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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Lattice Dynamics of Colloidal Crystals During Photopolymerization of Acrylic Monomer Matrix

Polymerization process are the major contributors for observed lattice compression and lattice disorder of the Crystalline Colloidal Arrays (CCA) of silica spheres in polymerized acrylic/methacrylic ester films. The effect of orientation of photocell with respect to the readiation source on Bragg diffraction of CCA indicated the presence of convective stirring in thin fluid system during the photopolymerization that deleteriously affect the periodic array structures. To devise reproducible and more efficient optical filters, experimental methods to minimize or eliminate convective instabilities in monomeric dispersions during polymerization are suggested.

Sunkara, Hari B.↗
DOE OSTI2015

The Bi{sub 2}O{sub 3}–Fe{sub 2}O{sub 3}–Sb{sub 2}O{sub 5} system phase diagram refinement, Bi{sub 3}FeSb{sub 2}O{sub 11} structure peculiarities and magnetic properties

The refinement of the Bi{sub 2}O{sub 3}–Fe{sub 2}O{sub 3}–Sb{sub 2}O{sub 5} system phase diagram has been performed and the existence of the two ternary compounds has been confirmed. The first one with a pyrochlore-type structure (sp. gr. Fd 3-barm) exists in the wide solid solution region, (Bi{sub 2−x}Fe{sub x})Fe{sub 1+y}Sb{sub 1−y}O{sub 7±δ}, where x=0.1–0.4 and y=−0.13–0.11. The second one, Bi{sub 3}FeSb{sub 2}O{sub 11}, corresponds to the cubic KSbO{sub 3}-type structure (sp. gr. Pn 3-bar) with unit cell parameter a=9.51521(2) Å. The Rietveld structure refinement showed that this compound is characterized by disordered structure. The Bi{sub 3}FeSb{sub 2}O{sub 11} factor group analysis has been carried out and a Raman spectrum has been investigated. According to magnetization measurements performed at the temperature range 2–300 K it may be concluded that the Bi{sub 3}FeSb{sub 2}O{sub 11} magnetic properties can be substantially described as a superposition of strong short-range antiferromagnetic exchange interactions realizing inside the [(FeSb{sub 2})O{sub 9}] 3D-framework via different pathways. - Graphical abstract: The refinement of the Bi{sub 2}O{sub 3}–Fe{sub 2}O{sub 3}–Sb{sub 2}O{sub 5} system phase diagram has been performed and the existence of the solid solution with a pyrochlore-type structure (sp. gr. Fd 3-barm) and Bi{sub 3}FeSb{sub 2}O{sub 11}, correspond of the cubic KSbO{sub 3}-type structure (sp. gr. Pn 3-bar has been confirmed. The structure refinement, Raman spectroscopy as well as magnetic measurements data of Bi{sub 3}FeSb{sub 2}O{sub 11} are presented. - Highlights: • The Bi{sub 2}O{sub 3}–Fe{sub 2}O{sub 3}–Sb{sub 2}O{sub 5} system phase diagram refinement has been performed. • The Bi{sub 3}FeSb{sub 2}O{sub 11} existence along with pyrochlore structure compound is shown. • It was determined that the Bi{sub 3}FeSb{sub 2}O{sub 11} is of disordered cubic KSbO{sub 3}-type structure. • Factor group analysis of Bi{sub 3}FeSb{sub 2}O{sub 11} vibrational spectrum has been performed. • Short-range antiferromagnetic interactions govern Bi{sub 3}FeSb{sub 2}O{sub 11} magnetic behavior.

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

Diffusion Dynamics of Charged Dust Particles in Capacitively Coupled RF Discharge System

Dusty plasma is loosely defined as electron-ion plasma with additional charged components of micron-sized dust particles. In this study, we developed a particle diagnostic technique based on light scattering and particle tracking velocimetry to investigate the dynamics of micron-sized titanium oxide particles in Argon gas capacitively coupled rf-discharge. The particle trajectories are constructed from sequence of image frames and treated as sample paths of charged Brownian motion. At specific sets of plasma parameters, disordered liquid-like dust particle configuration are observed. Mean-square-displacement of the particle trajectories are determined to characterize the transport dynamics. We showed that the dust particles in disordered liquid phase exhibit anomalous diffusion with different scaling exponents for short and large time scales, indicating the presence of slow and fast modes which can be related to caging effect and dispersive transport, respectively.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Two spatially separated phases in semiconducting Rb 0.8 Fe 1.5 S 2

