Fast Exchange Rate Research Articles

A novel chemical composite strategy for synergistically enhancing the activity and stability of strontium-containing air electrode for protonic ceramic cells

Cation segregation on perovskite electrode surfaces usually leads to a decrease in the catalytic activity of electrodes and a shortened cell life. Herein, we have developed a simple and novel chemical composite strategy to achieve selectively surface reconstruction of the Sr-rich regions in perovskite Pr0.4Sr0.6Fe0.9Nb0.1O3-δ (PSFN), constructing strongly interacting interfaces between PSFN and Fe2O3 phases. The enhanced charge transfer between the two phases in PSFN/Fe2O3 is revealed by soft X-ray absorption spectroscopy. Meanwhile, fast surface oxygen exchange rate accelerates the migration and adsorption/desorption of oxygen species. The chemical composite electrode PSFN/Fe2O3 exhibits a smaller polarization resistance (0.35 Ω cm2) than that (0.61 Ω cm2) of the physical composite electrode PSFN + Fe2O3 at 600 °C in air. The distribution of relaxation time analyses demonstrate that enhanced reaction kinetics related to charge transfer and mass transfer are key factors in achieving stable operation of symmetric cell within 1000 h. Importantly, this study provides a simple and universal strategy for surface modification of perovskite oxides with cation separation issue, and highlights the crucial role of constructing strongly interacting interfaces in achieving high activity and stability of air electrodes.

A nucleoid-associated protein is involved in the emergence of antibiotic resistance by promoting the frequent exchange of the replicative DNA polymerase in Mycobacterium smegmatis.

Unlike many other pathogens, Mycobacterium tuberculosis has limited ability for horizontal gene transfer, a major mechanism for developing antibiotic resistance. Thus, the mechanisms that facilitate chromosomal mutagenesis are of particular importance in mycobacteria. Here, we show that Lsr2, a nucleoid-associated protein, has a novel role in DNA replication and mutagenesis in the model mycobacterium Mycobacterium smegmatis. We find that Lsr2 promotes the fast exchange rate of the replicative DNA polymerase, DnaE1, at the replication fork and is important for the effective loading of the DnaE2-ImuA'-ImuB translesion complex. Without lsr2, M. smegmatis replicates its chromosome more faithfully and acquires resistance to rifampin at a lower rate, but at the cost of impaired survival to DNA damaging agents. Together, our work establishes Lsr2 as a potential factor in the emergence of mycobacterial antibiotic resistance.

Surprising Impact of Donor Charge on the Water Exchange Rates of Gd(III) AAZTA Derivatives

AbstractWe report the synthesis of three new heptadentate ligands derived from H4AAZTA (6‐amino‐6‐methylperhydro‐1,4‐diazepinetetraacetic acid) that contain a dimethyl‐amide group (AAZTA‐MA)3−, two dimethyl‐amide groups (AAZTA‐BMA)2− or two acetylglycine groups (AAZTA(Gly)2)4−. The corresponding Gd(III) complexes were investigated using 1H NMR relaxometry and 17O NMR chemical shifts and transverse relaxation rates. A computational DFT study was also performed to aid the analysis of the NMR data. The Gd(III) complexes contain two water molecules coordinated to the metal ion. In contrast to the prevailing trend, the amide derivatives discussed in this context exhibit comparatively rapid water exchange rates that do not align with the changes in the overall electric charge of the complexes: kex298=14.4×106 s−1, 14.5×106 s−1 and 9.56×106 s−1 for [Gd(AAZTA‐MA)(H2O)2], [Gd(AAZTA‐BMA)(H2O)2]+ and [Gd(AAZTA(Gly)2(H2O)2]−, respectively. The analysis of the data and the computational work suggest that the relatively fast water exchange rates could be linked to the occurrence of associatively activated mechanisms, which is somewhat unexpected for nine‐coordinated complexes.

Exploring Residue-Level Interactions between the Biofilm-Driving R2ab Protein and Polystyrene Nanoparticles.

