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NASA NTRS1997

Peptide Folding and Translocation Across the Water-Membrane Interface

The ability of small peptides to organize at aqueous interfaces was examined by performing a series of large-scale, molecular dynamics computer simulations of several peptides composed of two amino acids, nonpolar leucine (L) and polar glutamine (Q). The peptides differed in size and sequence of the amino acids. Studies on dipeptides LL, LQ, QL and QQ were extended to two heptamers, LQQLLQL and LQLQLQL, designed to maximize interfacial stability of an alpha-helix and a beta-strand, respectively, by exposing polar side chains to water and nonpolar side chains to a nonpolar phase. Finally, a transition of an undecamer, composed entirely of leucine residues, from a disordered structure in water to an alpha-helix in a nonpolar phase representing the interior of the membrane was investigated. Complete folding of a peptide in solution was accomplished for the first time in computer simulations. The simulations revealed several basic principles governing the sequence-dependent organization of peptides at interfaces. Short peptides tend to accumulate at interfaces and acquire ordered structures, providing that they have a proper sequence of polar and nonpolar amino acids. The dominant factor determining the interfacial structure of peptides is the hydrophobic effect, which is manifested at aqueous interfaces as a tendency for polar and nonpolar groups of the solute to segregate into the aqueous and nonpolar phases, respectively. If peptides consist of nonpolar residue's only, they become inserted into the nonpolar phase. As demonstrated by the example of the leucine undecamer, such peptides fold into an alpha-helix as they partition into the nonpolar medium. The folding proceeds through an intermediate, called 3-10-helix, which remains in equilibrium with the alpha-helix. Once in the nonpolar environment, the peptides can readily change their orientation with respect to the interface from parallel to perpendicular, especially in response to local electric fields. The ability of nonpolar peptides to modify both the structure and orientation with respect to the interface from parallel to perpendicular, especially in response to local electric fields. The ability of nonpolar peptides to modify both the structure and orientation with changing external conditions may have provided a simple mechanism of transmitting signals from the environment to the interior of a cell.

Pohorille, Andrew↗

NASA NTRS1984

Application of concurrent processing to structural dynamic response computations

Described are the experiences gained from solving for the dynamic response of two simple structures on an experimental Multiple Instruction Multiple Data (MIMD) computer called the finite element machine. Introduced are MIMD computing concepts, describing how the concurrent algorithmic techniques implemented and giving results for the two example problems. The results show computational speedups of up to 7.83 using eight of the finite element machine processors and indicate that significant computational speedups are possible for large order structural computations.

Ransom, J.↗

DOE OSTI2021

Condensation of collective polar vortex modes

The dynamics of extended objects such as domain walls, domain bubbles, vortex structures, etc., can be described by their equations of motion associated with their effective mass and spring constant. Here we analytically derive the equations of motion for the polarization dynamics and elastodynamics for the structural responses of ferroelectric polar vortices, and theoretically extract their effective mass, spring constant, and mode frequencies. We demonstrate two subterahertz phonon modes and predicted their frequencies, both consistent with our recent experimental measurements and phase-field simulations. We show that elastic modulation of the energy function and spring constants leads to a condensation of a collective mode upon a second-order structural transition from symmetric to asymmetric vortices at a critical strain, analogous to the ferroelectric soft phonon mode at a ferroelectric transition. Furthermore, the present work offers a theoretical framework for predicting and manipulating the ultrafast collective dynamics of polar nanostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1987

Effects of sequential treatment with fluorine and bromine on graphite fibers

Three pitch based graphite fibers with different degrees of graphitization and one polyacryonitrile (PAN) based carbon fiber from Amoco Corporation were treated with 1 atm, room temperature fluorine gas for 90 hrs. Fluorination resulted in higher electrical conductivity for all pitch fibers. Further bromination after ambient condition defluorination resulted in further increases in electrical defluorination conductivity for less graphitized, less structurally ordered pitch fibers (P-55) which contain about 3% fluorine by weight before bromination. This product can be stable in 200 C air, or 100% humidity at 60 C. Due to its low cost, this less graphitized fiber may be useful for industrial application, such as airfoil deicer materials. The same bromination process, however, resulted in conductivity decreases for fluorine rich, more graphitized, structurally oriented pitch fibers (P-100 and P-75). Such decreases in electrical conductivity were partially reversed by heating the fibers at 185 C in air. Differential scanning calorimetric (DSC) data indicated that the more graphitized fibers (P-100) contained BrF3, whereas the less graphitized fibers (P-55) did not.

