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Magic angle spinning (MAS) dynamic nuclear polarization (DNP) increases the signal intensity of solid-state nuclear magnetic resonance. DNP typically uses continuous wave (CW) microwave irradiation close to the resonance frequency of unpaired electron spins. In this study, we demonstrate that frequency-chirped microwaves improve DNP performance under MAS. By modulating the gyrotron anode potential, we generate a train of microwave chirps with a maximum bandwidth of 310 MHz and a maximum incident power on the spinning sample of 18 W. We characterize the efficiency of chirped DNP using the following polarizing agents: TEMTriPol-1, AsymPolPOK, AMUPol, and Finland trityl. The effects of different chirp widths and periods are analyzed at different MAS frequencies and microwave powers. Furthermore, we show that chirped DNP can be combined with electron decoupling to improve signal intensity by 59%, compared to CW DNP without electron decoupling, using Finland trityl as a polarizing agent.
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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.
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Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 °C. Insight into the atomic-scale structure of these shells was obtained by high-field 71Ga solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and 29Si dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere ([4]Ga(Si)) and, according to 15N DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Brønsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-µ2-OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaOx), i.e., an increased relative fraction of pentacoordinate sites ([5]Ga(Ga)), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO2-500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10-SiO2-500. In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomic-scale mixing of Al sites with silica, leading to Al-µ2-OH-Si sites. Our results suggest that [4]Ga(Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of [5]Ga(Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaOx-based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Brønsted acidity (nature, strength, and abundance).
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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 9.5â¯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 ±1â¯Hz. At a field of 7â¯T and spinning at 2.0â¯kHz with a sample temperature of 105-107â¯K, DNP enhancements of 256 and 200 were observed for 124 and 223⯵L sample volumes, respectively, each consisting of 4â¯M 13C, 15N-labeled urea and 20â¯mM AMUPol in a glycerol-water glassy matrix.
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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.
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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.
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Imageamento por Ressonância Magnética , Ondas de Rádio , Espectroscopia de Ressonância Magnética/métodos , Micro-Ondas , Simulação por ComputadorRESUMO
We demonstrate for the first time in-cell dynamic nuclear polarization (DNP) in conjunction with flow cytometry sorting to address the cellular heterogeneity of in-cell samples. Utilizing a green fluorescent protein (GFP) reporter of HIV reactivation, we correlate increased 15N resonance intensity with cytokine-driven HIV reactivation in a human cell line model of HIV latency. As few as 10% GFP+ cells could be detected by DNP nuclear magnetic resonance (NMR). The inclusion of flow cytometric sorting of GFP+ cells prior to analysis by DNP-NMR further boosted signal detection through increased cellular homogeneity with respect to GFP expression. As few as 3.6 million 15N-labeled GFP+ cells could be readily detected with DNP-NMR. Importantly, cell sorting allowed for the comparison of cytokine-treated GFP+ and GFP- cells in a batch-consistent way. This provides an avenue for normalizing NMR spectral contributions from background cellular processes following treatment with cellular modulators. We also demonstrate the remarkable stability of AMUPol (a nitroxide biradical) in Jurkat T cells and achieved in-cell enhancements of 46 with 10 mM AMUPol, providing an excellent model system for further in-cell DNP-NMR studies. This represents an important contribution to improving in-cell methods for the study of endogenously expressed proteins by DNP-NMR.
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Citometria de Fluxo/métodos , Infecções por HIV/diagnóstico por imagem , Ressonância Magnética Nuclear Biomolecular/métodos , Humanos , Células Jurkat , Imageamento por Ressonância Magnética , Espectroscopia de Ressonância Magnética/métodos , Estrutura Molecular , Óxidos de Nitrogênio/farmacologia , Ativação Viral/fisiologiaRESUMO
Continuous wave (CW) dynamic nuclear polarization (DNP) is used with magic angle spinning (MAS) to enhance the typically poor sensitivity of nuclear magnetic resonance (NMR) by orders of magnitude. In a recent publication we show that further enhancement is obtained by using a frequency-agile gyrotron to chirp incident microwave frequency through the electron resonance frequency during DNP transfer. Here we characterize the effect of chirped MAS DNP by investigating the sweep time, sweep width, center-frequency, and electron Rabi frequency of the chirps. We show the advantages of chirped DNP with a trityl-nitroxide biradical, and a lack of improvement with chirped DNP using AMUPol, a nitroxide biradical. Frequency-chirped DNP on a model system of urea in a cryoprotecting matrix yields an enhancement of 142, 21% greater than that obtained with CW DNP. We then go beyond this model system and apply chirped DNP to intact human cells. In human Jurkat cells, frequency-chirped DNP improves enhancement by 24% over CW DNP. The characterization of the chirped DNP effect reveals instrument limitations on sweep time and sweep width, promising even greater increases in sensitivity with further technology development. These improvements in gyrotron technology, frequency-agile methods, and in-cell applications are expected to play a significant role in the advancement of MAS DNP.
