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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Comparative Study of the Effect of Defects on Selective Adsorption of Butanol from Butanol/Water Binary Vapor Mixtures in Silicalite-1 Films

A promising route for sustainable 1-butanol (butanol) production is ABE (acetone, butanol, ethanol) fermentation. However, recovery of the products is challenging because of the low concentrations obtained in the aqueous solution, thus hampering large-scale production of biobutanol. Membrane and adsorbent-based technologies using hydrophobic zeolites are interesting alternatives to traditional separation techniques (e.g., distillation) for energy-efficient separation of butanol from aqueous mixtures. To maximize the butanol over water selectivity of the material, it is important to reduce the number of hydrophilic adsorption sites. This can, for instance, be achieved by reducing the density of lattice defect sites where polar silanol groups are found. The density of silanol defects can be reduced by preparing the zeolite at neutral pH instead of using traditional synthesis solutions with high pH. In this work, binary adsorption of butanol and water in two silicalite-1 films was studied using in situ attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy under equal experimental conditions. One of the films was prepared in fluoride medium, whereas the other one was prepared at high pH using traditional synthesis conditions. The amounts of water and butanol adsorbed from binary vapor mixtures of varying composition were determined at 35 and 50 °C, and the corresponding adsorption selectivities were also obtained. Both samples showed very high selectivities (100–23 000) toward butanol under the conditions studied. The sample having low density of defects, in general, showed ca. a factor 10 times higher butanol selectivity than the sample having a higher density of defects at the same experimental conditions. This difference was due to a much lower adsorption of water in the sample with low density of internal defects. Analysis of molecular simulation trajectories provides insights on the local selectivities in the zeolite channel network and at the film surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

How a Formate Dehydrogenase Responds to Oxygen: Unexpected O 2 Insensitivity of an Enzyme Harboring Tungstopterin, Selenocysteine, and [4Fe–4S] Clusters

The reversible two-electron interconversion of formate and CO 2 is catalyzed by both nonmetallo- and metallo-formate dehydrogenases (FDHs). The latter group comprises molybdenum- or tungsten-containing enzymes with the metal coordinated by two equivalents of a pyranopterin cofactor, a cysteinyl or selenocysteinyl (Sec) ligand supplied by the polypeptide, and a catalytically essential terminal sulfido ligand. In addition, these biocatalysts incorporate one or more [4Fe–4S] clusters for facilitating long-distance electron transfer. However, an interesting dichotomy arises when attempting to understand how the metallo-FDHs react with O 2 . Whereas existing scholarship portrays these enzymes as being unable to perform in air due to extreme O 2 lability of their metal centers, studies dating as far back as the 1930s emphasize that some of these systems exhibit formate oxidase (FOX) activity, coupling formate oxidation to O 2 reduction. Therefore, to reconcile these conflicting views, we explored context-dependent functional linkages between metallo-FDHs and their cognate electron acceptors within the same organism vis-à-vis catalysis under atmospheric O 2 . Here, we report the discovery and characterization of an O 2 -insensitive FDH2 from the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH) that ligates tungsten, Sec, and four [4Fe–4S] clusters. By advancing a robust expression platform for its recombinant production, we eliminate both the requirement of nitrate or azide during purification and reductive activation with thiols and/or formate prior to catalysis. Because the distinctive spectral signatures of formate-reduced DvH-FDH2 remain invariant under anaerobic and aerobic conditions, we benchmarked the enzyme activity in air, identifying CO 2 as the catalytic product. Full reaction progress curve analysis discloses a high catalytic efficiency when probed with a high-potential artificial electron acceptor. Furthermore, we show that DvH-FDH2 enables near-stoichiometric hydrogen peroxide production without superoxide release to achieve O 2 insensitivity. Notably, simultaneous electron transfer to cytochrome c and O 2 reveals that metal-based electron bifurcation is operational in this system. Taken together, our work proves the co-occurrence of redox bifurcated FDH and FOX activities within a metalloenzyme scaffold. These findings set the stage for uncovering previously unknown O 2 -insensitive flavin-based electron bifurcation mechanisms, as well as for developing authentic formate/air biofuel cells, engineering O 2 -stable FDHs and biohybrid metallocatalysts, and discerning formate bioenergetics of gut microbiota.

