Dynamic nuclear polarization at high magnetic fields

Probing the Molecular and Macroscopic Structure of Solid Solutions by Dynamic Nuclear Polarization (DNP) Enhanced 13C and 15N Solid-State NMR Spectroscopy

Crystallization is a widely used purification technique in the manufacture of active pharmaceutical ingredients (APIs) and precursor molecules. However, when impurities and desired compounds have similar molecular structures, separation by crystallization may become challenging. In such cases, some impurities may form crystalline solid solutions with the desired product during recrystallization. Understanding the molecular structure of these recrystallized solid solutions is crucial to devise methods for effective purification. Unfortunately, there are limited analytical techniques that provide insights into the molecular structure or spatial distribution of impurities that are incorporated within recrystallized products. In this study, we investigated model solid solutions formed by recrystallizing salicylic acid (SA) in the presence of anthranilic acid (AA). These two molecules are known to form crystalline solid solutions due to their similar molecular structures. To overcome challenges associated with the long 1H longitudinal relaxation times (T1(1H)) of SA and AA, we employed dynamic nuclear polarization (DNP) and 15N isotope enrichment to enable solid-state NMR experiments. Results of solid-state NMR experiments and DFT calculations revealed that SA and AA are homogeneously alloyed as a solid solution. Heteronuclear correlation (HETCOR) experiments and plane-wave DFT structural models provide further evidence of the molecular-level interactions between SA and AA. This research provides valuable insights into the molecular structure of recrystallized solid solutions, contributing to the development of effective purification strategies and an understanding of the physicochemical properties of solid solutions.

Exploring tunneling ESEEM beyond methyl groups in nitroxides at low temperatures

Tunneling of methyl rotors coupled to an electron spin causes magnetic field independent electron spin echo envelope modulation (ESEEM) at low temperatures. For nitroxides containing alkyl substituents, we observe this effect as a contribution at the beginning of the Hahn echo decay signal occurring on a faster time scale than the matrix-induced decoherence. The tunneling ESEEM contribution includes information on the local environment of the methyl rotors, which manifests as a distribution of rotation barriers P(V3) when measuring the paramagnetic species in a glassy matrix. Here, we investigate the differences in tunneling behaviour of geminal methyl and ethyl group rotors in nitroxides while exploring different levels of theory in our previously introduced methyl quantum rotor (MQR) model. Moreover, we extend the MQR model to analyze the tunneling ESEEM originating from two different rotor types coupled to the same electron spin. We find that ethyl groups in nitroxides give rise to stronger tunneling ESEEM contributions than methyl groups because the difference between hyperfine couplings of their methyl protons better matches the tunneling frequency. The methyl rotors of both ethyl and propyl groups exhibit distributions at lower rotation barriers compared to geminal methyl groups. This is in good agreement with density functional theory (DFT) calculations of their rotation barriers and showcases that conformational flexibility impacts the hindrance of rotation. Using Monte-Carlo based fitting in combination with an identifiability analysis of the MQR model parameter space, we extract rotation barrier distributions for the individual rotor types in mixed-rotor nitroxides as well as identify which rotors dominate the observed tunneling contribution in the Hahn echo decay signal.

Cocrystalline Matrices for Hyperpolarization at Room Temperature Using Photoexcited Electrons

We propose using cocrystals as effective polarization matrices for triplet dynamic nuclear polarization (DNP) at room temperature. The polarization source can be uniformly doped into cocrystals formed through acid-acid, amide-amide, and acid-amide synthons. The dense-packing crystal structures, facilitated by multiple hydrogen bonding and π-π interactions, result in extended T1 relaxation times, enabling efficient polarization diffusion within the crystals. Our study demonstrates the successful polarization of a DNP-magnetic resonance imaging molecular probe, such as urea, within a cocrystal matrix at room temperature using triplet-DNP.

Dipolar Order Induced Electron Spin Hyperpolarization

The structure of coupled electron spin systems is of fundamental interest to many applications, including dynamic nuclear polarization (DNP), enhanced nuclear magnetic resonance (NMR), the generation of electron spin qubits for quantum information science (QIS), and quantitative studies of paramagnetic systems by electron paramagnetic resonance (EPR). However, the characterization of electron spin coupling networks is nontrivial, especially at high magnetic fields. This study focuses on a system containing high concentrations of trityl radicals that give rise to a DNP enhancement profile of 1H NMR characteristic of the presence of electron spin clusters. When this system is subject to selective microwave saturation through pump-probe ELectron DOuble Resonance (ELDOR) experiments, electron spin hyperpolarization is observed. We show that the generation of an out-of-equilibrium longitudinal dipolar order is responsible for the transient hyperpolarization of electron spins. Notably, the coupled electron spin system needs to form an AX-like system (where the difference in the Zeeman interactions of two spins is larger than their coupling interaction) such that selective microwave irradiation can generate signatures of electron spin hyperpolarization. We show that the extent of dipolar order, as manifested in the extent of electron spin hyperpolarization generated, can be altered by tuning the pump or probe pulse length, or the interpulse delay in ELDOR experiments that change the efficiency to generate or readout longitudinal dipolar order. Pump-probe ELDOR with selective saturation is an effective means for characterizing coupled electron spins forming AX-type spin systems that are foundational for DNP and quantum sensing.

Magic Angle Spinning Solid-State 13C Photochemically Induced Dynamic Nuclear Polarization by a Synthetic Donor-Chromophore-Acceptor System at 9.4 T

Solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) is a nuclear magnetic resonance spectroscopy technique in which nuclear spin hyperpolarization is generated upon optical irradiation of an appropriate donor-acceptor system. Until now, solid-state photo-CIDNP at high magnetic fields has been observed only in photosynthetic reaction centers and flavoproteins. In the present work, we show that the effect is not limited to such biomolecular samples, and solid-state 13C photo-CIDNP can be observed at 9.4 T under magic angle spinning using a frozen solution of a synthetic molecular system dissolved in an organic solvent. Signal enhancements for the source molecule larger than a factor of 2300 are obtained. In addition, we show that bulk 13C hyperpolarization of the solvent can be generated via spontaneous 13C-13C spin diffusion at natural abundance.

