Local Structure Dynamics Research Articles

Abstract 4283: A structure network approach to predict the dynamics and structural stability effects of germline PTEN mutations associated with cancer versus autism phenotypes

Abstract Individuals with germline mutations in the tumor suppressor gene phosphatase and tensin homolog (PTEN), irrespective of clinical presentation, are referred to as PTEN hamartoma tumor syndrome (PHTS). PHTS confers a high risk of breast, thyroid and other cancers. Yet, PTEN mutations are found in up to 20% of individuals with autism spectrum disorder (ASD) and macrocephaly. What remains unclear is why mutations in one gene can lead to seemingly disparate phenotypes. Thus, we sought to identify differences in cancer- vs. ASD-associated PTEN mutations by investigating putative structural effects. We utilized a computational theoretical approach combining protein structure network (PSN), normal mode analysis (NMA), and elastic network models (ENMs) to interrogate 6 PTEN mutations from patients with PHTS-associated cancer, 6 mutations from those with ASD only, 4 mutations shared across both phenotypes, and 1 mutation with both ASD and cancer. Combined with in silico prediction methods we calculated structural stability changes induced by each mutation. All 6 cancer-associated mutations showed decreases in structure stability and increased dynamics across domain interface. In contrast, 5 of 6 ASD-associated mutations showed only localized destabilization. We coupled ENM-NMA with PSN and found PTEN R130G (cancer only) and R173C (shared in patients with both ASD and cancer), play a role in inter-residue signal propagation, indicating both residue positions are crucial to structural stability. Our data suggest that ASD-associated mutations affect local structural dynamics, whereas cancer-associated mutations mediate long-range perturbations significantly altering structural stability and salient communication pathways. These approaches lend insight into altered structural effects of germline PTEN mutations associated with PTEN-ASD and PTEN-cancer, potentially aiding in identification of specific mutations that contribute to each phenotype. Moreover, this work may illuminate the shared and separate molecular features that contribute to autism or cancer – providing a deeper understanding of genotype-phenotype relationships for germline PTEN mutations. The ability to accurately predict PTEN-ASD may spare patients the need to undergo high-risk cancer surveillance that is now performed in all with PHTS. Citation Format: Iris N. Smith, Stetson Thacker, Charis Eng. A structure network approach to predict the dynamics and structural stability effects of germline PTEN mutations associated with cancer versus autism phenotypes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4283.

Local Structure and Relaxation Dynamics in the Brush of Polymer-Grafted Silica Nanoparticles.

When grafted to spherical nanoparticles at high grafting densities, polymers adopt a variety of conformations. Because of strong confinement by neighboring chains, portions of the polymer near the nanoparticle core are highly stretched in the concentrated polymer brush region (CPB) of the polymer layer. Farther away from the core, where the polymer is less confined, the conformation becomes more ideal in the semidilute polymer brush (SDPB) region. Using a combination of small-angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy, we directly characterized both the structure and dynamics of the CPB and SDPB on poly(methyl acrylate) (PMA) grafted SiO2 nanoparticles (NPs). Analysis of SANS measurements using a new core-chain-chain (CCC) model confirmed that the portion of the chain in the CPB region is highly stretched, and transitions to a more random conformation. Dynamics in the CPB region were found to be much slower than the SDPB region.

Anisotropic particles strengthen granular pillars under compression.

We probe the effects of particle shape on the global and local behavior of a two-dimensional granular pillar, acting as a proxy for a disordered solid, under uniaxial compression. This geometry allows for direct measurement of global material response, as well as tracking of all individual particle trajectories. In general, drawing connections between local structure and local dynamics can be challenging in amorphous materials due to lower precision of atomic positions, so this study aims to elucidate such connections. We vary local interactions by using three different particle shapes: discrete circular grains (monomers), pairs of grains bonded together (dimers), and groups of three bonded in a triangle (trimers). We find that dimers substantially strengthen the pillar and the degree of this effect is determined by orientational order in the initial condition. In addition, while the three particle shapes form void regions at distinct rates, we find that anisotropies in the local amorphous structure remain robust through the definition of a metric that quantifies packing anisotropy. Finally, we highlight connections between local deformation rates and local structure.