We report neutron scattering and transport measurements on semiconducting Rb 0.8 Fe 1.5 S 2 , a compound isostructural and isoelectronic to the well-studied A 0.8 Fe y Se 2 (A = K, Rb, Cs, Tl/K) superconducting systems. Both resistivity and DC susceptibility measurements reveal a magnetic phase transition at T = 275 K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, the stripe antiferromagnetic phase interdigitates mesoscopically with an ubiquitous phase with √5 x√5 iron vacancy order. This phase has a magnetic transition at T N = 425 K and an iron vacancy order-disorder transition at T S = 600 K. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb 0.8 Fe 1.5 S 2 and K 0.81 Fe 1.58 Se 2 , we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the A 0.8 Fe y Se 2₋ z S z system, is absent in A 0.8 Fe 1.5 S 2 , which has a semiconducting ground state. We discuss the implied relationship between stripe and block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Temperature effects on the atomic structure and kinetics in single crystal electrochemistry

The influence of temperature on the atomic structure at the electrochemical interface has been studied using in-situ surface x-ray scattering (SXS) during the formation of metal monolayers on a Au(111) electrode. For the surface reconstruction of Au(111), higher temperatures increase the mobility of surface atoms in the unreconstructed phase which then determines the surface ordering during the formation of the reconstruction. For the underpotential deposition (UPD) systems, the surface diffusion of the depositing metal adatoms is significantly reduced at low temperatures which results in the frustration of ordered structures in the case of Cu UPD, occurring on a Br-modified surface, and in the formation of a disordered Ag monolayer during Ag UPD. The results indicate that temperature changes affect the mass transport and diffusion of metal adatoms on the electrode surface. This demonstrates the importance of including temperature as a variable in studying surface structure and reactions at the electrochemical interface.

Gründer, Yvonne↗
DOE OSTI2021

Defects in Layered van der Waals Heterostructures: Implications for Thermoelectrics

Layered van der Waals heterostructures provide extraordinary opportunities for applications such as thermoelectrics and allow for tunability of optical and electronic properties. The performance of devices made from these heterostructures will depend on their properties, which are sensitive to the nanoarchitecture (constituent layer thicknesses, layer sequence, etc.). However, performance will also be impacted by defects, which will vary in concentration and identity with the nanoarchitecture and preparation conditions. Here, we identify several types of defects and propose mechanisms for their formation, focusing on compounds in the ([SnSe] 1+δ ) m (TiSe 2 ) n system prepared using the modulated elemental reactants method. The defects were observed by atomic resolution high-angle annular dark-field scanning transmission electron microscopy and can be broadly categorized into those that form domain boundaries as a result of rotational disorder from the self-assembly process and those that are layer-thickness-related and result from local or global deviations in the amount of material deposited. Furthermore, defect type and density were found to depend on the nanoarchitecture of the heterostructure. Categorizing the defects provides insights into defect formation in these van der Waals layered heterostructures and suggests strategies for controlling their concentrations. Strategies for controlling defect type and concentration are proposed, which would have implications for transport properties for applications in thermoelectrics.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Colloidal quasicrystals engineered with DNA

In principle, designing and synthesizing almost any class of colloidal crystal is possible. Nonetheless, the deliberate and rational formation of colloidal quasicrystals has been difficult to achieve. Here we describe the assembly of colloidal quasicrystals by exploiting the geometry of nanoscale decahedra and the programmable bonding characteristics of DNA immobilized on their facets. Further, this process is enthalpy-driven, works over a range of particle sizes and DNA lengths, and is made possible by the energetic preference of the system to maximize DNA duplex formation and favour facet alignment, generating local five- and six-coordinated motifs. This class of axial structures is defined by a square–triangle tiling with rhombus defects and successive on-average quasiperiodic layers exhibiting stacking disorder which provides the entropy necessary for thermodynamic stability. Taken together, these results establish an engineering milestone in the deliberate design of programmable matter.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Acoustic wave propagation and stochastic effects in metamaterial absorbers

