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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 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Field angle dependent vortex lattice phase diagram in MgB 2

Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in MgB 2 as the applied magnetic field is rotated away from the c axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high-symmetry directions in the hexagonal basal plane for H || c. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. Finally, the evolution towards the discontinuous transition can be parametrized by a vanishing twelvefold anisotropy term in the VL free energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Selective Detection of Intermediate-Amplitude Motion by Solid-State NMR

The coexistence of rigid and mobile molecules or molecular segments abounds in biomolecular assemblies. Examples include the carbohydrate-rich cell walls of plants and intrinsically disordered proteins that contain rigid β-sheet cores. In solid-state nuclear magnetic resonance (NMR) spectroscopy, dipolar polarization transfer experiments are well suited for detecting rigid components, whereas scalar-coupling experiments are well suited for detecting highly mobile components. However, few NMR methods are available to detect the segments that undergo intermediate-amplitude fast motion. Here, we introduce two NMR experiments, a two-dimensional T 2H -filtered CP-hCH correlation and a three-dimensional J-INADEQUATE CCH correlation, to observe this intermediate-amplitude motion. Both experiments involve 1 H detection under fast magic-angle spinning (MAS). By combining 1 H transverse relaxation (T 2H ) filters with dipolar polarization transfer, we suppress the signals of both highly rigid and highly mobile species, thus revealing the signals of intermediate mobile species. 1 H detection under fast MAS is crucial for distinguishing the different motional amplitudes. Here, we demonstrate these techniques on several plant cell wall samples and show that they allow the selective detection and resolution of certain hemicellulose and pectin signals, which are usually masked by the signals of the rigid cellulose and the highly dynamic pectins in purely dipolar and scalar NMR spectra.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2013

Oxidase uncoupling in heme monooxygenases: Human cytochrome P450 CYP3A4 in Nanodiscs

Highlights: ► Substantial reducing equivalents are lost in human P450 CYP3A4 via an oxidase channel. ► Substrate binding has a pronounced effect on uncoupling in cytochrome P450. ► Anionic phospholipids improve the overall coupling in CYP3A4 Nanodiscs. -- Abstract: The normal reaction mechanism of cytochrome P450 operates by utilizing two reducing equivalents to reduce atmospheric dioxygen, producing one molecule of water and an oxygenated product in an overall stoichiometry of 2 electrons:1 dioxygen:1 product. However, three alternate unproductive pathways exist where the intermediate iron–oxygen states in the catalytic cycle can yield reduced oxygen products without substrate metabolism. The first involves release of superoxide from the oxygenated intermediate while the second occurs after input of the second reducing equivalent. Superoxide rapidly dismutates and hence both processes produce hydrogen peroxide that can be cytotoxic to the organism. In both cases, the formation of hydrogen peroxide involves the same overall stoichiometry as oxygenases catalysis. The key step in the catalytic cycle of cytochrome P450 involves scission of the oxygen–oxygen bond of atmospheric dioxygen to produce a higher valent iron-oxo state termed “Compound I”. This intermediate initiates a radical reaction in the oxygenase pathway but also can uptake two additional reducing equivalents from reduced pyridine nucleotide (NADPH) and the flavoprotein reductase to produce a second molecule of water. This non-productive decay of Compound I thus yields an overall oxygen to NADPH ratio of 1:2 and does not produce hydrocarbon oxidation. This water uncoupling reaction provides one of a limited means to study the reactivity of the critical Compound I intermediate in P450 catalysis. We measured simultaneously the rates of NADPH and oxygen consumption as a function of substrate concentration during the steady-state hydroxylation of testosterone catalyzed by human P450 CYP3A4 reconstituted in Nanodiscs. We discovered that the “oxidase” uncoupling pathway is also operating in the substrate free form of the enzyme with rate of this pathway substantially increasing with the first substrate binding event. Surprisingly, a large fraction of the reducing equivalents used by the P450 system is wasted in this oxidase pathway. In addition, the overall coupling with testosterone and bromocryptine as substrates is significantly higher in the presence of anionic lipids, which is attributed to the changes in the redox potential of CYP3A4 and reductase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Structural Evidence for a Dehydrated Intermediate in Green Fluorescent Protein Chromophore Biosynthesis