Amyloid β (1–40) Toxicity Depends on the Molecular Contact between Phenylalanine 19 and Leucine 34

The formation of the hydrophobic contact between phenylalanine 19 (F19) and leucine 34 (L34) of amyloid β (1-40) (Aβ(1-40)) is known to be an important step in the fibrillation of Aβ(1-40) peptides. Mutations of this putatively early molecular contact were shown to strongly influence the toxicity of Aβ(1-40) ( Das et al. ( 2015 ) ACS Chem. Neurosci. 6 , 1290 - 1295 ). Any mutation of residue F19 completely abolished the toxicity of Aβ(1-40), suggesting that a proper F19-L34 contact is crucial also for the formation of transient oligomers. In this work, we investigate a series of isomeric substitutions of L34, namely, d-leucine, isoleucine, and valine, to study further details of this molecular contact. These replacements represent very minor alterations in the Aβ(1-40) structure posing the question how these alterations challenge the fibrillation kinetics, structure, dynamics, and toxicity of the Aβ(1-40) aggregates. Our work involves kinetic studies using thioflavin T, transmission electron microscopy, X-ray diffraction for the analysis of the fibril morphology, and nuclear magnetic resonance experiments for local structure and molecular dynamics investigations. Combined with cell toxicity assays of the mutated Aβ(1-40) peptides, the physicochemical and biological importance of the early folding contact between F19 and L34 in Aβ(1-40) is underlined. This implies that the F19-L34 contact influences a broad range of different processes including the initiation of fibrillation, oligomer stability, fibril elongation, local fibril structure, and dynamics and cellular toxicity. These processes do not only cover a broad range of diverse mechanisms, but also proved to be highly sensitive to minor modulations of this crucial contact. Furthermore, our work shows that the contact is not simply mediated by general hydrophobic interactions, but also depends on stereospecific mechanisms.

Structural Dynamics of Al2O3/NiAl(110) During Film Growth in NO2.

While continuum descriptions of oxide film growth are well established, the local structural dynamics during oxide growth are largely unexplored. Here, we investigate this using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) for the example of alumina film growth on NiAl(110) following NO2 exposure. To maintain a well-defined system, we have adopted a cyclic growth approach of NO2 adsorption and annealing. NO2 adsorption at 693 K results in the formation of a vacancy island pattern in the NiAl(110) substrate, which is filled with AlOx by diffusion of O through the alumina film. The patches of AlOx coalesce to form smooth terraces upon annealing to 1200 K. By repeated cycling, we have grown films of up to 0.9 nm thick. While peak shifts in the XPS spectra indicate that the film maintains its insulating character upon thickening, our STM data show that there is a finite density of states within the band gap. The thickening of the alumina film is accompanied by the formation of trenches in the surface, which we interpret to be the result of film stress relief.

Effects of Cd vacancies and unconventional spin dynamics in the Dirac semimetal Cd3As2.

Cd3As2 is a Dirac semimetal that is a 3D analog of graphene. We investigated the local structure and nuclear-spin dynamics in Cd3As2 via 113Cd NMR. The wideline spectrum of the static sample at 295 K is asymmetric and its features are well described by a two-site model with the shielding parameters extracted via Herzfeld-Berger analysis of the magic-angle spinning spectrum. Surprisingly, the 113Cd spin-lattice relaxation time (T1) is extremely long (T1 = 95 s at 295 K), in stark contrast to conductors and the effects of native defects upon semiconductors; but it is similar to that of 13C in graphene (T1 = 110 s). The temperature dependence of 1/T1 revealed a complex bipartite mechanism that included a T2 power-law behavior below 330 K and a thermally activated process above 330 K. In the high-temperature regime, the Arrhenius behavior is consistent with a field-dependent Cd atomic hopping relaxation process. At low temperatures, a T2 behavior consistent with a spin-1/2 Raman-like process provides evidence of a time-dependent spin-rotation magnetic field caused by angular oscillations of internuclear vectors due to lattice vibrations. The observed mechanism does not conform to the conventional two-band model of semimetals, but is instead closer to a mechanism observed in high-Z element ionic solids with large magnetorotation constant [A. J. Vega et al., Phys. Rev. B 74, 214420 (2006)].