Hung, Ching-Cheh↗

NASA NTRS2023

STXM-C-XANES, IR Microspectroscopy, and Raman Microspectroscopy Analysis of Organic Matter in the Chwichiya 002 C3.00-Ungrouped Chondrite

Chwichiya 002 is a C3.00-ungrouped carbonaceous chondrite discovered in Western Sahara in 2018. The presence of hydrated phases in Chwichiya 002 was not confirmed by XRD analysis, and only a faint water and Si-O band attributed to phyllosilicate were observed by infrared transmission spectroscopy, indicating a very low degree of aqueous alteration (Meteoritical Bulletin, 2020). The structural order of polycyclic aromatic hydrocarbons evaluated by Raman spectroscopy indicates that the effect of thermal metamorphism is less severe than in the Semarkona meteorite, an LL3.00 ordinary chondrite (Meteoritical Bulletin, 2020). Chiwichiya 002 is a very primitive carbonaceous chondrite and an important information source for the early stage of the evolution of organic matter in the solar system. In this study, we investigated the molecular structure and distribution of organic matter in Chiwichiya 002 by Carbon X-ray Absorption Near Edge Structure (C-XANES) analysis using a Scanning Transmission X-ray Microscope (STXM). The STXM-C-XANES analysis has a spatial resolution of 20~100 nm and can observe the spatial distribution of organic materials in microstructures. In addition, to investigate the degree of thermal metamorphism of the organic matter, micro-IR, and micro-Raman spectroscopic analyses were performed. In addition, the Murchison CM2 chondrite was also analyzed for comparison. To evaluate contamination during analytical procedures, antigorite, which was heated in air at 500°C for 5 hours to eliminate organic matter was also analyzed in the same manner. The samples were milled into 100 nm-thick, thin sections using a Focused Ion Beam (FIB), and C-XANES spectra were obtained using the STXM at the High Energy Accelerator Research Organization, Photon Factory (PF), BL-19B. The obtained C-XANES spectra were smoothed and then fitted with Gaussian functions using the method of Le Guillou et al. (2018). The C-XANES showed that the ratio of oxygenated functional group (ketone, phenolic) carbons in Chwichiya 002 was lower than in Murchison, whereas the ratio of aliphatic carbons was almost the same. This may reflect the low degree of aqueous alteration of Chwichiya 002. In addition, contamination peaks were detected at 287.3-287.7 eV and 288.3-288.7 eV in the C-XANES spectra of some of the antigorite samples. The source of these peaks is under investigation. Raman spectroscopic analysis showed that the intensity ratio of the D band to the G band of Chwichiya 002 was slightly higher than that of Murchison. IR absorption spectra showed aromatic absorption at 1630 cm -1 , but there was little absorption due to aliphatic C-H around 2900 cm -1 . These results suggest that the organic matter in Chwichiya 002 may be more heated than in Murchison.

Chwichiya 002↗

NASA NTRS1994

Multifractal characterizations of nonstationarity and intermittency in geophysical fields: Observed, retrieved, or simulated

