Pulsed Field Gradient NMR Research Articles

Modular MPS3-Based Frameworks for Superionic Conduction of Monovalent and Multivalent Ions.

Next-generation batteries based on more sustainable working ions could offer improved performance, safety, and capacity over lithium-ion batteries while also decreasing the cost. Development of next-generation battery technology using "beyond-Li" mobile ions, especially multivalent ions, is limited due to a lack of understanding of solid state conduction of these ions. Here, we introduce ligand-coordinated ions in MPS3-based (M = Mn, Cd) solid host crystals to simultaneously increase the size of the interlayer spacing, through which the ions can migrate, and screen the charge-dense ions. The ligand-assisted conduction mechanism enables ambient temperature superionic conductivity of various next-generation mobile ions in the electronically insulating MPS3-based solid. Without the coordinating ligands, all of the compounds show little to no ionic conductivity. Pulsed-field gradient nuclear magnetic resonance spectroscopy suggests that the ionic conduction occurs through a hopping mechanism, where the cations are moving between H2O molecules, instead of a vehicular mechanism which has been observed in other hydrated layered solids. This modular system not only facilitates tailoring to different potential applications but also enables us to probe the effect of different host structures, mobile ions, and coordinating ligands on the ionic conductivity. This research highlights the influence of cation charge density, diffusion channel size, and effective charge screening on ligand-assisted solid state ionic conductivity. The insights gained can be applied in the design of other ligand-assisted solid state ionic conductors, which will be especially impactful in realizing solid state multivalent ionic conductors. Additionally, the ion-intercalated MPS3-based frameworks could potentially serve as a universal solid state electrolyte for various next-generation battery chemistries.

Translational Diffusion and Self-Association of an Intrinsically Disordered Protein κ-Casein Using NMR with Ultra-High Pulsed-Field Gradient and Time-Resolved FRET.

Much attention has been given to studying the translational diffusion of globular proteins, whereas the translational diffusion of intrinsically disordered proteins (IDPs) is less studied. In this study, we investigate the translational diffusion and how it is affected by the self-association of an IDP, κ-casein, using pulsed-field gradient nuclear magnetic resonance and time-resolved Förster resonance energy transfer. Using the analysis of the shape of diffusion attenuation and the concentration dependence of κ-casein diffusion coefficients and intermolecular interactions, we demonstrate that κ-casein exhibits continuous self-association. When the volume fraction of κ-casein is below 0.08, we observe that κ-casein self-association results in a macroscopic phase separation upon storage at 4 °C. At κ-casein volume fractions above 0.08, self-association leads to the formation of labile gel-like networks without subsequent macroscopic phase separation. Unlike α-casein, which shows a strong concentration dependence and extensive gel-like network formation, only one-third of κ-casein molecules participate in the gel network at a time, resulting in a more dynamic and less extensive structure. These findings highlight the unique association properties of κ-casein, contributing to a better understanding of its behavior under various conditions and its potential role in casein micelle formation.

Unraveling the NH3-SCR-DeNOx Mechanism of Cu-SSZ-13 Variants by Spectroscopic and Transient Techniques.

Commercial SSZ-13 zeolite with different n(Si)/n(Al) ratios and from different suppliers were subjected to a post-synthetic treatment in order to create mesopores of up to 15 nm. Furthermore, the materials were modified with copper ions and thoroughly physico-chemically characterized. The modified textural properties varied the nature of copper species, and thus, activity in the selective catalytic reduction of NOx with ammonia (NH3-SCR-DeNOx). Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) studies with hexane as probe liquid revealed improved intracrystalline diffusion for some Cu-containing SSZ-13 materials. The NH3-SCR-DeNOx pathways are verified via in situ DR UV-Vis, in situ FT-IR and EPR, temperature-programmed studies as well as SSITKA studies that provide a mechanistic understanding of the reaction. Kinetic modelling results demonstrate the highest NH3-SCR-DeNOx reaction rates and up to 20 % lower energy barriers with n(Si)/n(Al) ratio of 6.5 for all modified forms (i. e., (NH4)Cu-SSZ-13_6.5 and Cu-SSZ-13_6.5_NaOH/0.1) and cause only negligible parasitic ammonia oxidation. The modelling of the stop-flow experiments further demonstrates that the SCR pathway via the HONO surface intermediate is present but barely contributes to the overall NO conversion compared to the dominant path between adsorbed NH3 and NO from the gas phase.

