Solid-state NMR spectroscopy

Revealing the structure-activity relationship of Pt1/CeO2 with 17O solid-state NMR spectroscopy and DFT calculations

Single-atom catalysts (SACs) have attracted significant interest due to their exceptional and tunable performance, enabled by diverse coordination environments achieved through innovative synthetic strategies. However, various local structures of active sites pose significant challenges for precise characterization, a prerequisite for developing structure-activity relationships. Here, we combine 17O solid-state NMR spectroscopy and DFT calculations to elucidate the detailed structural information of Pt/CeO2 SACs and their catalytic behaviors. The NMR data reveal that single Pt atoms, dispersed from clusters with water vapor, exhibit a square planar geometry embedded in CeO2 (111) surface, distinct from the original clusters and other conventionally generated Pt single atoms. The square planar Pt/CeO2 SAC demonstrates improved CO oxidation performance compared to Pt/CeO2 SAC with octahedral coordination, due to moderately strong CO adsorption and low energy barriers. This approach can be extended to other oxide-supported SACs, enabling spatially resolved characterization and offering comprehensive insights into their structure-activity relationships.

Direct Detection of Natural-Abundance Low-γ Nuclei NMR Signals of Minute Quantities of Organic Solids

Low-γ nuclei signal enhancement in solid-state NMR spectroscopy is typically achieved via cross-polarization (CP) using abundant 1H polarization in organic solids. Nevertheless, direct low-γ nuclei signal detection via a single CP process is quite challenging with minute quantities of samples due to the extremely limited signal-to-noise ratio (SNR) of the acquired spectra. Herein, we demonstrated the robust performance of a multiple-contact CP experiment with multiple acquisition periods (MCP) in each transient scan, leading to several-fold SNR enhancement over a conventional single-CP experiment at fast MAS conditions with slightly increased experimental time. Spin thermodynamic analysis was further performed to achieve maximum SNR by adding the obtained Nmax CP spectra from each transient, where Nmax ∼ T1ρ/τcw. Here, T1ρ is the proton spin-lattice relaxation time in the rotating frame, and τcw is the total time of CP and a heteronuclear decoupling period. The theoretical analysis is in good agreement with experimental results, and more than 4.5-fold SNR enhancement can be achieved for the pharmaceutical danazol/vanillin cocrystals. Besides, MCP was also used for proton T1 and T1ρ measurement with high-resolution 13C detection, where both proton T1 and T1ρ can serve as the spectral-editing basis to identify different immiscible components in complex molecular systems.

NASICON-NaV0.25Al0.25Nb1.5(PO4)3/C: A High-Rate and Robust Anode for Fast Charging and Long-Life Sodium-Ion Batteries

Nb-based NAtrium Super Ionic CONductor (NASICON) frameworks (e.g., Nb2(PO4)3 and Na1.5V0.5Nb1.5(PO4)3) are emerging as the attractive Na-ion anodes due to their lower intercalation voltage (≈1.4-1.2 V vs Na+/Na0) and higher storage capacities (≈140-160 mAh g-1). However, their practical realization is limited by moderate cycle life and rate performances. In this work, a carbon-coated NASICON-NaV0.25Al0.25Nb1.5(PO4)3 (NaVAlNb/C) anode is unveiled for fast charging Na-ion batteries. The incorporation of Al3+ increases covalency of NASICON and creates disordered Na-ion sublattice as observed by X-ray diffraction and nuclear magnetic resonance spectroscopy measurements. Besides, the carbon-coating and particle downsizing produces facile electron and ion percolation network. Accordingly, the NaVAlNb/C anode renders extraordinary rate performances (80 mAh g-1 at 20C) with longer cycling stability (95.2% retention after 5000 cycles at 10C). Climbing image nudged elastic band calculations reveal reduced Na-ion migration barrier (202 meV) for NaVAlNb/C. Most importantly, a full Na-ion cell based on Na4V2(PO4)3 cathode and NaVAlNb/C anode is demonstrated with a high-power density (6493 W kg-1) and long-cycle life (3000 cycles at 20C), which are far excellent compared to the state-of-the-art NASICON-based cells. This work demonstrates the significance of carbon coating and chemical tuning to tailor high-rate NASICON anodes, which can produce fast-charging Na-ion batteries.

Strain-induced rubidium incorporation into wide-bandgap perovskites reduces photovoltage loss

A-site cation mixing can enhance the photovoltaic performance of a wide-bandgap (WBG) perovskite, but rubidium (Rb) cation mixing generally forms a nonperovskite phase. We report that lattice strain locks Rb ions into the α-phase of the lattice of a triple-halide WBG perovskite, preventing phase segregation into a nonperovskite Rb-cesium-rich phase. This process cooperates with chloride accommodation and promotes halide homogenization across the entire film volume. The resulting 1.67-electron volt WBG perovskite exhibits photoluminescence quantum yields exceeding 14% under 1-sun-equivalent irradiation, corresponding to a quasi-Fermi level splitting of ~1.34 electron volts. A WBG perovskite solar cell with an open-circuit voltage (VOC) of 1.30 volts was prepared, corresponding to 93.5% of the radiative VOC limit and representing the lowest photovoltage loss relative to the theoretical limit observed in WBG perovskites.

Optimal control-based nuclear spin cross-polarization in the presence of complicating anisotropic interactions

Cross-polarization is an indispensable part of solid state nuclear magnetic resonance spectroscopy to enhance sensitivity and extract structural information. However, the presence of certain anisotropic interactions, including chemical shift anisotropy and quadrupolar coupling, makes the inter-nuclear spin correlation experiments challenging. This impedes characterization of numerous materials and pharmaceutical compounds containing isotopes, such as 19F with large chemical shift anisotropy and 6/7Li, 23Na, 27Al, etc., with quadrupolar coupling. To address this problem, we introduce a new optimal control simulation-generated pulse sequence for Optimal Polarization Transfer In the presence of Anisotropic Nuclear Spin interactions (OPTIANS). Numerical simulations show high efficiency and robustness against experimental imperfections under a broad range of anisotropic interaction strengths for 19F-7Li, 19F-23Na, 19F-27Al, and 19F-13C polarization transfers. The polarization transfer curves show transient oscillations, which make the pulse sequence a quantitative method for dipolar coupling measurements. Experiments on a multi-metal fluoride system validate the predictions of the simulations by showing efficient PT in three spin pairs at varying experimental conditions. Remarkably, this method shows 50% better 19F-7Li PT efficiency at 14.1 T compared to the ramped cross-polarization experiment. The underlying polarization transfer mechanism is analyzed using the Fourier transform of the polarization transfer curves revealing that this optimal control method utilizes the chemical shift anisotropy and quadrupolar coupling to facilitate robust and efficient cross-polarization.

Deuteron-proton isotope correlation spectroscopy at high magnetic fields

A cross-polarization 2H-1H isotope correlation spectroscopy (CP-iCOSY) approach is presented for characterizing a deuterated amino acid, pharmaceutical compound and a solid formulation. This can be achieved by isotopic enrichment in conjunction with high magnetic field (28.2 T) and fast magic-angle spinning (MAS), enabling the rapid detection of 2H NMR spectra in a few seconds to minutes. Specifically, two-dimensional (2D) 2H-1H CP-iCOSY experiment allows the local structures and through-space interactions in a partially deuterated compounds to be elucidated. In doing so, we compare conventional spin-lock and rotor-echo-short-pulse-irradiation RESPIRATIONCP sequences for acquiring 2D 1H-2H correlation spectra. The RESPIRATIONCP sequence allows the detection of 2D peaks at lower CP contact times (0.1-1 ms) than the conventional CP (0.2-4 ms) sequence. Analysis of partially deuterated L-histidine·HCl·H2O and dopamine.HCl is presented, in which the detection of 2D peaks corresponding to 2H-1H pairs separated by greater than 4 Å distance demonstrates the potential of the presented approach for the characterization of packing interactions. These results are corroborated by NMR crystallography analysis using the Gauge-Including Projector Augmented-Wave (GIPAW) approach.