We show how stochastic variations of the effective parameters of anisotropic structured metamaterials can lead to increased absorption of sound. For this, we derive an analytical model based on the Bourret approximation and illustrate the immediate connection between material disorder and attenuation of the averaged field. We demonstrate numerically that broadband absorption persists at oblique irradiation and that the influence of red noise comprising short spatial correlation lengths increases the absorption beyond what can be archived with a structured but ordered system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Molecular Processes Underlying the Structure and Assembly of Thin Films and Nanoparticles at Complex interfaces

Since 1995 we have pursued a number of different studies that are quite diverse in nature but with the common theme of using novel laser based methods to study important processes at buried interfaces. Studies of Corrosion, Passivation on n-GaAs(100)Methanol Photoelectrochemical Cell In these studies we have used picosecond photoluminescence and electrochemical studies to understand the GaAs/methanol interface. In our most extensive set of studies we conducted photo-illumination and XPS experiments to understand the chemistry occurring in the GaAs/methanol photoelectrochemical during photoexcitation. An important distinction between photocorrosion and photoetching of GaAs is elucidated by these studies. The dependence of GaAs photocorrosion on light intensity has been explored to better understand intrinsic differences between the lamplight studies and the picosecond photoluminescence studies. The effect of coating the GaAs with a sulfide layer prior to immersion in the cell has also been explored. This last result has led us to examine n-GaAs as a function of crystallographic orientation after exposure to aqueous Na 2 S containing solutions has been studied as a function of crystallographic orientation of the GaAs surface. The (100) and (110) surfaces are relatively similar, with significant amounts of As-S species present at the interface. The (111)B surface lacks this constituent, but shows significant amounts of metallic As. The XPS results have been correlated with the results of previous photocorrosion and passivation studies conducted in a photoelectrochemical cell. The studies indicate that the metallic As present at (111)B surface contributes strongly to the large surface recombination velocity found there, and to the inability of Na 2 S to passivate the (111)B surface. SAMS Under Water: Water Molecular Structure and Bonding at Hydrophobic Surfaces In these DOE sponsored studies we have been interested in learning the similarities and differences in how water behaves at hydrophobic self-assembled monolayer (SAMS)/water interfaces relative to the organic liquid/water interfaces. Several monolayer films have been examined in these studies using a combination of vibrational sum frequency spectroscopy (VSFS), contact angle measurements and AFM. At the hydrocarbon monolayer/water interface we find that water has a weak bonding interaction with the monolayer film that results in an orientation of water at the terminus of these hydrocarbon chains. The water-film interaction is still present for fluorinated films but it is found to be considerably weaker. Hydration and Surfactant Adsorption at Salt/Water Interfaces This set of studies has examined the molecular characteristics of the CaF 2 /water interface using VSFS. Our first studies detailed the structure and orientation of water molecules adsorbed at this mineral surfaces including studies of the surface in the presence of aqueous solutions of salts. These studies have been followed by a series of static and time-resolved studies of the adsorption of carboxylic acid containing organics at this surface, specifically carboxylic acid surfactants and acetic acid. In the latter we have developed a new method for time resolved studies that involve sequential wavelength tuning and automated control of spatial beam overlap at the target can probe amplitude changes of sum-frequency resonances in widely spaced infrared regions. This offers great advantages for the study of the synchronism of molecular processes at interfaces. This approach is particularly suitable to investigate the synchronization of interfacial processes such as surfactant adsorption at charged mineral surfaces. Macromolecular Assembly at Liquid/Liquid Interfaces Macromolecular assembly at the interface between water and a hydrophobic surface underlies some of the most important biological and environmental processes on the planet. Our work has examined polymer adsorption and assembly of carboxylic acid-containing polyelectrolytes at the carbon tetrachloride–water interface, a model system for an oil–water interface. Using VSFS and interfacial tension techniques, these unique set of studies identify the factors that dictate whether or not polyelectrolytes will adsorb to the oil–water interface and also describe the specifics of the adsorption process that depend upon factors such as polymer size, charge density, hydrophobicity, conformation, and the effect of metal ion electrostatics and bonding. The systems studied include polyelectrolytes polyacrylic acid (PAA) and polymethylacrylic acid (PMA) of different polymer sizes and under different aqueous solution conditions. The studies are the first to show the highly ordered nature of the adsorption of the first monolayer with subsequent monolayers disordered. The second set of studies have examined how peptoid nanosheets assemble at the oil/water interface. Peptoid nanosheets are a recently discovered class of two-dimensional (2D) nanomaterial, which form from the self-assembly of a sequence-specific peptoid polymer at an air-water interface. Nanosheet formation occurs first through the assembly of a peptoid monolayer and subsequent compression into a bilayer structure. In these highly successful studies we have shown that the oil-water interface provides another opportunity for growth of these unique and highly ordered peptoid sheets. The monolayer formed at this interface are found through surface spectroscopic measurements to be highly ordered and electrostatic interactions between the charged moieties, namely carboxylate and ammonium residues, of the peptoid are essential in the ability of these peptoids to form ordered nanosheets at the oil-water interface.

