Low-energy Sites Research Articles

Deuterium occupation of vacancy-type defects in argon-damaged tungsten exposed to high flux and low energy deuterium plasma

Doppler broadening spectroscopy in the positron annihilation technique (DBS-PA) has been employed to investigate the defect properties in argon-damaged tungsten exposed to low-energy and high flux deuterium plasma. Argon ion irradiations with energy 500 keV are performed for tungsten samples with various levels of damage. The remarkable increment of the S parameter in DBS-PA indicates the introduction of vacancy-type defects in argon irradiated tungsten. An increase of ion fluence results in a continuous increase of the S parameter until saturation. Unexpectedly, a much higher fluence leads to a decrease of the S parameter in the near surface, and the (S,W) slope changes greatly. This should be associated with the formation of argon-vacancy complexes in the near surface produced by the excessive implanted argon ions. With deuterium plasma exposure, a significant decrease of the S parameter occurs in the pre-irradiated tungsten, suggesting the sharp reduction of the number and density of the vacancy-type defects. The thermal desorption spectroscopy results demonstrate that the argon-damaged tungsten, compared to the pristine one, exhibits an enhanced low-temperature desorption peak and an additional and broad high-temperature desorption peak, which indicates that deuterium atoms are trapped in both low-energy and high-energy sites. All these observations directly indicate the deuterium occupation of irradiation-induced vacancy defects in damaged tungsten, which is responsible for the remarkable increase of the deuterium retention in comparison with the pristine one.

Thermal migration of deuterium implanted in graphite: Influence of free surface proximity and structure

This paper is a contribution to the study of the behavior of activation products produced in irradiated nuclear graphite, graphite being the moderator of the first French generation of CO2 cooled nuclear fission reactors. This paper is focused on the thermal release of Tritium, a major contributor to the initial activity, taking into account the role of the free surfaces (open pores and graphite surface). Two kinds of graphite were compared. On one hand, Highly Oriented Pyrolitic Graphite (HOPG), a model well graphitized graphite, and on the other hand, SLA2, a porous less graphitized nuclear graphite. Deuterium ion implantation at three different energies 70, 200 and 390keV allows simulating the presence of Tritium at three different depths, corresponding respectively to projected ranges Rp of 0.75, 1.7 and 3.2μm. The D isotopic tracing is performed thanks to the D(3He,p)4He nuclear reaction. The graphite structure is studied by Raman microspectrometry. Thermal annealing is performed in the temperature range 200–1200°C up to 300h annealing time. As observed in a previous study, the results show that the D release occurs according to three kinetic regimes: a rapid permeation through open pores, a transient regime corresponding to detrapping and diffusion of D located at low energy sites correlated to the edges of crystallites and finally a saturation regime attributed to detrapping of interstitial D located at high energy sites inside the crystallites. Below 600°C, D release is negligible whatever the implantation depth and the graphite type. The present paper clearly puts forward that above 600°C, the D release decreases at deeper implantation depths and strongly depends on the graphite structure. In HOPG where high energy sites are more abundant, the D release is less dependent on the surface proximity compared to SLA2. In SLA2, in which the low energy sites prevail, the D release curves are clearly shifted towards lower temperatures when D is located close to free surfaces. Extrapolating our data to Tritium mobility in irradiated graphite, we show that thermal selective extraction of T would be all the more so efficient as the graphite structure is more disordered, which means in the most irradiated and damaged graphite zones in the reactor.

Quantifying turbulence from field measurements at a mixed low tidal energy site

This study explores typical characteristics of the mean and turbulent profiles at a mixed low tidal energy site (40 m mean water depth) where the waves have limited effects on the currents. The turbulence profiles were derived from secondary current data using a 5-beam ADCP which was optimised for wave measurements. The tidal currents have peak flows of ∼1 m/s during spring tide. The turbulence intensity is no less than 10% at peak flows and compares well with values at other tidal channels (at ∼5 m from seabed). The Reynolds stresses show symmetry at the neap tide but less so for the spring tide. Although the qualitative profiles of TKE are similar between the neap and spring tides, the values of TKE for flood flow are the largest throughout the deployment. The integral length scales are in good agreement with theory, and with estimates based on the mixing length concept. The measured turbulence parameters are sensitive to flow inhomogeneity, Doppler noise, and ADCP tilt. The findings demonstrate the practical benefits of exploiting secondary current data at a mixed low tidal energy site for estimating typical turbulence characteristics; such information can be used to define design standards and protocols for marine energy devices.