Formulation of zeolite-mesoporous silica composite catalysts for light olefin production from catalytic cracking

Framework materials such as zeolites and mesoporous silicas are commonly used for many applications, especially catalysis and separation. Here zeolite-mesoporous silica composite catalysts (employing zeolite Y, ZSM-5, KIT-6, SBA-15 and MCM-41 mesoporous silica) were prepared (with different weight percent of zeolite Y and ZSM-5) and assessed for catalytic cracking (using n-heptane, as the model compound at 550 °C) with the aim to improve the selectivity/yield of light olefins of ethylene and propylene from n-heptane. Physicochemical properties of the parent zeolites and the prepared composites were characterized comprehensively using several techniques including X-ray diffraction, nitrogen physisorption, scanning electron microscopy, fourier transform infrared spectroscopy, pulsed-field gradient nuclear magnetic resonance and thermogravimetric analysis. Catalytic cracking results showed that the ZY/ZSM-5/KIT-6 composite (20:20:60 wt %) achieved a high n-heptane conversion of 85% with approximately 6% selectivity to ethylene/propylene. In contrast, the ZY/ZSM-5/SBA-15 composite achieved a higher conversion of 95% and an ethylene/propylene ratio of 8%, indicating a more efficient process in terms of both conversion and selectivity. Magnetic resonance relaxation analysis of the ZY/ZSM-5/KIT-6 (20:20:60) catalyst confirmed a micro-mesoporous environment that influences n-heptane diffusion and mass transfer. As zeolite Y and ZSM-5 have micropores, n-heptane can move and undergo hydrogen transfer reactions, whereas KIT-6 has mesopores that facilitate n-heptane’s accessibility to the active sites of zeolite Y and ZSM-5.

Dimensionality Control of Li Transport by MOFs Based Quasi‐Solid to Solid Electrolyte (Q‐SSEs) for Li−Metal Batteries

AbstractNowadays, lithium‐ion batteries (LIBs) are widely used in all walks of life and play a very important role. As complex systems composed of multiple materials with diverse chemical compositions, where different electrochemical reactions take place, battery interfaces are essential for determining the operation, performance, durability and safety of the battery. This work, set out to study the incorporation of lithium bis(fluorosulfonyl)amide (LiFSI) doped 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) ionic liquid into an archetype Ti‐based Metal Organic Framework (MOF) ((Ti) MIL125−NH2) to create a solid to quasi‐solid (depending on the amount of IL in the system), and how it affects not only ionic transport but also the structural properties of the IL/MOF electrolyte. Remarkably high ionic conductivity values (2.13×10−3 S ⋅ cm−1 at room temperature) as well as a lithium transference number (tLi=0.58) were achieved, supported by pulsed field gradient (PFG) NMR experiments. Electrochemical characterization revealed reversible plating‐stripping of lithium and lower overpotential after 750 h at 50 °C. Additionally, a proof‐of‐concept solid state battery was fabricated resulting in a discharge capacity of 160 mAh ⋅ g−1 at 50 °C and 0.1 C rate after 50 cycles. This work presents a suitable strategy to dendrite suppression capability, allowing its implementation as interface modifiers in next‐generation solid‐state batteries.