Towards a unified picture of polarization transfer - pulsed DNP and chemically equivalent PHIP

Nuclear spin hyperpolarization techniques, such as dynamic nuclear polarization (DNP) and parahydrogen-induced polarization (PHIP), have revolutionized nuclear magnetic resonance and magnetic resonance imaging. In these methods, a readily available source of high spin order, either electron spins in DNP or singlet states in hydrogen for PHIP, is brought into close proximity with nuclear spin targets, enabling efficient transfer of spin order under external quantum control. Despite vast disparities in energy scales and interaction mechanisms between electron spins in DNP and nuclear singlet states in PHIP, a pseudo-spin formalism allows us to establish an intriguing equivalence. As a result, the important low-field polarization transfer regime of PHIP can be mapped onto an analogous system equivalent to pulsed-DNP. This establishes a correspondence between key polarization transfer sequences in PHIP and DNP, facilitating the transfer of sequence development concepts. This promises fresh insights and significant cross-pollination between DNP and PHIP polarization sequence developers.

Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry

This review gives an overview of current trends in the investigation of confined molecules such as water, small and higher alcohols, carbonic acids, ethylene glycol, and non-ionic surfactants, such as polyethylene glycol or Triton-X, as guest molecules in neat and functionalized mesoporous silica materials employing solid-state NMR spectroscopy, supported by calorimetry and molecular dynamics simulations. The combination of steric interactions, hydrogen bonds, and hydrophobic and hydrophilic interactions results in a fascinating phase behavior in the confinement. Combining solid-state NMR and relaxometry, DNP hyperpolarization, molecular dynamics simulations, and general physicochemical techniques, it is possible to monitor these confined molecules and gain deep insights into this phase behavior and the underlying molecular arrangements. In many cases, the competition between hydrogen bonding and electrostatic interactions between polar and non-polar moieties of the guests and the host leads to the formation of ordered structures, despite the cramped surroundings inside the pores.

Advanced solid-state NMR spectroscopy and its applications in zeolite chemistry

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host-guest/guest-guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host-guest/guest-guest interactions, and catalytic reaction mechanisms.

Soliton Based Dynamic Nuclear Polarization: An Overhauser Effect in Cyclic Polyacetylene at High Field and Room Temperature

Polyacetylene, a versatile material with an electrical conductivity that can span 7 orders of magnitude, is the prototypical conductive polymer. In this letter, we report the observation of a significant Overhauser effect at the high magnetic field of 14.1 T that operates at 100 K and room temperature in both linear and cyclic polyacetylene. Significant NMR signal enhancements ranging from 24 to 45 are obtained. The increased sensitivity enabled the characterization of the polymer chain defects at natural abundance. The absence of end methyl group carbon-13 signals provides proof of the closed-loop molecular structure of cyclic polyacetylene. The remarkable efficiency of the soliton based Overhauser effect DNP mechanism at high temperature and high field holds promise for applications and extension to other conductive polymer systems.

SABRE hyperpolarization of nicotinamide derivatives and their molecular dynamics properties

Signal amplification by reversible exchange hyperpolarization explores the chemical structure and kinetic properties of nicotinamide derivatives. N-Benzyl nicotinamide and nicotinic acid hydrazide compounds display relatively fast dissociation rates of approximately 7-8 s-1 and long proton T1 relaxation times of 5-20 s, respectively. Consequently, these substrates exhibit remarkable signal enhancements, reaching approximately 175 and 102 fold, respectively, underscoring the efficacy of the hyperpolarization technique in elucidating the behavior of these compounds.

Multivalent Fluorinated Nanorings for On-Cell 19F NMR

The design of imaging agents with a high fluorine content is necessary for overcoming the challenges of low sensitivity in 19F magnetic resonance imaging (MRI)-based molecular imaging. Chemically self-assembled nanorings (CSANs) provide a strategy to increase the fluorine content through multivalent display. We previously reported an 19F NMR-based imaging tracer, in which case a CSAN-compatible epidermal growth factor receptor (EGFR)-targeting protein E1-dimeric dihydrofolate (E1-DD) was bioconjugated to a highly fluorinated peptide. Despite good 19F NMR performance in aqueous solutions, a limited signal was observed in cell-based 19F NMR using this monomeric construct, motivating further design. Here, we design several new E1-DD proteins bioconjugated to peptides of different fluorine contents. Flow cytometry analysis was used to assess the effect of variable fluorinated peptide sequences on the cellular binding characteristics. Structure-optimized protein, RTC-3, displayed an optimal spectral performance with high affinity and specificity for EGFR-overexpressing cells. To further improve the fluorine content, we next engineered monomeric RTC-3 into CSAN, η-RTC-3. With an approximate eightfold increase in the fluorine content, multivalent η-RTC-3 maintained high cellular specificity and optimal 19F NMR spectral behavior. Importantly, the first cell-based 19F NMR spectra of η-RTC-3 were obtained bound to EGFR-expressing A431 cells, showing a significant amplification in the signal. This new design illustrated the potential of multivalent fluorinated CSANs for future 19F MRI molecular imaging applications.

Accelerated acquisition of wideline solid-state NMR spectra of spin 3/2 nuclei by frequency-stepped indirect detection experiments

73% of all NMR-active nuclei are quadrupolar nuclei with a nuclear spin I > 1/2. The broadening of the solid-state NMR signals by the quadrupolar interaction often leads to poor sensitivity and low resolution. In this work we present experimental and theoretical investigations of magic angle spinning (MAS) 1H{X} double-echo resonance-echo saturation-pulse double-resonance (DE-RESPDOR) and Y{X} J-resolved solid-state NMR experiments for the indirect detection of spin 3/2 quadrupolar nuclei (X = spin 3/2 nuclei, Y = spin 1/2 nuclei). In these experiments, the spectrum of the quadrupolar nucleus is reconstructed by plotting the observed dephasing of the detected spin as a function of the transmitter offset of the indirectly detected spin. Numerical simulations were used to investigate the achievable levels of dephasing and to predict the lineshapes of indirectly detected NMR spectra of the quadrupolar nucleus. We demonstrate 1H, 31P and 207Pb detection of 35Cl, 81Br, and 63Cu (I = 3/2) nuclei in trans-Cl2Pt(NH3)2 (transplatin), (CH3NH3)PbCl3 (methylammonium lead chloride, MAPbCl3), (CH3NH3)PbBr3 (methylammonium lead bromide, MAPbBr3) and CH3C(CH2PPh2)3CuI (1,1,1-tris(diphenylphosphinomethyl)ethane copper(I) iodide, triphosCuI), respectively. In all of these experiments, we were able to detect megahertz wide central transition or satellite transition powder patterns. Significant time savings and gains in sensitivity were attained in several test cases. Additionally, the indirect detection experiments provide valuable structural information because they confirm the presence of dipolar or scalar couplings between the detected nucleus and the quadrupolar nucleus of interest. Finally, numerical simulations suggest these methods are also potentially applicable to abundant spin 5/2 and spin 7/2 quadrupolar nuclei.