Capturing the Conformational Heterogeneity of HSPB1 Chaperone Oligomers at Atomic Resolution

Small heat shock proteins (sHSPs), including HSPB1, are essential regulators of cellular proteostasis that interact with unfolded and partially folded proteins to prevent aberrant misfolding and aggregation. These proteins fulfill a similar role in biological condensates, where they interact with intrinsically disordered proteins to modulate their liquid-liquid and liquid-to-solid phase transitions. Characterizing the sHSP structure, dynamics, and client interactions is challenging due to their partially disordered nature, their tendency to form polydisperse oligomers, and their diverse range of clients. In this work, we leverage various biophysical methods, including fast 1H-based magic angle spinning (MAS) NMR spectroscopy, molecular dynamics (MD) simulations, and modeling, to shed new light on the structure and dynamics of HSPB1 oligomers. Using split-intein-mediated segmental labeling, we provide unambiguous evidence that in the oligomer context, the N-terminal domain (NTD) of HSPB1 is rigid and adopts an ensemble of heterogeneous conformations, the α-Crystallin domain (ACD) forms dimers and experiences multiple distinct local environments, while the C-terminal domain (CTD) remains highly dynamic. Our computational models suggest that the NTDs participate in extensive NTD-NTD and NTD-ACD interactions and are sequestered within the oligomer interior. We further demonstrate that HSPB1 higher order oligomers disassemble into smaller oligomeric species in the presence of a client protein and that an accessible NTD is essential for HSPB1 partitioning into condensates and interactions with client proteins. Our integrated approach provides a high-resolution view of the complex oligomeric landscape of HSPB1 and sheds light on the elusive network of interactions that underlies the function of HSPB1 in biological condensates.

From Ab Initio to Instrumentation: A Field Guide to Characterizing Multivalent Liquid Electrolytes

In this field guide, we outline empirical and theory-based approaches to characterize the fundamental properties of liquid multivalent-ion battery electrolytes, including (i) structure and chemistry, (ii) transport, and (iii) electrochemical properties. When detailed molecular-scale understanding of the multivalent electrolyte behavior is insufficient we use examples from well-studied lithium-ion electrolytes. In recognition that coupling empirical and theory-based techniques is highly effective, but often nontrivial, we also highlight recent electrolyte characterization efforts that uncover a more comprehensive and nuanced understanding of the underlying structures, processes, and reactions that drive performance and system-level behavior. We hope the insights from these discussions will guide the design of future electrolyte studies, accelerate development of next-generation multivalent-ion batteries through coupling of modeling with experiments, and help to avoid pitfalls and ensure reproducibility of modeling results.

New insights into bioactive Ga(III) hydroxyquinolinate complexes from UV-vis, fluorescence and multinuclear high-field NMR studies

There is current interest in the anticancer and antimicrobial activities of Ga(III) tris-hydroxyquinolinate complexes, and hence their solution and solid-state chemistry. Here, we have studied the formation, stability and structure of a novel tris-5,7-dibromo-8-hydroxyquinolinate Ga(III) complex [Ga(Br2-HQ)3]. Reactions of 5,7-dibromo-8-hydroxyquinoline with Ga(NO)3 in DMSO were followed using electronic absorption and emission spectroscopy, and revealed the slow but concerted coordination of three chelated ligands, with ligand deprotonation being the apparent rate-limiting step, facilitated by basic Ga(III) hydroxido species. The emissive excited state of [Ga(Br2-HQ)3] in DMSO had a short half-life of 1.2 ns, and the fluorescence (550 nm, λex = 400 nm) was characterized by TDDFT calculations as arising from a ligand-centred singlet S1 state. We compared the structures of [Ga(Br2-HQ)3] and the clinical tris-hydroxyquinolinate complex [Ga(HQ)3] using high-field magic-angle-spinning solid-state 1D and 2D 850 MHz and 1 GHz 1H, 13C and 71Ga NMR spectroscopy. The similarity of their coordination spheres was confirmed by their 71Ga chemical shifts of 101 and 98 ppm, respectively, and quadrupolar coupling constants of 9.265 MHz and 9.282 MHz. 1H-1H 2D NOESY experiments revealed second coordination sphere interactions between an acetic acid solvent molecule and the bound hydroxyquinolinate ligands of [Ga(HQ)3]·0.5CH3CO2H. This finding suggests that carboxylic acids could play a role in modifying the formulation properties of this drug for clinical use.

Proton-Detected Solid-State NMR for Deciphering Structural Polymorphism and Dynamic Heterogeneity of Cellular Carbohydrates in Pathogenic Fungi

Carbohydrate polymers in their cellular context display highly polymorphic structures and dynamics essential to their diverse functions, yet they are challenging to analyze biochemically. Proton-detection solid-state NMR spectroscopy offers high isotopic abundance and sensitivity, enabling rapid and high-resolution structural characterization of biomolecules. Here, an array of 2D/3D 1 H-detection solid-state NMR techniques are tailored to investigate polysaccharides in fully protonated or partially deuterated cells of three prevalent pathogenic fungi: Rhizopus delemar , Aspergillus fumigatus , and Candida albicans , representing filamentous species and yeast forms. Selective detection of acetylated carbohydrates reveals fifteen forms of N-acetylglucosamine units in R. delemar chitin, which coexists with chitosan as separate domains or polymers and associates with proteins only at limited sites. This is supported by distinct order parameters and effective correlation times of their motions, analyzed through relaxation measurements and model-free analysis. Five forms of α-1,3-glucan with distinct structural origins and dynamics were identified in A. fumigatus , important for this buffering polysaccharide to perform diverse roles of supporting wall mechanics and regenerating soft matrix under antifungal stress. Eight α-1,2-mannan sidechain variants in C. albicans were resolved, highlighting the crucial role of mannan sidechains in maintaining interactions with other cell wall polymers to preserve structural integrity. These methodologies provide novel insights into the functional structures of key fungal polysaccharides and create new opportunities for exploring carbohydrate biosynthesis and modifications across diverse organisms.

Verdazyl-Based Radicals for High-Field Dynamic Nuclear Polarization NMR

High-field dynamic nuclear polarization nuclear magnetic resonance (DNP NMR) spectroscopy transfers polarization from unpaired electrons in polarizing agents to nuclei of interest to boost NMR sensitivity. Verdazyl biradicals are a promising choice as polarizing agents because they have been found to generate narrower electron paramagnetic resonance (EPR) signals compared to nitroxide biradicals; an advantageous characteristic for high-field DNP when operating above 400 MHz/263 GHz. The use of verdazyl radicals as DNP polarizing agents has been very limited to date, yet, recent numerical simulations have predicted that verdazyl-nitroxide hybrid biradicals could be more effective polarizing agents than nitroxide-nitroxide biradicals. Herein, the syntheses of a series of verdazyl mono- and biradicals, as well as verdazyl-nitroxide biradicals are described. These radicals were examined in high-field DNP NMR experiments (600 MHz/395 GHz), by measuring 1H signal enhancements directly and through 13C{1H} cross-polarization experiments. X-band EPR, 1H DNP field profiles, and experiments to determine the nuclear build-up times were performed for verdazyl-nitroxide biradicals VerTEMPol and VerTEKol. These hybrid biradicals provide enhancements of up to 100-fold increased signal intensities (i.e., representing >104-fold time savings), approximately four times higher than that of the nitroxide biradical TEKPol, a commonly used polarizing agent in the field.