Weakly solvating ester electrolyte for high voltage sodium-ion batteries

Ethyl acetate (EA) was identified as a promising electrolyte solvent for sodium-ion batteries (SIBs), exhibiting low viscosity, cost-effectiveness, and low toxicity. Despite a significant portion of aggregation being linked to the weak solvation of Na+/EA as revealed by molecular dynamics (MD) simulations, pulsed-field gradient nuclear magnetic resonance (pfg-NMR) analysis identified a noteworthy Na+ diffusion coefficient of 3.95×10−10 m2 s−1 at 25°C in the presence of 1 m NaPF6 salt. Employing fluoroethylene carbonate (FEC) as a film-forming additive to create electrode-electrolyte interphase, this electrolyte surprisingly made ∼210 mAh Na0.97Ca0.03[Mn0.39Fe0.31Ni0.22Zn0.08]O2 (NCMFNZO)/hard carbon (HC) pouch cells achieve a lengthy cycling lifetime of 250 cycles with ∼80 % capacity retention, cycled up to 4.0 V at 40°C. X-ray photoelectron spectroscopy (XPS) revealed increasing interphasial organic species over cycling, augmenting charge transfer resistance on both cathode and anode, particularly during fast charging or low temperatures (<10°C), promoting Na plating. Gas chromatography-mass spectrometry combined with density functional theory identified CO2 as the major gas generated from charged cathode/electrolyte interactions, exhibiting temperature/voltage dependence.

Cell wall permeability in relation to in vitro starch digestion of pea cotyledon cells

The role of cell wall permeability and rate of starch digestibility in intact cotyledon cells from four different varieties of pea seeds was studied. Pulsed-field gradient nuclear magnetic resonance (PFG NMR) coupled with light and confocal microscopy were employed to evaluate the cotyledon cells' diffusion coefficients and cell wall permeability. The cotyledon cells' diffusion coefficients and cell wall permeability followed a decreasing trend: White/yellow pea > Marrowfat pea > Maple pea > Blue pea. The varying size of internal cavities in the microstructure in the cotyledon cells, as observed by the light and confocal micrographs, may be responsible for this trend. The extent of starch hydrolysis recorded from the cotyledon cells followed the same trend of the cell wall permeability except for Blue pea cotyledon cells. Thus, indicating that the more permeable the cotyledon cell to the starch-degrading enzymes, the higher the extent of intracellular starch hydrolysis. The microstructure changes in the cotyledon cells during digestion also confirmed this observation.

Molecular localization and exchange kinetics in pharmaceutical liposome and mRNA lipoplex nanoparticle products determined by small angle X-ray scattering and pulsed field gradient NMR diffusion measurements

We have used pulsed field gradient (PFG)-NMR diffusion experiments, also known as DOSY, in combination with small angle X-ray scattering measurements to investigate structure and molecular exchange dynamics between pharmaceutical lipid nanoparticles and the bulk phase. Using liposomes and lipoplexes formed after complexation of the liposomes with messenger mRNA as test systems, information on dynamics of encapsulated water molecules, lipids and excipients was obtained. The encapsulated fraction, having a diffusivity similar to that of the liposomes, could be clearly identified and quantified by the NMR diffusion measurements. The unilamellar liposome membranes allowed a fast exchange of water molecules, while sucrose, used as an osmolyte and model solute, showed very slow exchange. Upon interactions with mRNA a topological transition from a vesicular to a lamellar organization took place, where the mRNA was inserted in repeating lipid bilayer stacks. In the lipoplexes, a small fraction of tightly bound water molecules was present, with a diffusivity that was influenced by the additional presence of sucrose. This extended information on dynamic coherencies inside pharmaceutical nanoparticle products, provided by the combined application of SAXS and PFG-NMR diffusion measurements, can be valuable for evaluation of quality and comparability of nanoscaled pharmaceuticals.

Elastin-Like Peptide as a Model for Disordered Proteins: Diffusion Behaviour in Self-Crowding Conditions.

Despite the current high interest, there is limited information on diffusion data for intrinsically disordered proteins (IDPs). This study investigates the effect of crowding on the diffusion behaviour of an elastin-like peptide (ELP), by combined pulse field gradient (PFG) and static field gradient (SFG) NMR techniques. We interpret our findings in terms of highly dynamic chain assemblies with weak interactions, resulting in ELP diffusion that is primarily governed by the viscous flow of the solvent. The diffusion behaviour of the peptide appears to resemble that of globular proteins rather than flexible linear polymers over a wide concentration range.

