Investigation of plasma opening switch conduction and openingmechanisms

Plasma opening switch techniques have been developed for pulsed power applications to exploit the advantages of electrical energy storage in a vacuum inductor compared to conventional, capacitive-based energy storage. Experiments are described that demonstrate the successful application of these techniques in conduction time ranges from 50 ns to over 1 μs. Physics understanding of the conduction and opening mechanisms is far from complete; however, many insights have been gained from experiments and theory. Measurements of current distribution, plasma density, and ion emission indicate that conduction and opening mechanisms differ for the 50 ns and 1 μs conduction times. For the 50 ns conduction time case, switching begins at a current level close to the bipolar emission limit, and opening could occur primarily by erosion. In the 1 μs conduction time case, limited hydrodynamic plasma displacement implies far higher plasma density than is required by the bipolar emission limit. Magnetic pressure is required to augment erosion to generate the switch gap inferred from experiments     

Thiocarbonyl ylides : Reactions and stereochemical properties of some 4-t-butylcyclohexyl derivatives

The azine from 4-t-butylcyclohexanone on treatment with hydrogen sulfide under pressure is converted to a mixture of stereoisomeric 1,3,4-thiadiazolidines. Dehydrogenation of this mixture with an alkyl azodicarboxylate afforded in quantitative yield (based on azine) a mixture of the three possible Δ³-1,3,4-thiadiazolines. These three isomers have been isolated and their structures have been established as trans,trans-3,11-di-[1,1-dimethylethyl]-14,15-diaza-7-thiadispiro[5.1.5.2]pentadeca-14-ene(5); the cis,cis-isomer (6), and the cis,trans-isomer (7). Pyrolysis of either 5 or 6 leads in quantitative yield to cis,trans-3,10-di-[1,1-dimethylethyl]thiadispiro[5.0.5.2]tridecane (8), the formation of which is rationalized by conrotatory ring closure of the same thiocarbonyl ylide (24) formed from either 5 or 6. Pyrolysis of 7 leads exclusively to a thiirane isomeric with 10 and which is assigned the cis,cis-structure (9). The thiiranes 8 and 9 are desulfurized by tri-n-butylphosphine to the anti- and syn-1-(1,1-dimethylethyl)-4-[4-(1,1-dimethylethyl)cyclohexylidene]cyclohexanes (11 and 12), respectively. The cycloadditions of the thiocarbonyl ylides derived from 5–7 with dimethyl acetylenedicarboxylate and dimethylazodicarboxylate have been examined but stereochemistries have not been assigned to these products. Cis additions to the olefins (11 and 12) have been investigated with the most attention having been paid to the reactions with osmium tetroxide. The configurations of the glycols expected from these reactions have been correlated by comparison with the three possible glycols (16–18) obtained from pinacol reduction of 4-t-butylcyclohexanone. These three glycols have been separated and their configurations assigned from PMR and IR data. A discussion of the stereochemical implications of the various results is given.     

Energy-based prediction of amino acid-nucleotide base recognition

Despite decades of investigations, it is not yet clear whether there are rules dictating the specificity of the interaction between amino acids and nucleotide bases. This issue was addressed by determining, in a dataset consisting of 100 high-resolution protein-DNA structures, the frequency and energy of interaction between each amino acid and base, and the energetics of water-mediated interactions. The analysis was carried out using HINT, a non-Newtonian force field encoding both enthalpic and entropic contributions, and Rank, a geometry-based tool for evaluating hydrogen bond interactions. A frequency- and energy-based preferential interaction of Arg and Lys with G, Asp and Glu with C, and Asn and Gln with A was found. Not only favorable, but also unfavorable contacts were found to be conserved. Water-mediated interactions strongly increase the probability of Thr-A, Lys-A, and Lys-C contacts. The frequency, interaction energy, and water enhancement factors associated with each amino acid-base pair were used to predict the base triplet recognized by the helix motif in 45 zinc fingers, which represents an ideal case study for the analysis of one-to-one amino acid-base pair contacts. The model correctly predicted 70.4% of 135 amino acid-base pairs, and, by weighting the energetic relevance of each amino acid-base pair to the overall recognition energy, it yielded a prediction rate of 89.7%.     