AsymPol-TEKs as efficient polarizing agents for MAS-DNP in glass matrices of non-aqueous solvents

Two polarizing agents from the AsymPol family, AsymPol-TEK and cAsymPol-TEK (methyl-free version) are introduced for MAS-DNP applications in non-aqueous solvents. The performance of these new biradicals is rationalized in detail using a combination of electron paramagnetic resonance spectroscopy, density functional theory, molecular dynamics and quantitative MAS-DNP spin dynamics simulations. By slightly modifying the experimental protocol to keep the sample temperature low at insertion, we are able to obtain reproducable DNP-NMR data with 1,1,2,2-tetrachloroethane (TCE) at 100 K, which facilitates optimization and comparison of different polarizing agents. At intermediate magnetic fields, AsymPol-TEK and cAsymPol-TEK provide 1.5 to 3-fold improvement in sensitivity compared to TEKPol, one of the most widely used polarizing agents for organic solvents, with significantly shorter DNP build-up times of ∼1 s and ∼2 s at 9.4 and 14.1 T respectively. In the course of the work, we also isolated and characterized two diastereoisomers that can form during the synthesis of AsymPol-TEK; their difference in performance is described and discussed. Finally, the advantages of the AsymPol-TEKs are demonstrated by recording 2D 13C-13C correlation experiments at natural 13C-abundance of proton-dense microcrystals and by polarizing the surface of ZnO nanocrystals (NCs) coated with diphenyl phosphate ligands. For those experiments, cAsymPol-TEK yielded a three-fold increase in sensitivity compared to TEKPol, corresponding to a nine-fold time saving.

Surface Sites and Ligation in Amine-capped CdSe Nanocrystals

Converting colloidal nanocrystals (NCs) into devices for various applications is facilitated by designing and controlling their surface properties. One key strategy for tailoring surface properties is thus to choose tailored surface ligands. In that context, amines have been universally used, with the goal to improve NCs synthesis, processing and performances. However, understanding the nature of surface sites in amine-capped NCs remains challenging, due to the complex surface compositions as well as surface ligands dynamic. Here, we investigate both surface sites and amine ligation in CdSe NCs by combining advanced NMR spectroscopy and computational modelling. Notably, dynamic nuclear polarization (DNP) enhanced 113 Cd and 77 Se 1D NMR helps to identify both bulk and surface sites of NCs, while 113 Cd 2D NMR spectroscopy enables to resolve amines terminated sites on both Se-rich and nonpolar surfaces. In addition to directly bonding to surface sites, amines are shown to also interact through hydrogen-bonding with absorbed water as revealed by 15 N NMR, augmented with computations. The characterization methodology developed for this work provides unique molecular-level insight into the surface sites of a range of amine-capped CdSe NCs, and paves the way to identify structure-function relationships and rational approaches towards colloidal NCs with tailored properties.

Relativistic DFT Calculations of Cadmium and Selenium Solid-State NMR Spectra of CdSe Nanocrystal Surfaces

Solid-state NMR spectra have been used to probe the structure of CdSe nanocrystals and propose detailed models of their surface structures. Density functional theory (DFT)-optimized cluster models that represent probable molecular structures of carboxylate-coordinated surface sites have been proposed. However, to the best of our knowledge, 113Cd and 77Se chemical shifts have not been calculated for these surface models. We performed relativistic DFT calculations of cadmium and selenium magnetic shielding tensors on model compounds with previously measured solid-state NMR spectra with (i) the four-component Dirac-Kohn-Sham (DKS) Hamiltonian and (ii) the scalar and (iii) spin-orbit levels within the ZORA Hamiltonian. Molecular clusters with Cd and Se sites in varying bonding environments were used to model CdSe (100) and CdSe(111) surfaces capped with carboxylic acid ligands. Our calculations identify the observed 113Cd isotropic chemical shifts δ(iso) of -465, -318, and -146 ppm arising from CdSeO3, CdSe2O2, and CdSe3O surface groups, respectively, with very good agreement with experimental measurements. The 113Cd chemical shifts linearly decrease with the number of O-neighbors. The calculated spans (δ11 - δ33) encompass the experimental values for CdSe3O and CdSe2O2 clusters but are slightly larger than the measured value for CdSeO3 clusters. Relativistic DFT calculations predicted a one-bond 113Cd-77Se scalar coupling of 258 Hz, which is in good agreement with the experimental values of 250 Hz. With a dense coverage of carboxylic acid ligands, the CdSe (100) surface shows a distribution of Cd-Se bond lengths and J-couplings. Relativistic DFT simulations thus aid in interpretation of NMR spectra of CdSe nanocrystals and related nanomaterials.

Structural characterization of tin in toothpaste by dynamic nuclear polarization enhanced 119Sn solid-state NMR spectroscopy

Stannous fluoride (SnF2) is an effective fluoride source and antimicrobial agent that is widely used in commercial toothpaste formulations. The antimicrobial activity of SnF2 is partly attributed to the presence of Sn(II) ions. However, it is challenging to directly determine the Sn speciation and oxidation state within commercially available toothpaste products due to the low weight loading of SnF2 (0.454 wt% SnF2, 0.34 wt% Sn) and the amorphous, semi-solid nature of the toothpaste. Here, we show that dynamic nuclear polarization (DNP) enables 119Sn solid-state NMR experiments that can probe the Sn speciation within commercially available toothpaste. Solid-state NMR experiments on SnF2 and SnF4 show that 19F isotropic chemical shift and 119Sn chemical shift anisotropy (CSA) are highly sensitive to the Sn oxidation state. DNP-enhanced 119Sn magic-angle turning (MAT) 2D NMR spectra of toothpastes resolve Sn(II) and Sn(IV) by their 119Sn chemical shift tensor parameters. Fits of DNP-enhanced 1D 1H → 119Sn solid-state NMR spectra allow the populations of Sn(II) and Sn(IV) within the toothpastes to be estimated. This analysis reveals that three of the four commercially available toothpastes contained at least 80% Sn(II), whereas one of the toothpaste contained a significantly higher amount of Sn(IV).