Probing native metal ion association sites through quenching of fluorophores in the nucleotide-binding domains of the ABC transporter MsbA.

ATP-binding cassette (ABC) transporters are ubiquitously present in prokaryotic and eukaryotic cells. Binding of ATP to the nucleotide-binding domains (NBDs) elicits major conformational changes of the transporters resulting in the transport of the substrate across the membrane. The availability of a crystal structure of the NBDs enabled us to elucidate the local structure and small-scale dynamics in the NBDs. Here, we labeled the ABC transporter MsbA, a homodimeric flippase from Escherichia coli, with a fluorescent probe, Alexa532, within the NBDs. ATP application elicited collisional quenching, whereas no quenching was observed after the addition of ATP analogs or ATP hydrolysis inhibitors. The Alexa532-conjugated MsbA variants exhibited transition metal ion Förster resonance energy transfer (tmFRET) after the addition of Ni2+, and ATP decreased this Ni2+-mediated FRET of the NBDs. Structure modeling developed from crystallographic data and examination of tmFRET measurements of MsbA variants in the absence of ATP revealed the presence of metal ion-associated pockets (MiAPs) in the NBDs. Three histidines were predicted to participate in chelating Ni2+ in the two possible MiAPs. Performing histidine-substitution experiments with the NBDs showed that the dissociation constant for Ni2+ of MiAP2 was smaller than that of MiAP1. The structural allocation of the MiAPs was further supported by showing that the addition of Cu2+ resulted in higher quenching than Ni2+ Taken together, the present study showed that the NBDs contain two native binding sites for metal ions and ATP addition affects the Ni2+-binding activity of the MiAPs.

Delay-induced depinning of localized structures in a spatially inhomogeneous Swift-Hohenberg model.

We report on the dynamics of localized structures in an inhomogeneous Swift-Hohenberg model describing pattern formation in the transverse plane of an optical cavity. This real order parameter equation is valid close to the second-order critical point associated with bistability. The optical cavity is illuminated by an inhomogeneous spatial Gaussian pumping beam and subjected to time-delayed feedback. The Gaussian injection beam breaks the translational symmetry of the system by exerting an attracting force on the localized structure. We show that the localized structure can be pinned to the center of the inhomogeneity, suppressing the delay-induced drift bifurcation that has been reported in the particular case where the injection is homogeneous, assuming a continuous wave operation. Under an inhomogeneous spatial pumping beam, we perform the stability analysis of localized solutions to identify different instability regimes induced by time-delayed feedback. In particular, we predict the formation of two-arm spirals, as well as oscillating and depinning dynamics caused by the interplay of an attracting inhomogeneity and destabilizing time-delayed feedback. The transition from oscillating to depinning solutions is investigated by means of numerical continuation techniques. Analytically, we use an order parameter approach to derive a normal form of the delay-induced Hopf bifurcation leading to an oscillating solution. Additionally we model the interplay of an attracting inhomogeneity and destabilizing time delay by describing the localized solution as an overdamped particle in a potential well generated by the inhomogeneity. In this case, the time-delayed feedback acts as a driving force. Comparing results from the later approach with the full Swift-Hohenberg model, we show that the approach not only provides an instructive description of the depinning dynamics, but also is numerically accurate throughout most of the parameter regime.