13C NMR↗
DOE OSTI2017

Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution [Surface-active metal dichalcogenide electrocatalysts with self-improving performance for hydrogen evolution]

Low-cost, layered transition-metal dichalcogenides (MX 2 ) based on molybdenum and tungsten have attracted substantial interest as alternative catalysts for the hydrogen evolution reaction (HER). These materials have high intrinsic per-site HER activity; however, a significant challenge is the limited density of active sites, which are concentrated at the layer edges. Here we unravel electronic factors underlying catalytic activity on MX 2 surfaces, and leverage the understanding to report group-5 MX 2 (H-TaS 2 and H-NbS 2 ) electrocatalysts whose performance instead mainly derives from highly active basal-plane sites, as suggested by our first-principles calculations and performance comparisons with edge-active counterparts. Beyond high catalytic activity, they are found to exhibit an unusual ability to optimize their morphology for enhanced charge transfer and accessibility of active sites as the HER proceeds, offering a practical advantage for scalable processing. In conclusion, the catalysts reach 10 mA cm –2 current density at an overpotential of ~50–60 mV with a loading of 10–55 μg cm –2 , surpassing other reported MX 2 candidates without any performance-enhancing additives.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2014

A new Cu–cysteamine complex: structure and optical properties

Here we report the structure and optical properties of a new Cu–cysteamine complex (Cu–Cy) with a formula of Cu3Cl(SR)2 (R ¼ CH2CH2NH2). This Cu–Cy has a different structure from a previous Cu–Cy complex, in which both thio and amine groups from cysteamine bond with copper ions. Single-crystal X-ray diffraction and solid-state nuclear magnetic resonance results show that the oxidation state of copper in Cu3Cl(SR)2 is +1 rather than +2. Further, Cu3Cl(SR)2 has been observed to show intense photoluminescence and X-ray excited luminescence. More interesting is that Cu3Cl(SR)2 particles can produce singlet oxygen under irradiation by light or X-ray. This indicates that Cu3Cl(SR)2 is a new photosensitizer that can be used for deep cancer treatment as X-ray can penetrate soft tissues. All these findings mean that Cu3Cl(SR)2 is a new material with potential applications for lighting, radiation detection and cancer treatment.

Ma, Lun↗
DOE OSTI2016

Mechanistic investigation of the Zn/Pd/C catalyzed cleavage and hydrodeoxygenation of lignin

While current biorefinery processes use lignin only for its heat value, the conversion of lignin to high value chemicals is an area of increasing interest. In this article we present a detailed mechanistic study of the hydrodeoxygenation (HDO) of lignin by using a synergistic Pd/C and Zn II catalyst through use of both lignin model compounds and lignocellulosic biomass. Spectroscopic data coupled with the study of lignin model compounds suggest that Zn II activates and facilitates removal of the hydroxyl group at the Cγ position of the β-O-4 ether linkage. Activation is proposed to occur through formation of a six-membered ring complex of Zn II coordinated to the oxygen atoms at Cα and Cγ of the lignin model compound guaiacylglycerol-β-guaiacyl.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Soft particles and infinite-dimensional geometry

Abstract In the sigma model, soft insertions of moduli scalars enact parallel transport ofS-matrix elements about the finite-dimensional moduli space of vacua, and the antisymmetric double-soft theorem calculates the curvature of the vacuum manifold. We explore the analogs of these statements in gauge theory and gravity in asymptotically flat spacetimes, where the relevant moduli spaces are infinite-dimensional. These models have spaces of vacua parameterized by (trivial) flat connections on the celestial sphere, and soft insertions of photons, gluons, and gravitons parallel transportS-matrix elements about these infinite-dimensional manifolds. We argue that the antisymmetric double-soft gluon theorem ind + 2 bulk dimensions computes the curvature of a connection on the infinite-dimensional space Map ( S d , G ) / G , whereGis the global part of the gauge group. The analogous metrics in abelian gauge theory and gravity are flat, as indicated by the vanishing of the antisymmetric double-soft theorems in those models. In other words, Feynman diagram calculations not only know about the vacuum manifold of Yang–Mills theory, they can also be used to compute its curvature. The results have interesting implications for flat space holography.