36 MATERIALS SCIENCE↗
NASA NTRS2001

What can posturography tell us about vestibular function?

Patients with balance disorders want answers to the following basic questions: (1) What is causing my problem? and (2) What can be done about my problem? Information to fully answer these questions must include status of both sensory and motor components of the balance control systems. Computerized dynamic posturography (CDP) provides quantitative assessment of both sensory and motor components of postural control along with how the sensory inputs to the brain interact. This paper reviews the scientific basis and clinical applications of CDP. Specifically, studies describing the integration of vestibular inputs with other sensory systems for postural control are briefly summarized. Clinical applications, including assessment, rehabilitation, and management are presented. Effects of aging on postural control along with prevention and management strategies are discussed.

NASA Discipline Neuroscience↗
DOE OSTI2015

Let7a involves in neural stem cell differentiation relating with TLX level

Neural stem cells (NSCs) have the potential for differentiation into neurons known as a groundbreaking therapeutic solution for central nervous system (CNS) diseases. To resolve the therapeutic efficiency of NSCs, recent researchers have focused on the study on microRNA's role in CNS. Some micro RNAs have been reported significant functions in NSC self-renewal and differentiation through the post-transcriptional regulation of neurogenesis genes. MicroRNA-Let7a (Let7a) has known as the regulator of diverse cellular mechanisms including cell differentiation and proliferation. In present study, we investigated whether Let7a regulates NSC differentiation by targeting the nuclear receptor TLX, which is an essential regulator of NSC self-renewal, proliferation and differentiation. We performed the following experiments: western blot analysis, TaqMan assay, RT-PCR, and immunocytochemistry to confirm the alteration of NSCs. Our data showed that let7a play important roles in controlling NSC fate determination. Thus, manipulating Let-7A and TLX could be a novel strategy to enhance the efficiency of NSC's neuronal differentiation for CNS disorders. - Highlights: • Let7a influences on NSC differentiation and proliferation. • Let7a involves in mainly NSC differentiation rather than proliferation. • Let7a positively regulates the TLX expression.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Grain growth of nanocrystalline 3C-SiC under Au ion irradiation at elevated temperatures

Nanocrystalline silicon carbide (SiC) represents an excellent model system for a fundamental study of interfacial (grain boundary) processes under nuclear radiation, which are critical to the understanding of the response of nanostructured materials to high-dose irradiation. This study reports on a comparison of irradiation effects in cubic phase SiC (3C-SiC) grains of a few nanometers in size and single-crystal 3C-SiC films under identical Au ion irradiation to a range of doses at 700 K. In contrast to the latter, in which lattice disorder is accumulated to a saturation level without full amorphization, the average grain size of the former increases with dose following a power-law trend. In addition to coalescence, the grain grows through atomic jumps and mass transport, where irradiation induced vacancies at grain boundaries assist the processes. It is found that a higher irradiation temperature leads to slower grain growth and a faster approach to a saturation size of SiC nanograins. The results could potentially have a positive impact on structural components of advanced nuclear energy systems.