The Interplay of Structure and Dynamics in Mixed Lipid Bilayers

The implication of lipids in membrane mediated processes has motivated significant theoretical and experimental research interest to understand the functional significance of lipid chemical structure variations on the biomembrane structure and dynamics. In particular, hydrophobic mismatch between lipids with different acyl chain lengths is expected to play an important role in determining the local membrane structure and collective membrane dynamics. Herein we investigated these phenomena in large unilamellar vesicles composed of binary mixtures of lipids with a 4 carbon mismatch in tail length, dimyristoylphosphatidylcholine (DMPC, 14:0 C) and distearoylphosphatidycholine (DSPC, 18:0 C), using neutron scattering techniques. Structural studies using small angle neutron scattering (SANS) revealed that the mixed lipid bilayers were thinner than expected for a simple composition-weighted average of the pure component bilayers. The structural differences were accompanied by dramatic differences in the mixed lipid membrane dynamics. Neutron spin echo spectroscopy (NSE) experiments showed that the mixed lipid bilayers were more dynamic than their single component analogs, with the mixed bilayers having a lower bending modulus and increased thickness fluctuation amplitude than the pure DMPC or DSPC bilayers. Both of these enhanced dynamics were consistent with a decrease in the area expansion modulus of the mixed systems. Together our results demonstrate the influence of lipid composition on the bilayer biophysical properties and highlight the complex interplay between structure and dynamics in lipid membranes.

Electrical addressing and temporal tweezing of localized pulses in passively-mode-locked semiconductor lasers

We show that the pumping current is a convenient parameter for manipulating the temporal Localized Structures (LSs), also called localized pulses, found in passively mode-locked Vertical-Cavity Surface-Emitting Lasers. While short electrical pulses can be used for writing and erasing individual LSs, we demonstrate that a current modulation introduces a temporally evolving parameter landscape allowing to control the position and the dynamics of LSs. We show that the localized pulses drifting speed in this landscape depends almost exclusively on the local parameter value instead of depending on the landscape gradient, as shown in quasi-instantaneous media. This experimental observation is theoretically explained by the causal response time of the semiconductor carriers that occurs on an finite timescale and breaks the parity invariance along the cavity, thus leading to a new paradigm for temporal tweezing of localized pulses. Different modulation waveforms are applied for describing exhaustively this paradigm. Starting from a generic model of passive mode-locking based upon delay differential equations, we deduce the effective equations of motion for these LSs in a time-dependent current landscape.

Multidomain structure and correlated dynamics determined by self-consistent FRET networks.

We present an approach that allows us to simultaneously access structure and dynamics of a multi-domain protein in solution. Dynamic domain arrangements are experimentally determined by combining self-consistent networks of distance distributions with known domain structures. Local structural dynamics are correlated with the global arrangements by analyzing networks of time-resolved single-molecule fluorescence parameters. The strength of this hybrid approach is shown by an application to the flexible multi-domain Hsp90. The average solution structure of Hsp90’s closed state resembles the known x-ray crystal structure with Angstrom precision. The open state is represented by an ensemble of conformations with inter-domain fluctuations of up to 25 Å. The data reveal a state-specific suppression of the sub-millisecond fluctuations by dynamic protein-protein interaction. Finally, the method enables localization and functional characterization of dynamic elements and domain interfaces.

Vibrational dynamics of crystalline silicon dioxide with charged Ge impurities

The effects of differently charged Ge impurities on the local atomic structure and lattice dynamics of [Formula: see text]-quartz were studied. We have determined the equilibrium structures and calculated the symmetrized local density of vibrational states for the Ge-doped [Formula: see text]-quartz. The frequencies of localized vibrations of [Formula: see text]- and [Formula: see text]-symmetries induced by Ge impurities were obtained. Besides, we have analyzed what contribution the vibrations of atoms located around the Ge impurities make to the localized symmetrized vibrations.