In biological systems, proteins can bind to nanoparticles to form a "corona" of adsorbed molecules. The nanoparticle corona is of significant interest because it impacts an organism's response to a nanomaterial. Understanding the corona requires knowledge of protein structure, orientation, and dynamics at the surface. A residue-level mapping of protein behavior on nanoparticle surfaces is needed, but this mapping is difficult to obtain with traditional approaches. Here, we have investigated the interaction between R2ab and polystyrene nanoparticles (PSNPs) at the level of individual residues. R2ab is a bacterial surface protein from Staphylococcus epidermidis and is known to interact strongly with polystyrene, leading to biofilm formation. We have used mass spectrometry after lysine methylation and hydrogen-deuterium exchange (HDX) NMR spectroscopy to understand how the R2ab protein interacts with PSNPs of different sizes. Lysine methylation experiments reveal subtle but statistically significant changes in methylation patterns in the presence of PSNPs, indicating altered protein surface accessibility. HDX rates become slower overall in the presence of PSNPs. However, some regions of the R2ab protein exhibit faster than average exchange rates in the presence of PSNPs, while others are slower than the average behavior, suggesting conformational changes upon binding. HDX rates and methylation ratios support a recently proposed "adsorbotope" model for PSNPs, wherein adsorbed proteins consist of unfolded anchor points interspersed with partially structured regions. Our data also highlight the challenges of characterizing complex protein-nanoparticle interactions using these techniques, such as fast exchange rates. While providing insights into how R2ab adsorbs onto PSNP surfaces, this research emphasizes the need for advanced methods to comprehend residue-level interactions in the nanoparticle corona.

Stress‐induced dynamics of glassy vitrimers with fast bond exchange rate

AbstractVitrimers are dynamic covalent networks, showing considerable promise in self‐healing and reprocessability applications. In this work, molecular simulations are used to study the shear stress‐induced dynamics of vitrimers in the glassy regime and compare their response with that of permanent networks, providing valuable insights into the macroscopic properties of these networks. Results show that vitrimers undergo bond exchange reactions induced by the application of the shear stress, and these events result in accelerated dynamics and subsequently, higher shear compliance of vitrimer compared to the thermoset. When the applied shear stress is high and deformation enters the nonlinear regime, the vitrimer network shows large shear creep compliance and enhanced non‐affine dynamics. Furthermore, the analysis of the self‐particle dynamics demonstrates that the dynamical behavior of vitrimers is highly heterogeneous at short observation times under low and intermediate stress values, while they become homogeneous in the regime of the shear compliance where it shows significantly higher compliance than the permanent network. The significance of the rate of deformation on the dynamics of glassy vitrimers is successfully demonstrated.

Niobium-Tungsten Bimetallic Oxide Electrodes with High Dual-Band Electrochromic Performance Prepared by Hydrothermal Method

Dual-band electrochromic (EC) smart windows, capable of selectively regulating visible (VIS) and near-infrared (NIR) light, are one of the most attractive candidates for significantly reducing the energy consumption of buildings. Niobium-tungsten bimetallic oxides exhibit a fast ion exchange rate and high cycle stability in the electrochemical field because of various structural chemistry in the pseudobinary system Nb2O5/WO3, showing excellent dual-band EC potential. Herein, we employ a facile hydrothermal technique to synthesize Nb18W16O93 (NWO) films with low temperature and facile procedure. The obtained NWO films exhibit impressive dual-band electrochromic performances, including high optical regulation (radiation blocking rates of 33.1% and 77.5% in the VIS and NIR band) and short coloring/bleaching time (12 s/2.6 s at 633 nm and 6.5 s/6.1 s at 1600 nm). These results demonstrate that NWO is a high-performance dual-band EC material and provide a practical strategy to synthesize the NWO electrodes with nanorod stacking architectures in a facile and low-cost way.

Adjustment of rotation and saturation effects (AROSE) for CEST imaging.

