Ultraintense Lasers Applied to Laser Fusion Material Testing: Production of Ions, X rays and Neutrons

The ability of ultraintese lasers to create short pulses of energetic particles and high fluences is addressed as a solution to reproduce ion and X-ray ICF bursts for the characterization and validation of plasma facing components. The possibility of using a laser neutron source for material testing will also be discussed.

Tailoring the Optical Properties of Silica Irradiated with Swift Heavy Ions

Irradiation with swift heavy ions (SHI), roughly defined as those having atomic masses larger than 15 and energies exceeding 1 MeV/amu, may lead to significant modification of the irradiated material in a nanometric region around the (straight) ion trajectory (i.e., latent tracks). In the case of amorphous silica it has been reported that SHI irradiation originates nano-tracks of either higher density than the virgin material (for low electronic stopping powers, Se < 7 keV/nm) [1] or having a low-density core and a dense shell (Se > 12 keV/nm) [2]. The intermediate region has not been studied in detail but we will show in this work that essentially no changes in density occur in this zone. An interesting effect of the compaction is that the refractive index is increased with respect to that of the surroundings. In the first Se region it is clear that track overlapping leads to continuous amorphous layers that present a significant contrast with respect to the pristine substrate and this has been used to produce optical waveguides. The optical effects of intermediate and high stopping powers, on the other hand, are largely unknown so far. In this work we have studied theoretically (molecular dynamics and optical simulations) and experimentally (irradiation with SHI and optical characterization) the dependence of the macroscopic optical properties (i.e., the refractive index of the effective medium, n_EMA) on the electronic stopping power of the incoming ions. Our results show that the refractive index of the irradiated silica is not increased in the intermediate region, as expected; however, the core-shell tracks of the high-Se region produce a quite effective enhancement of n_EMA that could prove attractive for the fabrication of optical waveguides at ultralow fluences (as low as 1E11 cm^-2). 1. J. Manzano, J. Olivares, F. Agulló-López, M. L. Crespillo, A. Moroño, and E. Hodgson, "Optical waveguides obtained by swift-ion irradiation on silica (a-SiO2)," Nucl. Instrum. Meth. B 268, 3147-3150 (2010). 2. P. Kluth, C. S. Schnohr, O. H. Pakarinen, F. Djurabekova, D. J. Sprouster, R. Giulian, M. C. Ridgway, A. P. Byrne, C. Trautmann, D. J. Cookson, K. Nordlund, and M. Toulemonde, "Fine structure in swift heavy ion tracks in amorphous SiO2," Phys. Rev. Lett. 101, 175503 (2008).

On the production of N-2(+) ions at the N 1s edge of the nitrogen molecule

The N+2 ion yield of the N2 molecule has been measured at the N 1s → Rydberg excitations. It displays Fano-type line shapes due to interference between direct outer-valence photoionization and participator decay of the core-excited Rydberg states. The N+2 ion yield is compared with the total intensity of the outer-valence photoelectron lines obtained recently with electron spectroscopy (Kivimäki et al 2012 Phys. Rev. A 86 012516). The increasing difference between the two curves at the higher core-to-Rydberg excitations is most likely due to soft x-ray emission processes that are followed by autoionization. The results also suggest that resonant Auger decay from the core–valence doubly excited states contributes to the N+2 ion yield at the photon energies that are located on both sides of the N 1s ionization limit.

Fast ignition driven by quasi-monoenergetic ions: Optimal ion type and reduction of ignition energies with an ion beam array

Fast ignition of inertial fusion targets driven by quasi-monoenergetic ion beams is investigated by means of numerical simulations. Light and intermediate ions such as lithium, carbon, aluminum and vanadium have been considered. Simulations show that the minimum ignition energies of an ideal configuration of compressed Deuterium-Tritium are almost independent on the ion atomic number. However, they are obtained for increasing ion energies, which scale, approximately, as Z2, where Z is the ion atomic number. Assuming that the ion beam can be focused into 10 ?m spots, a new irradiation scheme is proposed to reduce the ignition energies. The combination of intermediate Z ions, such as 5.5 GeV vanadium, and the new irradiation scheme allows a reduction of the number of ions required for ignition by, roughly, three orders of magnitude when compared with the standard proton fast ignition scheme.