Robust Heteronuclear Correlations for Sub-milligram Protein in Ultrafast Magic-Angle Spinning Solid-State NMR

Proton-detected solid-state nuclear magnetic resonance (ssNMR) under ultrafast magic-angle spinning (MAS) has become a powerful tool for elucidating the structures of proteins with sub-milligram quantities, where establishing 13C-15N correlations is essential. However, traditional 13C-15N cross-polarization (CP), effective at lower MAS frequencies, suffers diminished efficiency under ultrafast MAS conditions. To overcome this limitation, we developed a robust method for selective polarization between insensitive nuclei (SPINE). This approach significantly enhances the heteronuclear 13C-15N correlation efficiency over CP, with gain factors of 1.75 for 13CA-15N and 1.9 and 13CO-15N transfers. SPINE's efficacy was validated on four diverse proteins: the microcrystalline β1 immunoglobulin binding domain of protein G (GB1), the large-conductance mechanosensitive ion channel from Methanosarcina acetivorans (MaMscL), fibrillar septum-forming protein (SepF), and the vertex protein of the β-carboxysome shell (CcmL). This enhancement can reduce the duration of current multidimensional experiments to about one-third of that using a single 13C-15N CP and to about one-tenth with dual 13C-15N transfers. Our findings underscore the practical utility and versatility of SPINE in ssNMR spectroscopy, making it a valuable approach for advancing structural biology studies of sub-milligram protein.

Electrochemical Random-Access Memory: Progress, Perspectives, and Opportunities

Non-von Neumann computing using neuromorphic systems based on analogue synaptic and neuronal elements has emerged as a potential solution to tackle the growing need for more efficient data processing, but progress toward practical systems has been stymied due to a lack of materials and devices with the appropriate attributes. Recently, solid state electrochemical ion-insertion, also known as electrochemical random access memory (ECRAM) has emerged as a promising approach to realize the needed device characteristics. ECRAM is a three terminal device that operates by tuning electronic conductance in functional materials through solid-state electrochemical redox reactions. This mechanism can be considered as a gate-controlled bulk modulation of dopants and/or phases in the channel. Early work demonstrating that ECRAM can achieve nearly ideal analogue synaptic characteristics has sparked tremendous interest in this approach. More recently, the realization that electrochemical ion insertion can be used to tune the electronic properties of many types of materials including transition metal oxides, layered two-dimensional materials, organic and coordination polymers, and that the changes in conductance can span orders of magnitude has further attracted interest in ECRAM as the basis for analogue synaptic elements for inference accelerators as well as for dynamical devices that can emulate a wide range of neuronal characteristics for implementation in analogue spiking neural networks. At its core, ECRAM shares many fundamental aspects with rechargeable batteries, where ion insertion materials are used extensively for their ability to reversibly store charge and energy. Computing applications, however, present drastically different requirements: systems will require many millions of devices, scaled down to tens of nanometers, all while achieving reliable electronic-state tuning at scaled-up rates and endurances, and with minimal energy dissipation and noise. In this review, we discuss the history, basic concepts, recent progress, as well as the challenges and opportunities for different types of ECRAM, broadly grouped by their primary mobile ionic charge carrier, including Li, protons, and oxygen vacancies.

Characterizing Electrode Materials and Interfaces in Solid-State Batteries

Solid-state batteries (SSBs) could offer improved energy density and safety, but the evolution and degradation of electrode materials and interfaces within SSBs are distinct from conventional batteries with liquid electrolytes and represent a barrier to performance improvement. Over the past decade, a variety of imaging, scattering, and spectroscopic characterization methods has been developed or used for characterizing the unique aspects of materials in SSBs. These characterization efforts have yielded new understanding of the behavior of lithium metal anodes, alloy anodes, composite cathodes, and the interfaces of these various electrode materials with solid-state electrolytes (SSEs). This review provides a comprehensive overview of the characterization methods and strategies applied to SSBs, and it presents the mechanistic understanding of SSB materials and interfaces that has been derived from these methods. This knowledge has been critical for advancing SSB technology and will continue to guide the engineering of materials and interfaces toward practical performance.

Diffuse scattering from correlated electron systems

The role of inhomegeneity in determining the properties of correlated electron systems is poorly understood because of the dearth of structural probes of disorder at the nanoscale. Advances in both neutron and x-ray scattering instrumentation now allow comprehensive measurements of diffuse scattering in single crystals over large volumes of reciprocal space, enabling structural correlations to be characterized over a range of length scales from 5 to 200 angstroms or more. When combined with new analysis tools, such as three-dimensional difference pair-distribution functions, these advanced capabilities have produced fresh insights into the interplay of structural fluctuations and electronic properties in a broad range of correlated electron materials. This review describes recent investigations that have demonstrated the importance of understanding structural inhomogeneity pertaining to phenomena as diverse as superconductivity, charge density wave modulations, metal-insulator transitions, and multipolar interactions.

Tailoring Ion Transport in Li3-3yHo1+yCl6-xBrx via Transition-Metal Free Structural Planes and Charge Carrier Distribution

Localized atomistic disorder in halide-based solid electrolytes (SEs) can be leveraged to boost Li+ mobility. In this study, Li+ transport in structurally modified Li3HoCl6, via Br- introduction and Li+ deficiency, is explored. The optimized Li3-3 yHo1+ yCl6- xBrx achieves an ionic conductivity of 3.8 mS cm-1 at 25 °C, the highest reported for holmium halide materials. 6,7Li nuclear magnetic resonance and relaxometry investigations unveil enhanced ion dynamics with bromination, attaining a Li+ motional rate neighboring 116 MHz. X-ray diffraction analyses reveal mixed-anion-induced phase transitions with disproportionate octahedral expansions and distortions, creating Ho-free planes with favorable energetics for Li+ migration. Bond valence site energy analysis highlights preferred Li+ transport pathways, particularly in structural planes devoid of Ho3+ blocking effects. Molecular dynamics simulations corroborate enhanced Li+ diffusion with Br- introduction into Li3HoCl6. Li-Ho electrostatic repulsions in the (001) plane presumably drive Li+ diffusion into the Ho-free (002) layer, enabling rapid intraplanar Li+ motion and exchange between the 2d and 4h sites. Li3-3 yHo1+ yCl6- xBrx also demonstrates good battery cycling stability. These findings offer valuable insights into the intricate correlations between structure and ion transport and will help guide the design of high-performance fast ion conductors for all-solid-state batteries.