Endogenous CEST signal usually has low specificity due to contaminations from the magnetization transfer contrast (MTC) and other labile protons with overlapping or close Larmor frequencies. We propose to improve CEST signal specificity with adjustment of rotation and saturation effects (AROSE). The AROSE approach measures the difference between CEST signals acquired with the same average irradiation power but largely different duty cycles, for example, a continuous wave or a high duty cycle pulse train versus a low duty cycle pulse train with a flip angle φ. Simulation, phantom, and in vivo rodent studies were performed to evaluate the characteristics of the AROSEφ signal. Simulation and experimental results show that AROSE2π is a low-pass filter that can suppress fast exchanging processes (e.g., >3000 s-1 ), whereas AROSEπ is a band-pass filter suppressing both fast and slow exchange (e.g., <30 s-1 ) rates. For other φ angles, the sensitivity and the exchange-rate filtering effect of AROSEφ falls between AROSEπ and AROSE2π . AROSE can also minimize MTC and improve the Larmor frequency selectivity of the CEST signal. The linewidth of the AROSE1.5π spectrum is about 60% to 65% when compared to the CEST spectrum measured by continuous wave. Depending on the needs of an application, the sensitivity, exchange-rate filtering, and Larmor frequency selectivity can be adjusted by varying the flip angle, duty cycle, and average irradiation power. Compared to conventional CEST signals, AROSE can minimize MTC and improve exchange rate filtering and Larmor frequency specificity.

Direct and Catalyst-Free Ester Metathesis Reaction for Covalent Adaptable Networks.

Thermosetting polymers possess excellent environmental resistance and mechanical properties but cannot be reprocessed due to their covalently cross-linked structures. Recycling of thermosets via the implantation of dynamic covalent bonds offers a promising solution. Here, we report the direct and catalyst-free ester metathesis of N-acyloxyphthalimide (NAPI) at about 100 °C without the requirement of hydroxyl groups and its utilization for the fabrication of covalent adaptable networks (CANs). NAPI metathesis has interesting sigmoid kinetics with a fast exchange rate, which proceeds via a free radical chain mechanism, guaranteeing a fast associative exchange under a rather low dissociation. The bifunctional molecule of NAPI as both the radical precursor and substrate is the key to the dissociatively initiated associative (DAssociative) mechanism and kinetic behavior. Based on the efficient NAPI metathesis, polyester networks, poly(N-acyloxyphthalimides) (PNAPIs), show excellent malleability. Notably, PNAPIs exhibit exceptional solvent resistance and mechanical stability at elevated temperatures owing to the unique DAssociative mechanism, suggesting exciting opportunities for designing recyclable thermosetting polymers.

Tuning the Zn2+ solvation structure in dual-salts hybrid aqueous electrolyte to stabilize the Zn metal anode

Aqueous Zn-ion battery is expected to become a substitute for Li-ion battery due to its inherent safety, low cost, and environmental friendliness. Dendrite growth and side reaction problems during electroplating lead to its low Coulombic efficiency and unsatisfactory life, which greatly limits its practical application. Here, we propose a dual-salts hybrid electrolyte, which alleviates the above issues by mixing Zn(OTf)2 to ZnSO4 solution. Extensive tests and MD simulations have shown that the dual-salts hybrid electrolyte can regulate the solvation structure of Zn2+, facilitating uniform Zn deposition, and inhibiting side reactions and dendrite growth. Hence, the dual-salts hybrid electrolyte exhibits good reversibility in Zn//Zn batteries, which can provide a lifetime of more than 880 h at 1 mA cm−2 and 1 mAh cm−2. Moreover, the average Coulombic efficiency of Zn//Cu cells in hybrid system can reach 98.2% after 520 h, much better than that of 90.7% in pure ZnSO4 electrolyte and 92.0% in pure Zn(OTf)2 electrolyte. Benefiting from the fast ion exchange rate and high ion conductivity, Zn-ion hybrid capacitor in hybrid electrolyte also displays excellent stability and capacitive performance. This effective strategy for dual-salts hybrid electrolytes provides a promising direction for designing aqueous electrolytes for Zn-ion batteries.

A monetary model of exchange rate beats the random walk forecast even at a short horizon: Evidence from the Serbian hyperinflation at daily frequency

This paper finds that an out-of-sample forecast of a monetary model of exchange rate (MMER) in hyperinflation decisively beats a random walk one particularly at the most challenging one step ahead forecast, thus outperforming standard results previously obtained for low inflation episodes. The findings refer to the Serbian hyperinflation at daily frequency, and are robust with respect to various tests. Fast adjustment of the exchange rate to its fundamental value and the low, well below one discount factor found in the Serbian episode, as opposed to low-inflation ones, can account for divergent results in the respective inflation environments. The low discount factor appears in other hyperinflation episodes, while fast adjustment is due to the absence of nominal rigidities in hyperinflation thus both suggesting that reported findings for one episode might generalize.