Geophysical data rarely show any smoothness at any scale, and this often makes comparison with theoretical model output difficult. However, highly fluctuating signals and fractal structures are typical of open dissipative systems with nonlinear dynamics, the focus of most geophysical research. High levels of variability are excited over a large range of scales by the combined actions of external forcing and internal instability. At very small scales we expect geophysical fields to be smooth, but these are rarely resolved with available instrumentation or simulation tools; nondifferentiable and even discontinuous models are therefore in order. We need methods of statistically analyzing geophysical data, whether measured in situ, remotely sensed or even generated by a computer model, that are adapted to these characteristics. An important preliminary task is to define statistically stationary features in generally nonstationary signals. We first discuss a simple criterion for stationarity in finite data streams that exhibit power law energy spectra and then, guided by developments in turbulence studies, we advocate the use of two ways of analyzing the scale dependence of statistical information: singular measures and qth order structure functions. In nonstationary situations, the approach based on singular measures seeks power law behavior in integrals over all possible scales of a nonnegative stationary field derived from the data, leading to a characterization of the intermittency in this (gradient-related) field. In contrast, the approach based on structure functions uses the signal itself, seeking power laws for the statistical moments of absolute increments over arbitrarily large scales, leading to a characterization of the prevailing nonstationarity in both quantitative and qualitative terms. We explain graphically, step by step, both multifractal statistics which are largely complementary to each other. The geometrical manifestations of nonstationarity and intermittency, 'roughness' and 'sparseness', respectively, are illustrated and the associated analytical (differentiability and continuity) properties are discussed. As an example, the two techniques are applied to a series of recent measurements of liquid water distributions inside marine stratocumulus decks; these are found to be multifractal over scales ranging from approximately 60 m to approximately 60 km. Finally, we define the 'mean multifractal plane' and show it to be a simple yet comprehensive tool with many applications including data intercomparison, (dynamical or stochastic) model and retrieval validations.

Davis, Anthony↗

DOE OSTI2021

Driving magnetic domains at the nanoscale by interfacial strain-induced proximity

Here, we investigate the local nanoscale changes of the magnetic anisotropy of a Ni film subject to an inverse magnetostrictive effect by proximity to a V 2 O 3 layer. Using temperature-dependent photoemission electron microscopy (PEEM) combined with X-ray magnetic circular dichroism (XMCD), direct images of the Ni spin alignment across the first-order structural phase transition (SPT) of V 2 O 3 were obtained. We find an abrupt temperature-driven reorientation of the Ni magnetic domains across the SPT, which is associated with a large increase of the coercive field. Moreover, angular dependent ferromagnetic resonance (FMR) shows a remarkable change in the magnetic anisotropy of the Ni film across the SPT of V 2 O 3 . Micromagnetic simulations based on these results are in quantitative agreement with the PEEM data. Direct measurements of the lateral correlation length of the Ni domains from XMCD images show an increase of almost one order of magnitude at the SPT compared to room temperature, as well as a broad spatial distribution of the local transition temperatures, thus corroborating the phase coexistence of Ni anisotropies caused by the V 2 O 3 SPT. We show that the rearrangement of the Ni domains is due to strain induced by the oxide layers’ structural domains across the SPT. Our results illustrate the use of alternative hybrid systems to manipulate magnetic domains at the nanoscale, which allows for engineering of coercive fields for novel data storage architectures.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Structural and magnetic transitions in the planar antiferromagnet Ba 4 Ir 3 O 10

We report the structural and magnetic ground state properties of the monoclinic compound barium iridium oxide Ba 4 Ir 3 O 10 using a combination of resonant x-ray scattering, magnetometry, and thermodynamic techniques. Magnetic susceptibility exhibits a pronounced antiferromagnetic transition at T N ≈ 25 K, a weaker anomaly at T S ≈ 142 K, and strong magnetic anisotropy at all temperatures. Resonant elastic x-ray scattering experiments reveal a second order structural phase transition at T S and a magnetic transition at T N . Both structural and magnetic superlattice peaks are observed at L = half integer values. The magnetization anomaly at T S implies the presence of magnetoelastic coupling, which conceivably facilitates the symmetry lowering. Mean field critical scattering is observed above T S . The magnetic structure of the antiferromagnetic ground state is discussed based on the measured magnetic superlattice peak intensity. Furthermore, our study not only presents essential information for understanding the intertwined structural and magnetic properties in Ba 4 Ir 3 O 10 but also highlights the necessary ingredients for exploring novel ground states with octahedra trimers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Theoretical antiferromagnetism of ordered face-centered cubic Cr-Ni alloys

Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semilocal density-functional theory. The optimization of local magnetic moments significantly increases the driving force for the formation of CrNi 2 , the only experimentally observed intermetallic phase. This structure's ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin fluctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, specifically a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. In conclusion, implications for related commercial and high-entropy alloys are discussed for each case.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Open-Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