Control of grain boundary connectivity based on fractal analysis for improvement of intergranular corrosion resistance in SUS316L austenitic stainless steel

The connectivity of high energy random boundaries was investigated on the basis of the fractal analyses of grain boundary microstructures in SUS316L stainless steel, to prove the usefulness of a refined approach to grain boundary engineering (GBE) for more precise prediction and control of intergranular corrosion in polycrystalline materials. It was found that the maximum connectivity for random boundary network, termed the maximum random boundary connectivity (MRBC) had a fractal nature in the studied specimens of SUS316L stainless steel. The fractal dimension of MRBC tended to decrease with decreasing fraction of random boundaries, or in other words with increasing fraction of low-energy low-Σ coincidence site lattice (CSL) boundaries. The lower coefficient of variation of grain size distribution suggesting a more homogeneous grain structure, was found to result in the lower fractal dimension of MRBC for the specimens with a similar grain boundary character distribution (GBCD). The optimum grain boundary microstructure for enhanced intergranular corrosion resistance in the SUS316L stainless steel was discussed based on the results from the fractal analyses of MRBC for different grain boundary microstructures.

Atomistic modelling of zirconium and silicon segregation at twist and tilt grain boundaries in molybdenum

We investigate the influence of Zr and Si segregation on the cohesive strength of grain boundaries (GBs) in molybdenum using density functional theory calculations. A tilt \(\Sigma \)5(310)[001] and twist \(\Sigma \)5[001] GB in bicrystal geometry are chosen as structural models. We determine the site preference of Zr and Si for segregation in these GBs and define the segregation energy. We quantify the effect of solutes on the stability of the GBs against brittle fracture by means of the Griffith criterion (work of separation). Additionally, the intrinsic bond strength of the GB containing a solute is quantified by means of the theoretical strength. The results show that Zr and Si tend to segregate at the GBs if the low-energy insertion sites are available. However, the work of separation is decreased by the presence of Zr and Si and even in the presence of oxygen, there is no increase of the Griffith energy. Contributions of strain and chemical energy are analysed in order to explain our findings.

The Effect of Nb on the Continuous Cooling Transformation Curves of Ultra-Thin Strip CASTRIP© Steels

The effect of Nb on the hardenability of ultra-thin cast strip (UCS) steels produced via the unique regime of rapid solidification, large austenite grain size, and inclusion engineering of the CASTRIP© process was investigated. Continuous cooling transformation (CCT) diagrams were constructed for 0, 0.014, 0.024, 0.04, 0.06 and 0.08 wt% Nb containing UCS steels. Phase nomenclature for the identification of lower transformation product in low carbon steels was reviewed. Even a small addition of 0.014 wt% Nb showed a potent effect on hardenability, shifting the ferrite C-curve to the right and expanding the bainitic ferrite and acicular ferrite phase fields. Higher Nb additions increased hardenability further, suppressed the formation of ferrite to even lower cooling rates, progressively lowered the transformation start and finish temperatures and promoted the transformation of bainite instead of acicular ferrite. The latter was due to Nb suppressing the formation of allotriomorphic ferrite and allowing bainite to nucleate at prior austenite grain boundaries, a lower energy site than that for the intragranular nucleation of acicular ferrite at inclusions. Strength and hardness increased with increasing Nb additions, largely due to microstructural strengthening and solid solution hardening, but not from precipitation hardening.