Watch-sized 12 Tesla all-high-temperature-superconducting magnet

We demonstrate the construction of 7 Tesla and 12 Tesla all high-temperature-superconducting (HTS) magnets, small enough to fit on your wrist. The size of the magnet reduces the cost of fabrication, decreases the fringe field to permit facile siting of magnets, and decreases the stored energy of high field magnets. These small HTS-based magnets are being developed for gyrotron microwave sources for use in high-field nuclear magnetic resonance applications. The 7 Tesla and 12 Tesla magnets employ a no-insulation winding technique and are cooled to 4.2 Kelvin in a liquid helium cryostat. The 7 Tesla magnet is a single pancake coil, made of only 9.4 m of HTS tape, with an inner diameter of 8 mm and an outer diameter of 24 mm. This magnet was charged up to 1168 Amperes, generating a field of 7.3 Tesla. The 12 Tesla magnet is comprised of two pancake coils (inner diameter of 10 mm and outer diameter of 27 mm) connected in series. This magnet reached its maximum field at a current of 850 Amperes.

Hierarchy of the Components in Spray-Dried, Protein-Excipient Particles Using DNP-Enhanced NMR Spectroscopy

Protein-based drugs are becoming increasingly important, but there are challenges associated with their formulation (for example, formulating stable inhalable aerosols while maintaining the proper long-term stability of the protein). Determining the morphology of multicomponent, protein-based drug formulations is particularly challenging. Here, we use dynamic nuclear polarization (DNP) solid-state NMR spectroscopy to determine the hierarchy of components within spray-dried particles containing protein, trehalose, leucine, and trileucine. DNP NMR was applied to these formulations to assess the localization of the components within the particles. We found a consistent scheme, where trehalose and the protein are co-located within the same phase in the core of the particles and leucine and trileucine are distributed in separate phases at the surface of the particles. The description of the hierarchy of the organic components determined by DNP NMR enables the rationalization of the performance of the formulation.

A focus on detection of polymorphs by dynamic nuclear polarization solid-state nuclear magnetic resonance spectroscopy

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy has found increasing application as a method for quantification and structure determination of solid forms (polymorphs) of organic solids and active pharmaceutical ingredients (APIs). However, ssNMR spectroscopy suffers from low sensitivity and resolution, making it challenging to detect dilute solid forms that may be present after recrystallization or reaction with co-formers. Cousin et al. (S. F. Cousin et al., Chem. Sci., 2023, https://doi.org/10.1039/D3SC02063K) have demonstrated that dynamic nuclear polarization (DNP) enhanced 13C cross-polarization (CP) saturation recovery experiments can be used to detect dilute polymorphic forms that are present within a mixture of solid forms. Enhancement of the NMR signal by DNP and differences in signal build-up rates for different polymorphs provide the sensitivity and contrast needed to resolve NMR signals from minor polymorphic forms. This method demonstrated by Cousin et al. should aid the discovery of solid drug forms.

Water-Compatible and Recyclable Heterogeneous SABRE Catalyst for NMR Signal Amplification

A water-compatible and recyclable catalyst for nuclear magnetic resonance (NMR) hyperpolarization via signal amplification by reversible exchange (SABRE) was developed. The [Ir(COD)(IMes)Cl] catalyst was attached to a polymeric resin of bis(2-pyridyl)amine (heterogeneous SABRE catalyst, HET-SABRE catalyst), and it amplified the 1H NMR signal of pyridine up to (-) 4455-fold (43.2%) at 1.4 T in methanol and (-) 50-fold (0.5%) in water. These are the highest amplification factors ever reported among HET-SABRE catalysts and for the first time in aqueous media. Moreover, the HET-SABRE catalyst demonstrated recyclability by retaining its activity in water after more than three uses. This newly designed polymeric resin-based heterogeneous catalyst shows great promise for NMR signal amplification for biomedical NMR and MRI applications in the future.

Electron-to-nuclear spectral mapping via dynamic nuclear polarization

We report on a strategy to indirectly read out the spectrum of an electronic spin via polarization transfer to nuclear spins in its local environment. The nuclear spins are far more abundant and have longer lifetimes, allowing for repeated polarization accumulation in them. Subsequent nuclear interrogation can reveal information about the electronic spectral density of states. We experimentally demonstrate the method by reading out the ESR spectrum of nitrogen vacancy center electrons in diamond via readout of lattice 13C nuclei. Spin-lock control on the 13C nuclei yields a significantly enhanced signal-to-noise ratio for the nuclear readout. Spectrally mapped readout presents operational advantages in being background-free and immune to crystal orientation and optical scattering. We harness these advantages to demonstrate applications in underwater magnetometry. The physical basis for the "one-to-many" spectral map is itself intriguing. To uncover its origin, we develop a theoretical model that maps the system dynamics, involving traversal of a cascaded structure of Landau-Zener anti-crossings, to the operation of a tilted "Galton board." This work points to new opportunities for "ESR-via-NMR" in dilute electronic systems and in hybrid electron-nuclear quantum memories and sensors.

Three Decades of REDOR in Protein Science: A Solid-State NMR Technique for Distance Measurement and Spectral Editing

Solid-state NMR (ss-NMR) is a powerful tool to investigate noncrystallizable, poorly soluble molecular systems, such as membrane proteins, amyloids, and cell walls, in environments that closely resemble their physical sites of action. Rotational-echo double resonance (REDOR) is an ss-NMR methodology, which by reintroducing heteronuclear dipolar coupling under magic angle spinning conditions provides intramolecular and intermolecular distance restraints at the atomic level. In addition, REDOR can be exploited as a selection tool to filter spectra based on dipolar couplings. Used extensively as a spectroscopic ruler between isolated spins in site-specifically labeled systems and more recently as a building block in multidimensional ss-NMR pulse sequences allowing the simultaneous measurement of multiple distances, REDOR yields atomic-scale information on the structure and interaction of proteins. By extending REDOR to the determination of 1H-X dipolar couplings in recent years, the limit of measurable distances has reached ~15-20 Å, making it an attractive method of choice for the study of complex biomolecular assemblies. Following a methodological introduction including the most recent implementations, examples are discussed to illustrate the versatility of REDOR in the study of biological systems.

Large cross-effect dynamic nuclear polarisation enhancements with kilowatt inverting chirped pulses at 94 GHz

Dynamic nuclear polarisation (DNP) is a process that transfers electron spin polarisation to nuclei by applying resonant microwave radiation, and has been widely used to improve the sensitivity of nuclear magnetic resonance (NMR). Here we demonstrate new levels of performance for static cross-effect proton DNP using high peak power chirped inversion pulses at 94 GHz to create a strong polarisation gradient across the inhomogeneously broadened line of the mono-radical 4-amino TEMPO. Enhancements of up to 340 are achieved at an average power of a few hundred mW, with fast build-up times (3 s). Experiments are performed using a home-built wideband kW pulsed electron paramagnetic resonance (EPR) spectrometer operating at 94 GHz, integrated with an NMR detection system. Simultaneous DNP and EPR characterisation of other mono-radicals and biradicals, as a function of temperature, leads to additional insights into limiting relaxation mechanisms and give further motivation for the development of wideband pulsed amplifiers for DNP at higher frequencies.