OPTO: Automated Optimization for Solid-State NMR Spectroscopy

NMR spectroscopy presents boundless opportunities for understanding the structure, dynamics, and function for a broad range of scientific applications. Solid-state NMR (SSNMR), in particular, provides novel insights into biological and material systems that are not amenable to other approaches. However, a major bottleneck is the extent of user training and the difficulty of obtaining reproducible, high-quality experimental results, especially for the sophisticated multidimensional pulse sequences that are essential to provide site-resolved measurements in large biomolecules. Here, we present OPTO, a software operating environment that addresses these challenges and enhances the performance of many types of commonly utilized SSNMR experiments. OPTO is compatible with Varian OpenVnmrJ and Bruker Topspin, with a front-end graphical user interface that presents the instrument operator with access to powerful underlying optimization algorithms, including simplex and grid searches of the dozens of parameter settings required for optimal performance. Therefore, OPTO efficiently leverages instrument time and enables instrument operators to find optimal experimental conditions reliably. We demonstrate examples including improvements in (1) resolution, with an automated, global search of 21 shimming parameters to achieve a 12 parts per billion line width; (2) sensitivity, with searches and refinements of several cross-polarization conditions dependent on 16 parameters in triple resonance experiments; and (3) robustness, with results from protein samples on several spectrometers operating at different magnetic field strengths and magic-angle spinning rates.

Highly efficient heteronuclear polarization transfer using dipolar-echo edited R-symmetry sequences in solid-state NMR

In solid-state NMR, dipolar-based heteronuclear polarization transfer has been extensively used for sensitivity enhancement and multidimensional correlations, but its efficiency often suffers from undesired spin interactions and hardware limitations. Herein, we propose a novel dipolar-echo edited R-symmetry (DEER) sequence, which is further incorporated into the INEPT-type scheme, dubbed DEER-INEPT, for achieving highly efficient heteronuclear polarization transfer. Numerical simulations and NMR experiments demonstrate that DEER-INEPT offers significantly improved robustness, enabling efficient polarization transfer under a wide range of MAS conditions, from slow to ultrafast rates, outperforming existing methods. Its high efficiency leads to noticeably enhanced sensitivity in both 1H → X and X → 1H transfers, applicable to both spin-1/2 and spin-half-integer quadrupolar nuclei. DEER-INEPT is expected to be widely used in various systems, offering advantages in both sensitivity enhancement and structural analysis.

Formamidinium Incorporates into Rb-based Non-Perovskite Phases in Solar Cell Formulations

Organic-inorganic hybrid perovskite materials, such as formamidinium lead iodide (FAPbI3), are among the most promising emerging photovoltaic materials. However, the spontaneous phase transition from the photoactive perovskite phase to an inactive non-perovskite phase complicates the application of FAPbI3 in solar cells. To remedy this, alkali metal cations, most often Cs+, Rb+ or K+, are included during perovskite synthesis to stabilize the photoactive phase. The atomic-level mechanisms of stabilization are complex. While Cs+ dopes directly into the perovskite lattice, Rb+ does not, but instead forms an additional non-perovskite phase, and the mechanism by which Rb confers increased stability remains unclear. Here, we use 1H-87Rb double resonance NMR experiments to show that FA+ incorporates into the Rb-based non-perovskite phases (FAyRb1-yPb2Br5 and δ-FAyRb1-yPbI3) for both bromide and iodide perovskite formulations. This is demonstrated by changes in the 1H and 87Rb chemical shifts, 1H-87Rb heteronuclear correlation spectra, and 87Rb{1H} REDOR spectra. Simulation of the REDOR dephasing curves suggests up to ~60 % FA+ incorporation into the inorganic Rb-based phase for the bromide system. In light of these results, we hypothesize that the substitution of FA+ into the non-perovskite phase may contribute to the greater stability conferred by Rb salts in the synthesis of FA-based perovskites.

Defects vibrations engineering for enhancing interfacial thermal transport in polymer composites

To push upper boundaries of thermal conductivity in polymer composites, understanding of thermal transport mechanisms is crucial. Despite extensive simulations, systematic experimental investigation on thermal transport in polymer composites is limited. To better understand thermal transport processes, we design polymer composites with perfect fillers (graphite) and defective fillers (graphite oxide), using polyvinyl alcohol (PVA) as a matrix model. Measured thermal conductivities of ~1.38 ± 0.22 W m-1 K-1 in PVA/defective filler composites is higher than those of ~0.86 ± 0.21 W m-1 K-1 in PVA/perfect filler composites, while measured thermal conductivities in defective fillers are lower than those of perfect fillers. We identify how thermal transport occurs across heterogeneous interfaces. Thermal transport measurements, neutron scattering, quantum mechanical modeling, and molecular dynamics simulations reveal that vibrational coupling between PVA and defective fillers at PVA/filler interfaces enhances thermal conductivity, suggesting that defects in polymer composites improve thermal transport by promoting this vibrational coupling.

Deciphering the Topology of Sitagliptin Using an Integrated Approach

Determining the structure of sitagliptin is crucial for ensuring its effectiveness and safety as a DPP-4 inhibitor used to treat type 2 diabetes. Accurate structure determination is vital for both drug development and maintaining quality control in manufacturing. This study integrates the advanced techniques of solid-state nuclear magnetic resonance (NMR) spectroscopy, three-dimensional (3D) electron diffraction, and density functional theory (DFT) calculations to investigate the structural intricacies of sitagliptin. Solid-state NMR provides detailed information on the molecular environment, revealing insights into the atomic-level structure. The DFT calculations complement these experimental findings by offering theoretical insights into the electronic structure and helping validate the NMR data. Dynamic nuclear polarization has recently emerged as a cornerstone approach to enhance the sensitivity of solid-state NMR spectroscopy under magic angle spinning (MAS), opening unprecedented analytical opportunities. In this work, we incorporated the latest state-of-the art dynamic nuclear polarization NMR into 3D ED NMR crystallography. The findings from this study have important implications for the pharmaceutical industry, particularly in enhancing the precision of drug development and ensuring the high quality of diabetes treatments. Overall, this combined methodological approach not only advances the structural characterization of sitagliptin but also sets a precedent for analyzing other pharmaceutical compounds of similar complexity.

Indirect Detection of the Protons in and around Biradicals and their Mechanistic Role in MAS-DNP

The contribution of protons in or near biradical polarizing agents in Dynamic Nuclear Polarization (DNP) has recently been under scrutiny. Results from selective deuteration and simulations have previously suggested that the role of protons in the biradical molecule depends on the strength of the electron-electron coupling. Here we use the cross effect DNP mechanism to identify and acquire 1H solid-state NMR spectra of the protons that contribute to propagation of the hyperpolarization, via an experimental approach dubbed Nuclear-Nuclear Double Resonance (NUDOR).

Analysis of the MODIST Sequence for Selective Proton-Proton Recoupling

Theoretical and simulated analyses of selective homonuclear dipolar recoupling sequences serve as primary tools for understanding and determining the robustness of these sequences under various conditions. In this article, we investigate the recently proposed first-order dipolar recoupling sequence known as MODIST (Modest Offset Difference Internuclear Selective Transfer). We evaluate the MODIST transfer efficiency, assessing its dependence on rf-field strengths and the number of simulated spins, extending up to 10 spins. This helps to identify conditions that enhance polarization transfer among spins that are nearby in frequency, particularly among aliphatic protons. The exploration uncovers a novel effect for first-order selective recoupling sequences that we term "facilitated dipolar recoupling". This effect amplifies the recoupled dipolar interaction between distant spins due to the presence of additional strongly dipolar-coupled spins. Unlike the third spin-assisted recoupling mechanism, facilitated dipolar recoupling only requires a coupling to one of the two distant spins of interest. Experimental demonstration of MODIST, including at different rf-field strengths, was carried out with the membrane protein influenza A M2 in lipid bilayers using 55 kHz magic-angle spinning (MAS). Reducing MODIST rf-field strength by a factor of 2 unveils possibilities for detecting Hα-Hα and HMeth-HMeth correlations with a 3D (H)C(H)(H)CH experiment under fast MAS rates, all achievable without specific spin labeling.