Elucidation of μs dynamics of domain-III of human serum albumin during the chemical and thermal unfolding: A fluorescence correlation spectroscopic investigation

The local structural dynamics and denaturation profile of domain-III of HSA against guanidine hydrochloride (GnHCl) and temperature has been studied using a coumarin based solvatochromic fluorescent probe p-nitrophenyl coumarin ester (NPCE), covalently tagged to Tyr-411 residue. By the steady state, time-resolved and single molecular level fluorescence studies it has been established that the domain-III of HSA is very sensitive to GnHCl but somewhat resistant to temperature and the domain specific unfolding proceeds in an altered way as compared to the overall unfolding of HSA. While the overall denaturation of HSA is a two-state process for both GnHCl and heat, domain-III adopts two intermediate states for GnHCl induced denaturation and one intermediate state for temperature induced denaturation. Fluorescence correlation spectroscopic investigation divulges the conformational dynamics of domain-III of HSA in the native, intermediates and denatured state.

Differentiating Two Nitrosylruthenium Isomeric Complexes by Steady-State and Ultrafast Infrared Spectroscopies.

The [Ru(II)-NO+] group affects the structure and chemical reactivity of nitrosylruthenium(II) complexes. A characteristic infrared absorption band due to the nitrosyl (NO) stretching motion is shown in the frequency region 1800-1900 cm-1. In this work, linear infrared (IR) and nonlinear IR methods, including pump-probe and two-dimensional (2D) IR, were utilized to study the structures and dynamics of two isomeric nitrosylruthenium complexes [Ru(OAc)(2mqn)2NO] (H2mqn = 2-methyl-8-quinolinol) in cis and trans isomeric configurations in a weak polar solvent (CDCl3). Using the NO stretching mode as a vibrational probe, information about local structural dynamics of the Ru complex as well as solvent fluctuation dynamics was obtained. In particular, a "structured" solvent environment is believed to form in the vicinity of the NO group in the case of the cis isomer with the aid of a neighboring OAc ligand, which is the reason for more efficient vibrational relaxation but more inhomogeneously distributed solvent and thus associated slower spectral diffusion. Our results also suggest a more anharmonic potential surface for the NO stretching mode in the less stable trans isomer.

Local electronic structure, work function, and line defect dynamics of ultrathin epitaxial ZnO layers on a Ag(1 1 1) surface

Using combined low-temperature scanning tunneling microscopy and Kelvin probe force microscopy we studied the local electronic structure and work function change of the (0 0 0 1)-oriented epitaxial ZnO layers on a Ag(1 1 1) substrate. Scanning tunneling spectroscopy (STS) revealed that the conduction band minimum monotonically downshifts as the number of the ZnO layers increases up to 4 monolayers (ML). However, it was found by field emission resonance (FER) spectroscopy that the local work function of Ag(1 1 1) slightly decreases for 2 ML thick ZnO but it dramatically changes and drops by about 1.2 eV between 2 and 3 ML, suggesting a structural transformation of the ZnO layer. The spatial variation of the conduction band minimum and the local work function change were visualized at the nanometer scale by mapping the STS and FER intensities. Furthermore, we found that the ZnO layers contained line defects with a few tens of nm long, which can be removed by the injection of a tunneling electron into the conduction band.

Toward a Designable Extracellular Matrix: Molecular Dynamics Simulations of an Engineered Laminin-Mimetic, Elastin-Like Fusion Protein.