Conductive polymers largely derive their electronic functionality from chemical doping, processes by which redox and charge-transfer reactions form mobile carriers. While decades of research have demonstrated fundamentally new technologies that merge the unique functionality of these materials with the chemical versatility of macromolecules, doping and the resultant material properties are not ideal for many applications. Here, it is demonstrated that open-shell conjugated polymers comprised of alternating cyclopentadithiophene and thiadiazoloquinoxaline units can achieve high electrical conductivities in their native “undoped” form. Spectroscopic, electrochemical, electron paramagnetic resonance, and magnetic susceptibility measurements demonstrate that this donor–acceptor architecture promotes very narrow bandgaps, strong electronic correlations, high-spin ground states, and long-range π-delocalization. A comparative study of structural variants and processing methodologies demonstrates that the conductivity can be tuned up to 8.18 S cm -1 . This exceeds other neutral narrow bandgap conjugated polymers, many doped polymers, radical conductors, and is comparable to commercial grades of poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene). X-ray and morphological studies trace the high conductivity to rigid backbone conformations emanating from strong π-interactions and long-range ordered structures formed through self-organization that lead to a network of delocalized open-shell sites in electronic communication. The results offer a new platform for the transport of charge in molecular systems.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Structure and dynamics of $\mathrm{Fe_{90}}$$\mathrm{Si_3}$$\mathrm{O_7}$ liquids close to Earth's liquid core conditions

Using an artificial neural-network machine learning interatomic potential, we have performed molecular dynamics simulations to study the structure and dynamics of $\mathrm{Fe_{90}}$$\mathrm{Si_3}$$\mathrm{O_7}$ liquid close to the Earth's liquid core conditions. Here, the simulation results reveal that the short-range structural order (SRO) in the $\mathrm{Fe_{90}}$$\mathrm{Si_3}$$\mathrm{O_7}$ liquid is very strong. About 80% of the atoms are arranged in crystallinelike SRO motifs. In particular, ~ $70$% of Fe-centered clusters can be classified as either hexagonal-close-packed-like or icosahedrallike SRO motifs. The SRO clusters centered on Fe, Si, or O atoms are strongly intermixed and homogenously distributed throughout the liquid. The atomic structure of the liquid and the fractions of dominant SRO clusters are not sensitive to pressure/temperature used in the simulations except that the SRO of the O-centered clusters is enhanced close to inner core pressures. The O-diffusion coefficient is about two to three times larger than the Fe and Si ions and increases more rapidly in the deeper core regions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO 2

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl 2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO 2 and H 2 O reached as high as ∼10 to 50 mA cm −2 , and remained stable for >2 days. Selectivity for the reduction of CO 2 to CO vs. H 2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO 2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO 2 reduction at sizable current densities.

Chemistry↗

NASA NTRS1997

Energetics of the lattice: packing elements in crystals of four-stranded intercalated cytosine-rich DNA molecules

Condensation of single molecules from solution into crystals represents a transition between distinct energetic states. In solution, the atomic interactions within the molecule dominate. In the crystalline state, however, a set of additional interactions are formed between molecules in close contact in the lattice--these are the packing interactions. The crystal structures of d(CCCT), d(TAACCC), d(CCCAAT), and d(AACCCC) have in common a four-stranded intercalated cytosine segment, built by stacked layers of cytosine.cytosine+ (C.C+) base pairs coming from two parallel duplexes that intercalate into each other with opposite polarity. The intercalated cytosine segments in these structures are similar in their geometry, even though the sequences crystallized in different space groups. In the crystals, adenine and thymine residues of the sequences are used to build the three-dimensional crystal lattice by elaborately interacting with symmetry-related molecules. The packing elements observed provide novel insight about the copious ways in which nucleic acid molecules can interact with each other--for example, when folded in more complicated higher order structures, such as mRNA and chromatin.

