Increasing ¹³C CP-MAS NMR Resolution Using Single Crystals: Application to Model Octaethyl Porphyrins

Octaethyl porphyrin (OEP) and its Ni and Zn derivatives are considered as model compounds in biochemical, photophysical, and fossil fuel chemistry. They have thus been investigated by high-resolution solid-state ¹³C NMR using powders, but peak assignment has been difficult because of large line widths. Arguing that a significant cause of broadening might be the anisotropic bulk magnetic susceptibility, we utilized single crystals in our ¹³C cross-polarization magic angle spinning (CP-MAS) measurements and observed a nearly 2-fold line narrowing. This enhanced resolution enabled us to assign chemical shifts to each carbon for all the three compounds. The new assignments are now in agreement with X-ray structural data and allowed us to probe the motional dynamics of the methyl and methylene carbons of the OEP side chains. It is apparent that the use of single crystals in ¹³C CP-MAS measurements has a significantly wider impact than previously thought

Probing Hydronium Ion Histidine NH Exchange Rate Constants in the M2 Channel via Indirect Observation of Dipolar-Dephased ¹⁵N Signals in Magic-Angle-Spinning NMR

Water–protein chemical exchange in membrane-bound proteins is an important parameter for understanding how proteins interact with their aqueous environment, but has been difficult to observe in membrane-bound biological systems. Here, we demonstrate the feasibility of probing specific water–protein chemical exchange in membrane-bound proteins in solid-state MAS NMR. By spin-locking the ¹H magnetization along the magic angle, the ¹H spin diffusion is suppressed such that a water–protein chemical exchange process can be monitored indirectly by dipolar-dephased ¹⁵N signals through polarization transfer from ¹H. In the example of the Influenza A full length M2 protein, the buildup of dipolar-dephased ¹⁵N signals from the tetrad of His37 side chains have been observed as a function of spin-lock time. This confirms that hydronium ions are in exchange with protons in the His37 NH bonds at the heart of the M2 proton conduction mechanism, with an exchange rate constant of ∼1750 s^–1 for pH 6.2 at −10 °C

Long- and Local-Range Structural Changes in Flexible Titanium Silicates with Variable Faulting upon Thermal Treatment and Corresponding Adsorption and Particle Size Polydispersity-Corrected Diffusion Parameters for CO₂/CH₄ Separation

Sr²⁺-UPRM-5 is a titanosilicate containing adjustable structural faulting that prescribes changes in textural properties with temperature. In this work, we studied thermally induced structural changes in Sr²⁺-UPRM-5 variants prepared using tetrapropylammonium (TPA⁺) and tetrabutylammonium (TBA⁺) and their correlation to the diffusion of CO₂ and CH₄ at 25 °C. Both Sr²⁺-UPRM-5 materials contained different amounts of structural faulting that are correlated to the formation of 12-MR pores. In situ high-temperature X-ray diffraction revealed structural changes corresponding to orthorhombic phases up to 300 °C. Analysis of in situ high-temperature ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed new silicon environments surrounding the archetypical Si­(2Si, 2Ti_oct) and Si­(3Si, 1Ti_semioct) coordination centers. MAS NMR data analysis indicated that the Si environment in Sr²⁺-UPRM-5 (TPA) appears to be more susceptible to changes upon thermal treatment. A phenomenological volumetric transport model corrected for particle size polydispersity was used to estimate diffusion constants at 25 °C in adsorbents preactivated at different temperatures. At the optimal conditions, the CO₂/CH₄ kinetic selectivities were 41 and 30 for Sr²⁺-UPRM-5 (TBA) and (TPA), respectively

Determination of the Apparent Crystal Structure of a Highly Faulted UPRM‑5 Type Flexible Porous Titanium Silicate via a Polymorph Based Superposition Model, a Rietveld Refinement and a Pair Distribution Function