Environmental Molecular Sciences Laboratory↗
NASA NTRS2009

Complexity for Survival of Living Systems

A logical connection between the survivability of living systems and the complexity of their behavior (equivalently, mental complexity) has been established. This connection is an important intermediate result of continuing research on mathematical models that could constitute a unified representation of the evolution of both living and non-living systems. Earlier results of this research were reported in several prior NASA Tech Briefs articles, the two most relevant being Characteristics of Dynamics of Intelligent Systems (NPO- 21037), NASA Tech Briefs, Vol. 26, No. 12 (December 2002), page 48; and Self-Supervised Dynamical Systems (NPO- 30634) NASA Tech Briefs, Vol. 27, No. 3 (March 2003), page 72. As used here, living systems is synonymous with active systems and intelligent systems. The quoted terms can signify artificial agents (e.g., suitably programmed computers) or natural biological systems ranging from single-cell organisms at one extreme to the whole of human society at the other extreme. One of the requirements that must be satisfied in mathematical modeling of living systems is reconciliation of evolution of life with the second law of thermodynamics. In the approach followed in this research, this reconciliation is effected by means of a model, inspired partly by quantum mechanics, in which the quantum potential is replaced with an information potential. The model captures the most fundamental property of life - the ability to evolve from disorder to order without any external interference. The model incorporates the equations of classical dynamics, including Newton s equations of motion and equations for random components caused by uncertainties in initial conditions and by Langevin forces. The equations of classical dynamics are coupled with corresponding Liouville or Fokker-Planck equations that describe the evolutions of probability densities that represent the uncertainties. The coupling is effected by fictitious information-based forces that are gradients of the information potential, which, in turn, is a function of the probability densities. The probability densities are associated with mental images both self-image and nonself images (images of external objects that can include other agents). The evolution of the probability densities represents mental dynamics. Then the interaction between the physical and metal aspects of behavior is implemented by feedback from mental to motor dynamics, as represented by the aforementioned fictitious forces. The interaction of a system with its self and nonself images affords unlimited capacity for increase of complexity. There is a biological basis for this model of mental dynamics in the discovery of mirror neurons that learn by imitation. The levels of complexity attained by use of this model match those observed in living systems. To establish a mechanism for increasing the complexity of dynamics of an active system, the model enables exploitation of a chain of reflections exemplified by questions of the form, "What do you think that I think that you think...?" Mathematically, each level of reflection is represented in the form of an attractor performing the corresponding level of abstraction with more details removed from higher levels. The model can be used to describe the behaviors, not only of biological systems, but also of ecological, social, and economics ones.

Zak, Michail↗
DOE OSTI2018

Spin freezing into a disordered state in CaFeT i 2 O 6 synthesized under high pressure

Double perovskites exhibit a variety of interesting phenomena due to numerous combinations of cation orderings and distortions. CaMnTi 2 O 6 and CaFeTi 2 O 6 are the only two discovered double perovskites that have the columnar type A-site ordering associated with the a + a + c – titling system. The former perovskite, synthesized recently, has a polar structure below T c and shows an unusual ferroelectricity. The latter, while synthesized decades ago, received less investigation. Here, we have synthesized CaFeTi 2 O 6 under high pressure and carried out thorough characterizations through measurements of x-ray powder diffraction (XRD), second harmonic generation (SHG), dc and ac magnetization, Mössbauer spectra, resistivity, thermoelectric power, and specific heat. Although the structural refinement of the XRD pattern cannot distinguish two possible tetragonal phases, SHG measurements help to rule out the possibility of a polar structure observed in CaMnTi 2 O 6 . Fitting the paramagnetic susceptibility to a Curie-Weiss law gives a negligible Weiss constant. The weak exchange interaction gives way to a dipole-dipole interaction and single-ion anisotropy. An anomaly at 1.2 K in the specific heat is too broad for a magnetic transition, but suggestive of a spin-ice-like state as in the pyrochlores. Mössbauer spectroscopy shows two distinct iron sites, as expected for the crystal structure. Both sites are occupied by high-spin Fe 2+ as indicated by the isomer shift. The sharply different temperature dependence of the quadrupole splitting at the two iron sites can be well explained based on the local structural distortions from the refinement of XRD patterns. In conclusion, the line broadening associated with long spin-relaxation times is observed at 6 K as the spin freezing transition is approached from high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Configurational-Bias Monte Carlo Back-Mapping Algorithm for Efficient and Rapid Conversion of Coarse-Grained Water Structures into Atomistic Models