Native extracellular matrices (ECMs) exhibit networks of molecular interactions between specific matrix proteins and other tissue components. Guided by these naturally self-assembling supramolecular systems, we have designed a matrix-derived protein chimera that contains a laminin globular-like (LG) domain fused to an elastin-like polypeptide (ELP). This bipartite design offers a flexible protein engineering platform: (i) laminin is a key multifunctional component of the ECM in human brains and other neural tissues, making it an ideal bioactive component of our fusion, and (ii) ELPs, known to be well-tolerated in vivo, provide a self-assembly scaffold with tunable physicochemical (viscoelastic, thermoresponsive) properties. Experimental characterization of novel proteins is resource-intensive, and examining many conceivable designs would be a formidable challenge in the laboratory. Computational approaches offer a way forward: molecular dynamics (MD) simulations can be used to analyze the structural/physical behavior of candidate LG-ELP fusion proteins, particularly in terms of conformational properties salient to our design goals, such as assembly propensity in a temperature range spanning the inverse temperature transition. As a first step in examining the physical characteristics of a model LG-ELP fusion protein, including its temperature-dependent structural behavior, we simulated the protein over a range of physiologically relevant temperatures (290-320 K). We find that the ELP region, built upon the archetypal (VPGXG)5 scaffold, is quite flexible and has a propensity for β-rich secondary structures near physiological (310-315 K) temperatures. Our trajectories indicate that the temperature-dependent burial of hydrophobic patches in the ELP region, coupled to the local water structure dynamics and mediated by intramolecular contacts between aliphatic side chains, correlates with the temperature-dependent structural transitions in known ELP polymers. Because of the link between compaction of ELP segments into β-rich structures and differential solvation properties of this region, we posit that future variation of ELP sequence and composition can be used to systematically alter the phase transition profiles and, thus, the general functionality of our LG-ELP fusion protein system.

Local structure and translational dynamics of NMF (N-methylformamide)–DMF (N,N-dimethylformamide) mixtures, via molecular dynamics simulation

In this work, the liquid amide mixture of N-methylformamide (NMF) with N,N-dimethylformamide (DMF) has been investigated by means of the molecular dynamics simulation method. The intermolecular structure and local mole fractions around any kind of the component molecules have been studied in terms of the calculated site-site correlation functions at three solvent compositions, XDMF, and ambient conditions. The hydrogen bonding network of the system has been extensively explored and the several hydrogen bond types formed between NMF-NMF, DMF-DMF and NMF-DMF in each mixture per molecule have been obtained with increasing XDMF. The results obtained reveal the notable effect of DMF molecules upon the formation of the hydrogen bonding network in each system. The investigated diffusion coefficients of both mixture components show clearly a linear increase of the diffusivity of all molecules while the DMF fraction increases. The intermolecular structure and hydrogen bonding properties studied validate the results from previous small angle neutron scattering studies on the system and support their conclusion that this amide mixture may be considered as an almost ideal one. Also, it may be concluded that DMF does not self-aggregate, because it acts as a hydrogen-bond breaker over the whole range of solvent composition. This finding is in accordance with the translational diffusive behavior of the molecules in the mixtures mentioned above. The detailed analysis of the hydrogen bonding network in DMF- rich and NMF-rich mixtures has revealed important information regarding the short range molecular aggregation in these diluted mixtures. This result is in general found to be comparable with corresponding predictions from previous experimental low-frequency raman spectroscopic studies of these mixtures.

Local Structure and Li Ion Dynamics in Garnet Li7-2xGaxLa3Zr2O12 Solid Electrolytes