Noble gas diffusivity hindered by low energy sites in amphibole

The diffusion kinetics of He and Ne in four amphibole specimens have been experimentally determined using stepwise degassing analysis of samples previously irradiated with energetic protons, and Arrhenius relationships have been fit to these data. The primary finding is that He and Ne diffusivities are systematically lower in amphiboles that have higher concentrations of unoccupied ring sites, suggesting that unoccupied ring sites act as traps for migrating noble gases. Ring site influence of noble gas diffusivity in amphiboles has substantial implications for 40Ar/39Ar thermochronology applied to these phases and the efficiency of noble gas recycling in subduction zones. These findings are consistent with the correlation between noble gas solubility and the concentration of unoccupied ring sites in amphibole (Jackson et al., 2013a, 2015) but are inconsistent with the ionic porosity model for noble gas diffusion (Fortier and Giletti, 1989; Dahl, 1996). Rather, these findings suggest that the topology of ionic porosity and absolute volume of ionic porosity compete in determining the rate at which noble gases diffuse.

Nanoscale transport of charge-transfer states in organic donor-acceptor blends.

Charge-transfer (CT) states, bound combinations of an electron and a hole on separate molecules, play a crucial role in organic optoelectronic devices. We report direct nanoscale imaging of the transport of long-lived CT states in molecular organic donor-acceptor blends, which demonstrates that the bound electron-hole pairs that form the CT states move geminately over distances of 5-10 nm, driven by energetic disorder and diffusion to lower energy sites. Magnetic field dependence reveals a fluctuating exchange splitting, indicative of a variation in electron-hole spacing during diffusion. The results suggest that the electron-hole pair of the CT state undergoes a stretching transport mechanism analogous to an 'inchworm' motion, in contrast to conventional transport of Frenkel excitons. Given the short exciton lifetimes characteristic of bulk heterojunction organic solar cells, this work confirms the potential importance of CT state transport, suggesting that CT states are likely to diffuse farther than Frenkel excitons in many donor-acceptor blends.

Synthesis of few layer single crystal graphene grains on platinum by chemical vapour deposition

The present competition of graphene electronics demands an efficient route which produces high quality and large area graphene. Chemical vapour deposition technique, where hydrocarbons dissociate in to active carbon species and form graphene layer on the desired metal catalyst via nucleation is considered as the most suitable method. In this study, single layer graphene with the presence of few layer single crystal graphene grains were grown on Pt foil via chemical vapour deposition. The higher growth temperature changes the surface morphology of the Pt foil so a delicate process of hydrogen bubbling was used to peel off graphene from Pt foil samples with the mechanical support of photoresist and further transferred to SiO2/Si substrates for analysis. Optical microscopy of the graphene transferred samples showed the regions of single layer along with different oriented graphene domains. Two type of interlayer stacking sequences, Bernal and twisted, were observed in the graphene grains. The presence of different stacking sequences in the graphene layers influence the electronic and optical properties; in Bernal stacking the band gap can be tunable and in twisted stacking the overall sheet resistance can be reduced. Grain boundaries of Pt provides low energy sites to the carbon species, therefore the nucleation of grains are more at the boundaries. The stacking order and the number of layers in grains were seen more clearly with scanning electron microscopy. Raman spectroscopy showed high quality graphene samples due to very small D peak. 2D Raman peak for single layer graphene showed full width half maximum (FWHM) value of 30cm−1. At points A, B and C, Bernal stacked grain showed FWHM values of 51.22, 58.45 and 64.72cm−1, while twisted stacked grain showed the FWHM values of 27.26, 28.83 and 20.99cm−1, respectively. FWHM values of 2D peak of Bernal stacked grain showed an increase of 20–30cm−1 as compare to single layer graphene which showed its dispersive nature and confirmed Bernal sequence. On the other hand, the slightest decrease in FWHM values of 2D peak in twisted grain comparing to single layer confirmed the twisted sequence of grains. Atomic force microscopy analysis showed an increasing trend in grain height profile with an increase in the number of layers.