Development of low-viscosity phosphonium-based ionic liquid electrolytes for lithium-ion batteries: Charge–discharge performance and ionic transport properties

Phosphonium cation-based ionic liquids (phosphonium ILs) have several advantages for electrochemical device applications compared with ammonium cation-based ionic liquids because of their unique features, such as higher ionic conductivity and a larger electrochemical potential window. In this study, we prepared various phosphonium ILs with different substituents (alkyl, alkyl ether, or allyl) on the cation, and bis(fluorosulfonyl)amide (FSA) or bis(trifluoromethanesulfonyl)amide (TFSA) as the anion. This study describes these phosphonium ILs used as electrolytes for lithium-ion batteries and investigates their performance as typical Li/LiCoO2 cells. We demonstrate that discharge capacity increases with increasing lithium-ion conductivity determined by electrochemical impedance and pulsed-field gradient nuclear magnetic resonance. A discharging rate test conducted in constant current (CC) mode also displays good high conductivity phosphonium IL electrolyte performance. Furthermore, the difference in cycle performance between FSA-based and TFSA-based phosphonium IL electrolytes is explained in terms of solid electrolyte interphase formation, similar to the performance of other cation-based ILs.

Application of pfg NMR diffusometry to study diffusion and ionization phenomena in plasticized polymeric membranes doped with ionic liquids

Ionic liquids (ILs) are increasingly used as promising organic solvents for a variety of applications, including electrochemical processes, novel separation techniques and innovative methods of synthesis, due to their low volatility, excellent solvation properties, high ionic conductivity and electrochemical stability. Moreover, the tunable properties of ILs determined by their composition make them favorable candidates as dopants for composite and/or functional polymeric materials, e.g. in the membranes of chemical sensors. To effectively use ILs in polymeric materials and control the characteristics of the latter, one should develop a complete understanding of the properties of ILs in the polymeric phase, such as their diffusion and ionic behavior.In our previous work, we applied pulsed field gradient (PFG) NMR technique to verify the results of a chronopotentiometric method conventionally used to determine the diffusion coefficients and electrolytic properties of electrolytes in plasticized poly(vinyl chloride) (PVC) membranes of chemical sensors. The results obtained with these two independent methods were in good agreement, confirming that PEG NMR diffusometry is a viable technique for assessing the diffusion properties of species even in highly viscous polymeric media of low polarity.Here we report a comprehensive study of the transport and ionic behavior of IL 1-hexyl-3-methyl-1H-imidazolium bis[(trifluoromethyl)sulfonyl]amide ([C6Meim][NTf2]) in a poly(vinyl chloride) matrix, performed in situ by means of PFG NMR diffusometry. A number of sets of spectra for the 1H and 19F nuclei of the [C6Meim][NTf2] ionic liquid in PVC membranes doped with the plasticizer bis(2-ethylhexyl)sebacate were registered at different amplitudes of the magnetic field gradient. The mobility of both individual ions of the IL and the formed ion associates was investigated over a broad range of IL concentrations in the PVC matrix; in addition, the ionization degree and dissociation constant of [C6Meim][NTf2] were determined and their dependence on the IL content was traced.

Understanding Ion Dynamics in Closoborate-Type Lithium-Ion Conductors on Different Time-Scales.

The lithium-ion transport mechanism in 0.7Li(CB9H10)-0.3Li(CB11H12) complex hydride solid electrolyte was studied over a wide time-scale (ns-ms) by choosing appropriate techniques for assessing ionic motion on the desired time-scale using nuclear magnetic resonance (NMR) relaxation, AC impedance, and pulsed field gradient-NMR (PFG-NMR) measurements. The 7Li NMR line width decreased with increasing temperature, and the spin-lattice relaxation time T1 for the cation and anions showed a minimum near 303 K, indicating that the lithium ions and the anions were highly mobile. The activation energy estimated from the analysis of the NMR relaxation time matched well with the values estimated from the AC impedance and PFG-NMR. This confirms that the lithium-ion motion in 0.7Li(CB9H10)-0.3Li(CB11H12) is the same over a wide time-scale, suggesting steady Li-ion motion over a wide transport range. This understanding offers insights into strategies for designing complex hydride lithium superionic conductors.

Apparent Anomalous Temperature Dependence of Self-Diffusion Studied by Pulsed-Field Gradient Nuclear Magnetic Resonance and Thermodynamic Modeling.