Astronomy & Astrophysics↗
DOE OSTI2026

Genome-resolved insights into microbial diversity and elemental cycling in Winogradsky columns

We retained 18 MAGs with ≥50% completion and <10% contamination (i.e., at least medium quality). Of these, 10 had >90% completion and <5% contamination; however, only one (Paceibacteria Bin.003_MG) can be described as high-quality, as the others lacked a full suite of 5S, 16S, and 23S rRNA genes. To maximize the diversity of our recovered MAGs, we also retained one MAG (Chromatiaceae Bin.008_AM) with >40% (but less than 50%) completion and <5% contamination, as well as one (Rhodopseudomonas Bin.015_MK) with >90% completion and <20% (but>10%) contamination. Interestingly, significant chimerism was not detected in this MAG (40) , suggesting that the elevated contamination (20%) may instead reflect two closely related strains collapsing into a single bin. Consistent with this, contig coverage was bimodal, with roughly 17% of the assembly at ~115x and the remaining 83% at ~282x, while GC content remained uniform across both groups (~64%), arguing against contamination from a taxonomically distinct source.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1974

Estimation and detection of signals in multiplicative noise

We define a class of detection-estimation problems on matrix Lie groups in which the observation noise is multiplicative in nature. By examining the differential versions of the hypotheses, which are bilinear, we are able to derive the relevant likelihood ratio formula and the associated optimal estimation equations for the signal given the observations and the assumption that the signal is present. These estimation equations are of interest in their own right, in that they represent a finite-dimensional optimal solution to a nonlinear estimation problem and consist of a Kalman-Bucy filter along with the on-line computation of the solution of the associated Riccati equation, which is driven by the observations. The usefulness of these results is illustrated via an example concerning the detection of an actuator failure in a rigid-body rotational control system.

Willsky, A. S.↗
NASA NTRS1993

New monomers for high performance polymers

This laboratory has been concerned with the development of new polymeric materials with high thermo-oxidative stability for use in the aerospace and electronics industries. Currently, there is special emphasis on developing matrix resins and composites for the high speed civil transport (HSCT) program. This application requires polymers that have service lifetimes of 60,000 hr at 350 F (177 C) and that are readily processible into void-free composites, preferably by melt-flow or powder techniques that avoid the use of high boiling solvents. Recent work has focused on copolymers which have thermally stable imide groups separated by flexible arylene ether linkages, some with trifluoromethyl groups attached to the aromatic rings. The presence of trifluoromethyl groups in monomers and polymers often improves their solubility and processibility. The goal of this research was to synthesize several new monomers containing pendant trifluoromethyl groups and to incorporate these monomers into new imide/arylene ether copolymers. Initially, work was begun on the synthesis of three target compounds. The first two, 3,5-dihydroxybenzo trifluoride and 3-amino 5-hydroxybenzo trifluoride, are intermediates in the synthesis of more complex monomers. The third, 3,5-bis (3-amino-phenoxy) benzotrifluoride, is an interesting diamine that could be incorporated into a polyimide directly.

Gratz, Roy F.↗
NASA NTRS2004

From Present Surveying to Future Prospecting of the Asteroid Belt

We have applied a future mission architecture, the Autonomous Nano-Technology Swarm (ANTS), to a proposed mission for in situ survey, or prospecting, of the asteroid belt, the Prospecting Asteroid Mission (PAM) as part of a NASA 2003 Revolutionary Aerospace Concept (RASC) study. ANTS architecture builds on and advances recent trends in robotics, artificial intelligence, and materials processing to minimize costs and maximize effectiveness of space operations. PAM and other applications have been proposed for the survey of inaccessible, high surface area populations of great interest from the standpoint of resources and/or solar system origin. The ANTS architecture is inspired by the success of social insect colonies, a success based on the division of labor within the colonies in two key ways: 1) within their specialties, individual specialists generally outperform generalists, and 2) with sufficiently efficient social interaction and coordination, the group of specialists generally outperforms the group of generalists. Thus systems designed as ANTS are built from potentially very large numbers of highly autonomous, yet socially interactive, elements. The architecture is self-similar in that elements and sub-elements of the system may also be recursively structured as ANTS on scales ranging from microscopic to interplanetary distances. Here, we analyze requirements for the mission application at the low gravity target end of the spectrum, the Prospecting Asteroid Mission (PAM), and for specialized autonomous operations which would support this mission. ANTS as applied to PAM involves the activities of hundreds of individual specialist 'sciencecraft'. Most of them, called Workers, carry and operate eight to nine different scientific instruments, as listed in the table, including spectrometers, ranging and radio science devices, and imagers. The remaining specialists, Messenger/Rulers, provide communication and coordination functions among specialists operating autonomously as individuals, team members, and subswarms.

