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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 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Active region of a functionally integrated laser – modulator

A method for the functional integration of injection lasers and modulators of generated light in a single heterostructure with spatially displaced quantum wells in the conduction and valence bands, formed by the second-type heterojunctions, is proposed. The structures and energy band diagrams of the active regions of functionally integrated lasers – modulators with amplitude and frequency modulation are considered. A physical – topological model and a technique for numerical simulation of lasers – modulators is proposed. It is shown that the maximum modulation frequencies for the proposed heterostructures, in contrast to the same frequencies implemented under pump current modulation, are not limited by the duration of transients in the laser power supply circuit and the carrier lifetime in the laser active region but are determined by the photon lifetime in the cavity. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Collaborative atomic-scale design, analysis, and nanofabrication for record breaking, single-crystal Zn(x)Cd(1-x)Te solar cell arrays

Cadmium telluride (CdTe) based photovoltaics provide a viable solar energy solution because of their short payback time, low manufacturing cost, and high theoretical efficiency ~29%. However, the highest record laboratory efficiency is only 22.1% and long-term performance degradation is ~1 % per year which hinders greater economic viability. Higher open circuit voltage (Voc) and fill factor (FF) are needed to increase the efficiency of the solar cell. Cells with record energy conversion efficiencies report short current (Jsc) values close to their theoretical maximum. However, Voc and FF in these devices remain well below their anticipated optimum. These electrical parameters depend on the quality of the various layers in the solar cell. In particular they depend on the crystalline quality including the defect and impurity concentrations. Therefore understanding how defects and impurities form and evolve in the material layers is important to design devices with higher efficiency and stability. The aim of this project is to create a novel path around the fundamental barriers to achieve high open-circuit voltages (VOC ≥ 1.0 V) in CdTe/CdS solar cells. The idea of the project was to combine nanopatterned crystal growth and alloy compositional grading (with concomitant lattice parameter and energy band gap grading) to reduce the defect density and dramatically improve spatial uniformity of ZnxCd(1-x)Te/CdS cells. The lattice mismatch between ZnTe and CdS is 3.9% compared to a much larger 9.2% between CdTe and CdS. Importantly, when crystal growth is contained to the nanoscale, the strain in each layer is reduced by a factor of two due to strain partitioning and 3D strain relief mechanisms that come into play. This reduces the strain in the ZnTe from 3.9% to 1.95% which is within the range of metamorphic crystal growth. In preliminary studies, molecular dynamics simulation of ZnTe/CdS islands predicted dislocation-free material when the patterning was reduced to ~80 nm in diameter. The energy band gap of ZnxCd(1-x)Te is also graded in conjunction with alloy composition. ZnTe has an energy band gap of 2.3 eV. Finally, an added benefit of patterned growth is that it enables direct one-to-one study of the relationships between fabrication, microstructure and performance of individual crystal grains within the array. Experimental characterization techniques such as atomic probe tomography (APT) and transmission electron microscopy (TEM) have elucidated much about the microstructure of the layers. However, the techniques are destructive, costly and have limitations on the information they convey. For example, TEM gives information with atomic scale resolution but on 2-dimensional substrates. On the other hand, APT gives 3-dimensional information but is not able to resolve atomic scale structure. Alternatively, Molecular Dynamics (MD) simulations offer a solution to rapidly generate time-resolved, 3-dimensional, atomic scale structures of crystal growth. Direct MD simulations of microstructures of Cu-doped II-VI compound multilayers can impact the research on CdTe/CdS solar cells. Such MD simulations require an interatomic potential database that includes at least four elements Cu, Cd, Te, and S. The Stillinger-Weber (SW) potential is the most widely used interatomic potential for semiconductors. However, while SW potentials have been developed for Ga-In-As-Se-Te, Al-Ga-In-P-As-Sb, Cd-Te-Zn-As-Si, In-Ga-N systems, no potentials existed to simulate Cd-Zn-Hg-Se-S-Te-Cu. The goal of this effort was to fill the knowledge gap by developing a Zn-Cd-Hg-S-Se-Te-Cu SW potential database to enable MD simulations of microstructures of Cu-doped CdTe/CdS solar cells. The new potential was then applied, via MD crystal growth simulations, to study the growth mechanisms and atomic structures of the ZnTe/Cu/CdTe/CdS multilayers under various conditions. The objective of the project was to inform the solar cell community on crystal growth and defect formation at the interface between CdTe-based epilayers grown on CdS substrates. MD simulations offered a unique opportunity to analyze the 3-dimensional crystal growth evolution of Cu doped ZnxCd(1-x)TeyS(1-y) alloys with atomic scale resolution under a variety of conditions. The effect of substrate orientation and polycrystallinity on crystal growth was studied. Substrates with grain boundaries were created and the growth evolution on these substrates were analyzed. Another focus was to elucidate the effect of dopant incorporation on the nature of polycrystallinity crystal growth. Dopants available through the Stillinger-Weber potential developed in this project included Cu, Zn, S, Se, Hg and native vacancies.