The self-diffusivity of cyclohexane and n-octane adsorbed in hierarchical zeolite monoliths has been investigated by using PFG-NMR. In these samples, the intrinsic FAU-X zeolite microporosity combines with a complex macroporous network composed of aggregated zeolite nanocrystals. As temperature is increased, cyclohexane self-diffusivity apparently decreases, reaches a minimum, and then starts increasing upon further increasing the temperature. Such striking, i.e., non-Arrhenius, temperature dependence is not observed for n-octane in the same samples and for cyclohexane adsorbed in purely microporous FAU-X. Through thermodynamic modeling, we show that this anomalous behavior can be rationalized by considering the evolution in the adsorbate populations when changing the temperature. In more detail, we show that the slow and fast diffusing species present in the microporosity and secondary porosity arising from the packing of zeolite nanocrystals vary significantly with a strong impact on the effective diffusivity. Applying the temperature evolution of their relative fractions to a simple two-phase diffusion model helps obtain insights into the physicochemical factors responsible for the complex behavior of effective self-diffusivity in hierarchical zeolites.

Charge Regulation at the Nanoscale as Evidenced from Light-Responsive Nanoemulsions.

Emulsions are indispensable in everyday life, and the demand for emulsions' diversity and control of properties is therefore substantial. As emulsions possess a high internal surface area, an understanding of the oil/water (o/w) interfaces at the molecular level is fundamental but often impaired by experimental limitations to probe emulsion interfaces in situ. Here, we have used light-responsive surfactants (butyl-AAP) that can photoisomerize between E and Z isomers by visible and UV light irradiation to tune the emulsion interfaces. This causes massive changes in the interface tension at the extended o/w interfaces in macroemulsions and a drastic shift in the surfactants' critical micelle concentration, which we show can be used to control both the stability and phase separation. Strikingly different from macroemulsions are nanoemulsions (RH ∼90 nm) as these are not susceptible to E/Z photoisomerization of the surfactants in terms of changes in their droplet size or ζ-potential. However, in situ second-harmonic scattering and pulsed-field gradient nuclear magnetic resonance (NMR) experiments show dramatic and reversible changes in the surface excess of surfactants at the nanoscopic interfaces. The apparent differences in ζ-potentials and surface excess provide evidence for a fixed charge to particle size ratio and the need for counterion condensation to renormalize the particle charge to a critical charge, which is markedly different compared to the behavior of very large particles in macroemulsions. Thus, our findings may have broader implications as the electrostatic stabilization of nanoparticles requires much lower surfactant concentrations, allowing for a more sustainable use of surfactants.

Nonstoichiometric Protic Ionic Liquids: The Role of Excess Acid in Charge Transport Mechanisms.

A study of charge transport mechanisms in an electric field was conducted on nonstoichiometric protic ionic liquids (PIL) based on triethylamine (TEA), in combination with an excess of either trifluoroacetic acid (TFA) or trifluoromethanesulfonic acid (TfO). The addition of excess precursor acid adds proton-donor sites to the system to support potential structural proton transport, which could, for example, enable the use in fuel cells. Transport measurements by pulsed field gradient (PFG) NMR diffusion and, in particular, electrophoretic NMR (eNMR) are supported by NMR chemical shifts and Raman spectroscopy, where the latter techniques elucidate the local solvation structures. Migration of the acidic proton of the excess acid in the electric field occurs toward the cathode with a velocity larger than that of the anions. This intriguing feature of a rapid drift of a neutral molecule is explained by the interplay of strong correlations between anion and cation as well as between anion and acid. The neutral acid is subject to vehicular transport with the anion, while the anion is partitioning between anion-acid and anion-cation clusters, resulting in a lower average drift velocity. The negative drift direction of the neutral acid and its proton is superimposed to and thus counteracts the vehicular transport of protons with the cation. The study sheds light on the role of excess acid in PIL and reveals the versatile interactions between anion, cation, and excess acid within a PIL determining its charge transport properties.