Tidal energy leasing and tidal phasing

In addition to technical and economic constraints, tidal energy leasing is generally governed by demand for sites which contain the highest tidal streams, and does not take into account the phase relationship (i.e. the time lag) between sites. Here, the outputs of a three-dimensional tidal model are analysed to demonstrate that there is minimal phase diversity among the high tidal stream regions of the NW European shelf seas. It is therefore possible, under the current leasing system, that the electricity produced by the first generation of tidal stream arrays will similarly be in phase. Extending the analysis to lower tidal stream regions, we demonstrate that these lower energy sites offer more potential for phase diversity, with a mean phase difference of 1.25 h, compared to the phase of high energy sites, and hence more scope for supplying firm power to the electricity grid. We therefore suggest that a state-led leasing strategy, favouring the development of sites which are complementary in phase, and not simply sites which experience the highest current speeds, would encourage a sustainable tidal energy industry.

A novel strategy for accelerating the recovery of an anammox reactor inhibited by copper(II): EDTA washing combined with biostimulation via low-intensity ultrasound

Excessive heavy metals are inhibitory or even toxic to microorganisms in biological wastewater treatment systems. Given the promising prospects of anaerobic ammonium oxidation (anammox), this work details a novel remediation strategy for preventing anammox failure due to heavy metal overload (in this study, Cu(II) overload). The strategy involves combining ethylenediamine tetraacetic acid (EDTA) washing with bioactivity stimulation via low-intensity ultrasound. Kinetic experiments and batch activity assays were employed to determine the optimal concentration of EDTA and washing time, and changes in reactor performance and granule characteristics were tracked by continuous-flow monitoring to evaluate the long-term effects. The two-step desorption process revealed that the Cu in anammox granules was primarily introduced via adsorption (approximately 84.9%) and that low-energy adsorption sites were predominant (accounting for 58.6%). The Cu internalized in the cells (approximately 12.5%) could then diffuse out of the cells and be gradually washed out of the reactor within 20days. Thereafter, ultrasound irradiation at intervals of 8days contributed to accelerated performance, with a nitrogen removal rate (NRR) increasing rate of 0.1971kgNm−3d−2. The NRR was able to return to normal levels within 64days, whereas the recovery of dehydrogenase activity (DHA) was hysteretic, requiring approximately 110days. Gradual improvements in granule diameter, settling properties, extracellular polymeric substances and heme content were also detected. Overall, these results demonstrate that external removal of the adsorbed Cu followed by internal repair of the metabolic system may represent an important and useful recovery method in wastewater treatment plant operation.

Theoretical Modeling of Defect Segregation and Space-Charge Formation in Proton-Conducting Barium Zirconate

Introduction Acceptor-doped barium zirconate (BaZrO3) shows considerable potential as an electrolyte material for intermediate temperature solid oxide fuel cells due to its high bulk proton conductivity and chemical stability. However, in polycrystalline materials the grain boundaries (GBs) display high proton resistivity, severely limiting the overall proton transport in the material. The low GB conductivity has been thought to be due to an intrinsic mechanism, and recently the so called space charge model has been used to interpret and model the GB blocking behavior in BaZrO3 by several research groups [1-5]. Modeling According to the space charge model, charged defects accumulate in the structurally distorted region at the GB plane, the GB core. The resulting core charge gives rise to an electrostatic potential barrier and causes depletion of mobile charge carriers of the same polarity in the regions adjacent to the core, the space charge layers. The existence of a positive core charge is crucial to the application of the space charge model to BaZrO3. In a previous study, we used density functional theory (DFT) to investigate oxygen vacancy segregation to a symmetric tilt GB and we found that oxygen vacancies segregate to the GB and create a positive core charge [4]. Here, we extend our previous studies [4,6] to several other tilt GBs. Segregation energies of both oxygen vacancies and protons are determined and used in a one-dimensional thermodynamic space charge model. We also investigate the effect of segregation of dopants as well as interaction between oxygen vacancies in the GB core. Our first-principles electronic-structure calculations are performed within the framework of DFT using the generalized gradient PBE functional to approximate exchange and a layer-by-layer space charge model [6] is employed to determine the potential barrier and core charge resulting from defect segregation. Results and Discussion Using DFT proton and oxygen vacancy segregation have been investigated for an extensive set of tilt GBs. Both defect species are found to segregate to the GB core, with typical segregation energies of -1.5 eV and -1.0 eV for oxygen vacancies and protons, respectively. Below 900 K and at wet conditions potential barrier heights around 0.6V are obtained, which is consistent with experimental results. For many GBs we find the protons to be responsible for the major part of the excess core charge and thereby the electrostatic potential. We have also investigated dopant segregation and its effect on the potential barrier. Experimental evidence suggests that an increased dopant content should decrease the potential barrier and that this effect should be especially pronounced if the dopants are able to segregate to the GB. Indeed, we find that if low-energy sites in the core are saturated with positive defects, an increase in the dopant concentration does lower the potential barrier. However, if the low-energy sites are not saturated an increased dopant concentration in the core could also lead to an increase in the concentration of positive defects and the core concentration is instead limited by the electrostatic potential. An increase in the number of negative defects is therefore followed by an increase in the number of positive defects, keeping the total core charge constant. Furthermore, our results suggest that interactions among oxygen vacancies may effectively reduce the number of available sites in the GB core. This has consequences for the effects of dopant segregation since it implies that the core can become saturated with positive defects at an earlier stage and segregation of dopant ions may then actually decrease the core charge, and reduce the potential barrier height. Conclusions To conclude, segregation of oxygen vacancies and protons plays an important role in the formation of space charge potentials at the grain boundary interface. Additionally, our results show that protons may be the main source to the space-charge potentials under conditions relevant for fuel cell applications.