Concluding remarks: Faraday Discussion on NMR crystallography

This Faraday Discussion explored the field of NMR crystallography, and considered recent developments in experimental and theoretical approaches, new advances in machine learning and in the generation and handling of large amounts of data. Applications to a wide range of disordered, amorphous and dynamic systems demonstrated the range and quality of information available from this approach and the challenges that are faced in exploiting automation and developing best practice. In these closing remarks I will reflect on the discussions on the current state of the art, questions about what we want from these studies, how accurate we need results to be, how we best generate models for complex materials and what machine learning approaches can offer. These remarks close with thoughts about the future direction of the field, who will be carrying out this type of research, how they might be doing it and what their focus will be, along with likely possible challenges and opportunities.

High-throughput calculations and machine learning modeling of 17O NMR in non-magnetic oxides

The only NMR-active oxygen isotope, oxygen-17 (17O), serves as a sensitive probe due to its large chemical shift range, the electric field gradient at the oxygen site, and the quadrupolar interaction. Consequently, 17O solid-state NMR offers unique insights into local structures and finds significant applications in the studies of disorder, reactivity, and host-guest chemistry. Despite recent advances in sensitivity enhancement, isotopic labeling, and NMR crystallography, the application of 17O solid-state NMR is still hindered by low natural abundance, costly enrichment, and challenges in handling spectrum signals. Density functional theory calculations and machine learning techniques offer an alternative approach to mapping the local crystal structures to NMR parameters. However, the lack of high-quality data remains a challenge, despite the establishment of some datasets. In this study, we implement and execute a high-throughput workflow combining AiiDA and CASTEP to evaluate the NMR parameters. Focusing on non-magnetic oxides, we have chosen over 7100 binary, ternary, and quaternary compounds from the Materials Project database and performed calculations. Furthermore, using various descriptors for the local crystalline environments, we model the 17O NMR parameters using machine learning techniques, further enhancing our ability to predict and understand 17O NMR parameters in oxide crystals.

The EFG Rosetta Stone: translating between DFT calculations and solid state NMR experiments

We present a comprehensive study on the best practices for integrating first principles simulations in experimental quadrupolar solid-state nuclear magnetic resonance (SS-NMR), exploiting the synergies between theory and experiment for achieving the optimal interpretation of both. Most high performance materials (HPMs), such as battery electrodes, exhibit complex SS-NMR spectra due to dynamic effects or amorphous phases. NMR crystallography for such challenging materials requires reliable, accurate, efficient computational methods for calculating NMR observables from first principles for the transfer between theoretical material structure models and the interpretation of their experimental SS-NMR spectra. NMR-active nuclei within HPMs are routinely probed by their chemical shielding anisotropy (CSA). However, several nuclear isotopes of interest, e.g.7Li and 27Al, have a nuclear quadrupole and experience additional interactions with the surrounding electric field gradient (EFG). The quadrupolar interaction is a valuable source of information about atomistic structure, and in particular, local symmetry, complementing the CSA. As such, there is a range of different methods and codes to choose from for calculating EFGs, from all-electron to plane wave methods. We benchmark the accuracy of different simulation strategies for computing the EFG tensor of quadrupolar nuclei with plane wave density functional theory (DFT) and study the impact of the material structure as well as the details of the simulation strategy. Especially for small nuclei with few electrons, such as 7Li, we show that the choice of physical approximations and simulation parameters has a large effect on the transferability of the simulation results. To the best of our knowledge, we present the first comprehensive reference scale and literature survey for 7Li quadrupolar couplings. The results allow us to establish practical guidelines for developing the best simulation strategy for correlating DFT to experimental data extracting the maximum benefit and information from both, thereby advancing further research into HPMs.

Spiers Memorial Lecture: NMR crystallography

Chemical function is directly related to the spatial arrangement of atoms. Consequently, the determination of atomic-level three-dimensional structures has transformed molecular and materials science over the past 60 years. In this context, solid-state NMR has emerged to become the method of choice for atomic-level characterization of complex materials in powder form. In the following we present an overview of current methods for chemical shift driven NMR crystallography, illustrated with applications to complex materials.

Crystal structure validation of verinurad via proton-detected ultra-fast MAS NMR and machine learning

The recent development of ultra-fast magic-angle spinning (MAS) (>100 kHz) provides new opportunities for structural characterization in solids. Here, we use NMR crystallography to validate the structure of verinurad, a microcrystalline active pharmaceutical ingredient. To do this, we take advantage of 1H resolution improvement at ultra-fast MAS and use solely 1H-detected experiments and machine learning methods to assign all the experimental proton and carbon chemical shifts. This framework provides a new tool for elucidating chemical information from crystalline samples with limited sample volume and yields remarkably faster acquisition times compared to 13C-detected experiments, without the need to employ dynamic nuclear polarization.

Atomic-level structure of the amorphous drug atuliflapon via NMR crystallography

We determine the complete atomic-level structure of the amorphous form of the drug atuliflapon, a 5-lipooxygenase activating protein (FLAP) inhibitor, via chemical-shift-driven NMR crystallography. The ensemble of preferred structures allows us to identify a number of specific conformations and interactions that stabilize the amorphous structure. These include preferred hydrogen-bonding motifs with water and with other drug molecules, as well as conformations of the cyclohexane and pyrazole rings that stabilize structure by indirectly allowing for optimization of hydrogen bonding.

Resolving Structures of Paramagnetic Systems in Chemistry and Materials Science by Solid-State NMR: The Revolving Power of Ultra-Fast MAS

Ultra-fast magic-angle spinning (100+kHz) has revolutionized solid-state NMR of biomolecular systems but has so far failed to gain ground for the analysis of paramagnetic organic and inorganic powders, despite the potential rewards from substantially improved spectral resolution. The principal blockages are that the smaller fast-spinning rotors present significant barriers for sample preparation, particularly for air/moisture-sensitive systems, and are associated with low sensitivity from the reduced sample volumes. Here, we demonstrate that the sensitivity penalty is less severe than expected for highly paramagnetic solids and is more than offset by the associated improved resolution. While previous approaches employing slower MAS are often unsuccessful in providing sufficient resolution, we show that ultra-fast 100+kHz MAS allows site-specific assignments of all resonances from complex paramagnetic solids. Combined with more reliable rotor materials and handling methods, this opens the way to the routine characterization of geometry and electronic structures of functional paramagnetic systems in chemistry, including catalysts and battery materials. We benchmark this approach on a hygroscopic luminescent Tb3+ complex, an air-sensitive homogeneous high-spin Fe2+ catalyst, and a series of mixed Fe2+/Mn2+/Mg2+ olivine-type cathode materials.

Physiological markers for immunotherapeutics: a review

Immunotherapy has been advanced through multiple approaches, including immunogenic cytokines, monoclonal antibodies, therapeutic vaccinations, adoptive cell transfer, stem cell transplantation, and oncolytic viruses. This review analyses various strategies in genomics, transcriptomics, single-cell techniques, computational analysis, big data, and imaging technologies for the identification of tumour microbiota and microenvironments. Immunotherapy is becoming acknowledged as a feasible cancer treatment method, facilitating innovative cancer medicines and personalized medicine techniques.