Non-NASA Center↗

DOE OSTI2023

Guided anisotropic oxygen transport in vacancy ordered oxides

Abstract Anisotropic and efficient transport of ions under external stimuli governs the operation and failure mechanisms of energy-conversion systems and microelectronics devices. However, fundamental understanding of ion hopping processes is impeded by the lack of atomically precise materials and probes that allow for the monitoring and control at the appropriate time- and length- scales. In this work, using in-situ transmission electron microscopy, we directly show that oxygen ion migration in vacancy ordered, semiconducting SrFeO 2.5 epitaxial thin films can be guided to proceed through two distinctly different diffusion pathways, each resulting in different polymorphs of SrFeO 2.75 with different ground electronic properties before reaching a fully oxidized, metallic SrFeO 3 phase. The diffusion steps and reaction intermediates are revealed by means of ab-initio calculations. The principles of controlling oxygen diffusion pathways and reaction intermediates demonstrated here may advance the rational design of structurally ordered oxides for tailored applications and provide insights for developing devices with multiple states of regulation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Cryo-EM structure of a methanogen nitrogenase-PII protein supercomplex

Abstract Nitrogenases are metalloenzymes that catalyze the reduction of atmospheric dinitrogen to ammonia, sustaining the global nitrogen cycle. While bacterial nitrogenase has been extensively characterized, the architecture and regulation of archaeal nitrogenases remain unknown despite longstanding evidence of nitrogen fixation in methanogens. Here we report a 3.1 Å cryo-electron microscopy structure of a native nitrogenase–PII protein supercomplex from Methanosarcina acetivorans. The structure reveals an unprecedented assembly of three NifDK heterotetramers bridged by six NifI1,2 heterotrimeric PII complexes, which sterically block NifH association and lock the enzyme in an inactive state. The NifI complexes display asymmetric binding of ADP and 2-oxoglutarate, coupling nitrogenase inhibition directly to cellular energy and nitrogen status. Addition of 2-oxoglutarate and ATP releases the NifI complexes, stimulating a threefold increase in NifDK activity in vitro. This higher-order architecture uncovers a previously unrecognized regulatory strategy in methanogens, in which PII proteins drive nitrogenase oligomerization to control activity. The discovery that nitrogenase activity may be modulated through direct assembly into higher-order structures opens new avenues for exploring nitrogenase evolution, regulation, and biotechnological applications. One sentence summary Discovery of a nitrogenase-PII protein supercomplex in methanogens, uncovering a metabolite-gated assembly mechanism for nitrogenase inhibition.

Nitrogenase, Electron Transfer, Methanogen↗

DOE OSTI2023

Improving Carbonate-Promoted C–H Carboxylation Using Mesoporous Carbon Supports

C–H carboxylation is an attractive way to utilize CO 2 for chemical production provided that it does not consume resource-intensive reagents. Alkali carbonates dispersed into the pores of mesoporous supports display strongly basic reactivity under CO 2 , allowing them to be used as base promoters for C–H carboxylation of (hetero)arenes in the absence of other reagents or catalysts. Mesoporous oxides are convenient support materials, but only a relatively small fraction of the dispersed carbonate (ca. 10-20%) is converted to carboxylate products when metal oxide supports are used. Here we compare mesoporous oxide and carbon supports and investigate the dependence of carbonate reactivity on pore structure. We show that using mesoporous carbons supports can increase the carbonate conversion by 2–4x when compared to oxide supports. This improved carbonate reactivity is maintained across a variety of mesoporous carbons with different pore structures (ordered vs. disordered) and pore diameters, indicating that the dispersed carbonate is intrinsically more reactive on the surface of a carbon material compared to an oxide surface. Reaction of the carboxylate products with dimethyl carbonate yields isolable methyl esters as the final product and regenerates the dispersed carbonate. We show that mesoporous carbon supports are robust to at least 5 cycles of successive C–H carboxylation and methylation with dimethyl carbonate. Furthermore, understanding how the support structure affects dispersed carbonate reactivity is valuable for advancing C–H carboxylation toward practical application and utilizing these materials in other CO 2 transformations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Final Technical Report for DE-FG02-86ER45268