Reduced graphene oxide functionalized with Cu nanoparticles: Fabrication, structure, and sensing properties

We have fabricated the NO2 gas sensors by using reduced graphene oxide (RGO) functionalized with copper (Cu) nanoparticles, revealing that the sensitivity was significantly enhanced by the Cu functionalization. Transmission electron microscopy indicated that the nanoparticles with a cubic Cu phase were attached on the surface of RGO. Raman spectra revealed that the structural disorder of RGO was increased by the Cu-functionalization. We performed the NO2 gas sensing test, revealing that the sensor response of RGOs was increased by 326% by means of the Cu-functionalization. We have discussed the associated mechanisms. In addition, we have ascribed the functionalization-induced changes of response and recovery times to the increase of low-energy binding sites.

Site selection spectroscopy in Eu3 +-doped lanthanum fluorozirconate glass and glass-ceramic

The optical properties of Eu3+-doped glass and transparent glass-ceramic with composition (in mol%) 70.2ZrF4–23.4LaF3–0.6AlF3–5.8GaF3 were investigated by site selective spectroscopy in order to study the effect of ceramization on the symmetry of Eu3+ sites. The glass-ceramic obtained after thermal treatment of the glass ZLAG contains a unique crystalline phase of unknown structure. The analysis and comparison of FLN spectra in these materials are presented and different classes of sites are discussed. We found the presence of two main site distributions for Eu3+ ions of equivalent symmetry (C2v or lower) in the glass and two sites of different symmetry (C2v or lower and C4v or lower) in the glass-ceramic (GC). The estimated average crystal field strength for glass and GC decreases with the 5D0→7F0 energy. The results suggest that the two kinds of sites identified in the glass correspond to Eu3+ ions in and out of the former network. Decay-time measurements of 5D0 level of Eu3+ evidenced energy transfer between high and low energy sites and showed an increase of lifetime from the glass to the glass-ceramic.

Electrochemical preparation of nitrogen-doped graphene quantum dots and their size-dependent electrocatalytic activity for oxygen reduction