14 SOLAR ENERGY↗
DOE OSTI2019

Tricoupled hybrid lattice Boltzmann model for nonisothermal drying of colloidal suspensions in micropore structures

A tricoupled hybrid lattice Boltzmann model (LBM) is developed to simulate colloidal liquid evaporation and colloidal particle deposition during the nonisothermal drying of colloidal suspensions in micropore structures. An entropic multiple-relaxation-time multirange pseudopotential two-phase LBM for isothermal interfacial flow is first coupled to an extended temperature equation for simulating nonisothermal liquid drying. Then the coupled model is further coupled with a modified convection diffusion equation to consider the nonisothermal drying of colloidal suspensions. Two drying examples are considered. First, drying of colloidal suspensions in a two-pillar micropore structure is simulated in two dimensions (2D), and the final configuration of colloidal particles is compared with the experimental one. Good agreement is observed. Second, at the temperature of 343.15 K ( 70°C ), drying of colloidal suspensions in a complex spiral-shaped micropore structure containing 220 pillars is simulated (also in 2D). The drying pattern follows the designed spiral shape due to capillary pumping, i.e., transport of the liquid from larger pores to smaller ones by capillary pressure difference. Because the colloidal particles are passively carried with liquid, they accumulate at the small menisci as drying proceeds. As liquid evaporates at the small menisci, colloidal particles are deposited, eventually forming solid structures between the pillars (primarily), and at the base of the pillars (secondarily). As a result, the particle deposition conforms to the spiral route. Qualitatively, the simulated liquid and particle configurations agree well with the experimental ones during the entire drying process. Quantitatively, the model demonstrates that the evaporation rate and the particle accumulation rate slowly decrease during drying, similar to what is seen in the experimental results, which is due to the reduction of the liquid-vapor interfacial area. In conclusion, the hybrid model shows the capability and accuracy for simulating nonisothermal drying of colloidal suspensions in a complex micropore structure both qualitatively and quantitatively, as it includes all the required physics and captures all the complex features observed experimentally. Such a tricoupled LBM has a high potential to become an efficient numerical tool for further investigation of real and complex engineering problems incorporating drying of colloidal suspensions in porous media.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Quantum-mechanical study of energies, structures, and vibrational spectra of the H(D)Cl complexed with dimethyl ether

Interaction energies, molecular structure and vibrational frequencies of the binary complex formed between H(D)Cl and dimethyl ether have been obtained using quantum-chemical methods. Equilibrium and vibrationally averaged structures, harmonic and anharmonic wavenumbers of the complex and its deuterated isotopomer were calculated using harmonic and anharmonic second-order perturbation theory procedures with Density Functional Theory B3LYP and B2PLYP-D and ab initio Møller-Plesset second-order methods, and a 6-311++G(3d,3p) basis set. A phenomenological model describing anharmonic-type vibrational couplings within hydrogen bonds was developed to explain the unique broadening and fine structure, as well as the isotope effect of the Cl–H and Cl–D stretching IR absorption bands in the gaseous complexes with dimethyl ether, as an effect of hydrogen bond formation. Simulations of the rovibrational structure of the Cl–H and Cl–D stretching bands were performed and the results were compared with experimental spectra.