In situ synthesis of magnesium-doped hydroxyapatite aerogel for highly efficient U(VI) separation with ultra high adsorption capacity and excellent recyclability

Mg-doped HAP aerogel (MHAPA) was firstly in situ prepared via freeze-drying-calcination technology to capture U(VI). The U(VI) removal capacity by MHAPA even arrived 2685.6 mg g−1, which was about 2 times over purchased HAP, illustrating that the incorporation of Mg ions could greatly enhance the U(VI) removal capacity. Compared with HAP, MHAPA also showed better anti-ion interference ability and dynamic removal performances. In comparison with other HAP-based adsorbents, MHAPA possessed good recyclability and its desorption rate was up to 93.4% in the first cycle. The excellent U(VI) removal performances of MHAPA might be owing to its low crystallinity and grain size, fast ion exchange rate and partial ionization under acidic conditions, which would accelerate the process of electrostatic attraction, ion-exchange, and complexation to immobilize U(VI). To sum up, the prepared MHAPA was expected to be an environmentally friendly, recyclable and effective adsorbent to immobilize U(VI) in actual wastewater.

Tunable interactions of quasibound states in the continuum with cavity mode in a metasurface-microcavity hybrid

All-dielectric photonic crystals or metasurfaces supporting optical bound states in the continuum (BIC) are emerging as a promising platform for the study of light-matter interactions in the strong coupling regime. However, coupling of BIC to other optical excitations is generally weak, with the coupling strength limited in the range of several tens of milli electronvolts with short decoherence time. This greatly hinders their applications in solid-base quantum information devices that require a fast information exchange rate and long coherence time. Herein, a cavity-coupling strategy is used to extend the BIC-based coupling platforms. We propose a hybrid system containing an all-dielectric metasurface embedded in an optical microcavity, demonstrating the formation of BIC-cavity polaritons in the strong coupling regime with a surprisingly large Rabi splitting over 220 meV, far beyond the strong coupling regime. A flexible tuning of BIC-cavity coupling from weak to strong coupling regime is realized by controlling the shape of dielectric element of the metasurface. Importantly, a full quantum model based on Heisenberg-Langevin formalism reveals the temporal population dynamics of the coupling system, demonstrating the ultrafast Rabi socillation within 19 fs with a long decoherence time over 200 fs. Utilizing the multipolar expansion approach combined with full wave simulations, we also investigate the coupling-induced polariton characteristics in both the weak and strong coupling regimes. We find that the electric dipole component of the BIC mode dominates in the polariton states in weak coupling regime, which dramatically changes as the system is tuned into strong coupling regime. Such a cavity-coupling strategy in the present hybrid system is expected to be of great use for enhancing BIC-based light-matter interactions for more complex hybrid nanostructures such as three-mode systems with mixing BIC-cavity-exciton interactions.

Microscopic Dynamics and Viscoelasticity of Vitrimers

We implement a hybrid molecular dynamics/Monte Carlo simulation to study the microscopic dynamics and the macroscopic rheology of vitrimers with a fast bond exchange rate. We show that the linear viscoelastic properties and mean squared displacement of the vitrimers collapse onto master curves by applying the same shift factors that follow the Williams–Landel–Ferry equation at low temperatures and Arrhenius-like behavior at high temperatures. The linkage between the microscopic dynamics and the linear rheology of vitrimers is established using the generalized Stokes–Einstein relationship, which efficiently extends the timescale of simulations and predicts the viscoelasticity. The values of the shift factors are related to the characteristic decay time of the intermediate scattering function, which is accessible in scattering experiments. The same results hold in the case of an all-atom model of an ionic liquid. Our methodology provides a microscopic basis for the time-superposition principle and predicts the macroscopic rheology of thermo-rheologically simple vitrimers.