Clark, P. E.↗
DOE OSTI2015

Accumulation of gadolinium isotopes in used nuclear fuel - 14374

For the criticality safety control of damaged nuclear fuel, i.e., fuel debris, accounting for burnup credit (BUC) may become important if higher initial-uranium-enrichment fuel almost 5 wt% loaded into the reactor. In this case, if the presence of fission products (FP) could be confirmed, the negative reactivity effect of FP allows us to treat a substantially greater quantity of fuel debris in one removal and storage operation. In the technical development of the criticality safety control for fuel debris from the Fukushima accident in Japan, adopting BUC assuming the presence of the FP is being considered because the fuel used in the Fukushima Daiichi Nuclear Power Plant had a higher initial {sup 235}U enrichment, i.e., almost 5 wt%, than the fuel used at TMI-2. The Expert Group on Burnup Credit Criticality Safety (EGBUC) under the Working Party on Nuclear Criticality Safety (WPNCS) in the OECD/NEA Nuclear Science Committee carried out international burnup calculation benchmarks 'Phase-IIIB' and 'Phase-IIIC' for BWR fuel assemblies. In these benchmarks, the differences in the {sup 155}Gd calculation results of the participants generated keen interest because these differences were rather large. As the coordinators of the Phase-IIIC benchmark, the authors have conducted additional analyses on the accumulation of the gadolinium isotopes in used nuclear fuel during burnup. Without cooling time, the assembly-averaged amount of {sup 155}Gd against the burnup value depends on the burnout property of gadolinium in burnable poison rods. This is because the amount of gadolinium isotopes accumulated owing to the fission reaction is less than the residual gadolinium in the burnable poison rods. After the burnout of gadolinium isotopes in the burnable poison rods, the amount of {sup 155}Gd isotopes remains almost constant, which is defined by the balance between depletion due to the neutron capture reaction and generation due to beta-decay of {sup 155}Eu generated by the fission reaction. However, after a few years of cooling, the amount of {sup 155}Gd drastically increases owing to {sup 155}Eu decay. This effect is important for higher burnup fuel. In this case, the amount of gadolinium isotopes in the burnable poison rods is of less importance. This means that for better prediction of {sup 155}Gd, the adopted parameters and data pertaining to {sup 155}Eu generation have significantly greater importance than the burnup treatment of the burnable poison rods. The release behavior of europium as well as gadolinium during the severe accident should be examined. This paper provides basic information for considering {sup 155}Gd in the criticality safety evaluation of fuel debris. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Gas Sorption and Storage Properties of Calixarenes

Calixarenes, a class of organic macrocyclic molecules have shown interesting gas sorption properties towards industrially important gases such as carbon di-oxide, hydrogen, methane and acetylene. These macrocycles are involved in weak van der Waals interaction to form multidimensional supramolecular frameworks. The gas-diffusion and subsequent sorption occurs due to a cooperative behavior between neighboring macrocycles. Furthermore, the flexibility at the upper rim functional group also plays a key role in the overall gas uptake of calixarene. In this book chapter, we give a brief account of interaction and diffusion of gases in calixarene and selected derivatives.