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

Recent Hydrodynamic Modeling Enhancements in OpenFAST

To improve the hydrodynamic modeling capabilities of the offshore wind design and modeling tool OpenFAST, a restructuring of the HydroDyn hydrodynamics module was undertaken with several new features implemented. The generation of the wave field is now separated from HydroDyn into a new module called SeaState. Unlike previous versions of HydroDyn, which precomputed the wave kinematics at the undisplaced positions of the hydrodynamic nodes, the new SeaState module computes the complete time history of the wave field at the vertices of a user-defined wave grid. During the simulation, the wave kinematics at any point within the grid can be efficiently interpolated, allowing the strip-theory wave loads to be evaluated based on the wave kinematics at the displaced structure position. A phase correction was also implemented for the potential-flow wave-exciting loads to account for large structure displacements. Several wave-stretching methods were implemented for the strip-theory solution along with a wave-load redistribution method that ensures smooth variation of the nodal loads as the nodes of a discrete hydrodynamic mesh enter and exit the water. The load smoothing is included to avoid the excitation of unphysical high-frequency structural vibration during hydroelastic simulations. The MacCamy-Fuchs diffraction correction for the strip-theory solution and the capability to insert a constrained NewWave into extreme stochastic sea states with directional spreading were also added. The present article documents the formulations of the new improvements to HydroDyn with example applications and numerical results.

constrained wave↗
DOE OSTI2021

The Simons Observatory: modeling optical systematics in the Large Aperture Telescope

We present geometrical and physical optics simulation results for the Simons Observatory Large Aperture Telescope. This work was developed as part of the general design process for the telescope, allowing us to evaluate the impact of various design choices on performance metrics and potential systematic effects. The primary goal of the simulations was to evaluate the final design of the reflectors and the cold optics that are now being built. We describe nonsequential ray tracing used to inform the design of the cold optics, including absorbers internal to each optics tube. We discuss ray tracing simulations of the telescope structure that allow us to determine geometries that minimize detector loading and mitigate spurious near-field effects that have not been resolved by the internal baffling. We also describe physical optics simulations, performed over a range of frequencies and field locations, that produce estimates of monochromatic far-field beam patterns, which in turn are used to gauge general optical performance. Finally, we describe simulations that shed light on beam sidelobes from panel gap diffraction.

Gudmundsson, Jon E. (ORCID:0000000317600355)↗
DOE OSTI2020

The Simons Observatory: Modeling Optical Systematics in the Large Aperture Telescope

We present geometrical and physical optics simulation results for the Simons Observatory Large Aperture Telescope. This work was developed as part of the general design process for the telescope; allowing us to evaluate the impact of various design choices on performance metrics and potential systematic effects. The primary goal of the simulations was to evaluate the final design of the reflectors and the cold optics which are now being built. We describe non-sequential ray tracing used to inform the design of the cold optics, including absorbers internal to each optics tube. We discuss ray tracing simulations of the telescope structure that allow us to determine geometries that minimize detector loading and mitigate spurious near-field effects that have not been resolved by the internal baffling. We also describe physical optics simulations, performed over a range of frequencies and field locations, that produce estimates of monochromatic far field beam patterns which in turn are used to gauge general optical performance. Finally, we describe simulations that shed light on beam sidelobes from panel gap diffraction.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Recent Hydrodynamic Modeling Enhancements in OpenFAST: Preprint

To improve the hydrodynamic modeling capabilities of the offshore wind design and modeling tool OpenFAST, a restructuring of the HydroDyn hydrodynamics module was undertaken with several new features implemented. The generation of the wave field is now separated from HydroDyn into a new module called SeaState. Unlike previous versions of HydroDyn, which precomputed the wave kinematics at the undisplaced positions of the hydrodynamic nodes, the new SeaState module computes the complete time history of the wave field at the vertices of a user-defined wave grid. During the simulation, the wave kinematics at any point within the grid can be efficiently interpolated, allowing the strip-theory wave loads to be evaluated based on the wave kinematics at the displaced structure position. A phase correction was also implemented for the potential-flow wave-exciting loads to account for large structure displacements. Several wave-stretching methods were implemented for the strip-theory solution along with a wave-load redistribution method that ensures smooth variation of the nodal loads as the nodes of a discrete hydrodynamic mesh enter and exit the water. The load smoothing is included to avoid the excitation of unphysical high-frequency structural vibration during hydroelastic simulations. The MacCamy-Fuchs diffraction correction for the strip-theory solution and the capability to insert a constrained NewWave into extreme stochastic sea states with directional spreading were also added. The present article documents the formulations of the new improvements to HydroDyn with example applications and numerical results.