The acGFPL is the first-identified member of a novel, colorless and non-fluorescent group of green fluorescent protein (GFP)-like proteins. Its mutant aceGFP, with Gly replacing the invariant catalytic Glu-222, demonstrates a relatively fast maturation rate and bright green fluorescence ({lambda}{sub ex} = 480 nm, {lambda}{sub em} = 505 nm). The reverse G222E single mutation in aceGFP results in the immature, colorless variant aceGFP-G222E, which undergoes irreversible photoconversion to a green fluorescent state under UV light exposure. Here we present a high resolution crystallographic study of aceGFP and aceGFP-G222E in the immature and UV-photoconverted states. A unique and striking feature of the colorless aceGFP-G222E structure is the chromophore in the trapped intermediate state, where cyclization of the protein backbone has occurred, but Tyr-66 still stays in the native, non-oxidized form, with C{sup {alpha}} and C{sup {beta}} atoms in the sp{sup 3} hybridization. This experimentally observed immature aceGFP-G222E structure, characterized by the non-coplanar arrangement of the imidazolone and phenolic rings, has been attributed to one of the intermediate states in the GFP chromophore biosynthesis. The UV irradiation ({lambda} = 250-300 nm) of aceGFP-G222E drives the chromophore maturation further to a green fluorescent state, characterized by the conventional coplanar bicyclic structure with the oxidized double Tyr-66 C{sup {alpha}} = C{sup {beta}} bond and the conjugated system of {pi}-electrons. Structure-based site-directed mutagenesis has revealed a critical role of the proximal Tyr-220 in the observed effects. In particular, an alternative reaction pathway via Tyr-220 rather than conventional wild type Glu-222 has been proposed for aceGFP maturation.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1993

Optical and X-ray observations of the low-mass X-ray binary EXO 0748-676

Optical and X-ray observations of EXO-676 in late March 1989 are presented. Our optical observations provide some support for the previously observed correlation between the mean optical brightness and light curve morphology. Unexpectedly, the mean X-ray and optical flux levels during this period do not reflect similar system states. The optical counterpart is found to be in an intermediate to low state, while X-ray data imply a bright (high) state. The changed relationship between optical and X-ray fluxes is evidence showing that EXO 0748-676 has possibly evolved. We fail to find correlated variability in simultaneous X-ray and optical observations. The lack of covariability is attributed to the limited simultaneous coverage of the source and/or significant geometric modulation in the optical light curve.

Thomas, Brian↗
DOE OSTI2011

Photoelectron angular distributions from autoionizing 4s{sup 1}4p{sup 6}6p{sup 1} states in atomic krypton probed with femtosecond time resolution

Photoelectron angular distributions (PADs) are obtained for a pair of 4s{sup 1}4p{sup 6}6p{sup 1} (a singlet and a triplet) autoionizing states in atomic krypton. A high-order harmonic pulse is used to excite the pair of states and a time-delayed 801 nm ionization pulse probes the PADs to the final 4s{sup 1}4p{sup 6} continuum with femtosecond time resolution. The ejected electrons are detected with velocity map imaging to retrieve the time-resolved photoelectron spectrum and PADs. The PAD for the triplet state is inherently separable by virtue of its longer autoionization lifetime. Measuring the total signal over time allows for the PADs to be extracted for both the singlet state and the triplet state. Anisotropy parameters for the triplet state are measured to be {beta}{sub 2}= 0.55 {+-} 0.17 and {beta}{sub 4}=-0.01 {+-} 0.10, while the singlet state yields {beta}{sub 2}= 2.19 {+-} 0.18 and {beta}{sub 4}= 1.84 {+-} 0.14. For the singlet state, the ratio of radial transition dipole matrix elements, X, of outgoing S to D partial waves and total phase shift difference between these waves, {Delta}, are determined to be X= 0.56 {+-} 0.08 and {Delta}= 2.19 {+-} 0.11 rad. The continuum quantum defect difference between the S and D electron partial waves is determined to be -0.15 {+-} 0.03 for the singlet state. Based on previous analyses, the triplet state is expected to have anisotropy parameters independent of electron kinetic energy and equal to {beta}{sub 2}= 5/7 and {beta}{sub 4}=-12/7. Deviations from the predicted values are thought to be a result of state mixing by spin-orbit and configuration interactions in the intermediate and final states; theoretical calculations are required to quantify these effects.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Path optimization by a variational reaction coordinate method. I. Development of formalism and algorithms