Patil, Rahul S.↗
DOE OSTI2015

New vector-like fermions and flavor physics

We study renormalizable extensions of the standard model that contain vector-like fermions in a (single) complex representation of the standard model gauge group. There are 11 models where the vector-like fermions Yukawa couple to the standard model fermions via the Higgs field. These models do not introduce additional fine-tunings. They can lead to, and are constrained by, a number of different flavor-changing processes involving leptons and quarks, as well as direct searches. An interesting feature of the models with strongly interacting vector-like fermions is that constraints from neutral meson mixings (apart from CP violation in $ {K}^0-{\overline{K}}^0 $ mixing) are not sensitive to higher scales than other flavor-changing neutral-current processes. We identify order 1/(4πM) 2 (where M is the vector-like fermion mass) one-loop contributions to the coefficients of the four-quark operators for meson mixing, that are not suppressed by standard model quark masses and/or mixing angles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2026

Decorated Honeycomb Lattice and Successive Magnetic Transitions of the Delafossite Derivative K 4 Ni 5 Te 3 O 16

Here, we report the synthesis and characterization of the delafossite derivative K 4 Ni 5 Te 3 O 16 , which hosts a decorated honeycomb Ni 2+ lattice. Single-crystal X-ray diffraction, electron microscopy, and neutron diffraction establish a noncentrosymmetric Pmm2 crystal structure that combines regular honeycomb units with elongated motifs formed by Ni 2+ trimers along extended edges. Magnetic susceptibility reveals strong antiferromagnetic interactions, while heat capacity measurements identify two successive magnetic transitions at 30 and 12 K. Neutron diffraction shows that these transitions correspond to a progression from partial to full long-range magnetic order on distinct Ni sites, reflecting competition among the various exchange interactions. The resulting magnetic point group, mm2.1′, inherits the lattice polarity and permits high-order magnetoelectric coupling. Consistent with this symmetry, a quadratic magnetic-field-induced polarization is experimentally observed below the magnetic ordering temperature, likely arising from exchange-driven magnetostriction coupled to the polar lattice. These results establish K 4 Ni 5 Te 3 O 16 as an interesting platform for engineering B-site ordering in delafossite derivatives to realize complex quantum magnetism and functional responses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Stripe helical magnetism and two regimes of anomalous Hall effect in NdAlGe

Here, we report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate spin density wave (T inc =6.8 K) whose spins are predominantly pointing along the out-of-plane direction and have a small helical spin canting of 3°. The spin density wave has a long wavelength of ≈35 nm and transitions to a commensurate ferrimagnetic state below T com =5.1K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are, respectively, governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our paper suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAlX, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Elementary Particle Physics (Final Report)

vATLAS and the LHC are delivering on the promise of discovery physics at the high energy frontier. Using 4.8 fb -1 of 2011 $\sqrt{s}$=7 TeV data and the first 5.8 fb -1 of 2012 $\sqrt{s}$=8 TeV data, ATLAS published the observation of a new particle with a mass of 126 GeV with a significance of 5.9σ that is compatible with a Standard Model (SM) Higgs. The LHC is outperforming initial projections for the 2012 run, and ATLAS is on track to integrate ~20 fb -1 of proton-proton collisions in 2012 before Long Shutdown 1 (LS1) begins in Spring 2013. University of Texas at Dallas (UTD) physicists will complete work on two ATLAS analyses this fall. The first is the search for the gauge bosons of a hypothesized dark sector. For 2011 data, UTD is responsible for the dark photon search in the electron-jet channel, and we are looking forward to an expanded leadership role in the dark photon search using the full 2012 data set. Our second analysis interest is the study of X/Y/Z exotic states having cc content, which builds on our experience in this field from the BABAR experiment After completing a measurement of the Xc(3872) production cross section this fall, we will search for evidence of the Z(4430)+ which is reported by Belle but not confirmed by BABAR. The UTD group has played a strong role in ATLAS operations, with group members serving as Pixel Run Coordinator, ATLAS Shift Leader, and Pixel/Inner Detector Shifter. For most of the current 3-year funding cycle, a group member coordinated the development of the Pixel DAQ code, and another continues to build and maintain the data quality monitoring (DQM) application that is used by the Inner Detector control room shifter. Additionally, members of our group take Pixel on-call expert shifts for DQM and DAQ. We led an optoboard lifetime study to assess concerns of premature on-detector VCSEL failure using the Pixel working prototype detector at CERN. Physicists based at UTD participated through Pixel Offline DQM and ATLAS Distributed Computing Operations Shifts (ADCoS) During Summer 2012, UTD joined upgrade activities in preparation for LS1. We took a major role in the testing of Electro-Readout (ER) Bundle testing for new Service Quarter Panels (nSQPs), and we developed two utilities to measure the timing jitter and bit error rate of the Pixel readout chain for use commissioning Pixel detector upgrades. During BABAR's heyday, the UTD group pioneered the use of e + e - annihilation events with hard Initial State Radiation (ISR) to study the charm threshold region, and we carried out the first BABAR double-cc analysis. Our most recent ISR paper, written in collaboration with A. Palano (Bari) is Exclusive Production of Ds + Ds - , D S *+ Ds - , and Ds *+ Ds *- via e+ e- Annihilation with Initial-State-Radiation was published in Physical Review D 82, 052004 (2010). Work continues on a study of ISR Λc + Λc - production, and a new search to establish and study double-ss production is starting.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Analysis of Localized Deformation Processes in Highly Irradiated Austenitic Stainless Steel through In Situ Techniques

