Four millimeter spherical rotors spinning at 28 kHz with double-saddle coils for cross polarization NMR

High-field pulsed EPR spectroscopy under magic angle spinning

In this work, we demonstrate the first pulsed electron paramagnetic resonance (EPR) experiments performed under magic angle spinning (MAS) at high magnetic field. Unlike nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP), commonly performed at high magnetic fields and under MAS to maximize sensitivity and resolution, EPR is usually measured at low magnetic fields and, with the exception of the Spiess group work in the late 1990s, never under MAS, due to great instrumentational challenges. This hampers the investigation of DNP mechanisms, in which electron spin dynamics play a central role, because no experimental data about the latter under DNP-characteristic conditions are available. We hereby present our dedicated, homebuilt MAS-EPR probehead and show the pulsed MAS-EPR spectra of P1 center diamond defect recorded at 7 tesla. Our results reveal unique effects of MAS on EPR line shape, intensity, and signal dephasing. Time-domain simulations reproduce the observed changes in the line shapes and the trends in the signal intensity.

Cryogenic-compatible spherical rotors and stators for magic angle spinning dynamic nuclear polarization

Cryogenic magic angle spinning (MAS) is a standard technique utilized for dynamic nuclear polarization (DNP) in solid-state nuclear magnetic resonance (NMR). Here we describe the optimization and implementation of a stator for cryogenic MAS with 9.5 mm diameter spherical rotors, allowing for DNP experiments on large sample volumes. Designs of the stator and rotor for cryogenic MAS build on recent advancements of MAS spheres and take a step further to incorporate sample insert and eject and a temperature-independent spinning stability of ± 1  Hz. At a field of 7 T and spinning at 2.0 kHz with a sample temperature of 105-107 K, DNP enhancements of 256 and 200 were observed for 124 and 223 µ L sample volumes, respectively, each consisting of 4 M 13 C, 15 N-labeled urea and 20 mM AMUPol in a glycerol-water glassy matrix.

3D-Printable centrifugal devices for biomolecular solid state NMR rotors

The advent of magic angle spinning (MAS) rates exceeding 100 kHz has facilitated the acquisition of 1H-detected solid-state NMR spectra of biomolecules with high resolution. However, challenges can arise when preparing rotors for these experiments, due to the physical properties of biomolecular solid samples and the small dimensions of the rotors. In this study, we have designed 3D-printable centrifugal devices that facilitate efficient and consistent packing of crystalline protein slurries or viscous phospholipids into 0.7 mm rotors. We demonstrate the efficacy of these packing devices using 1H-detected solid state NMR at 105 kHz. In addition to devices for 0.7 mm rotors, we have also developed devices for other frequently employed rotor sizes and styles. We have made all our designs openly accessible, and we encourage their usage and ongoing development as a shared effort within the solid state NMR community.

High resolution solid-state NMR on the desktop

High-resolution low-field nuclear magnetic resonance (NMR) spectroscopy has found wide application for characterization of liquid compounds because of the low maintenance cost of modern permanent magnets. Solid-state NMR so far is limited to low-resolution measurements of static powders, because of the limited space available in this type of magnet. Magic-angle sample spinning and low-magnetic fields are an attractive combination to achieve high spectral resolution especially for paramagnetic solids. Here we show that magic angle spinning modules can be miniaturized using 3D printing techniques so that high-resolution solid-state NMR in permanent magnets becomes possible. The suggested conical rotor design was developed using finite element calculations and provides sample spinning frequencies higher than 20 kHz. The setup was tested on various diamagnetic and paramagnetic compounds including paramagnetic battery materials. The only comparable experiments in low-cost magnets known so far, had been done in the early times of magic angle spinning using electromagnets at much lower sample spinning frequency. Our results demonstrate that high-resolution low-field magic-angle-spinning NMR does not require expensive superconducting magnets and that high-resolution solid-state NMR spectra of paramagnetic compounds are feasible. Generally, this could introduce low-field solid-state NMR for abundant nuclei standard as a routine analytical tool.