constrained wave↗
DOE OSTI2017

Probing the Magnetic Field Structure in Sgr A* on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations

Magnetic fields are believed to drive accretion and relativistic jets in black hole accretion systems, but the magnetic field structure that controls these phenomena remains uncertain. We perform general relativistic (GR) polarized radiative transfer of time-dependent three-dimensional GR magnetohydrodynamical simulations to model thermal synchrotron emission from the Galactic Center source Sagittarius A* (Sgr A*). We compare our results to new polarimetry measurements by the Event Horizon Telescope (EHT) and show how polarization in the visibility (Fourier) domain distinguishes and constrains accretion flow models with different magnetic field structures. These include models with small-scale fields in disks driven by the magnetorotational instability as well as models with large-scale ordered fields in magnetically arrested disks. We also consider different electron temperature and jet mass-loading prescriptions that control the brightness of the disk, funnel-wall jet, and Blandford–Znajek-driven funnel jet. Our comparisons between the simulations and observations favor models with ordered magnetic fields near the black hole event horizon in Sgr A*, though both disk- and jet-dominated emission can satisfactorily explain most of the current EHT data. We also discuss how the black hole shadow can be filled-in by jet emission or mimicked by the absence of funnel jet emission. We show that stronger model constraints should be possible with upcoming circular polarization and higher frequency (349 GHz) measurements.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Molecular simulations and density functional theory calculations of bromine in clathrate hydrate phases

Bromine forms a tetragonal clathrate hydrate structure (TS-I) very rarely observed in clathrate hydrates of other guest substances. The detailed structure, energetics, and dynamics of Br{sub 2} and Cl{sub 2} in TS-I and cubic structure I (CS-I) clathrate hydrates are studied in this work using molecular dynamics and quantum chemical calculations. X-ray diffraction studies show that the halogen-water–oxygen distances in the cages of these structures are shorter than the sum of the van der Waals radii of halogen and oxygen atoms. This suggests that the stabilizing effects of halogen bonding or other non-covalent interactions (NCIs) may contribute to the formation of the unique tetragonal bromine hydrate structure. We performed molecular dynamics simulations of Br{sub 2} and Cl{sub 2} clathrate hydrates using our previously developed five-site charge models for the dihalogen molecules [Dureckova et al. Can. J. Chem. 93, 864 (2015)] which reproduce the computed electrostatic potentials of the dihalogens and account for the electropositive σ-hole of the halogen bond donor (the dihalogen). Analysis of the radial distribution functions, enthalpies of encapsulation, velocity and orientation autocorrelation functions, and polar angle distributions are carried out for Br{sub 2} and Cl{sub 2} guests in various cages to contrast the properties of these guests in the TS-I and CS-I phases. Quantum chemical partial geometry optimizations of Br{sub 2} and Cl{sub 2} guests in the hydrate cages using the M06-2X functional give short halogen-water distances compatible with values observed in X-ray diffraction experiments. NCI plots of guest-cage structures are generated to qualitatively show the relative strength of the non-bonding interactions between dihalogens and water molecules. The differences between behaviors of Br{sub 2} and Cl{sub 2} guests in the hydrate cages may explain why bromine forms the unique TS-I phase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Direct numerical simulation of mechanical response in synthetic additively manufactured microstructures

Additive manufacturing (AM) processes for metals can yield as-built microstructures that vary significantly from their cast or wrought counterparts. These microstructural variations can in turn, have profound effects on the properties of a component. Here, a modeling methodology is presented to investigate microstructurally-influenced mechanical response in additively manufactured structures via direct numeral simulation. Three-dimensional, synthetic voxelized microstructures are generated by kinetic Monte Carlo (kMC) additive manufacturing process simulations performed at four scan speeds to create a thin-wall cylindrical geometry notionally constructed using a concentric-pathed directed energy deposition AM process. The kMC simulations utilize a steady-state molten pool geometry that is held constant throughout the study. Resultant microstructures are mapped onto a highly-refined conformal finite-element mesh of a part geometry. A grain-scale anisotropic crystal elasticity model is then used to represent the constitutive response of each grain. Here, the response of the structure subjected to relatively simple load conditions is studied in order to provide understanding of both the influence of AM processing on microstructure as well as the microstructure's influence on the macroscale mechanical response.