The development of algorithms to optimize reaction pathways between reactants and products is an active area of study. Existing algorithms typically describe the path as a discrete series of images (chain of states) which are moved downhill toward the path, using various reparameterization schemes, constraints, or fictitious forces to maintain a uniform description of the reaction path. The Variational Reaction Coordinate (VRC) method is a novel approach that finds the reaction path by minimizing the variational reaction energy (VRE) of Quapp and Bofill. The VRE is the line integral of the gradient norm along a path between reactants and products and minimization of VRE has been shown to yield the steepest descent reaction path. In the VRC method, we represent the reaction path by a linear expansion in a set of continuous basis functions and find the optimized path by minimizing the VRE with respect to the linear expansion coefficients. Improved convergence is obtained by applying constraints to the spacing of the basis functions and coupling the minimization of the VRE to the minimization of one or more points along the path that correspond to intermediates and transition states. The VRC method is demonstrated by optimizing the reaction path for the Müller-Brown surface and by finding a reaction path passing through 5 transition states and 4 intermediates for a 10 atom Lennard-Jones cluster.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Intermediate-Valence Tautomerism in Decamethylytterbocene Complexes of Methyl-Substituted Bipyridines

Multiconfigurational, intermediate valent ground states are established in several methyl-substituted bipyridine complexes of bispentamethylcyclopentadienylytterbium, Cp*{sub 2} Yb(Me{sub x}-bipy). In contrast to Cp*{sub 2} Yb(bipy) and other substituted-bipy complexes, the nature of both the ground state and the first excited state are altered by changing the position of the methyl or dimethyl substitutions on the bipyridine rings. In particular, certain substitutions result in multiconfigurational, intermediate valent open-shell singlet states in both the ground state and the first excited state. These conclusions are reached after consideration of single-crystal x-ray diffraction (XRD), the temperature dependence of x-ray absorption near-edge structure (XANES), extended x-ray absorption fine-structure (EXAFS), and magnetic susceptibility data, and are supported by CASSCF-MP2 calculations. These results place the various Cp*{sub 2}Yb(bipy) complexes in a new tautomeric class, that is, intermediate-valence tautomers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Carbocation Stability in H-ZSM5 at High Temperature

Zeolites are common catalysts for multiple industrial applications, including alcohol dehydration to produce olefins, and given their commercial importance, reaction mechanisms in zeolites have long been proposed and studied. Some proposed reaction mechanisms for alcohol dehydration exhibit noncyclic carbocation intermediates or transition states that resemble carbocations, and several previous studies suggest that the tert-butyl cation is the only noncyclic cation more stable than the corresponding chemisorbed species with the hydrocarbon bound to the framework oxygen (i.e., an alkoxide). To determine if carbocations can exist at high temperatures in zeolites, where these catalysts are finding new applications for biomass vapor-phase upgrading (~500 °C), the stability of carbocations and the corresponding alkoxides were calculated with two ONIOM embedding methods (M06-2X/6-311G(d,p):M06-2X/3-21G) and (PBE-D3/6-311G(d,p):PBE-D3/3-21G) and plane-wave density functional theory (DFT) using the PBE functional corrected with entropic and Tkatchenko-Scheffler van der Waals corrections. The embedding methods tested are unreliable at finding minima for primary carbocations, and only secondary or higher carbocations can be described with embedding methods consistent with the periodic DFT results. The relative energy between the carbocations and alkoxides differs significantly between the embedding and the periodic DFT methods. The difference is between ~0.23 and 14.30 kcal/mol depending on the molecule, the model, and the functional chosen for the embedding method. At high temperatures, the pw-DFT calculations predict that the allyl, isopropyl, and sec-butyl cations exhibit negligible populations while acetyl and tertbutyl cations exhibit significant populations (>10%). Moreover, the periodic DFT results indicate that mechanisms including secondary and tertiary carbocations intermediates or carbocations stabilized by adjacent oxygen or double bonds are possible at high temperatures relevant to some industrial uses of zeolite catalysts, although as the minority species in most cases.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Imaging active site chemistry and protonation states: NMR crystallography of the tryptophan synthase α-aminoacrylate intermediate