Electron-decoupled MAS DNP with N@C60

Frequency-chirped microwaves decouple electron- and 13C-spins in magic-angle spinning N@C60:C60 powder, improving DNP-enhanced 13C NMR signal intensity by 12% for 7 s polarization, and 5% for 30 s polarization. This electron decoupling demonstration is a step toward utilizing N@C60 as a controllable electron-spin source for magic-angle spinning magnetic resonance experiments.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Improving the sensitivity of MAS spheres using a 9.5 mm spherical shell with 219 μL sample volume spinning in a spherical solenoid coil

Spherical rotors in magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments have potential advantages relative to cylindrical rotors in terms of ease of fabrication, low risk of rotor crash, easy sample exchange, and better microwave access. However, one major disadvantage so far of spherical rotors is poor NMR filling factor due to the small sample volume and large cylindrical radiofrequency (RF) coil. Here we present a novel NMR coil geometry in the form of a spherical coil. The spherical coil best fits the spherical sample to maximize sensitivity, while also providing excellent RF homogeneity. We further improve NMR sensitivity by employing a spherical shell as the rotor, thereby maximizing sample volume (219 μL in this case of 9.5 mm outer diameter spheres). The spinning gas is supplied by a 3D-printed ring stator external to the coil, thereby introducing a simplified form of MAS stators. In this apparatus, the RF field generated along the coil axis is perpendicular to the external magnetic field, regardless of rotor orientation. We observe a linear increase in sensitivity with increasing sample volume. We also simulate the RF performance of spherical and cylindrical solenoid coils with constant or variable pitch for spherical and cylindrical rotors, respectively. The simulation results show that spherical solenoid coils generate comparable B₁ field intensities but have better homogeneity than cylindrical solenoid coils do.

Field and magic angle spinning frequency dependence of proton resonances in rotating solids

Proton detection in solid state NMR is continuously developing and allows one to gain new insights in structural biology. Overall, this progress is a result of the synergy between hardware development, new NMR methodology and new isotope labeling strategies, to name a few factors. Even though current developments are rapid, it is worthwhile to summarize what can currently be achieved employing proton detection in biological solids. We illustrate this by analysing the signal-to-noise ratio (SNR) for spectra obtained for a microcrystalline α-spectrin SH3 domain protein sample by (i) employing different degrees of chemical dilution to replace protons by incorporating deuterons in different sites, by (ii) variation of the magic angle spinning (MAS) frequencies between 20 and 110 kHz, and by (iii) variation of the static magnetic field B₀. The experimental SNR values are validated with numerical simulations employing up to 9 proton spins. Although in reality a protein would contain far more than 9 protons, in a deuterated environment this is a sufficient number to achieve satisfactory simulations consistent with the experimental data. The key results of this analysis are (i) with current hardware, deuteration is still necessary to record spectra of optimum quality; (ii) 13CH3 isotopomers for methyl groups yield the best SNR when MAS frequencies above 100 kHz are available; and (iii) sensitivity increases with a factor beyond B0 3/2 with the static magnetic field due to a transition of proton-proton dipolar interactions from a strong to a weak coupling limit.

Solid-State NMR: Methods for Biological Solids

In the last two decades, solid-state nuclear magnetic resonance (ssNMR) spectroscopy has transformed from a spectroscopic technique investigating small molecules and industrial polymers to a potent tool decrypting structure and underlying dynamics of complex biological systems, such as membrane proteins, fibrils, and assemblies, in near-physiological environments and temperatures. This transformation can be ascribed to improvements in hardware design, sample preparation, pulsed methods, isotope labeling strategies, resolution, and sensitivity. The fundamental engagement between nuclear spins and radio-frequency pulses in the presence of a strong static magnetic field is identical between solution and ssNMR, but the experimental procedures vastly differ because of the absence of molecular tumbling in solids. This review discusses routinely employed state-of-the-art static and MAS pulsed NMR methods relevant for biological samples with rotational correlation times exceeding 100's of nanoseconds. Recent developments in signal filtering approaches, proton methodologies, and multiple acquisition techniques to boost sensitivity and speed up data acquisition at fast MAS are also discussed. Several examples of protein structures (globular, membrane, fibrils, and assemblies) solved with ssNMR spectroscopy have been considered. We also discuss integrated approaches to structurally characterize challenging biological systems and some newly emanating subdisciplines in ssNMR spectroscopy.

3D-printed stators & drive caps for magic-angle spinning NMR

3D printing has evolved into an invaluable tool for rapid and cost-effective production of intricate parts. In this paper we describe 3D printing and other rapid prototyping methods to fabricate 3.2 mm stators and drive caps for use in magic angle spinning (MAS) NMR experiments. These components can be fabricated with the assistance of computer-aided design (CAD) software and at a fraction of the cost of commercial parts. Additionally, we show that the performance of these 3D printed stators and drive caps is comparable to commercially available systems and that they have significant advantages over their machined counterparts.