Here we report a remarkable transformation of nitrogen-doped multiwalled carbon nanotubes (MWCNTs) to size selective nitrogen-doped graphene quantum dots (N-GQDs) by a two-step electrochemical method. The sizes of the N-GQDs strongly depend on the applied anodic potential, moreover increasing potential resulted in a smaller size of N-GQDs. These N-GQDs display many unusual size-dependant optoelectronic (blue emission) and electrocatalytic (oxygen reduction) properties. The presence of N dopants in the carbon framework not only causes faster unzipping of MWCNTs but also provides more low activation energy site for enhancing the electrocatalytic activity for technologically daunting reactions like oxygen reduction. The smaller size of N-GQDs has shown better performance as compared to the large N-GQDs. Interestingly, N-GQDs-3 (size = 2.5±0.3 nm, onset potential = 0.75 V) show a 30-mV higher positive onset potential shift compared to that of N-GQDs-2 (size = 4.7±0.3 nm, onset potential = 0.72 V) and 70 mV than that of N-GQDs-1 (size = 7.2±0.3, onset potential = 0.68 V) for oxygen reduction reaction (ORR) in a liquid phase. These result in the size-dependent electrocatalytic activity of N-GQDs for ORR as illustrated by the smaller sized N-GQDs (2.5±0.3 nm) undoubtedly promising metal-free electrocatalysts for fuel cell applications.

The Role of Exciton Delocalization in the Major Photosynthetic Light-Harvesting Antenna of Plants

In the major peripheral plant light-harvesting complex LHCII, excitation energy is transferred between chlorophylls along an energetic cascade before it is transmitted further into the photosynthetic assembly to be converted into chemical energy. The efficiency of these energy transfer processes involves a complicated interplay of pigment-protein structural reorganization and protein dynamic disorder, and the system must stay robust within the fluctuating protein environment. The final, lowest energy site has been proposed to exist within a trimeric excitonically coupled chlorophyll (Chl) cluster, comprising Chls a610-a611-a612. We studied an LHCII monomer with a site-specific mutation resulting in the loss of Chls a611and a612, and find that this mutant exhibits two predominant overlapping fluorescence bands. From a combination of bulk measurements, single-molecule fluorescence characterization, and modeling, we propose the two fluorescence bands originate from differing conditions of exciton delocalization and localization realized in the mutant. Disruption of the excitonically coupled terminal emitter Chl trimer results in an increased sensitivity of the excited state energy landscape to the disorder induced by the protein conformations. Consequently, the mutant demonstrates a loss of energy transfer efficiency. On the contrary, in the wild-type complex, the strong resonance coupling and correspondingly high degree of excitation delocalization within the Chls a610-a611-a612 cluster dampens the influence of the environment and ensures optimal communication with neighboring pigments. These results indicate that the terminal emitter trimer is thus an essential design principle for maintaining the efficient light-harvesting function of LHCII in the presence of protein disorder.

Pt atoms adsorbed onTiO2(110)−(1×1)studied with noncontact atomic force microscopy and first-principles simulations

We have studied the local properties of single Pt atoms adsorbed on hydroxylated ${\mathrm{TiO}}_{2}(110)$-($1\ifmmode\times\else\texttimes\fi{}1$) by combining noncontact atomic force microscopy (nc-AFM) and first-principles calculations. Room-temperature high-resolution nc-AFM images for the most frequently observed contrast modes reveal bright and elongated protrusions that can be traced back to the Pt atoms, and that are centered on the fivefold coordinated titanium rows, confined between two bridging oxygen rows. These observations are in line with the theoretical results, as the lowest energy sites for the Pt atom on the ${\mathrm{TiO}}_{2}(110)$ surface are in the neighborhood of the titanium rows, and high energy barriers have to be overcome to displace the Pt atom over the bridging oxygen rows. Single Pt atoms can be distinguished from H adsorbates (OH defects) due to their characteristic shape and binding site and, because they appear as the brightest surface features in all of the contrast modes. Force spectroscopy data over the protrusion and hole imaging modes and the corresponding tip-sample forces, simulated with O and OH terminated ${\mathrm{TiO}}_{2}$ nanoclusters, provide an explanation for this puzzling result in terms of the intrinsic strength of the interaction with the Pt adatom and the adatom and tip apex relaxations induced by the tip-sample interaction. These imaging mechanisms can be extended to other electropositive metal dopants and support the use of nc-AFM not only to characterize their adsorption structure but also to directly probe their chemical reactivity.