NMR-assisted crystallography-the integrated application of solid-state NMR, X-ray crystallography, and first-principles computational chemistry-holds significant promise for mechanistic enzymology: by providing atomic-resolution characterization of stable intermediates in enzyme active sites, including hydrogen atom locations and tautomeric equilibria, NMR crystallography offers insight into both structure and chemical dynamics. Here, this integrated approach is used to characterize the tryptophan synthase α-aminoacrylate intermediate, a defining species for pyridoxal-5'-phosphate-dependent enzymes that catalyze β-elimination and replacement reactions. For this intermediate, NMR-assisted crystallography is able to identify the protonation states of the ionizable sites on the cofactor, substrate, and catalytic side chains as well as the location and orientation of crystallographic waters within the active site. Most notable is the water molecule immediately adjacent to the substrate β-carbon, which serves as a hydrogen bond donor to the ε-amino group of the acid-base catalytic residue βLys87. From this analysis, a detailed three-dimensional picture of structure and reactivity emerges, highlighting the fate of the L-serine hydroxyl leaving group and the reaction pathway back to the preceding transition state. Reaction of the α-aminoacrylate intermediate with benzimidazole, an isostere of the natural substrate indole, shows benzimidazole bound in the active site and poised for, but unable to initiate, the subsequent bond formation step. When modeled into the benzimidazole position, indole is positioned with C3 in contact with the α-aminoacrylate C $\bar{β}$ and aligned for nucleophilic attack. Here, the chemically detailed, three-dimensional structure from NMR-assisted crystallography is key to understanding why benzimidazole does not react, while indole does.

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

Charge transfer as a mechanism for chlorophyll fluorescence concentration quenching

Highly concentrated solutions of chlorophyll display rapid fluorescence quenching. The same devastating energy loss is not seen in photosynthetic light-harvesting antenna complexes, despite the need for chromophores to be in close proximity to facilitate energy transfer. A promising, though unconfirmed mechanism for the observed quenching is energy transfer from an excited chlorophyll monomer to a closely associated chlorophyll pair that subsequently undergoes rapid nonradiative decay to the ground state via a short-lived intermediate charge-transfer state. In this work, we make use of newly emerging fast methods in quantum chemistry to assess the feasibility of this proposed mechanism. We calculate rate constants for the initial charge separation, based on Marcus free-energy surfaces extracted from molecular dynamics simulations of solvated chlorophyll pairs, demonstrating that this pathway will compete with fluorescence (i.e., drive quenching) at experimentally measured quenching concentrations. We show that the rate of charge separation is highly sensitive to interchlorophyll distance and the relative orientations of chromophores within a quenching pair. We discuss possible solvent effects on the rate of charge separation (and consequently the degree of quenching), using the light-harvesting complex II (LH2) protein from rps. acidophila as a specific example of how this process might be controlled in a protein environment. Crucially, we reveal that the LH2 antenna protein prevents quenching, even at the high chlorophyll concentrations required for efficient energy transfer, by restricting the range of orientations that neighboring chlorophyll pairs can adopt.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Na x CoO 2 phase stability and hierarchical orderings in the O3/P3 structure family

The operation of Na-ion batteries is closely linked to the occurrence of phase transitions and Na ordering within the electroactive materials of their electrodes. Here, we performed a first-principles study of Na x CoO 2 (0 ≤ x ≤ 1) to establish phase stability among O3, P3, O1, and staged hybrid O1-O3 host structures and to determine low-energy Na orderings within each host. We predict several staircases of ground-state orderings, many of which are found to remain ordered at room temperature. At high Na content, the ground states belong to a family of vacancy row orderings in O3 with distorted CoO 2 layers. In P3, where the available Na sites form a honeycomb network, we discover three families of hierarchical ground-state orderings at intermediate compositions that essentially consist of domains of the x = 1/2 ground state periodically separated by antiphase boundaries. Our results agree with the experimentally reported voltage profile and the observed changes in stacking sequence with Na concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Spin-Controllable Dynamics in Defect-Engineered Carbon Nanotubes as Single Photon Emitters: Data-Driven Modeling and Computations