Resonant Mixing Dynamic Nuclear Polarization

We propose a mechanism for dynamic nuclear polarization that is different from the well-known Overhauser effect, solid effect, cross effect, and thermal mixing processes. We term it Resonant Mixing (RM), and we show that it arises from the evolution of the density matrix for a simple electron-nucleus coupled spin pair subject to weak microwave irradiation, the same interactions as the solid effect. However, the SE is optimal when the microwave field is off-resonance, whereas RM is optimal when the microwave field is on-resonance and involves the mixing of states by the microwave field together with the electron-nuclear coupling. Finally, we argue that this mechanism is responsible for the observed dispersive-shaped DNP field profile for trityl samples near the electron paramagnetic resonance center.

Early events in amyloid-β self-assembly probed by time-resolved solid state NMR and light scattering

Self-assembly of amyloid-β peptides leads to oligomers, protofibrils, and fibrils that are likely instigators of neurodegeneration in Alzheimer's disease. We report results of time-resolved solid state nuclear magnetic resonance (ssNMR) and light scattering experiments on 40-residue amyloid-β (Aβ40) that provide structural information for oligomers that form on time scales from 0.7 ms to 1.0 h after initiation of self-assembly by a rapid pH drop. Low-temperature ssNMR spectra of freeze-trapped intermediates indicate that β-strand conformations within and contacts between the two main hydrophobic segments of Aβ40 develop within 1 ms, while light scattering data imply a primarily monomeric state up to 5 ms. Intermolecular contacts involving residues 18 and 33 develop within 0.5 s, at which time Aβ40 is approximately octameric. These contacts argue against β-sheet organizations resembling those found previously in protofibrils and fibrils. Only minor changes in the Aβ40 conformational distribution are detected as larger assemblies develop.

Coherent Dynamic Nuclear Polarization using Chirped Pulses

This paper presents a study of coherent dynamic nuclear polarization (DNP) using frequency swept pulses at 94 GHz which optimize the polarization transfer efficiency. Accordingly, an enhancement ε ∼ 496 was observed using 10 mM trityl-OX063 as the polarizing agent in a standard 6:3:1 d₈-glycerol/D₂O/H₂O glassing matrix at 70 K. At present, this is the largest DNP enhancement reported at this microwave frequency and temperature. Furthermore, the frequency swept pulses enhance the nuclear magnetic resonance (NMR) signal and reduce the recycle delay, accelerating the NMR signal acquisition.

Mechanistic origins of methyl-driven Overhauser DNP

The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1H and 2H Overhauser effects in H3C- or D3C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. We predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60 K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0 K, predominantly due to increases in nuclear relaxation times.

Real-Time Reaction Monitoring of Azide-Alkyne Cycloadditions Using Benchtop NMR-Based Signal Amplification by Reversible Exchange (SABRE)

Rufinamide, possessing a triazole ring, is a new antiepileptic drug (AED) relatively well-absorbed in the lower dose range (10 mg/kg per day) and is currently being used in antiepileptic medications. Triazole derivatives can interact with various enzymes and receptors in biological systems via diverse non-covalent interactions, thus inducing versatile biological effects. Strain-promoted azide-alkyne cycloaddition (SPAAC) is a significant method for obtaining triazoles, even under physiological conditions, in the absence of a copper catalyst. To confirm the progress of chemical reactions under biological conditions, research on reaction monitoring at low concentrations is essential. This promising strategy is gaining acceptance for applications in fields such as drug development and nanoscience. We investigated the optimum Ir catalyst and magnetic field for achieving maximum proton hyperpolarization transfer in triazole derivatives. These reactions were analyzed using signal amplification by reversible exchange (SABRE) to overcome the limitations of low sensitivity in nuclear magnetic resonance spectroscopy, when monitoring copper-free click reactions in real time. Finally, a more versatile copper-catalyzed click reaction was monitored in real time, using a 60 MHz benchtop NMR system, in order to analyze the reaction mechanism.

High-Resolution NMR Spectroscopy at Large Fields with Nitrogen Vacancy Centers

Ensembles of nitrogen-vacancy (NV) centers are used as sensors to detect nuclear magnetic resonance signals from micron-sized samples at room temperature. In this scenario, the regime of large magnetic fields is especially interesting as it leads to a large nuclear thermal polarization-thus, to a strong sensor response even in low concentration samples-while chemical shifts and J couplings become more accessible. Nevertheless, this regime remains largely unexplored owing to the difficulties of coupling NV-based sensors with high-frequency nuclear signals. In this Letter, we circumvent this problem with a method that maps the relevant energy shifts in the amplitude of an induced nuclear spin signal that is subsequently transferred to the sensor. This stage is interspersed with free-precession periods of the sample nuclear spins where the sensor does not participate. Thus, our method leads to high spectral resolutions ultimately limited by the coherence of the nuclear spin signal.

General Strategy for Incorporation of Functional Group Handles into Covalent Organic Frameworks via the Ugi Reaction

The library of imine-linked covalent organic frameworks (COFs) has grown significantly over the last two decades, featuring a variety of morphologies, pore sizes, and applications. An array of synthetic methods has been developed to expand the scope of the COF functionalities; however, most of these methods were designed to introduce functional scaffolds tailored to a specific application. Having a general approach to diversify COFs via late-stage incorporation of functional group handles would greatly facilitate the transformation of these materials into platforms for a variety of useful applications. Herein, we report a general strategy to introduce functional group handles in COFs via the Ugi multicomponent reaction. To demonstrate the versatility of this approach, we have synthesized two COFs with hexagonal and kagome morphologies. We then introduced azide, alkyne, and vinyl functional groups, which could be readily utilized for a variety of post-synthetic modifications. This facile approach enables the functionalization of any COFs containing imine linkages.

The Effect of Disorder on Endogenous MAS-DNP: Study of Silicate Glasses and Crystals

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li₂OCaO·2SiO₂ and Li₂CaSiO₄, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both ⁶Li and ²⁹Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

1H chemical shift anisotropy: a high sensitivity solid-state NMR dynamics probe for surface studies?