THE EXCHANGE RATE SYSTEM REFORM IN CHINA: SOME IMPORTANT RESULTS

We provide a review and empirical study on the exchange rate system reform in China. In the initial stage of the reform the Chinese central bank PBoC’s implicit promise of gradual appreciation helped to contain the appreciation rate and volatility of the renminbi. Subsequently, under US pressure for faster appreciation and hence the PBoC’s moderate violation of the implicit promise, there was a significant rise in the appreciation rate and the volatility of the renminbi. The moderate violation deteriorated further, forming a vicious cycle of speculative flows and faster exchange rate changes. Upon the onset of the global financial crisis in 2008, the threatening appreciation and volatility alleviated when the PBoC returned to the implicit stance of gradual appreciation or stable renminbi. We found that in the first ten years of the reform the PBoC did not shift its monitoring target from the RMB–USD rate to the nominal effective exchange rate. This may be one of the underlying causes of the renminbi crisis in 2015.

Creep and Recovery Behavior of Vitrimers with Fast Bond Exchange Rate.

Vitrimers encompass the desirable mechanical properties of thermosets with the recyclability of thermoplastics. This ability arises from the rearrangement of the vitrimer covalent network upon heating via a bond shuffling mechanism while its cross-link density remains preserved. This unique feature makes vitrimers interesting candidates for the design of materials that combine dimensional stability at high temperatures and solvent resistance with the ability to be reshaped and processed. Despite these advantages, vitrimer exhibits significant creep at operating conditions where thermosets show little or no creep. As the mechanical properties of vitrimers not only depend on their chemical composition but also on the dynamics of the polymer chains, molecular dynamics (MD) simulations can provide detailed molecular mechanisms of the system of interest under macroscopic stress-induced deformations. In this regard, the recently developed MD/Monte Carlo simulation methodology capable of capturing the bond exchange mechanics in vitrimers is used to study the creep and recovery response of a coarse-grained model thermoset and vitrimer with a fast bond exchange rate. The time-stress superposition principle is then successfully applied to the creep response. The resulting universal curves enable us to predict the long-time creep behavior of both systems extending the timescale from 4 to over 10 orders of magnitude.

Investigations of pH-dependent dynamic properties of OmpG-16SL, an outer membrane protein G mutant by ATR-FTIR spectroscopy

In this paper, the dynamic properties of outer membrane protein G mutant (OmpG-16SL) are investigated with ATR-FTIR spectroscopy. While OmpG-WT has 14 β-strands in its structure, the mutant is designed to have 16 β-strands with the intention of creating an enlarged pore. Loop L6 is elongated by introducing six residues, two of which are negatively charged. The solvent accessibility of the OmpG-16SL mutant is compared with WT and a previously reported mutant OmpG-16S by tracking the 1H/2H exchange kinetics in acidic and neutral buffer conditions. The exchange kinetics and dynamics in the fast and slow exchange phases are separately investigated using the 2DCOS technique, which enables the tracking of the structural changes at each phase of the exchange process. The results suggest that the mutant OmpG-16SL is equally exposed to buffer in both acidic and neutral pH conditions. Additionally, the time range in the fast phase is very short - one-tenth of that for WT - and most of the exchange is completed in this phase. This fast exchange within minutes is also indicative of the presence of highly flexible and/or unstructured regions. In all, the fast exchange rates independent of the buffer pH justify the assumption that there is an altered interaction among the charged residues, which leads to a steadily-open pore. The role of the side-chain interactions within the pore and between the loops involving the loop L6 is also discussed.

Structural stability of Cutibacterium acnes acyl carrier protein studied using CD and NMR spectroscopy

To survive in diverse environments, bacteria adapt by changing the composition of their cell membrane fatty acids. Compared with aerobic bacteria, Cutibacterium acnes has much greater contents of branched-chain fatty acids (BCFAs) in the cell membrane, which helps it survive in anaerobic environments. To synthesize BCFAs, C. acnes acyl carrier protein (CaACP) has to transfer growing branched acyl intermediates from its hydrophobic cavity to fatty acid synthases. CaACP contains an unconserved, distinctive Cys50 in its hydrophobic pocket, which corresponds to Leu in other bacterial acyl carrier proteins (ACPs). Herein, we investigated the substrate specificity of CaACP and the importance of Cys50 in its structural stability. We mutated Cys50 to Leu (C50L mutant) and measured the melting temperatures (Tms) of both CaACP and the C50L mutant by performing circular dichroism experiments. The Tm of CaACP was very low (49.6 °C), whereas that of C50L mutant was 55.5 °C. Hydrogen/deuterium exchange experiments revealed that wild-type CaACP showed extremely fast exchange rates within 50 min, whereas amide peaks of the C50L mutant in the heteronuclear single quantum coherence spectrum remained up to 200 min, thereby implying that Cys50 is the key residue contributing to the structural stability of CaACP. We also monitored chemical shift perturbations upon apo to holo, apo to butyryl, and apo to isobutyryl conversion, confirming that CaACP can accommodate isobutyryl BCFAs. These results provide a preliminary understanding into the substrate specificity of CaACPs for the production of BCFAs necessary to maintain cell membrane fluidity under anaerobic environments.