Calculated 11B–13C NMR chemical shift relationship in hypercoordinate methonium and boronium ions

The boronium-carbonium continuum was extended to include hypercoordinated protonated methanes and their boron analogs. The 11B NMR chemical shifts of the hypercoordinated hydriodo boron compounds and the 13C NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations were calculated by using the GIAO-MP2 method. The data show good linear correlation between 11B and 13C NMR chemical shifts, which indicates that the same factors that determine the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei of these hypercoordinated hydriodo compounds.

Three metal ions at the active site of the Tetrahymena group I ribozyme

Metal ions are critical for catalysis by many RNA and protein enzymes. To understand how these enzymes use metal ions for catalysis, it is crucial to determine how many metal ions are positioned at the active site. We report here an approach, combining atomic mutagenesis with quantitative determination of metal ion affinities, that allows individual metal ions to be distinguished. Using this approach, we show that at the active site of the Tetrahymena group I ribozyme the previously identified metal ion interactions with three substrate atoms, the 3′-oxygen of the oligonucleotide substrate and the 3′- and 2′-moieties of the guanosine nucleophile, are mediated by three distinct metal ions. This approach provides a general tool for distinguishing active site metal ions and allows the properties and roles of individual metal ions to be probed, even within the sea of metal ions bound to RNA.

Detection of alkali metal ions in DNA crystals using state-of-the-art X-ray diffraction experiments

The observation of light metal ions in nucleic acids crystals is generally a fortuitous event. Sodium ions in particular are notoriously difficult to detect because their X-ray scattering contributions are virtually identical to those of water and Na+…O distances are only slightly shorter than strong hydrogen bonds between well-ordered water molecules. We demonstrate here that replacement of Na+ by K+, Rb+ or Cs+ and precise measurements of anomalous differences in intensities provide a particularly sensitive method for detecting alkali metal ion-binding sites in nucleic acid crystals. Not only can alkali metal ions be readily located in such structures, but the presence of Rb+ or Cs+ also allows structure determination by the single wavelength anomalous diffraction technique. Besides allowing identification of high occupancy binding sites, the combination of high resolution and anomalous diffraction data established here can also pinpoint binding sites that feature only partial occupancy. Conversely, high resolution of the data alone does not necessarily allow differentiation between water and partially ordered metal ions, as demonstrated with the crystal structure of a DNA duplex determined to a resolution of 0.6 Å.

Monitoring the structure of Escherichia coli RNase P RNA in the presence of various divalent metal ions

Lead(II)-induced cleavage can be used as a tool to probe conformational changes in RNA. In this report, we have investigated the conformation of M1 RNA, the catalytic subunit of Escherichia coli RNase P, by studying the lead(II)-induced cleavage pattern in the presence of various divalent metal ions. Our data suggest that the overall conformation of M1 RNA is very similar in the presence of Mg2+, Mn2+, Ca2+, Sr2+ and Ba2+, while it is changed compared to the Mg2+-induced conformation in the presence of other divalent metal ions, Cd2+ for example. We also observed that correct folding of some M1 RNA domains is promoted by Pb2+, while folding of other domain(s) requires the additional presence of other divalent metal ions, cobalt(III) hexamine or spermidine. Based on the suppression of Pb2+ cleavage at increasing concentrations of various divalent metal ions, our findings suggest that different divalent metal ions bind with different affinities to M1 RNA as well as to an RNase P hairpin–loop substrate and yeast tRNAPhe. We suggest that this approach can be used to obtain information about the relative binding strength for different divalent metal ions to RNA in general, as well as to specific RNA divalent metal ion binding sites. Of those studied in this report, Mn2+ is generally among the strongest RNA binders.

Phosphorylated Nitrate Reductase and 14-3-3 Proteins1 : Site of Interaction, Effects of Ions, and Evidence for an AMP-Binding Site on 14-3-3 Proteins

The inactivation of phosphorylated nitrate reductase (NR) by the binding of 14-3-3 proteins is one of a very few unambiguous biological functions for 14-3-3 proteins. We report here that serine and threonine residues at the +6 to +8 positions, relative to the known regulatory binding site involving serine-543, are important in the interaction with GF14ω, a recombinant plant 14-3-3. Also shown is that an increase in ionic strength with KCl or inorganic phosphate, known physical effectors of NR activity, directly disrupts the binding of protein and peptide ligands to 14-3-3 proteins. Increased ionic strength attributable to KCl caused a change in conformation of GF14ω, resulting in reduced surface hydrophobicity, as visualized with a fluorescent probe. Similarly, it is shown that the 5′ isomer of AMP was specifically able to disrupt the inactive phosphorylated NR:14-3-3 complex. Using the 5′-AMP fluorescent analog trinitrophenyl-AMP, we show that there is a probable AMP-binding site on GF14ω.