Coarse-grained molecular dynamics (MD) simulations represent a powerful approach to simulate longer time scale and larger length scale phenomena than those accessible to all-atom models. The gain in efficiency, however, comes at the cost of atomistic details. The reverse transformation, also known as back-mapping, of coarse grained beads into their atomistic constituents represents a major challenge. Most existing approaches are limited to specific molecules or specific force-fields and often rely on running a long time atomistic MD of the back-mapped configuration to arrive at an optimal solution. Such approaches are problematic when dealing with systems with high diffusion barriers. Here, we introduce a new extension of the configurational-bias-Monte-Carlo (CBMC) algorithm, which we term the crystalline-configurational-bias-Monte-Carlo (C-CBMC) algortihm, that allows rapid and efficient conversion of a coarse-grained model back into its atomistic representation. Although the method is generic, we use a coarse-grained water model as a representative example and demonstrate the back-mapping or reverse transformation for model systems ranging from the ice-liquid water interface to amorphous and crystalline ice configurations. A series of simulations using the TIP4P/Ice model are performed to compare the new CBMC method to several other standard Monte Carlo and Molecular Dynamics based back-mapping techniques. In all the cases, the C-CBMC algorithm is able to find optimal hydrogen bonded configuration many thousand evaluations/steps sooner than the other methods compared within this paper. For crystalline ice structures such as a hexagonal, cubic, and cubic-hexagonal stacking disorder structures, the C-CBMC was able to find structures that were between 0.05 and 0.1 eV/water molecule lower in energy than the ground state energies predicted by the other methods. Detailed analysis of the atomistic structures show a significantly better global hydrogen positioning when contrasted with the existing simpler back-mapping methods. The error in the RDFs of backmapped relative to reference configuration for the CCBMC, MD, and MC were found to be 6.9, 8.7, and 12.9 respectively for the hexagonal system. For the cubic system the relative error of the RDFs for the CCBMC, MD, and MC were found to be 18.2, 34.6, and 39.0 respectively. Finally, our results demonstrate the efficiency and efficacy of our new back-mapping approach, especially for crystalline systems where simple force-field based relaxations have a tendency to get trapped in local minima.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Fe Vacancy Order and Domain Distribution in A{sub x}Fe{sub 2−y}Se{sub 2}

The Fe vacancy and its ordering pattern is a unique property of the A{sub x}Fe{sub 2−y}Se{sub 2} (A = K, Rb, Cs) superconductors. Below the structural transition temperature T{sub S} ∼ 550 K, the system phase separates into the I4/mmm phase with no Fe vacancy and the I4/m phase with the √5×√5×1 vacancy order. In this work, an Ising-like Hamiltonian defined on the Fe-sublattice is constructed and employed in a Metropolis Monte Carlo simulation to study the Fe vacancy order and the domain distribution under different thermal treatments. A third phase with randomly distributed Fe vacancies and space group I4/mmm is observed in the simulation. The response of the volume fraction of this simulated phase under thermal treatment shows a direct connection to the change of the superconducting shielding fraction under annealing and quenching processes, indicating a close relation between the vacancy disordered I4/mmm phase and the superconducting phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Ion-selective conformational stabilization of a disordered repeats-in-toxin protein domain

Ion-binding intrinsically disordered proteins (IDPs) recruit and bind to specific metal ions to perform critical biological functions. In proteins where ion binding and structural transitions are coupled, interactions with off-target toxic metals can dramatically disrupt protein structure and function, exemplified by lead and mercury poisoning. Understanding the complex mechanisms underlying how IDPs exclude or allow binding to different ionic species is crucial for addressing the origins of metal toxicity in biological systems. Here, we elucidate mechanisms of ion selectivity in an IDP that adopts a structure upon Ca 2+ binding. We probed ion-induced conformational changes of a repeats-in-toxin (RTX) protein domain in the presence of different ion ligands—Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ —with chemical similarities but drastically different ionic radii. RTX adopts ion-selective conformations measured by x-ray crystallography, small-angle x-ray scattering, and circular dichroism. High-resolution x-ray structures reveal that Sr 2+ induces a nearly identical RTX structure as natively binding Ca 2+ , enabled by the intrinsic flexibility and disorder of the protein. Small-angle x-ray scattering and circular dichroism indicate that smaller Mg 2+ does not induce a significant conformational change in RTX, whereas larger Ba 2+ induces a partially folded structure. These results highlight the importance of geometric constraints imposed by protein structure in determining metal ion selectivity, yielding insights into how off-target ion binding may result in protein misfolding and malfunction.

Gudinas, Alana P. [Stanford Univ., CA (United Stat↗