Carbonate build-ups in lacustrine, hydrothermal and fluvial settings: comparing depositional geometry, fabric types and geochemical signature

Abstract Carbonate build-ups in lakes, hydrothermal and fluvial settings are characterized by distinctive geometry, spatial distribution, fabrics and geochemical signature but also by some comparable features. Lake margin bioherms form continuous belts for hundreds of metres to kilometres, subparallel to shorelines. Sublacustrine spring mounds are spaced at hundreds of metres to kilometres and aligned along faults. Hydrothermal travertine mounds and aprons with planar clinoforms or terraced slopes are controlled by faults, thermal water discharge and substrate topography. Fluvial tufa barrages, cascades and terraced slopes are controlled by climate, vegetation and substrate gradient. The wide spectrum of carbonate microfabrics ranges from clotted peloidal micrite and laminated boundstone to crystalline dendrite cementstone. Non-marine carbonate microfabrics cannot be linked to specific depositional environments, and are not deterministic proxies for the interpretation of build-up architecture. Microfabric associations can be indicative, but not exclusive, of specific depositional environments and geometry. Stable isotope geochemistry is a useful tool to distinguish between hydrothermal, karstic freshwater and evaporative lake carbonates. Carbonate precipitation results from a continuum of abiotic and biologically influenced/induced processes in settings where carbonate supersaturation is largely driven by physico-chemical mechanisms and microbial biofilms, even if acting as passive low-energy surface sites for nucleation, are widely present.

The Intrinsically Disordered C. Trachomatis Tarp Binds Actin with a Partially Preformed Helix

Tarp (Translocated actin recruiting protein) is an intrinsically disordered protein (IDP) from Chlamydia trachomatis, which is translocated into the host cell via type III secretion to initiate the infection process. Once exposed to the cytosol, Tarp recruits host-cell actin, forming actin pedestal-like structures that contribute to host cell invasion (Clifton et al., PNAS101,10166). Tarp is capable of directly nucleating the formation of actin filaments, and its actin-binding region was mapped to the region between residues 726 and 825 (Jewett et al., PNAS103, 15599). We have acquired and assigned nuclear magnetic resonance (NMR) spectra of Tarp726-825, which show the narrow spectral distribution and chemical shifts characteristic of an IDP. Tarp shows some homology to the actin binding WH2 motif from WAVE2, and therefore it is expected to bind in a similar manner, forming a helix upon binding. Indeed, NMR relaxation experiments of Tarp726-825 show slightly lower flexibility in the 10-residue region homologous to the WH2 motif. However, chemical shift index calculations show that this helix is only partially formed before binding. In the presence of globular actin, the majority of peaks in the Tarp NMR spectra are affected, indicating that the binding affects most of the amino acid residues in this Tarp fragment. Isothermal titration calorimetry experiments determined the dissociation constant for the binding to be 1.02 (± 0.32) × 10−7 M, and showed a significant decrease in entropy upon binding, indicating increased order upon binding. Based on this information, the binding of Tarp to actin was modelled in silico. Two possible low energy sites of interaction have been identified, both of which could account for these observations. It is therefore possible that Tarp binds actin through a “fuzzy” interface, with contribution from both conformations.

Insertion energetics of lithium, sodium, and magnesium in crystalline and amorphous titanium dioxide: A comparative first-principles study

Titanium dioxide (TiO2) has been proposed as a potential electrode material for lithium, sodium, and magnesium ion batteries. Among the phases of TiO2, anatase, rutile, and (B)–TiO2 are the most commonly used phases for electrochemical storage, while the amorphous phase has also been shown to be a promising candidate. We present a comparative density functional theory study of the insertion energetics of Li, Na, and Mg into anatase, rutile, and (B)–TiO2, as well as into the amorphous phase. Our results show that among the crystalline phases, (B)–TiO2 provides the strongest binding between TiO2 and the inserted Li/Na/Mg atom. We also find that for all Li, Na, and Mg, the amorphous phase provides insertion sites well-dispersed in energies, with a lowest energy site more thermodynamically favorable than insertion sites in the crystalline phases. We also obtain the localized Ti3+ states together with the formation of the defect states in the band gap, which are induced by the insertion, at the GGA level of theory (without the Hubbard correction).