Quantum technologies, such as quantum computing and sensing, require efficient single-photon emission (SPE) sources that operate at room temperature in telecom wavelengths. While several materials can serve as SPE sources, no single platform meets all the criteria for efficiency, ambient operation, and scalability. Single-walled carbon nanotubes (SWCNTs) with covalently attached molecules offer a promising solution. Their SPE can be easily tuned via modifications of the SWCNT's diameter, chirality, and bonded molecules, enabling emission across near-IR to telecom wavelengths at ambient conditions. However, to fully realize the potential of SWCNTs and unlock their quantum capabilities, a deeper understanding of how structural defects from molecular adducts affect their emission and competing photoexcited processes is essential. To address this gap in our knowledge, this project combined quantum chemistry calculations with data-driven methods of cheminformatics (QSAR) and machine learning (ML). The developed computational approaches have provided several design strategies for covalent functionalization of SWCNTs to improve their optical response. The collaboration with Los Alamos National Lab (LANL) enabled direct comparison of computational and experimental data, facilitating method validation. This partnership was enhanced through access to LANL's Center for Integrated Nanotechnologies (CINT) utilizing User Facility Program and summer internships, which provided three NDSU graduate students with hands-on experience at LANL. The outcomes of this project included (1) Advancing the current stage of computational methods in accurate modeling of non-adiabatic spin-dependent photoexcited dynamics and its applicability to nanosystems consisting of thousands of atoms, realized as open-access codes linked to existing DFT-based software; (2) Establishing the relationship between the structure of adducts and SWCNTs and intrinsic excitonic and spin properties of defect states for guiding novel synthetic strategies and experimental probes of chemically functionalized SWCNTs as near-IR emitting materials; (3) Generating virtual libraries of hypothetical functionalized SWCNTs for virtual screening of their chemical structures and optical properties, leveraging new functionalities of SWCNTs; (4) Offering a unique experience for NDSU graduate students that prepared them for future scientific careers related to materials modeling and big data processing. These results were summarized in 12 published journal papers and 3 recently submitted papers. One of a key finding is that the position of defect sites on the SWCNT surface primarily drives the emission redshift (up to 100 meV), while the polarity of the defect-inducing molecules has a much smaller effect (~10 meV). However, the electron-donating or withdrawing properties of a molecule influence selecting reactivity of defect sites. These insights important for optimizing synthetic protocols for desired emissions in SWCNTs. We also revealed that the interaction between two defects at various positions on the SWCNT enhances the redshift and optical activity of states, favoring strong near-IR emission. This suggests that manipulations in defect concentrations is a promising strategy for controlling efficient emission. Mostly important, the defect position was found controllable by the spin states of photoexcited intermediates: Excited aromatic molecules form ortho defects with SWCNTs at their singlet states in the presence of oxygen, while oxygen-free conditions favor para defects via the triplet-state mechanism. Additionally, a heat-activated [2+2] cycloaddition reaction facilitates divalent defect formation with fewer bonding positions that narrows emission bands. These groundbreaking findings have been experimentally validated and significantly advance our understanding of defect chemistry in SWCNTs. Using a novel encoding technique and 3D-MoRSE descriptors, we developed highly accurate ML/QSAR models to predict both the 3D structure and optical properties of SWCNTs with chemical defects. This model enabled the creation of a virtual library of 125,556 structures, providing new insights into the relationship between SWCNT-defect structure and emission.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Topological Crystalline Insulator Candidate ErAsS with Hourglass Fermion and Magnetic-Tuned Topological Phase Transition

Topological crystalline insulators (TCIs) with hourglass fermion surface state have attracted a lot of attention and are further enriched by crystalline symmetries and magnetic order. Here, the emergence of hourglass fermion surface state and exotic phases in the newly discovered, air-stable ErAsS single crystals are shown. In the paramagnetic phase, ErAsS is expected to be a TCI with hourglass fermion surface state protected by the nonsymmorphic symmetry. Dirac-cone-like bands and nearly linear dispersions in large energy range are experimentally observed, consistent well with theoretical calculations. Below T N ≈ 3.27 K, ErAsS enters a collinear antiferromagnetic state, which is a trivial insulator breaking the time-reversal symmetry. An intermediate incommensurate magnetic state appears in a narrow temperature range (3.27–3.65 K), exhibiting an abrupt change in magnetic coupling. Overall, the results reveal that ErAsS is an experimentally available TCI candidate and provide a unique platform to understand the formation of hourglass fermion surface state and explore magnetic-tuned topological phase transitions.

36 MATERIALS SCIENCE↗