Dynamics play significant roles in chemistry and biochemistry-molecular motions impact both large- and small-scale chemical reactions in addition to biochemical processes. In many systems, including heterogeneous catalysts, the characterization of dynamics remains a challenge. The most common approaches involve the solid-state NMR measurement of anisotropic interactions, in particular ²H quadrupolar coupling and ¹H-X dipolar coupling, which generally require isotope enrichment. Due to the high sensitivity of ¹H NMR, ¹H chemical shift anisotropy (CSA) is a particularly enticing, and underexplored, dynamics probe. We carried out ¹H CSA and ¹H-¹³C dipolar coupling measurements in a series of model supported complexes to understand how ¹H CSA can be leveraged to gain dynamic information for heterogeneous catalysts. Mathematical descriptions are given for the dynamic averaging of the CSA tensor, and its dependence on orientation and asymmetry. The variability of the orientation of the tensor in the molecular frame, in addition to its magnitude and asymmetry, negatively impacts attempts to extract quantitative dynamic information. Nevertheless, ¹H CSA measurements can reveal useful qualitative insights into the motions of a particularly dilute site, such as from a surface species.

Nanomaterials for hyperpolarized nuclear magnetic resonance and magnetic resonance imaging

Nanomaterials play an important role in the development and application of hyperpolarized materials for magnetic resonance imaging (MRI). In this context they can not only act as hyperpolarized materials which are directly imaged but also play a role as carriers for hyperpolarized gases and catalysts for para-hydrogen induced polarization (PHIP) to generate hyperpolarized substrates for metabolic imaging. Those three application possibilities are discussed, focusing on carbon-based materials for the directly imaged particles. An overview over recent developments in all three fields is given, including the early developments in each field as well as important steps towards applications in MRI, such as making the initially developed methods more biocompatible and first imaging experiments with spatial resolution in either phantoms or in vivo studies. Focusing on the important features nanomaterials need to display to be applicable in the MRI context, a wide range of different approaches to that extent is covered, giving the reader a general idea of different possibilities as well as recent developments in those different fields of hyperpolarized magnetic resonance. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

DESPERATE: A Python library for processing and denoising NMR spectra

NMR spectroscopy is an inherently insensitive technique with respect to the amount of observable signal. A common element in all NMR spectra is random thermal noise that is often characterized by a signal-to-noise ratio (SNR). SNR can be generically improved experimentally with repetitive signal averaging or during post-processing with apodization; the former of which often results in long experimental times and the latter results in the loss of spectral resolution. Denoising techniques can instead be used during post-processing to enhance SNR without compromising resolution. The most common approach relies on the singular-value decomposition (SVD) to discard noisy components of NMR data. SVD-based approaches work well, such as Cadzow and PCA, but are computationally expensive when used for large datasets that are often encountered in NMR (e.g., Carr-Purcell/Meiboom-Gill and nD datasets). Herein, we describe the implementation of a new wavelet transform (WT) routine for the fast and robust denoising of 1D and 2D NMR spectra. Several simulated and experimental datasets are denoised with both SVD-based Cadzow or PCA and WT's, and the resulting SNR enhancements and spectral uniformity are compared. WT denoising offers similar and improved denoising compared with SVD and operates faster by several orders-of-magnitude in some cases. All denoising and processing routines used in this work are included in a free and open-source Python library called DESPERATE.

GTP-Bound Escherichia coli FtsZ Filaments Are Composed of Tense Monomers: a Dynamic Nuclear Polarization-Nuclear Magnetic Resonance Study Using Interface Detection

FtsZ filaments are the major structural component of the bacterial Z ring and are drivers of bacterial division. Crystal structures for FtsZ from some Gram-positive bacteria in the presence of GTP analogs suggest the possibility of a high-energy, "tense" conformation. It remains important to elucidate whether this tense form is the dominant form in filaments. Using dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) and differential isotopic labeling, we directly detected residues located at the intermonomer interface of GTP-bound wild-type (WT) Escherichia coli FtsZ filaments. We combined chemical shift prediction, homology modeling, and heteronuclear dipolar recoupling techniques to characterize the E. coli FtsZ filament interface and demonstrated that the monomers in active filaments assume a tense conformation. IMPORTANCE Bacterial replication is dependent on the cytoskeletal protein FtsZ, which forms filaments that scaffold and recruit other essential division proteins. While the FtsZ monomer is well studied across organisms, many questions remain about how the filaments form and function. Recently, a second monomer form was identified in Staphylococcus aureus that has far-reaching implications for FtsZ structure and function. However, to date, this form has not been directly observed outside S. aureus. In this study, we used solid-state NMR and dynamic nuclear polarization (DNP) to directly study the filaments of E. coli FtsZ to demonstrate that E. coli FtsZ filaments are primarily composed of this second, "tense" form of the monomer. This work is the first time GTP-bound, wild-type FtsZ filaments have been studied directly at atomic resolution and is an important step forward for the study of FtsZ filaments.

Geometrization for Energy Levels of Isotropic Hyperfine Hamiltonian Block and Related Central Spin Problems for an Arbitrarily Complex Set of Spin-1/2 Nuclei

Description of interacting spin systems relies on understanding the spectral properties of the corresponding spin Hamiltonians. However, the eigenvalue problems arising here lead to algebraic problems too complex to be analytically tractable. This is already the case for the simplest nontrivial (Kmax−1) block for an isotropic hyperfine Hamiltonian for a radical with spin-12 nuclei, where n nuclei produce an n-th order algebraic equation with n independent parameters. Systems described by such blocks are now physically realizable, e.g., as radicals or radical pairs with polarized nuclear spins, appear as closed subensembles in more general radical settings, and have numerous counterparts in related central spin problems. We provide a simple geometrization of energy levels in this case: given n spin-12 nuclei with arbitrary positive couplings ai, take an n-dimensional hyper-ellipsoid with semiaxes ai, stretch it by a factor of n+1 along the spatial diagonal (1, 1, …, 1), read off the semiaxes of thus produced new hyper-ellipsoid qi, augment the set {qi} with q0=0, and obtain the sought n+1 energies as Ek=−12qk2+14∑iai. This procedure provides a way of seeing things that can only be solved numerically, giving a useful tool to gain insights that complement the numeric simulations usually inevitable here, and shows an intriguing connection to discrete Fourier transform and spectral properties of standard graphs.