Proton-induced grain boundary segregation in stainless steel

Abstract

A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from 5.3 to 8.7 at %, over 80 times the bulk value. No radiation-induced grain boundary segregation of S was measured in the UHP + S alloy. Results also indicate that the presence of P or S may enhance radiation-induced segregation of major alloying elements at the boundary. Comparison of irradiated and unirradiated regions of the UHP + P alloy indicate that while a prior thermal treatment segregates P to the grain boundary to 5.3 at %, the major element concentrations at the grain boundary are completely different from those under irradiation.     

Regio- and stereoselective syntheses of the natural product CCR5 antagonist anibamine and its three olefin isomers

The syntheses of the natural product anibamine and its three olefin isomers have been achieved concisely and efficiently via highly regio- and stereoselective reactions. The crucial steps included a regioselective palladium-catalyzed alkynylation by Sonogashira coupling and a stereoselective Suzuki coupling. Further conformation analyses and in vitro calcium mobilization studies were carried out to characterize the compounds' biological properties.     

Quantum Monte Carlo study of the CO interaction with a dimer model surface for Cr(110)

The chemisorption of CO on a Cr (110) surface is investigated using the quantum Monte Carlo method in the diffusion Monte Carlo (DMC) variant and a model Cr₂CO cluster. The present results are consistent with the earlier ab initio HF study with this model that showed the tilted/near-parallel orientation as energetically favoured over the perpendicular arrangement. The DMC energy difference between the two orientations is larger (1.9 eV) than that computed in the previous study. The distribution and reorganization of electrons during CO adsorption on the model surface are analysed using the topological electron localization function method that yields electron populations, charge transfer and clear insight on the chemical bonding that occurs with CO adsorption and dissociation on the model surface.     

Characterization of aromatic-thiol π-type hydrogen bonding and phenylalanine-cysteine side chain interactions through ab initio calculations and protein database analyses

In this study, the aromatic-thiol π hydrogen bonding and phenylalanine-cysteine side chain interactions are characterized through both molecular orbital calculations on a C₆H₆-HSCH₃ model complex and database analyses of 609 X-ray protein structures. The aromatic-thiol π hydrogen bonding interaction can achieve a stabilization energy of 2.60 kcal mol^?1, and is stronger than the already documented aromatic-hydroxyl and aromatic-amino hydrogen bonds. However, the occurrence of the aromatic-thiol hydrogen bond is rather rare in proteins. This is because most of the thiol groups participate in the formation of either disulphide bonds or stronger S—H…O (or N) ‘normal’ hydrogen bonds in a protein environment. Interactions between the side chains of phenylalanine and cysteine residues are characterized as the phenyl(Phe)(HSCH₂-)(Cys) interaction. The bonding energy for such interactions is approximately 3.71 kcal mol-¹ and is achieved in a geometric arrangement with an optimal phenyl(Phe)-(HS-)(Cys) π-type hydrogen bonding interaction. The interaction is very sensitive to the orientation of the two lone electron pairs on the sulphur atom relative to the π electron cloud of the phenyl ring. Accordingly, the interaction configurations that can accomplish a significant bonding energy exist only within a narrow configurational space. The database analysis of 609 experimental X-ray protein structures demonstrates that only 268 of the 1620 cysteine residues involve such phenylalanine-cysteine side chain interactions. Most of these interactions occur in the form of π (aromatic)-lone pair(sulphur) attractions, and correspond to a bonding energy less than 1.5 kcal mol^?1. A few were identified as the aromatic-thiol hydrogen bond with a bonding energy of 2.0–3.6 kcal mol^?1.     

A compact high-efficiency cold atom beam source

We report on a compact high-efficiency Cs slow atom beam source based on a retro-reflected two-dimensional magneto-optical trap (2D MOT). Employing two laser beams in an angled retro-reflected setup, we achieve 3D MOT loading rates greater than 8?×?10⁹?atoms/s using only 20?mW of total laser power for the source.