Solid Effect Dynamic Nuclear Polarization Enhancement of >500 at 9.4 T

Efficient polarizing agents for dynamic nuclear polarization (DNP) enhanced magic angle spinning (MAS) NMR spectroscopy are of high current interest due to the potential to significantly boost NMR sensitivity. While most efforts have centered on cross-effect (CE) or Overhauser effect (OE) mechanisms, yielding enhancement factors up to ∼300 at 9.4 T, radicals yielding solid effect (SE) DNP have seen less development. Here we model the comparative performance of OE and SE mechanisms and then measure 1H enhancement factors up to 500 from 1,3-bisdiphenylene-2-phenylallyl (BDPA) in an ortho-terphenyl (OTP) matrix at 9.4 T, 100 K, achieved via increased microwave power across the sample volume. The measured SE and OE performances are in good agreement with the predictions. We note that both the experimental and theoretical analyses indicate that SE DNP remains saturation limited, particularly at elevated temperatures, and we envisage that further improvements in microwave power will further increase SE DNP enhancement factors.

ZnH2P4N8: Case Study on Topochemical Imidonitridophosphate High-Pressure Synthesis

Nitridophosphates are subject of current research, as they have a broad spectrum of properties and potential applications, such as ion conductors or luminescent materials. Yet, the subclass of imidonitridophosphates has been studied less extensively. The primary reason is that the controlled N-H functionalization of nitridophosphates is not straight forward, making targeted synthesis more challenging. Inspired by the high-pressure (HP) post-synthetic modification of nitridophosphates, we present the topochemical HP deprotonation of phosphorus nitride imides using the high-pressure polymorph β-PN(NH) as an example. Additional incorporation of Zn2+ results in the first quaternary transition metal imidonitridophosphate ZnH2P4N8. The crystal structure was elucidated by single-crystal X-ray diffraction (SCXRD), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD) and solid-state magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). In addition, the presence of H as part of an imide group was confirmed by IR spectroscopy. The potential of this defunctionalization approach for controlling the N-H content is demonstrated by the preparation of partially deprotonated intermediates ZnxH4-2xP4N8 (x≈0.5, 0.85). This topochemical high-pressure reaction represents a promising way to prepare, control and manipulate new imide-based materials without altering their overall anionic framework.

Mechanistic characterization of fast dissolving PVP-I powder with multipolymer approaches and investigation on their molecular interaction

Povidone-iodine (PVP-I) is widely used as an antiseptic in medical applications. However, its effectiveness is limited by certain drawbacks, such as low solubility in water and high volatility. Therefore, a formulation of a stable solid PVP-I is desirable. In this study, complexes of molecular PVP-I with polyethylene glycol-polyvinyl alcohol copolymer (PEG-PVA copolymer) were considered water-soluble iodophors. Two different methods were used to prepare the solids: physical mixtures and kneading. The physical characteristics of the obtained solids were evaluated using several spectroscopic methods. The presence of iodine was confirmed by a potentiometric titration and antimicrobial activity was tested. The results showed that the PEG-PVA copolymer interacted with povidone primarily through hydrogen bonding between the hydroxyl part of the PEG-PVA copolymer and the amide part of povidone with an estimated binding energy of 3.2 kcal/mol. The amide groups polarity in povidone made them more likely to form hydrogen bonds with the PEG-PVA copolymer. Also, the protonated pyrrolidone bonded to the triiodide anions by intermolecular hydrogen bonds, which increased PVP-I solubility in water. The kneading method provided a faster dissolution rate than physical mixing and pure PVP-I. The iodine contents were within an acceptable range (10-12%), and the antimicrobial activity proved effective against Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus mutans.

Manual and automatic assignment of two different Aβ40 amyloid fibril polymorphs using MAS solid-state NMR spectroscopy

Amyloid fibrils from Alzheimer's amyloid-beta peptides (Aβ) are found to be polymorphic. So far, 14 Aβ40 fibril structures have been determined. The mechanism of why one particular protein sequence adopts so many different three-dimensional structures is yet not understood. In this work, we describe the assignment of the NMR chemical shifts of two Alzheimer's disease fibril polymorphs, P1 and P2, which are formed by the amyloid-beta peptide Aβ40. The assignment is based on 13C-detected 3D NCACX and NCOCX experiments MAS solid-state NMR experiments. The fibril samples are prepared using an extensive seeding protocol in the absence and presence of the small heat shock protein αB-crystallin. In addition to manual assignments, we obtain chemical shift assignments using the automation software ARTINA. We present an analysis of the secondary chemical shifts and a discussion on the differences between the manual and automated assignment strategies.

Dipolar Recoupling in Rotating Solids

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) has evolved significantly over the past three decades and established itself as a vital tool for the structural analysis of biological macromolecules and materials. This review delves into the development and application of dipolar recoupling techniques in MAS NMR, which are crucial for obtaining detailed structural and dynamic information. We discuss a variety of homonuclear and heteronuclear recoupling methods which are essential for measuring spatial restraints and explain in detail the spin dynamics that these sequences generate. We also explore recent developments in high spinning frequency MAS, proton detection, and dynamic nuclear polarization, underscoring their importance in advancing biomolecular NMR. Our aim is to provide a comprehensive account of contemporary dipolar recoupling methods, their principles, and their application to structural biology and materials, highlighting significant contributions to the field and emerging techniques that enhance resolution and sensitivity in MAS NMR spectroscopy.

An Efficient and Stable Polarizing Agent for In-Cell Magic-Angle Spinning Dynamic Nuclear Polarization NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy would be a method of choice to follow biochemical events in cells because it can analyze molecules in complex environments. However, the intrinsically low sensitivity of NMR makes in-cell measurements challenging. Dynamic Nuclear Polarization (DNP) has emerged as a method to circumvent this limitation, but most polarizing agents developed for DNP are unstable in reducing cellular environments. Here, we introduce the use of Gd(III)-based DNP polarizing agents for in-cell NMR spectroscopy. Specifically, we show their persistent stability in cellular formulations, and we investigate the DNP performance of the Gd(III)-based complex [Gd(tpatcn)] in human embryonic kidney cell lysates and intact cells. For cell lysates, DNP enhancements up to -27 are obtained on the cellular signals, reproducible even after storage at room temperature for days. Mixing the [Gd(tpatcn)] solution with intact cells enables the observation of cellular signals with DNP, and DNP enhancement factors of about -40 are achieved.

The Role of Phase Separation and Local Mobility in the Stabilization of a Lyophilized IgG2 Formulation

The utility of employing solid-state NMR (SSNMR) to assess parameters governing the stability of a lyophilized IgG2 protein was the focus of the present work. Specifically, the interaction between the sugar stabilizer (sucrose) and protein component was measured using SSNMR and compared to physical and chemical stability data obtained from thermally stressed samples. 1H T1 and 1H T1⍴ relaxation times were measured by SSMNR for 5 different formulation conditions, and the resultant values were used to examine local mobility and phase separation, respectively. From the SSNMR measurements, it was found local mobility decreased as the sucrose to protein weight ratio increased. The decrease in local mobility corresponded to an increase in storage stability (both chemical and physical) of the lyophilized solids up to a critical weight ratio of sucrose to protein. Additionally, 1H T1⍴ measurements obtained on formulations having higher protein to sucrose weight ratios indicated phase separation of the protein and sucrose phases was occurring, at least on a small scale. Along with an increase in local mobility, phase separation in these specific formulations is thought to have played a role in their decreased storage stability in the solid state.