Room temperature DNP of diamond powder using frequency modulation

Dynamic nuclear polarization (DNP) is a method of enhancing NMR signals via the transfer of polarization from electron spins to nuclear spins using microwave (MW) irradiation. In most cases, monochromatic continuous-wave (MCW) MW irradiation is used. Recently, several groups have shown that frequency modulation of the MW irradiation can result in an additional increase in DNP enhancement above that obtained with MCW. The effect of frequency modulation on the solid effect (SE) and the cross effect (CE) has previously been studied using the stable organic radical 4-hydroxy TEMPO (TEMPOL) at temperatures under 20 K. Here, in addition to the SE and CE, we discuss the effect of frequency modulation on the Overhauser effect (OE) and the truncated CE (tCE) in the room-temperature ¹³C-DNP of diamond powders. We recently showed that diamond powders can exhibit multiple DNP mechanisms simultaneously due to the heterogeneity of P1 (substitutional nitrogen) environments within diamond crystallites. We explore how the two parameters that define the frequency modulation: (i) the Modulation frequency, f_m (how fast the microwave frequency is varied) and (ii) the Modulation amplitude, Δω (the magnitude of the change in microwave frequency) influence the enhancement obtained via each mechanism. Frequency modulation during DNP not only allows us to improve DNP enhancement, but also gives us a way to control which DNP mechanism is most active. By choosing the appropriate modulation parameters, we can selectively enhance some mechanisms while simultaneously suppressing others.

MAS-DNP enables NMR studies of insect wings

NMR is a valuable tool for studying insects. Solid-state NMR has been used to obtain the chemical composition and gain insight into the sclerotization process of exoskeletons. There is typically little difficulty in obtaining sufficient sample quantity for exoskeletons. However, obtaining enough sample of other insect components for solid-state NMR experiments can be problematic while isotopically enriching them is near impossible. This is especially the case for insect wing membranes which is of interest to us. Issues with obtaining sufficient sample are the thickness of wing membranes is on the order of microns, each membrane region is surrounded by veins and occupies a small area, and the membranes are separated from the wing by physical dissection. Accordingly, NMR signal enhancement methods are needed. MAS-DNP has a track record of providing significant signal enhancements for a wide variety of materials. Here we demonstrate that MAS-DNP is useful for providing high quality one-dimensional and two-dimensional solid-state NMR spectra on cicada wing membrane at natural isotopic abundance.

Biradical Polarizing Agents at High Fields

The sensitivity enhancements available from dynamic nuclear polarization (DNP) are rapidly reshaping the research landscape and expanding the field of nuclear magnetic resonance (NMR) spectroscopy as a tool for solving complex chemical and structural problems. The past decade has seen considerable advances in this burgeoning method, while efforts to further improve its capabilities continue along many avenues. In this report, we examine the influence of static magnetic field strength and temperature on the reported ¹H DNP enhancements from three conventional organic biradicals: TOTAPOL, AMUPol, and SPIROPOL. In contrast to the conventional wisdom, our findings show that at liquid nitrogen temperatures and 700 MHz/460.5 GHz, these three bisnitroxides all provide similar ¹H DNP enhancements, ε ≈ 60. Furthermore, we investigate the influence of temperature, microwave power, magnetic field strength, and protein sample deuteration on the NMR experimental results.

Proton-detected solution-state NMR at 14.1 T based on scalar-driven 13C Overhauser dynamic nuclear polarization

Overhauser dynamic nuclear polarization (ODNP) NMR of solutions at high fields is usually mediated by scalar couplings that polarize the nuclei of heavier, electron-rich atoms. This leaves ¹H-detected NMR outside the realm of such studies. This study presents experiments that deliver ¹H-detected NMR experiments on relatively large liquid volumes (60 ∼ 100 μL) and at high fields (14.1 T), while relying on ODNP enhancements. To this end ¹³C NMR polarizations were first enhanced by relying on a mechanism that utilizes e^--¹³C scalar coupling interactions; the nuclear spin alignment thus achieved was then passed on to neighboring ¹H for observation, by a reverse INEPT scheme relying on one-bond J_CH-couplings. Such ¹³C →¹H polarization transfer ported the ¹³C ODNP gains into the ¹H, permitting detection at higher frequencies and with higher potential sensitivities. For a model solution of labeled ¹³CHCl₃ comixed with a nitroxide-based TEMPO derivative as polarizing agent, an ODNP enhancement factor of ca. 5x could thus be imparted to the ¹H signal. When applied to bigger organic molecules like 2-¹³C-phenylacetylene and ¹³C₈-indole, ODNP enhancements in the 1.2-3x range were obtained. Thus, although handicapped by the lower γ of the ¹³C, enhancements could be imparted on the ¹H thermal acquisitions in all cases. We also find that conventional ¹H-¹³C nuclear Overhauser enhancements (NOEs) are largely absent in these solutions due to the presence of co-dissolved radicals, adding negligible gains and playing negligible roles on the scalar e^-→¹³C ODNP transfer. Potential rationalizations of these effects as well as extensions of these experiments, are briefly discussed.

Functionalized Controlled Porous Glasses for Producing Radical-Free Hyperpolarized Liquids by Overhauser DNP

Overhauser dynamic nuclear polarization (ODNP) can be used as a tool for NMR signal enhancement and happens on very short time scales. Therefore, ODNP is well suited for the measurement of fast-flowing samples, even in compact magnets, which is beneficial for the real-time monitoring of chemical reactions or processes. ODNP requires the presence of unpaired electrons in the sample, which is usually accomplished by the addition of stable radicals. However, radicals affect the nuclear relaxation times and can hamper the NMR detection. This is circumvented by immobilizing radicals in a packed bed allowing for the measurement of radical-free samples when using ex situ DNP techniques (DNP build-up and NMR detection happen at different places) and flow-induced separation of the hyperpolarized liquid from the radicals. Therefore, the synthesis of robust and chemically inert immobilized radical matrices is mandatory. In the present work, this is accomplished by immobilizing the radical glycidyloxy-tetramethylpiperidinyloxyl with a polyethyleneimine (PEI) linker on the surface of controlled porous glasses (CPG). Both the porosity of the CPGs and also the size of the PEI-linker were varied, resulting in a set of distinct radical matrices for continuous-flow ODNP. The study shows that CPGs with PEI-linkers provide robust, inert and efficient ODNP matrices.