The crystal structure of a UPRM-5 titanium silicate prepared using tetraethylammonium (TEA⁺) has been approximated using a superposition model product of a polymorph stacking and a dual-phase Rietveld refinement method. Na⁺-UPRM-5 polymorphs were employed to elucidate the level of polymorphism or faulting in the crystal structure. DIFFaX simulations revealed that it was impossible to match the experimental diffraction data based solely on “pure” polymorphs. Instead, an intergrowth of combinations of polymorphs in the <i>a</i> and <i>c</i> directions resulted in the best faulting simulation scenario. The most suitable model combined two (2) orthorhombic polymorphs with faulting of 90 and 10% in the <i>a</i> and <i>c</i> directions, respectively. A refinement using this model did not yield a reliable structure, but an approximation was possible after employing a combination of orthorhombic and faulted triclinic phases. The superposition model, however, was not able to predict the final configuration of the TiO₅ plausibly due to the unprecedented level of faulting in the structure. Upon convergence (χ² = 13.68), the triclinic phase accounted for ca. 14% (molar basis) of the overall phase, being this further evidence of the level of faulting present in UPRM-5. The refined structure also revealed Si–O and Ti–O distances and angles that contrast with those reported for a titanium silicate known as ETS-4, and related to structural distortion. These changes are plausibly attributed to the presence of the TEA⁺ cations and the strong interaction of the framework oxygen with sodium cations, which were also exposed to the 8MR pore channel as described by a pair distribution function (PDF) refinement. In general, the UPRM-5 structural features appear to commensurate well with the gas adsorption and thermal stability properties previously reported, which differ considerably from those exhibited by Zorite type titanium silicates prepared in the absence of a quaternary ammonium cation

Evidence from 900 MHz ¹H MAS NMR of Displacive Behavior of the Model Order–Disorder Antiferroelectric NH₄H₂AsO₄

NH₄H₂AsO₄ (ADA) is a model compound for understanding the mechanism of phase transitions in the KH₂PO₄ (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at <i>T</i>_N ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (¹H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, <i>T</i>₁, of the O–H···O and NH₄⁺ protons through the <i>T</i>_N. As <i>T</i> → <i>T</i>_N, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its <i>T</i>_N, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at <i>T</i>_N, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH₄⁺ protons. Both sets of protons exhibited order–disorder characteristics in their <i>T</i>₁ data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into <i>order–disorder</i> and <i>displacive</i> ones, should rather be “<i>order–disorder cum displacive</i>” type

Toward Understanding the Lithium Transport Mechanism in Garnet-type Solid Electrolytes: Li⁺ Ion Exchanges and Their Mobility at Octahedral/Tetrahedral Sites

The cubic garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ with aliovalent doping exhibits a high ionic conductivity, reaching up to ∼10^–3 S/cm at room temperature. Fully understanding the Li⁺ transport mechanism including Li⁺ mobility at different sites is a key topic in this field, and Li_{7–2<i>x</i>–3<i>y</i>}Al_<i>y</i>La₃Zr_2–<i>x</i>W_<i>x</i>O₁₂ (0 ≤ <i>x</i> ≤ 1) are selected as target electrolytes. X-ray and neutron diffraction as well as ac impedance results show that a low amount of aliovalent substitution of Zr with W does not obviously affect the crystal structure and the activation energy of Li⁺ ion jumping, but it does noticeably vary the distribution of Li⁺ ions, electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li⁺ ions. For the first time, high-resolution NMR results show evidence that the 24d, 96h, and 48g sites can be well-resolved. In addition, ionic exchange between the 24d and 96h sites is clearly observed, demonstrating a lithium transport route of 24d–96h–48g–96h–24d. The lithium mobility at the 24d sites is found to dominate the total ionic conductivity of the samples, with diffusion coefficients of 10^–9 m² s^–1 and 10^–12 m² s^–1 at the octahedral and tetrahedral sites, respectively

Gating Mechanism of Aquaporin Z in Synthetic Bilayers and Native Membranes Revealed by Solid-State NMR Spectroscopy