This report describes new experimental results obtained during in situ mechanical tests with neutron-irradiated (10.7 dpa) austenitic 304L steel specimens. The in situ tensile tests inside a scanning electron microscope (SEM) were accompanied by modern high-resolution electron backscatter diffraction (HR-EBSD) analysis. The HR-EBSD approach allowed for investigating and quantifying internal stresses and dislocation structures in nondeformed and deformed steels, materials used in light water reactor internal components. The literature regarding the HR-EBSD analysis of metals and alloys is very limited, and practically no publications on the HR-EBSD analysis coupled with in situ tests are available. Experimental work, performed at Oak Ridge National Laboratory’s Low Activation Materials Development and Analysis facility, is detailed in this report. Section 1 briefly describes strain and stress localization processes and underlines their importance and role in irradiated materials, materials used in nuclear power plants. The section demonstrates that very limited quantitative data exist regarding localized stresses and strains in the irradiated materials at the meso-level (i.e., inside the grain, for a group of interacting grains, or for channel–grain boundary [GB] interaction) whereas local stresses at GBs are key contributors to the stress corrosion crack initiation.Section 2 documents the key methods used to conduct the experimental work: the SEM/EBSD system, miniature tensile frame, and irradiated material. Irradiated specimen manufacturing is described, as this option may be of special interest for other projects. Section 3 provides details of the HR-EBSD in situ tests (e.g., tensile curves, geometry, dimensions of regions of interest, load, and deformation steps). Section 4 discusses the results related to the evolution of geometrically necessary dislocations (GNDs) and their density, spatial, and statistical distributions.Section 5 focuses on the HR-EBSD analysis with respect to a channel–GB interaction. Two aspects are discussed in detail: mapping and analyzing the GND fields before and after loading, and the possibility of investigating local stresses and elastic strains at the channel–GB interaction. The present work results are beneficial for understanding strain and stress localization processes leading to crack initiation, for analyzing the material after long-term in-service life, and for developing predictive models. Section 6 summarizes the work performed and discusses future activities within this research direction.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2015

Computational Study of Symmetric Methylation on Histone Arginine Catalyzed by Protein Arginine Methyltransferase PRMT5 through QM/MM MD and Free Energy Simulations

Protein arginine methyltransferases (PRMTs) catalyze the transfer of the methyl group from S-adenosyl-l-methionine (AdoMet) to arginine residues. There are three types of PRMTs (I, II and III) that produce different methylation products, including asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA) and monomethylarginine (MMA). Since these different methylations can lead to different biological consequences, understanding the origin of product specificity of PRMTs is of considerable interest. In this article, the quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) and free energy simulations are performed to study SDMA catalyzed by the Type II PRMT5 on the basis of experimental observation that the dimethylated product is generated through a distributive fashion. The simulations have identified some important interactions and proton transfers during the catalysis. Similar to the cases involving Type I PRMTs, a conserved Glu residue (Glu435) in PRMT5 is suggested to function as general base catalyst based on the result of the simulations. Moreover, our results show that PRMT5 has an energetic preference for the first methylation on N-1 followed by the second methylation on a different -guanidino nitrogen of arginine (N-2).The first and second methyl transfers are estimated to have free energy barriers of 19-20 and 18-19 kcal/mol respectively. The computer simulations suggest a distinctive catalytic mechanism of symmetric dimethylation that seems to be different from asymmetric dimethylation.

ARABIDOPSIS↗