42 ENGINEERING↗
DOE OSTI2016

Dependence of short and intermediate-range order on preparation in experimental and modeled pure a-Si

Variability in the short-to-intermediate range order of pure amorphous silicon prepared by different experimental and computational techniques is probed by measuring mass density, atomic coordination, bond-angle deviation, and dihedral angle deviation. It is found that there is significant variability in order parameters at these length scales in this archetypal covalently bonded, monoatomic system. This diversity strongly reflects preparation technique and thermal history in both experimental and simulated systems. Experiment and simulation do not fully quantitatively agree, partly due to differences in the way parameters are accessed. However, qualitative agreement in the trends is identified. Relaxed forms of amorphous silicon closely resemble continuous random networks generated by a hybrid method of bond-switching Monte Carlo and molecular dynamics simulation. As-prepared ion implanted amorphous silicon can be adequately modeled using a structure generated from amorphization via ion bombardement using energetic recoils. Preparation methods which narrowly avoid crystallization such as experimental pressure-induced amorphization or simulated melt-quenching result in inhomogeneous structures that contain regions with significant variations in atomic ordering. Ad hoc simulated structures containing small (1 nm) diamond cubic crystal inclusions were found to possess relatively high bond-angle deviations and low dihedral angle deviations, a trend that could not be reconciled with any experimental material.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electron diffraction intensity from single crystal silicon in a photoinjector

A method includes simulating diffraction in a transmission geometry of relativistic electron bunches from a crystallographic structure of a crystal thereby simulating diffraction of the relativistic electron bunches into a plurality of Bragg peaks. The method includes selecting a range of angles between a direction of propagation of the relativistic electron bunches and a normal direction of crystal including an angle at which a diffraction portion is maximized. The method includes sequentially accelerating a plurality of physical electron bunches to relativistic energies toward a physical crystal having the crystallographic structure and diffracting the plurality of physical electron bunches off the physical crystal at different angles and measuring the diffraction portion into the respective Bragg peak at the different angles. The method includes selecting a final angle based on the measured diffraction portion into the respective Bragg peak at the different angles and generating a pulse of light.

Graves, William↗
DOE OSTI2018

Collaborative Research: Tomographic imaging of laser-plasma structures

The interaction of intense short laser pulses with ionized gases, or plasmas, underlies many applications such as acceleration of elementary particles, production of energy by laser fusion, generation of x-ray and far-infrared “terahertz” pulses for medical and materials probing, remote sensing of explosives and pollutants, and generation of guide stars. Such laser-plasma interactions create tiny electron density structures (analogous to the wake behind a boat) inside the plasma in the shape of waves, bubbles and filaments that move at the speed of light, and evolve as they propagate. Prior to recent work by the PI of this proposal, detailed knowledge of such structures came exclusively from intensive computer simulations. Now “snapshots” of these elusive, light-velocity structures can be taken in the laboratory using dynamic variant of holography, the technique used to produce ID cards and DVDs, and dynamic variant of tomography, the technique used in medicine to image internal bodily organs. These fast visualization techniques are important for understanding, improving and scaling the above-mentioned applications of laser-plasma interactions. In this project, we accomplished three things: 1) We took holographic pictures of a laser-driven plasma-wave in the act of accelerating electrons to high energy, and used computer simulations to understand the pictures. 2) Using results from this experiment to optimize the performance of the accelerator, and the brightness of x-rays that it emits. These x-rays will be useful for medical and materials science applications. 3) We made technical improvements to the holographic technique that enables us to see finer details in the recorded pictures. Four refereed journal papers were published, and two students earned PhDs and moved on to scientific careers in US National Laboratories based on their work under this project.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Origin of structural analogies and differences between the atomic structures of GeSe{sub 4} and GeS{sub 4} glasses: A first principles study