One of the key component in solid-state Li ion rechargeable Li ion batteries is the solid electrolyte. Garnet Li7-xGaxLa3Zr2O12 is one of the solid electrolyte systems that has been reported to demonstrate conductivity values that almost reaches the level of conventional organic-based electrolytes (~10-3 S/cm).1 Interestingly, the dopant cation in this system, Ga3+, occupies Li sites and can directly impede Li ion movement. Meanwhile, experimental results were inconclusive and not unanimous in ascertaining the actual site preference and distribution of Ga3+ in the 24d tetrahedral (Td) and 48g/96hoctahedral (Oh) crystallographic sites. In this work, we attempted to clarify the aforementioned issues by performing computational modelling, emphasizing on the Ga dopant coordination environment and the effects of Ga towards the Li ion transport process. To study the local environment for Ga3+, we calculated the NMR parameters (chemical shift ∂iso, asymmetry factor ηQ, quadrupolar coupling CQ) at the two distinct Li sites using the augmented plane-wave method implemented in WIEN2k. Prior to NMR parameter fitting, model cells for the doped garnet (with Ga at the tetrahedral and octahedral cage, respectively) and several relevant Ga-containing compounds were relaxed by DFT using VASP. Meanwhile, we also analyzed the energy distribution with respect to Ga3+occupancy by uniformly sampling the Li-Ga-vacancy configurations within a 3 x 3 x 3 garnet supercell using fitted Buckingham interatomic potentials and GULP. For Li ion transport study, we employed molecular dynamics by using DL_POLY. Structural information of DFT-relaxed models showed good agreement with experimental results; ~1% difference for lattice constants and < 5% difference for volume. The calculated NMR parameters are as follows: ∂iso,Ga@Td = 231.7 ± 9.30 ppm, ηQ,Ga@Td = 0.4860 ± 0.0053, CQ,Ga@Td = 11.82 ± 0.34 MHz; ∂iso,Ga@Oh = 53.8 ± 9.30 ppm, ηQ,Ga@Oh = 0.4496 ± 0.0053, CQ,Ga@Oh = 5.84 ± 0.34 MHz. For the chemical shift ∂iso for the Td environment (four-fold coordination), our result is consistent with MAS NMR results for Ga-doped garnets (207 ± 10 and 244 ppm).1,2 On the other hand, the calculated asymmetry parameter ηQ is larger than the experimental value for the 24d tetrahedral cage (0.05 ± 0.05)1. However, when compared with the experimental value for LiGaO2 (0.37) in which Ga3+ also sits in a tetrahedral cage,3 our calculated value for ηQ is comparable, indicating that Ga may coordinate assymetrically as well in the 24d tetrahedral cage in the garnet Li sublattice. Based from configuration sampling for Li-Ga-vacancy arrangements (16,000 random configurations of 3 x 3 x 3 supercells, sampling for all-Td, all-Oh, and intermediate distribution for Ga), we found that the energy variation is small (< 15 meV/atom) which means that Ga has no site occupancy preference. Furthermore, Ga can exist in a four-fold coordination inside the octahedral cage (eg., when at the 96hsite). Hence, the inconsistency in experiments may be resolved by viewing the dopant Ga as sitting in a four-fold coordination environment either at the tetrahedral or the octahedral Li cage of the garnet structure. Our molecular dynamics results4 have shown that the first-nearest-neighbor site of Ga is energetically inaccessible for a migrating Li due to strong repulsion, possibly forming defect clusters according to the following association reactions: i) Ga·· Li + V'Li → {Ga·· LiV' Li }·, {Ga·· LiV' Li }· + V'Li → {Ga·· Li2V' Li }X, 2Ga·· Li + 4V'Li → {2Ga·· Li4V'Li}X. We also confirmed the stabilization effect of Ga for the high-conductivity cubic phase (from the low-conductivity tetragonal phase) as evidenced in the calculated thermal expansion profiles. Two distinct regions for conductivity were observed for 0 ≤ x ≤ 0.30 in Li7-xGaxLa3Zr2O12: an abrupt rise followed linear decrease for 0 ≤ x ≤ 0.10 (owing to competing effects of tetragonal → cubic transition and decreasing Li concentration) and a flattrend for 0.10 ≤ x ≤ 0.30 (due to increasing accessible Li vacancies which are crucial for initiating concerted Li motion). REFERENCES 1 Bernuy-Lopez et al., Chem. Mater. 2014, 26, 3610−3617. 2Rettenwander et al., Inorg. Chem. 2014, 53, 6264−6269. 3Ash et al., Magn. Reson. Chem. 2006, 44, 823−831. 4 Jalem et al., Chem. Mater. 2015, 27, 2821–2831.

