. The atomic contributions to the diamagnetic and paramagnetic parts of the magnetic susceptibility are derived by applying off-diagonal hypervirial relations which are exactly fulfilled if the state functions are exact eigenfunctions of a model Hamiltonian. The rationalization of the magnetic susceptibilities into atomic contributions is applied to some small molecules: HF, H2O, NH3 and CH4, and the sum of these contributions is compared to the corresponding calculated total values and the experimental data for the molecular magnetic susceptibility for the same compounds. Computations are performed using basis sets of increasing quality. A series of sum rules for gauge independence of the computed results and charge-current conservation have been tested to document the accuracy of the calculation of magnetic properties. © 1999 American Institute of Physics.

Descripción: In the frame of finite range density functional (FRDF) theory, we review the systematics of energetic features of 4HeN clusters doped with atomic or molecular impurities, as well as the instability scenario of their collective motions. It is shown that for drops up to N = 500, the predictions of FRDF theory are coincident with those arising from microscopic many-body calculations that employ variational or diffusion Monte Carlo methods. For larger drops, the present description predicts a smooth approach to zero of the collective energies, a scenario that appears to support the most recent experimental data and microscopic calculations that demonstrate that the impurity is located within the bulk of the drop. © 1997 American Institute of Physics.

Descripción: Secondary and tertiary RNA structures present in viral RNA genomes play essential regulatory roles during translation, RNA replication, and assembly of new viral particles. In the case of flaviviruses, RNA-RNA interactions between the 5′ and 3′ ends of the genome have been proposed to be required for RNA replication. We found that two RNA elements present at the ends of the dengue virus genome interact in vitro with high affinity. Visualization of individual molecules by atomic force microscopy reveled that physical interaction between these RNA elements results in cyclization of the viral RNA. Using RNA binding assays, we found that the putative cyclization sequences, known as 5′ and 3′ CS, present in all mosquito-borne flaviviruses, were necessary but not sufficient for RNA-RNA interaction. Additional sequences present at the 5′ and 3′ untranslated regions of the viral RNA were also required for RNA-RNA complex formation. We named these sequences 5′ and 3′ UAR (upstream AUG region). In order to investigate the functional role of 5′-3′ UAR complementarity, these sequences were mutated either separately, to destroy base pairing, or simultaneously, to restore complementarity in the context of full-length dengue virus RNA. Nonviable viruses were recovered after transfection of dengue virus RNA carrying mutations either at the 5′ or 3′ UAR, while the RNA containing the compensatory mutations was able to replicate. Since sequence complementarity between the ends of the genome is required for dengue virus viability, we propose that cyclization of the RNA is a required conformation for viral replication. Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Descripción: We investigate the importance of relativistic effects on NMR shielding constants and chemical shifts of linear HgL2 (L = Cl, Br, I, CH 3) compounds using three different relativistic methods: the fully relativistic four-component approach and the two-component approximations, linear response elimination of small component (LR-ESC) and zeroth-order regular approximation (ZORA). LR-ESC reproduces successfully the four-component results for the C shielding constant in Hg(CH3)2 within 6 ppm, but fails to reproduce the Hg shielding constants and chemical shifts. The latter is mainly due to an underestimation of the change in spin-orbit contribution. Even though ZORA underestimates the absolute Hg NMR shielding constants by ∼2100 ppm, the differences between Hg chemical shift values obtained using ZORA and the four-component approach without spin-density contribution to the exchange-correlation (XC) kernel are less than 60 ppm for all compounds using three different functionals, BP86, B3LYP, and PBE0. However, larger deviations (up to 366 ppm) occur for Hg chemical shifts in HgBr 2 and HgI2 when ZORA results are compared with four-component calculations with non-collinear spin-density contribution to the XC kernel. For the ZORA calculations it is necessary to use large basis sets (QZ4P) and the TZ2P basis set may give errors of ∼500 ppm for the Hg chemical shifts, despite deceivingly good agreement with experimental data. A Gaussian nucleus model for the Coulomb potential reduces the Hg shielding constants by ∼100-500 ppm and the Hg chemical shifts by 1-143 ppm compared to the point nucleus model depending on the atomic number Z of the coordinating atom and the level of theory. The effect on the shielding constants of the lighter nuclei (C, Cl, Br, I) is, however, negligible. © 2011 American Institute of Physics.

Descripción: Research done in the last years strongly support the hypothesis that PIP aquaporin can form heterooligomeric assemblies, specially combining PIP2 monomers with PIP1 monomers. Nevertheless, the structural elements involved in the ruling of homo versus heterooligomeric organization are not completely elucidated. In this work we unveil some features of monomer-monomer interaction in Beta vulgaris PIP aquaporins. Our results show that while BvPIP2;2 is able to interact with BvPIP1;1, BvPIP2;1 shows no functional interaction. The lack of functional interaction between BvPIP2;1 and BvPIP1;1 was further corroborated by dose-response curves of water permeability due to aquaporin activity exposed to different acidic conditions. We also found that BvPIP2;1 is unable to translocate BvPIP1;1-ECFP from an intracellular position to the plasma membrane when co-expressed, as BvPIP2;2 does. Moreover we postulate that the first extracellular loop (loop A) of BvPIP2;1, could be relevant for the functional interaction with BvPIP1;1. Thus, we investigate BvPIP2;1 loop A at an atomic level by Molecular Dynamics Simulation (MDS) and by direct mutagenesis. We found that, within the tetramer, each loop A presents a dissimilar behavior. Besides, BvPIP2;1 loop A mutants restore functional interaction with BvPIP1;1. This work is a contribution to unravel how PIP2 and PIP1 interact to form functional heterooligomeric assemblies. We postulate that BvPIP2;1 loop A is relevant for the lack of functional interaction with BvPIP1;1 and that the monomer composition of PIP assemblies determines their functional properties. © 2013 Jozefkowicz et al.