This work addresses several key issues for understanding the unusual superconductivity in IBS, already shown to be technologically useful materials. 1a). Are reactive gasses like fluorine, chlorine, or water vapor behaving simply as electron donors to cause the superconductivity (at T conset =21 K) in undoped IBS like BaFe 2 As 2 ? 1b). Does exposure to fluorine increase T c in electron-doped IBS materials that are underdoped, e. g. Ba(Fe 0.96 Co 0.04 ) 2 As 2 ? 1c). Can fluorine as a reactive electron-donor, with its relatively greater ease of introduction into the lattice, be used as a rapid tool to search for superconductivity in new materials under investigation? Method of investigation: Using electron probe microanalysis, x-ray photoemission spectroscopy, and dynamical mean field theory calculations, we found that fluorine substitutes for As in the BaFe 2 As 2 lattice, and should therefore behave as an electron donor. We will F-dope Ba(Fe 0.96 Co 0.04 ) 2 As 2 and several new compounds under investigation for superconductivity upon doping to determine fluorine’s effect. Potential impact: If – like for fluorine - a brief exposure to hydrogen indeed causes superconductivity to a depth of 10 microns, this technique could be useful in creating long lengths of superconducting tape without the chemical aggressiveness of fluorine. Using the F ion, which substitutes for As - away from the Fe planes, in underdoped IBS like Ba(Fe 0.96 Co 0.04 ) 2 As 2 may form optimally doped IBS with less impurity scattering of the superconducting Fe 3d electrons resulting in improved superconducting properties. Another potential impact is the speed of checking for superconductivity in new materials under investigation for superconductivity upon doping. 2. What are the effects of Be substitution into IBS, beginning with FeSe and its derivatives (FeSe 1-x S x and FeSe 1-x Te x ) and BaFe 2-x Co x As 2 ? Method of investigation: We recently succeeded in adding Be to the FeSe lattice and found, surprisingly, a factor of four improvement of the low temperature residual resistivity ratio, consistent with improved electronic ordering. Using single crystal x-ray diffraction and x-ray dispersive spectroscopy, determine the concentration and location of the Be dopant. Using resistivity, determine if there is improvement in the residual resistivity achieved by Be doping in the FeSe derivatives (using AlCl 3 :KCl flux growth) and Co-doped BaFe 2 As 2 (using self-flux single crystal growth). Measuring resistivity under strain, determine the changes of the nematic susceptibility at the structural ordering transition causes by the increase in electronic order caused by Be doping in FeSe. Potential impact: Further understanding of IBS, including FeSe which - under physical pressure - has a T c increase to 40 K and -prepared as a monolayer - has a T c reported to be over 100 K. 3. In collaboration, prepare light atom compounds involving Be and look for high pressure-induced superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1994

Three-dimensional structure of Schistosoma japonicum glutathione S-transferase fused with a six-amino acid conserved neutralizing epitope of gp41 from HIV

The 3-dimensional crystal structure of glutathione S-transferase (GST) of Schistosoma japonicum (Sj) fused with a conserved neutralizing epitope on gp41 (glycoprotein, 41 kDa) of human immunodeficiency virus type 1 (HIV-1) (Muster T et al., 1993, J Virol 67:6642-6647) was determined at 2.5 A resolution. The structure of the 3-3 isozyme rat GST of the mu gene class (Ji X, Zhang P, Armstrong RN, Gilliland GL, 1992, Biochemistry 31:10169-10184) was used as a molecular replacement model. The structure consists of a 4-stranded beta-sheet and 3 alpha-helices in domain 1 and 5 alpha-helices in domain 2. The space group of the Sj GST crystal is P4(3)2(1)2, with unit cell dimensions of a = b = 94.7 A, and c = 58.1 A. The crystal has 1 GST monomer per asymmetric unit, and 2 monomers that form an active dimer are related by crystallographic 2-fold symmetry. In the binding site, the ordered structure of reduced glutathione is observed. The gp41 peptide (Glu-Leu-Asp-Lys-Trp-Ala) fused to the C-terminus of Sj GST forms a loop stabilized by symmetry-related GSTs. The Sj GST structure is compared with previously determined GST structures of mammalian gene classes mu, alpha, and pi. Conserved amino acid residues among the 4 GSTs that are important for hydrophobic and hydrophilic interactions for dimer association and glutathione binding are discussed.

Schistosoma japonicum/enzymology/genetics/immunolo↗