Ssnmr Studies of Li3V2(PO4)3/C Cathode Reactions: Structural Evolution, Dynamics and LVP-Carbon Interface

Monoclinic lithium vanadium phosphate Li3V2 (PO4)3(LVP) is a highly promising cathode material for lithium-ion batteries. In the past decades, extensive efforts have been made to improve the electrochemical performance of LVP by carbon coating and cation doping[1]. In-depth understanding of the local structures and charge-discharge mechanism of LVP can provide valuable information for the design of cation doping or surface modification. SSNMR has been demonstrated a powerful tool of local structure analysis and dynamics. However, due to sever line broadening and signal coalescence caused by the heating effect of carbon in the cathode composite, SSNMR study of LVP material was only limited to samples prepared by chemical delithiation[2], which cannot fully represent the real electrochemical process. We hereby present the first SSNMR study of LVP/C cathode reaction. The electrodes have been prepared in a binder free and carbon additive free fashion, in order to reduce the heating effect to a minimum during NMR spectra acquisition. A variety of SSNMR techniques have been combined to obtain structural information from different aspects. The structural changes of the polyhedron framework upon delithiation-lithiation were traced by 7Li and 31P NMR. The dynamic processes involving Li exchange and migration were monitored by variable temperature 7Li NMR and EIS technique. 13C-7Li REDOR NMR and HRTEM were performed to investigate Li-O-C connectivity and the short range structure at the LVP-carbon interface. The NMR results have elucidated the driving force of the asymmetric phase formation during charge-discharge within the voltage window of 4.3-4.8 volts and provided experimental evidence of direct chemical bonding between LVP and carbon coating. Our study helps the battery community to understand the (de)lithiation mechanism of LVP cathode, and more important, the role of carbon coating. [1]. Rui X, Yan Q, Skyllas-Kazacos M, et al. Li3V2(PO4)3 cathode materials for lithium-ion batteries: A review[J]. J. Power Sources , 2014, 258(21):19-38. [2]. Yin S C, Grondey H, Strobel P, et al. Charge ordering in lithium vanadium phosphates: electrode materials for lithium-ion batteries.[J]. J. Am. Chem. Soc. , 2003, 125(2):326-7.

Trapping of Li(+) Ions by [ThFn](4-n) Clusters Leading to Oscillating Maxwell-Stefan Diffusivity in the Molten Salt LiF-ThF4.

A molten salt mixture of lithium fluoride and thorium fluoride (LiF-ThF4) serves as a fuel as well as a coolant in the most sophisticated molten salt reactor (MSR). Here, we report for the first time dynamic correlations, Onsager coefficients, Maxwell-Stefan (MS) diffusivities, and the concentration dependence of density and enthalpy of the molten salt mixture LiF-ThF4 at 1200 K in the composition range of 2-45% ThF4 and also at eutectic composition in the temperature range of 1123-1600 K using Green-Kubo formalism and equilibrium molecular dynamics simulations. We have observed an interesting oscillating pattern for the MS diffusivity for the cation-cation pair, in which ĐLi-Th oscillates between positive and negative values with the amplitude of the oscillation reducing as the system becomes rich in ThF4. Through the velocity autocorrelation function, vibrational density of states, radial distribution function analysis, and structural snapshots, we establish an interplay between the local structure and multicomponent dynamics and predict that formation of negatively charged [ThFn](4-n) clusters at a higher ThF4 mole % makes positively charged Li(+) ions oscillate between different clusters, with their range of motion reducing as the number of [ThFn](4-n) clusters increases, and finally Li(+) ions almost get trapped at a higher ThF4% when the electrostatic force on Li(+) exerted by various surrounding clusters gets balanced. Although reports on variations of density and enthalpy with temperature exist in the literature, for the first time we report variations of the density and enthalpy of LiF-ThF4 with the concentration of ThF4 (mole %) and fit them with the square root function of ThF4 concentration, which will be very useful for experimentalists to obtain data over a range of concentrations from fitting the formula for design purposes. The formation of [ThFn](4-n) clusters and the reduction in the diffusivity of the ions at a higher ThF4% may limit the percentage of ThF4 that can be used in the MSR to optimize the neutron economy.