Complementary Mass Transport Investigations in Open-Cell Foams: Full-Field Computational Fluid Dynamics Simulation with Random-Walk Microscopic Particle Tracking and Methane Nuclear Magnetic Resonance Displacement Measurements

Numerical simulation can provide detailed understanding of mass transport within complex structures. For this purpose, numerical tools are required that can resolve the complex morphology and consider the contribution of both convection and diffusion. Solving the Navier–Stokes equations alone, however, neglects self-diffusion. This influences the simulated displacement distribution of flow especially in porous media at low Péclet numbers (Pe < 16) and in near-wall regions where diffusion is the dominant mechanism. To address this problem, this study uses μCT-based computational fluid dynamics (CFD) simulations in OpenFOAM coupled with the random-walk particle tracking (PT) module disTrackFoam and cross-validated experimentally using pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements of gas flow within open-cell foams (OCFs). The results of the multi-scale simulations—with a resolution of 130–190 µm—and experimental PFG NMR data are compared in terms of diffusion propagators, which are microscopic displacement distributions of gas flows in OCFs during certain observation times. Four different flow rates with Péclet numbers in the range of 0.7–16 are studied in the laminar flow regime within 10 and 20 PPI OCFs, and axial dispersion coefficients were calculated. Cross-validation of PFG NMR measurements and CFD-PT simulations revealed a very good matching with integral differences below 0.04%, underpinning the capability of both complementary methods for multi-scale transport analysis.

Exploring the effect of molecular size and framework functionalisation on transport in metal-organic frameworks using pulsed-field gradient nuclear magnetic resonance.

Molecular transport is an important aspect in metal-organic frameworks (MOFs) as it affects many of their applications, such as adsorption/separation, drug delivery and catalysis. Yet probing the fundamental diffusion mechanisms in MOFs is challenging, and the interplay between the MOF's features (such as the pore structure and linker dynamics) and molecular transport remains mostly unexplored. Here, the pulsed-field gradient nuclear magnetic resonance (PFG NMR) technique is used to probe the diffusion of several probe molecules, i.e., water, xylenes and 1,3,5-triisopropylbenzene (TIPB), within the UiO-66 MOF and its derivatives (UiO-66NH2 and UiO-66Br). Exploiting differences in the size of probe molecules we were able to probe the diffusion rate selectively in the different pore environments of the MOFs. In particular, when relatively small molecules, such as water and small hydrocarbons, were used as probes, the PFG NMR log attenuation plots were non-linear with two distinctive diffusion regions, suggesting faster diffusion in the inter-crystalline space and slower diffusion within crystal aggregates, the latter occurring mostly inside the framework of the MOFs. Conversely, experiments with a larger probe molecule, i.e., TIPB, with a kinetic diameter of 0.95 nm, which makes it unable to access the framework windows of the MOF crystals, showed linear PFG NMR log attenuation plots, which indicates diffusion occurring in a single environment, most likely in the inter-crystalline space. Analysis of the apparent tortuosity values of the systems under investigation highlights the role of linker functionalisation in influencing the molecular diffusion of the probe molecules, which affects both intra-molecular interactions and pore accessibility within the MOF crystals. The findings of this work demonstrate that the diffusion behaviour of probe molecules within MOFs is influenced by the pore size, structure, functionalisation of the MOF linker and molecular interactions. Our study contributes to further advance the understanding of mass transport in MOFs by PFG NMR and provides insights that can inform the design and optimisation of MOF-based materials for various applications.

Catalytic Cracking of 1,3,5‐Triisopropylbenzene and Low‐Density Polyethylene over Hierarchical Y Zeolites and Al‐SBA‐15

AbstractCatalytic cracking of high molecular weight hydrocarbons underpins the production of fossil fuels from petroleum vapour and the recycling of polyolefin waste plastic. However, thermal cracking over conventional microporous solid acids is hindered by poor mass‐transport. Here we explore the performance of hierarchical H−Y zeolites and Al‐SBA‐15 for the catalytic cracking of 1,3,5‐triisopropylbenzene (1,3,5‐TIPB) and low‐density polyethylene (LDPE) in a continuous fixed‐bed flow reactor. Dealumination by acid washing was used to create hierarchical mesoporosity in H−Y zeolite and modify the solid acidity. Physicochemical properties were studied by X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), gas adsorption, in‐situ Fourier transform infrared (FTIR), ex‐situ pyridine DRIFT, 29Si and 27Al nuclear magnetic resonance (NMR), and 1H relaxation and pulsed field gradient (PFG) NMR diffusion studies. Despite weakening acidity, the introduction of hierarchical porosity promotes deep cracking of both feedstocks; HNO3 dealuminated H−Y produces five times more cumene and benzene from 1,3‐5‐TIPB, and 33 % more benzene and xylenes from LDPE, than the parent H−Y.