Insights into Ligand-Mediated Activation of an Oligomeric Ring-Shaped Gene-Regulatory Protein from Solution- and Solid-State NMR

The 91 kDa oligomeric ring-shaped ligand binding protein TRAP (trp RNA binding attenuation protein) regulates the expression of a series of genes involved in tryptophan (Trp) biosynthesis in bacilli. When cellular Trp levels rise, the free amino acid binds to sites buried in the interfaces between each of the 11 (or 12, depending on the species) protomers in the ring. Crystal structures of Trp-bound TRAP show the Trp ligands are sequestered from solvent by a pair of loops from adjacent protomers that bury the bound ligand via polar contacts to several threonine residues. Binding of the Trp ligands occurs cooperatively, such that successive binding events occur with higher apparent affinity but the structural basis for this cooperativity is poorly understood. We used solution methyl-TROSY NMR relaxation experiments focused on threonine and isoleucine sidechains, as well as magic angle spinning solid-state NMR 13C-13C and 15N-13C chemical shift correlation spectra on uniformly labeled samples recorded at 800 and 1200 MHz, to characterize the structure and dynamics of the protein. Methyl 13C relaxation dispersion experiments on ligand-free apo TRAP revealed concerted exchange dynamics on the µs-ms time scale, consistent with transient sampling of conformations that could allow ligand binding. Cross-correlated relaxation experiments revealed widespread disorder on fast timescales. Chemical shifts for methyl-bearing side chains in apo- and Trp-bound TRAP revealed subtle changes in the distribution of sampled sidechain rotameric states. These observations reveal a pathway and mechanism for induced conformational changes to generate homotropic Trp-Trp binding cooperativity.

Selective Methods Promote Protein Solid-State NMR

Solid-state nuclear magnetic resonance (ssNMR) is indispensable for studying the structures, dynamics, and interactions of insoluble proteins in native or native-like environments. While ssNMR includes numerous nonselective techniques for general analysis, it also provides various selective methods that allow for the extraction of precise details about proteins. This perspective highlights three key aspects of selective methods: selective signals of protein segments, selective recoupling, and site-specific insights into proteins. These methods leverage protein topology, labeling strategies, and the tailored manipulation of spin interactions through radio frequency (RF) pulses, significantly promoting the field of protein ssNMR. With ongoing advancements in higher magnetic fields and faster magic angle spinning (MAS), there remains an ongoing need to enhance the selectivity and efficiency of selective ssNMR methods, facilitating deeper atomic-level insights into complex biological systems.

Mechanism of Solid-State 1H Photochemically Induced Dynamic Nuclear Polarization in a Synthetic Donor-Chromophore-Acceptor at 0.3 T

1H photochemically induced dynamic nuclear polarization (photo-CIDNP) has recently emerged as a tool to enhance bulk 1H nuclear magnetic resonance (NMR) signals in solids at magnetic fields ranging from 0.3 to 21.1 T, using synthetic donor-chromophore-acceptor (D-C-A) molecules as optically active polarizing agents (PAs). However, the mechanisms at play for the generation of spin polarization in these systems have not been determined but are essential for an in-depth understanding and further development of the process. Here, we introduce site-selective deuteration to identify the 1H photo-CIDNP mechanisms at 85 K and 0.3 T in D-C-A molecule PhotoPol. We find that the protons on the acceptor moiety are essential for the generation of polarization, establishing differential relaxation as the main mechanism. These results establish selective deuteration as a tool to identify and suppress polarization transfer mechanisms, which opens up pathways for further optimization of the optical PA at both low and high magnetic fields.

Investigating Different Dynamic pHP1α States in Their KCl-Mediated Liquid-Liquid Phase Separation (LLPS) Using Solid-State NMR (SSNMR) and Molecular Dynamic (MD) Simulations

Chromatin phase separation is dynamically regulated by many factors, such as post-translational modifications and effector proteins, and plays a critical role in genomic activities. The liquid-liquid phase separation (LLPS) of chromatin and/or effector proteins has been observed both in vitro and in vivo. However, the underlying mechanisms are largely unknown, and elucidating the physicochemical properties of the phase-separated complexes remains technically challenging. In this study, we detected dynamic, viscous, and intermediate components within the phosphorylated heterochromatin protein 1α (pHP1α) phase-separated system by using modified solid-state NMR (SSNMR) pulse sequences. The basis of these sequences relies on the different time scale of motion detected by heteronuclear Overhauser effect (hetNOE), scalar coupling-based, and dipolar coupling-based transfer schemes in NMR. In comparison to commonly utilized scalar coupling-based methods for studying the dynamic components in phase-separated systems, hetNOE offers more direct insight into molecular dynamics. NMR signals from the three different states in the protein gel were selectively excited and individually studied. Combined with molecular dynamics (MD) simulations, our findings indicate that at low KCl concentration (30 mM), the protein gel displays reduced molecular motion. Conversely, an increase in molecular motion was observed at a high KCl concentration (150 mM), which we attribute to the resultant intermolecular electrostatic interactions regulated by KCl.

Light-Induced Metallic and Paramagnetic Defects in Halide Perovskites from Magnetic Resonance

Halide perovskites are promising next-generation solar cell materials, but their commercialization is hampered by their propensity to degrade under operating conditions, particularly under heat, humidity, and light. Identifying degradation products and linking them to the degradation mechanism at the atomic scale is necessary to design more stable perovskite materials. Here we use magnetic resonance methods to identify and characterize the formation of both metallic lead clusters and Pb3+ defects upon light-induced degradation of methylammonium lead halide perovskite using nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements. Paramagnetic relaxation enhancement (PRE) of the 1H NMR resonances demonstrates the presence of localized paramagnetic Pb3+ defects, a large Knight shift of the 207Pb NMR proves the presence of lead metal, and their relative proportions are determined by the differing temperature dependence in variable-temperature EPR. This work reconciles previous conflicting literature results, enabling the use of EPR spectroscopy to monitor photodegradation of perovskite devices.

Molecular architecture of chitin and chitosan-dominated cell walls in zygomycetous fungal pathogens by solid-state NMR

Zygomycetous fungal infections pose an emerging medical threat among individuals with compromised immunity and metabolic abnormalities. Our pathophysiological understanding of these infections, particularly the role of fungal cell walls in growth and immune response, remains limited. Here we conducted multidimensional solid-state NMR analysis to examine cell walls in five Mucorales species, including key mucormycosis causative agents like Rhizopus and Mucor species. We show that the rigid core of the cell wall primarily comprises highly polymorphic chitin and chitosan, with minimal quantities of β-glucans linked to a specific chitin subtype. Chitosan emerges as a pivotal molecule preserving hydration and dynamics. Some proteins are entrapped within this semi-crystalline chitin/chitosan layer, stabilized by the sidechains of hydrophobic amino acid residues, and situated distantly from β-glucans. The mobile domain contains galactan- and mannan-based polysaccharides, along with polymeric α-fucoses. Treatment with the chitin synthase inhibitor nikkomycin removes the β-glucan-chitin/chitosan complex, leaving the other chitin and chitosan allomorphs untouched while simultaneously thickening and rigidifying the cell wall. These findings shed light on the organization of Mucorales cell walls and emphasize the necessity for a deeper understanding of the diverse families of chitin synthases and deacetylases as potential targets for novel antifungal therapies.