Aquaporin Z (AqpZ) is an integral membrane protein that facilitates transport of water across <i>Escherichia coli</i> cells with a high rate. Previously, R189, a highly conserved residue of the selective filter of AqpZ, was proposed as a gate within the water channel on the basis of the observation of both open and closed conformations of its side chain in different monomers of an X-ray structure, and the observation of rapid switches between the two conformations in molecular dynamic simulations. However, the gating mechanism of the R189 side chain remains controversial since it is unclear whether the different conformations observed in the X-ray structure is due to different functional states or is a result of perturbation of non-native detergent environments. Herein, in native-like synthetic bilayers and native <i>E. coli</i> membranes, a number of solid-state NMR techniques are employed to examine gating mechanism of the R189 side chain of AqpZ. One R189 side-chain conformation is highly evident since only a set of peaks corresponding to the R189 side chain is observed in 2D ¹⁵N–¹³C spectra. The immobility of the R189 side chain is detected by ¹H–¹⁵N dipolar lineshapes, excluding the possibility of the rapid switches between the two side-chain conformations. High-resolution monomeric structure of AqpZ, determined by CS-Rosetta calculations using experimentally measured distance restraints related to the R189 side chain, reveals that this side chain is in an open conformation, which is further verified by its water accessibility. All the solid-state NMR experimental results, combining with water permeability essay, suggest a permanently open conformation of the R189 side chain in the synthetic bilayer and native membranes. This study provides new structural insights into the gating mechanism of aquaporins and highlights the significance of lipid bilayer environments in elucidating the molecular mechanism of membrane proteins

Exploring Highly Reversible 1.5-Electron Reactions (V³⁺/V⁴⁺/V⁵⁺) in Na₃VCr(PO₄)₃ Cathode for Sodium-Ion Batteries

The development of highly reversible multielectron reaction per redox center in sodium super ionic conductor-structured cathode materials is desired to improve the energy density of sodium-ion batteries. Here, we investigated more than one-electron storage of Na in Na₃VCr­(PO₄)₃. Combining a series of advanced characterization techniques such as ex situ ⁵¹V solid-state nuclear magnetic resonance, X-ray absorption near-edge structure, and in situ X-ray diffraction, we reveal that V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples in the materials can be accessed, leading to a 1.5-electron reaction. It is also found that a light change on the local electronic and structural states or phase change could be observed after the first cycle, resulting in the fast capacity fade at room temperature. We also showed that the irreversibility of the phase changes could be largely suppressed at low temperature, thus leading to a much improved electrochemical performance

Spherical Nanoparticle Supported Lipid Bilayers for the Structural Study of Membrane Geometry-Sensitive Molecules

Many essential cellular processes including endocytosis and vesicle trafficking require alteration of membrane geometry. These changes are usually mediated by proteins that can sense and/or induce membrane curvature. Using spherical nanoparticle supported lipid bilayers (SSLBs), we characterize how SpoVM, a bacterial development factor, interacts with differently curved membranes by magic angle spinning solid-state NMR. Our results demonstrate that SSLBs are an effective system for structural and topological studies of membrane geometry-sensitive molecules

Copper Phosphate as a Cathode Material for Rechargeable Li Batteries and Its Electrochemical Reaction Mechanism

In the search for new cathode materials for rechargeable lithium batteries, conversion-type materials have great potential because of their ability to achieve high specific capacities via the full utilization of transition metal oxidation states. Here, we report for the first time that copper phosphate can be used as a novel high-capacity cathode for rechargeable Li batteries, capable of delivering a reversible capacity of 360 mAh/g with two discharge plateaus of 2.7 and 2.1 V at 400 mA/g. The underlying reaction involves the formation as well as the oxidation of metallic Cu. The solid-state NMR, <i>in situ</i> XAFS, HR-TEM, and XRD results clearly indicate that Cu can react with Li₃PO₄ to form copper phosphate and Li_<i>x</i>Cu_<i>y</i>PO₄ during the charging process, largely determining the reversibility of Cu₃(PO₄)₂. This new reaction scheme provides a new venue to explore polyanion-type compounds as high-capacity cathode materials with conversion reaction processes