First-principles molecular dynamics simulations based on density functional theory are employed for a comparative study of structural and bonding properties of two stoichiometrically identical chalcogenide glasses, GeSe{sub 4} and GeS{sub 4}. Two periodic cells of 120 and 480 atoms are adopted. Both glasses feature a coexistence of Ge-centered tetrahedra and Se(S) homopolar connections. Results obtained for N = 480 indicate substantial differences at the level of the Se(S) environment, since Ge–Se–Se connections are more frequent than the corresponding Ge–S–S ones. The presence of a more prominent first sharp diffraction peak in the total neutron structure factor of glassy GeS{sub 4} is rationalized in terms of a higher number of large size rings, accounting for extended Ge–Se correlations. Both the electronic density of states and appropriate electronic localization tools provide evidence of a higher ionic character of Ge–S bonds when compared to Ge–Se bonds. An interesting byproduct of these investigations is the occurrence of discernible size effects that affect structural motifs involving next nearest neighbor distances, when 120 or 480 atoms are used.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Molecular Dynamics Simulation on the Effect of MgO/Al{sub 2}O{sub 3} Ratio on Structure and Properties of Blast Furnace Slag Under Different Basicity Conditions

The SiO{sub 2}-Al{sub 2}O{sub 3}-CaO-MgO is the basic structural system of blast furnace slag and the composition directly affects the performance of the slag. Molecular dynamics simulations were carried out to analyze the local structure, structural unit, bond angle, transport properties, and enthalpy of the slag with the increase of MgO/Al{sub 2}O{sub 3} mass ratio under different basicity conditions. It was found that the change of MgO/Al{sub 2}O{sub 3} ratio does not affect the short-range order structure, but it will reduce the overall stability of the network structure. Through the analysis of the structural unit of the slag, it was found that the polymerization degree of the system decreases with the increase of MgO/Al{sub 2}O{sub 3} ratio, indicating that the complex network structure of the system is partially depolymerized. Besides, the self-diffusion coefficients of each ion were concluded and the magnitudes were observed to be in the following order: Mg{sup 2+}>Ca{sup 2+}>Al{sup 3+}>O{sup 2-}>Si{sup 4+}. The diffusivity of the slag increases with the increase of MgO/Al{sub 2}O{sub 3} ratio and the corresponding viscosity decreases. And the enthalpy of the slag is increased with the increase of MgO/Al{sub 2}O{sub 3} ratio or basicity, therefore the fuel consumption should also be properly adjusted during the operation of blast furnace to ensure the target temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Crystallographic and Computational Analyses of AUUCU Repeating RNA That Causes Spinocerebellar Ataxia Type 10 (SCA10)

Spinocerebellar ataxia type 10 (SCA10) is caused by a pentanucleotide repeat expansion of r(AUUCU) within intron 9 of the ATXN10 pre-mRNA. The RNA causes disease by a gain-of-function mechanism in which it inactivates proteins involved in RNA biogenesis. Spectroscopic studies showed that r(AUUCU) repeats form a hairpin structure; however, there were no high-resolution structural models prior to this work. Herein, we report the first crystal structure of model r(AUUCU) repeats refined to 2.8 Å and analysis of the structure via molecular dynamics simulations. The r(AUUCU) tracts adopt an overall A-form geometry in which 3 × 3 nucleotide 5'UCU3'/3'UCU5' internal loops are closed by AU pairs. Helical parameters of the refined structure as well as the corresponding electron density map on the crystallographic model reflect dynamic features of the internal loop. The computational analyses captured dynamic motion of the loop closing pairs, which can form single-stranded conformations with relatively low energies. Overall, the results presented here suggest the possibility for r(AUUCU) repeats to form metastable A-from structures, which can rearrange into single-stranded conformations and attract proteins such as heterogeneous nuclear ribonucleoprotein K (hnRNP K). Here, the information presented here may aid in the rational design of therapeutics targeting this RNA.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