Electrospray-Induced Mass Spectrometry Is Not Suitable for Determination of Peptidic Cu(II) Complexes.

The toolset of mass spectrometry (MS) is still expanding, and the number of metal ion complexes researched this way is growing. The Cu(II) ion forms particularly strong peptide complexes of biological interest which are frequent objects of MS studies, but quantitative aspects of some reported results are at odds with those of experiments performed in solution. Cu(II) complexes are usually characterized by fast ligand exchange rates, despite their high affinity, and we speculated that such kinetic lability could be responsible for the observed discrepancies. In order to resolve this issue, we selected peptides belonging to the ATCUN family characterized with high and thoroughly determined Cu(II) binding constants and re-estimated them using two ESI-MS techniques: standard conditions in combination with serial dilution experiments and very mild conditions for competition experiments. The sample acidification, which accompanies the electrospray formation, was simulated with the pH–jump stopped-flow technique. Our results indicate that ESI-MS should not be used for quantitative studies of Cu(II)–peptide complexes because the electrospray formation process compromises the entropic contribution to the complex stability, yielding underestimations of complex stability constants.

Extreme iron isotope variation of pyrite in the Muping gold deposit, Jiaodong: Implication for tracing metal origin

Pyrite has complicated iron isotope fractionation behavior in the hydrothermal condition. Here we present a detailed in-situ iron isotope analyses of different generation pyrite from the Muping gold deposit, eastern China. δ56Fe values of pyrite investigated range from −4.25‰ to +1.78‰, unlike previous bulk analytic data (−0.78‰ to +0.79‰). Iron isotopic homogeneity of single pyrite grain, which can be verified by in-situ analysis, was recommended as the preferred and reliable criterion to judge the isotopic fractionation condition in the hydrothermal gold deposit. Inter-grain variations in Fe isotopic composition suggest occurrence of both kinetic and equilibrium fractionation for pyrite at Muping. Electron backscattered diffraction analysis shows that the replacement likely occurred via dissolution-reprecipitation pathway. The replacement process resulted in obvious kinetic isotope effect, in which pyrites are possessed in extreme negative Fe isotopic composition (~−4‰). Maximum δ56Fe of pyrite up to +1.78‰ implied that kinetic isotopic effect also play a significant role in increasing the Fe isotopic composition of ore-forming fluids in the Muping gold deposit. Further model calculations suggested the kinetic isotopic effect was dominated by pyrite fast precipitation rather than pyrrhotite precipitation or pyrite precipitation via “FeS” pathway. This kinetic isotope effect ultimately will be partly or completely erased due to fast isotope exchange rate at medium–high temperature condition. Pyrite with homogeneous Fe isotopic composition can be used to trace iron source of hydrothermal gold deposit but be cautious.

Facile in situ growth of ZIF-8 films onto aluminum for applications requiring fast thermal response

In this work, we report a simple approach for the synthesis of highly crystalline films of ZIF-8 directly grown onto non-functionalized common aluminum substrates through an easy, short in situ solvothermal methodology. Through several techniques such as XRD, SEM, EDS, AFM and XPS, it is shown that a simple, short pretreatment of the substrate with a dilute hydrochloric acid solution promotes the subsequent solvothermal nucleation and growth of continuous, homogeneous micrometer thickness and very adherent ZIF-8 films. The robustness and usefulness of ZIF-8/aluminum are shown by testing it in the highly exothermic catalytic reaction of carbon monoxide oxidation, in which it presented a high performance and durability, preserving the metal–organic framework integrity during reaction time. This behavior demonstrates that the novel ZIF-8/aluminum systems have a high potential for use in applications at moderate temperatures demanding a fast heat exchange rate between the metal–organic framework (MOF) film and the substrate.