Electron diffraction and solid-state NMR reveal the structure and exciton coupling in a eumelanin precursor

Eumelanin, a versatile biomaterial found throughout the animal kingdom, performs essential functions like photoprotection and radical scavenging. The diverse properties of eumelanin are attributed to its elusive and heterogenous structure with DHI (5,6-dihydroxyindole) and DHICA (5,6-dihydroxyindole-2-carboxylic acid) precursors as the main constituents. Despite DHICA being recognized as the key eumelanin precursor, its crystal structure and functional role in the assembled state remain unknown. Herein, we employ a synthesis-driven, bottom-up approach to elucidate the structure and assembly-specifics of DHICA, a critical building block of eumelanin. We introduce an interdisciplinary methodology to analyse the nanocrystalline assembly of DHICA, employing three-dimensional electron diffraction (3D ED), solid-state NMR and density functional theory (DFT), while correlating the structural aspects with the electronic spectroscopic features. The results underscore charge-transfer exciton delocalization as the predominant energy transfer mechanism within the π-π stacked and hydrogen-bonded crystal network of DHICA. Additionally, extending the investigation to the 13C-labelled DHICA-based polymer improves our understanding of the chemical heterogeneity across the eumelanin pigment, providing crucial insights into the structure of eumelanin.

Efficient DNP at high fields and fast MAS with antenna-sensitized dinitroxides

Dynamic Nuclear Polarization (DNP) can significantly enhance the sensitivity of solid-state NMR. In DNP, microwave irradiation induces polarization transfer from unpaired electron spins to 1H nuclear spins via hyperfine couplings and spin-diffusion. The structure of the polarizing agents that host the electron spins is key for DNP efficiency. Currently, only a handful of structures perform well at very high magnetic fields (≥18.8 T), and enhancements are significantly lower than those obtained at lower fields. Here, we introduce a new series of water-soluble nitroxide biradicals with a scaffold augmented by dihydroxypropyl antenna chains that perform significantly better than previous dinitroxides at 18.8 T. The new radical M-TinyPol(OH)4 yields enhancement factors of ∼220 at 18.8 T and 60 kHz MAS, which is a nearly factor 2 larger than for the previous best performing dinitroxides. The performance is understood through 2H ESEEM measurements to probe solvent accessibility, supported by Molecular Dynamics simulations, and by experiments on deuterated samples. We find that the deuterated glycerol molecules in the matrix are located mainly in the second solvation shell of the NO bond, limiting access for protonated water molecules, and restricting spin diffusion pathways. This provides a rational understanding of why the dihydroxypropyl chains present in the best-performing structures are essential to deliver the polarization to the bulk solution.

A metal-free cascaded process for efficient H2O2 photoproduction using conjugated carbonyl sites

Carbon-based metal-free catalysts are promising green catalysts for photocatalysis and electrocatalysis due to their low cost and environmental friendliness. A key challenge in utilizing these catalysts is identifying their active sites, given their poor crystallinity and complex structures. Here we demonstrate the key structure of the double-bonded conjugated carbon group as a metal-free active site, enabling efficient O2 photoreduction to H2O2 through a cascaded water oxidation - O2 reduction process. Using ethylenediaminetetraacetic acid as a precursor, we synthesized various carbon-based photocatalysts and analyzed their structural evolution. Under the polymerization conditions of 260 °C to 400 °C, an N-ethyl-2-piperazinone-like structure was formed on the surface of the catalyst, resulting in high photocatalytic H2O2 photoproduction (2884.7 μmol g-1h-1) under visible light. A series of control experiments and theoretical calculations further confirm that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts.

Nondissociative Activated Dihydrogen Binding on CeO2 Revealed by High-Pressure Operando Solid-State NMR Spectroscopy

Dihydrogen complexes, which retain the H-H bond, have been extensively studied in molecular science and found to be prevalent in homogeneous and enzymatic catalysis. However, their counterparts in heterogeneous catalysis, specifically nondissociative chemisorbed dihydrogen binding on the catalyst surface, are rarely reported experimentally. This scarcity is due to the complexity of typical material surfaces and the lack of effective characterization techniques to prove and distinguish various dihydrogen binding modes. Herein, using high-pressure operando solid-state NMR technology, we report the first unambiguous experimental observation of activated dihydrogen binding on a reduced ceria catalyst through interactions with surface oxygen vacancies. By employing versatile NMR structural and dynamical analysis methods, we establish a proportional relationship between the degree of ceria surface reduction and dihydrogen binding, as evidenced by NMR observations of H-D through-bond coupling (JHD), T1 relaxation, and proton isotropic chemical shifts. In situ NMR analysis further reveals the participation of bound dihydrogen species in a room-temperature ethylene hydrogenation reaction. The remarkable similarities between surface-activated dihydrogen in heterogeneous catalysis and dihydrogen model molecular complexes can provide valuable insights into the hydrogenation mechanism for many other solid catalysts, potentially enhancing hydrogen utilization.

A complete 3D-printed tool kit for Solid-State NMR sample and rotor handling

Solid state NMR (SSNMR) is a highly versatile and broadly applicable method for studying the structure and dynamics of biomolecules and materials. For scientists entering the field of SSNMR, the many quotidian activities required in the workflow to prepare samples for data collection can present a significant barrier to adoption. These steps include transfer of samples into rotors, marking the reflective surfaces for high sensitivity tachometer signal detection, inserting rotors into the magic-angle spinning (MAS) stator, achieving stable spinning, and removing and storing rotors to ensure reproducibility of data collection conditions. Even experienced spectroscopists experience occasional problems with these operations, and the cumulative probability of a delay to successful data collection is high enough to cause frequent disruptions to instrument schedules, particularly in the context of large facilities serving a diverse community of users. These problems are all amplified when utilizing rotors smaller than about 4 mm in diameter. Therefore, to improve the reliability and robustness of SSNMR sample preparation workflows, here we describe a set of tools for rotor packing, unpacking, tachometer marking, extraction and storage. Stereolithography 3D printing was employed as a cost-effective and convenient method for prototyping and manufacturing a full range of designs suitable for several types of probes and rotor geometries.

Studying large biomolecules as sedimented solutes with solid-state NMR

Sedimentation solid-state NMR is a novel method for sample preparation in solid-state NMR (ssNMR) studies. It involves the sedimentation of soluble macromolecules such as large protein complexes. By utilizing ultra-high centrifugal forces, the molecules in solution are driven into a high-concentrated hydrogel, resulting in a sample suitable for solid-state NMR. This technique has the advantage of avoiding the need for chemical treatment, thus minimizing the loss of sample biological activity. Sediment ssNMR has been successfully applied to a variety of non-crystalline protein solids, significantly expanding the scope of solid-state NMR research. In theory, using this method, any biological macromolecule in solution can be transferred into a sedimented solute appropriate for solid-state NMR analysis. However, specialized equipment and careful handling are essential for effectively collecting and loading the sedimented solids to solid-state NMR rotors. To improve efficiency, we have designed a series of loading tools to achieve the loading process from the solution to the rotor in one step. In this paper, we illustrate the sample preparation process of sediment NMR using the H1.4-NCP167 complex, which consists of linker histone H1.4 and nucleosome core particle, as an example.