DNA bending by hexamethylene-tethered ammonium ions.

DNA is bent when complexed with certain proteins. We are exploring the hypothesis that asymmetric neutralization of phosphate charges will cause the DNA double helix to collapse toward the neutralized face. We have previously shown that DNA spontaneously bends toward one face of the double helix when it is partially substituted with neutral methylphosphonate linkages. We have now synthesized DNA duplexes in which cations are tethered by hexamethylene chains near specific phosphates. Electrophoretic phasing experiments demonstrate that tethering six ammonium ions on one helical face causes DNA to bend by approximately 5 degrees toward that face, in qualitative agreement with predictions. Ion pairing between tethered cations and DNA phosphates provides a new model for simulating the electrostatic consequences of phosphate neutralization by proteins.

Excitation transfer spectroscopy with two metastable 138Ba+ ions in same trap.

In the absence of lasers approaching trapped ion clock transitions in sharpness we propose to replace the 12.49 m laser field exciting the D3/2-D5/2 transition of the single Ba+ ion A in D3/2 with the near-field of a close by identical ion B in the excited D5/2 state. We tune the frequency of the near-field by the differential Stark shift generated when the center of mass of the tuned ions is slightly moved out of the trap center by a small bias voltage. We demonstrate that the resultant resonant energy exchange can be made considerably faster than the natural lifetime of either metastable level and show how it might be detected.

Conformations and folding of lysozyme ions in vacuo.

Proton transfer reactivity of isolated charge states of the protein hen egg-white lysozyme shows that multiple distinct conformations of this protein are stable in the gas phase. The reactivities of the 9+ and 10+ charge state ions, formed by electrospray ionization of "native" (disulfide-intact) and "denatured" (disulfide-reduced) solutions, are consistent with values calculated for ions in their crystal structure and fully denatured conformations, respectively. Charge states below 8+ of both forms, formed by proton stripping, have similar or indistinguishable reactivities, indicating that the disulfide-reduced ions fold in the gas phase to a more compact conformation.

Crystallographic evidence for the action of potassium, thallium, and lithium ions on fructose-1,6-bisphosphatase.

Fructose-1,6-bisphosphatase (Fru-1,6-Pase; D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) requires two divalent metal ions to hydrolyze alpha-D-fructose 1,6-bisphosphate. Although not required for catalysis, monovalent cations modify the enzyme activity; K+ and Tl+ ions are activators, whereas Li+ ions are inhibitors. Their mechanisms of action are still unknown. We report here crystallographic structures of pig kidney Fru-1,6-Pase complexed with K+, Tl+, or both Tl+ and Li+. In the T form Fru-1,6-Pase complexed with the substrate analogue 2,5-anhydro-D-glucitol 1,6-bisphosphate (AhG-1,6-P2) and Tl+ or K+ ions, three Tl+ or K+ binding sites are found. Site 1 is defined by Glu-97, Asp-118, Asp-121, Glu-280, and a 1-phosphate oxygen of AhG-1,6-P2; site 2 is defined by Glu-97, Glu-98, Asp-118, and Leu-120. Finally, site 3 is defined by Arg-276, Glu-280, and the 1-phosphate group of AhG-1,6-P2. The Tl+ or K+ ions at sites 1 and 2 are very close to the positions previously identified for the divalent metal ions. Site 3 is specific to K+ or Tl+. In the divalent metal ion complexes, site 3 is occupied by the guanidinium group of Arg-276. These observations suggest that Tl+ or K+ ions can substitute for Arg-276 in the active site and polarize the 1-phosphate group, thus facilitating nucleophilic attack on the phosphorus center. In the T form complexed with both Tl+ and Li+ ions, Li+ replaces Tl+ at metal site 1. Inhibition by lithium very likely occurs as it binds to this site, thus retarding turnover or phosphate release. The present study provides a structural basis for a similar mechanism of inhibition for inositol monophosphatase, one of the potential targets of lithium ions in the treatment of manic depression.