Deuteration for High-Resolution Detection of Protons in Protein Magic Angle Spinning (MAS) Solid-State NMR

Proton detection developed in the last 20 years as the method of choice to study biomolecules in the solid state. In perdeuterated proteins, proton dipolar interactions are strongly attenuated, which allows yielding of high-resolution proton spectra. Perdeuteration and backsubstitution of exchangeable protons is essential if samples are rotated with MAS rotation frequencies below 60 kHz. Protonated samples can be investigated directly without spin dilution using proton detection methods in case the MAS frequency exceeds 110 kHz. This review summarizes labeling strategies and the spectroscopic methods to perform experiments that yield assignments, quantitative information on structure, and dynamics using perdeuterated samples. Techniques for solvent suppression, H/D exchange, and deuterium spectroscopy are discussed. Finally, experimental and theoretical results that allow estimation of the sensitivity of proton detected experiments as a function of the MAS frequency and the external B₀ field in a perdeuterated environment are compiled.

Stable magic angle spinning with Low-Cost 3D-Printed parts

A 3D-printed double-bearing magic angle spinning (MAS) system was developed with a home-built 4.0 mm MAS nuclear magnetic resonance (NMR) probe at 7 T. Various fused deposition modelling 3D printers were used to produce spinning modules of ignorable materials costs for rotors with a diameter of 7.0, 4.0, and 3.5 mm. High-performance MAS experiments on the 4.0 mm-diameter model using a pencil-type ceramic rotor and 3D-printed drive cap resulted in a high-resolution ¹H NMR signal of silicone grease. The 3.5 mm-diameter MAS system reached a spinning frequency of 23 kHz. Furthermore, 3D-printed inserts were designed for various rotor sizes which can isolate the sample from humidity for a duration of more than a week. Single crystal inserts for MAS rotors of commercial probes can readily be printed using two-color printers. Those developments enable customized low-cost MAS NMR for both adapting existing and manufacturing new probes, respectively.

Residual dipolar line width in magic-angle spinning proton solid-state NMR

Magic-angle spinning is routinely used to average anisotropic interactions in solid-state nuclear magnetic resonance (NMR). Due to the fact that the homonuclear dipolar Hamiltonian of a strongly coupled spin system does not commute with itself at different time points during the rotation, second-order and higher-order terms lead to a residual dipolar line broadening in the observed resonances. Additional truncation of the residual broadening due to isotropic chemical-shift differences can be observed. We analyze the residual line broadening in coupled proton spin systems based on theoretical calculations of effective Hamiltonians up to third order using Floquet theory and compare these results to numerically obtained effective Hamiltonians in small spin systems. We show that at spinning frequencies beyond 75 kHz, second-order terms dominate the residual line width, leading to a 1 / ω r dependence of the second moment which we use to characterize the line width. However, chemical-shift truncation leads to a partial ω r - 2 dependence of the line width which looks as if third-order effective Hamiltonian terms are contributing significantly. At slower spinning frequencies, cross terms between the chemical shift and the dipolar coupling can contribute in third-order effective Hamiltonians. We show that second-order contributions not only broaden the line, but also lead to a shift of the center of gravity of the line. Experimental data reveal such spinning-frequency-dependent line shifts in proton spectra in model substances that can be explained by line shifts induced by the second-order dipolar Hamiltonian.

Solid-state NMR spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopy is an atomic-level method used to determine the chemical structure, three-dimensional structure, and dynamics of solids and semi-solids. This Primer summarizes the basic principles of NMR as applied to the wide range of solid systems. The fundamental nuclear spin interactions and the effects of magnetic fields and radiofrequency pulses on nuclear spins are the same as in liquid-state NMR. However, because of the anisotropy of the interactions in the solid state, the majority of high-resolution solid-state NMR spectra is measured under magic-angle spinning (MAS), which has profound effects on the types of radiofrequency pulse sequences required to extract structural and dynamical information. We describe the most common MAS NMR experiments and data analysis approaches for investigating biological macromolecules, organic materials, and inorganic solids. Continuing development of sensitivity-enhancement approaches, including 1H-detected fast MAS experiments, dynamic nuclear polarization, and experiments tailored to ultrahigh magnetic fields, is described. We highlight recent applications of solid-state NMR to biological and materials chemistry. The Primer ends with a discussion of current limitations of NMR to study solids, and points to future avenues of development to further enhance the capabilities of this sophisticated spectroscopy for new applications.

Highly stable magic angle spinning spherical rotors

The use of spherical rotors for magic angle spinning offers a number of advantages, including improved sample exchange, efficient microwave coupling for dynamic nuclear polarization nuclear magnetic resonance (NMR) experiments, and, most significantly, high frequency and stable spinning with minimal risk of rotor crash. Here we demonstrate the simple retrofitting of a commercial NMR probe with MAS spheres for solid-state NMR. We analyze a series of turbine groove geometries to investigate the importance of the rotor surface for spinning performance. Of note, rotors lacking any surface modification spin rapidly and stably even without feedback control. The high stability of a spherical rotor about the magic angle is shown to be dependent on its inertia tensor rather than the presence of turbine grooves.