Enzyme kinetics by real-time quantitative NMR (qNMR) spectroscopy with progress curve analysis

This review article summarizes how the experimental data obtained using quantitative nuclear magnetic resonance (qNMR) spectroscopy can be combined with progress curve analysis to determine enzyme kinetic parameters. The qNMR approach enables following the enzymatic conversion of the substrate to the product in real-time by a continuous collection of spectra. The Lambert-W function, a closed-form solution to the time-dependent substrate/product kinetics of the rate equation, can estimate the Michaelis-Menten constant (K_M.) and the maximum velocity (V_max) from a single experiment. This article highlights how the qNMR data is well suited for analysis using the Lambert-W function with three different applications. Results from studies on acetylcholinesterase (acetylcholine to acetic acid and choline), β-Galactosidase (lactose to glucose and galactose), and invertase (sucrose to glucose and fructose) are presented. Furthermore, an additional example of how the progress curve analysis is applied to understand the inhibitory role of the artificial sweetener sucralose on sucrose's enzymatic conversion by invertase is discussed. With the wide availability of NMR spectrometers in academia and industries, including bench-top systems with permanent magnets, and the potential to enhance sensitivity using dynamic nuclear polarization in combination with ultrafast methods, the NMR-based enzyme kinetics could be considered a valuable tool for broader applications in the field of enzyme kinetics.

Exploring Biomolecular Self-Assembly with Far-Infrared Radiation

Physical engineering technology using far-infrared radiation has been gathering attention in chemical, biological, and material research fields. In particular, the high-power radiation at the terahertz region can give remarkable effects on biological materials distinct from a simple thermal treatment. Self-assembly of biological molecules such as amyloid proteins and cellulose fiber plays various roles in medical and biomaterials fields. A common characteristic of those biomolecular aggregates is a sheet-like fibrous structure that is rigid and insoluble in water, and it is often hard to manipulate the stacking conformation without heating, organic solvents, or chemical reagents. We discovered that those fibrous formats can be conformationally regulated by means of intense far-infrared radiations from a free-electron laser and gyrotron. In this review, we would like to show the latest and the past studies on the effects of far-infrared radiation on the fibrous biomaterials and to suggest the potential use of the far-infrared radiation for regulation of the biomolecular self-assembly.

Tools shaping drug discovery and development

Spectroscopic, scattering, and imaging methods play an important role in advancing the study of pharmaceutical and biopharmaceutical therapies. The tools more familiar to scientists within industry and beyond, such as nuclear magnetic resonance and fluorescence spectroscopy, serve two functions: as simple high-throughput techniques for identification and purity analysis, and as potential tools for measuring dynamics and structures of complex biological systems, from proteins and nucleic acids to membranes and nanoparticle delivery systems. With the expansion of commercial small-angle x-ray scattering instruments into the laboratory setting and the accessibility of industrial researchers to small-angle neutron scattering facilities, scattering methods are now used more frequently in the industrial research setting, and probe-less time-resolved small-angle scattering experiments are now able to be conducted to truly probe the mechanism of reactions and the location of individual components in complex model or biological systems. The availability of atomic force microscopes in the past several decades enables measurements that are, in some ways, complementary to the spectroscopic techniques, and wholly orthogonal in others, such as those related to nanomechanics. As therapies have advanced from small molecules to protein biologics and now messenger RNA vaccines, the depth of biophysical knowledge must continue to serve in drug discovery and development to ensure quality of the drug, and the characterization toolbox must be opened up to adapt traditional spectroscopic methods and adopt new techniques for unraveling the complexities of the new modalities. The overview of the biophysical methods in this review is meant to showcase the uses of multiple techniques for different modalities and present recent applications for tackling particularly challenging situations in drug development that can be solved with the aid of fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, atomic force microscopy, and small-angle scattering.

Time-resolved solid state NMR of biomolecular processes with millisecond time resolution

We review recent efforts to develop and apply an experimental approach to the structural characterization of transient intermediate states in biomolecular processes that involve large changes in molecular conformation or assembly state. This approach depends on solid state nuclear magnetic resonance (ssNMR) measurements that are performed at very low temperatures, typically 25-30 K, with signal enhancements from dynamic nuclear polarization (DNP). This approach also involves novel technology for initiating the process of interest, either by rapid mixing of two solutions or by a rapid inverse temperature jump, and for rapid freezing to trap intermediate states. Initiation by rapid mixing or an inverse temperature jump can be accomplished in approximately-one millisecond. Freezing can be accomplished in approximately 100 microseconds. Thus, millisecond time resolution can be achieved. Recent applications to the process by which the biologically essential calcium sensor protein calmodulin forms a complex with one of its target proteins and the process by which the bee venom peptide melittin converts from an unstructured monomeric state to a helical, tetrameric state after a rapid change in pH or temperature are described briefly. Future applications of millisecond time-resolved ssNMR are also discussed briefly.

1H Hyperpolarization of Solutions by Overhauser Dynamic Nuclear Polarization with 13C-1H Polarization Transfer

Dynamic nuclear polarization (DNP) is a method that can significantly increase the sensitivity of nuclear magnetic resonance. The only effective DNP mechanism for in situ hyperpolarization in solution is Overhauser DNP, which is inefficient for ¹H at high magnetic fields. Here we demonstrate the possibility of generating significant ¹H hyperpolarization in solution at room temperature. To counter the poor direct ¹H Overhauser DNP, we implement steady-state ¹³C Overhauser DNP in solutions and then transfer the ¹³C hyperpolarization to ¹H via a reverse insensitive nuclei enhanced by polarization transfer scheme. We demonstrate this approach using a 400 MHz gyrotron-equipped 3.2 mm magic angle spinning DNP system to obtain ¹H DNP enhancement factors of 48, 8, and 6 for chloroform, tetrachloroethane, and phenylacetylene, respectively, at room temperature.

Residue-Specific High-Resolution 17O Solid-State Nuclear Magnetic Resonance of Peptides: Multidimensional Indirect 1H Detection and Magic-Angle Spinning

Oxygen is an integral component of proteins but remains sparsely studied because its only NMR active isotope, ¹⁷O, has low sensitivity, low resolution, and large quadrupolar couplings. These issues are addressed here with efficient isotopic labeling, high magnetic fields, fast sample spinning, and ¹H detection in conjunction with multidimensional experiments to observe oxygen sites specific to each amino acid residue. Notably, cross-polarization at high sample spinning frequencies provides efficient ¹³C ↔ ¹⁷O polarization transfer. The use of ¹⁷O for initial polarization is found to provide better sensitivity per unit time compared to ¹H. Sharp isotropic ¹⁷O peaks are obtained by using a low-power multiple-quantum sequence, which in turn allows extraction of quadrupolar parameters for each oxygen site. Finally, the potential to determine sequential assignments and long-range distance restraints is demonstrated by using 3D ¹H/¹³C/¹⁷O experiments, suggesting that such methods can become an essential tool for biomolecular structure determination.