Structural and Dynamical Insights of Cu-Coordinated Cellulose Nanofibers for Li+ Battery by NMR Spectroscopy

In the quest of novel sustainable research to meet the global surge of clean energy demand, cellulose nanofibers (CNF) derived from natural wood materials offer exceptional properties for safe, cost-effective, efficient Li+ batteries. A new strategy has been established via introducing Cu2+ into the CNF to achieve a greater mobility of Li+, where Cu2+ opens up one-dimensional nanofibrils and creates molecular channels.1 Various experimental methods and molecular dynamic simulations have been used to characterize these materials. In this context, Nuclear Magnetic Resonance (NMR), a highly sensitive technique plays a significant role in providing new insights into the structure and dynamics of Li-Cu-CNF materials. In collaboration with Prof. Liangbing Hu, University of Maryland, current research focusses on studying highly conductive Li-Cu-CNF materials prepared with different Li-salt via different synthetic approaches in the ARPA-E project. Fast magic angle spinning (MAS) detecting 1H helps in estimating the residual solvent/H2O present in Li-Cu-CNF materials, which is crucial to determine the Li+ transport mechanism. Pulsed Field Gradient (PFG) NMR has been used to measure the diffusivity of Li+ and anion and to estimate the Li+ transference number (t Li+). We found that the estimated t Li+ of studied Li-Cu-CNF materials is much greater as compared to the other polymer-based electrolytes. Additionally, the anisotropic diffusion of Li+ and anion in aligned cellulose wood materials have been well-established through PFG NMR. Lastly, the local structure of cellulose fibers and their spatial interactions with ionic species are probed through multi-dimensional (1D and 2D) solid state NMR detecting 1H, 7Li, 19F, 13C nuclei. Reference. Yang, Q. Wu, W. Xie, X. Zhang, A. Brozena, J. Zheng, M. N. Garaga, B. H. Ko, Y. Mao, S. He, Y. Gao, P. Wang, M. Tyagi, F. Jiao, R. Briber, P. Albertus, C. Wang, S. Greenbaum, Y.-Y. Hu, A. Isogai, M. Winter, K. Xu, Y. Qi, and L. Hu, Copper-coordinated cellulose ion conductors for solid-state batteries. Nature (2021) 598, 590–596.

Using NMR diffusion data to validate MD models of disordered proteins: Test case of N-terminal tail of histone H4

MD simulations can provide uniquely detailed models of intrinsically disordered proteins (IDPs). However, these models need careful experimental validation. The coefficient of translational diffusion Dtr, measurable by pulsed field gradient NMR, offers a potentially useful piece of experimental information related to the compactness of the IDP’s conformational ensemble. Here, we investigate, both experimentally and via the MD modeling, the translational diffusion of a 25-residue N-terminal fragment from histone H4 (N-H4). We found that the predicted values of Dtr, as obtained from mean-square displacement of the peptide in the MD simulations, are largely determined by the viscosity of the MD water (which has been reinvestigated as a part of our study). Beyond that, our analysis of the diffusion data indicates that MD simulations of N-H4 in the TIP4P-Ew water give rise to an overly compact conformational ensemble for this peptide. In contrast, TIP4P-D and OPC simulations produce the ensembles that are consistent with the experimental Dtr result. These observations are supported by the analyses of the 15N spin relaxation rates. We also tested a number of empirical methods to predict Dtr based on IDP’s coordinates extracted from the MD snapshots. In particular, we show that the popular approach involving the program HYDROPRO can produce misleading results. This happens because HYDROPRO is not intended to predict the diffusion properties of highly flexible biopolymers such as IDPs. Likewise, recent empirical schemes that exploit the relationship between the small-angle x-ray scattering-informed conformational ensembles of IDPs and the respective experimental Dtr values also prove to be problematic. In this sense, the first-principle calculations of Dtr from the MD simulations, such as demonstrated in this work, should provide a useful benchmark for future efforts in this area.