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Peptídeos Catiônicos Antimicrobianos/química , Radicais Livres/química , Espectroscopia de Ressonância Magnética/métodos , Ureia/química , Isótopos de Carbono , Humanos , Células Jurkat , Micro-OndasRESUMO
Dynamic nuclear polarization (DNP) is used to improve the inherently poor sensitivity of nuclear magnetic resonance spectroscopy by transferring spin polarization from electrons to nuclei. However, DNP radicals within the sample can have detrimental effects on nuclear spins close to the polarizing agent. Chirped microwave pulses and electron decoupling (eDEC) attenuate these effects in model systems, but this approach is yet to be applied to intact cells or cellular lysates. Herein, we demonstrate for the first time exceptionally fast 1H T1DNP times of just 200 and 300 ms at 90 and 6 K, respectively, using a newly synthesized methylated trityl radical within intact human cells. We further demonstrate that eDEC can also be applied to intact human cells and human and bacterial cell lysates. We investigate eDEC efficiency at different temperatures, with different solvents, and with two trityl radical derivatives. At 90 K, eDEC yields a 13C signal intensity increase of 8% in intact human cells and 10% in human and bacterial cell lysates. At 6 K, eDEC provides larger intensity increases of 15 and 39% in intact human cells and cell lysates, respectively. Combining the manipulation of electron spins with frequency-chirped pulses and sample temperatures approaching absolute zero is a promising avenue for executing rapid, high-sensitivity magic-angle spinning DNP in complex cellular environments.
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Elétrons , Micro-Ondas , Humanos , Espectroscopia de Ressonância Magnética , TemperaturaRESUMO
We demonstrate that frequency-chirped dynamic nuclear polarization (DNP) with magic angle spinning (MAS) improves the enhancement of nuclear magnetic resonance (NMR) signal beyond that of continuous-wave (CW) DNP. Using a custom, frequency-agile gyrotron we implemented frequency-chirped DNP using the TEMTriPol-1 biradical, with MAS NMR at 7 T. Frequency-chirped microwaves yielded a DNP enhancement of 137, an increase of 19% compared to 115 recorded with CW. The chirps were 120â¯MHz-wide and centered over the trityl resonance, with 7â¯W microwave power incident on the sample (estimated 0.4â¯MHz electron spin Rabi frequency). We describe in detail the design and fabrication of the frequency-agile gyrotron used for frequency-chirped MAS DNP. Improvements to the interaction cavity and internal mode converter yielded efficient microwave generation and mode conversion, achieving >10â¯W output power over a 335â¯MHz bandwidth with >110â¯W peak power. Frequency-chirped DNP with MAS is expected to have a significant impact on the future of magnetic resonance.
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Espectroscopia de Ressonância Magnética/instrumentação , Campos Eletromagnéticos , Desenho de Equipamento , Espectroscopia de Ressonância Magnética/métodos , Micro-OndasRESUMO
Dynamic nuclear polarization (DNP) improves signal-to-noise in nuclear magnetic resonance (NMR) spectroscopy. Signal-to-noise in NMR can be further improved with cryogenic sample cooling. Whereas MAS DNP is commonly performed between 25 and 110 K, sample temperatures below 6â¯K lead to further improvements in sensitivity. Here, we demonstrate that solid effect MAS DNP experiments at 6â¯K, using trityl, yield 3.2× more sensitivity compared to 90â¯K. Trityl with solid effect DNP at 6â¯K yields substantially more signal to noise than biradicals and cross effect DNP. We also characterize cross effect DNP with AMUPol and TEMTriPol-1 biradicals for DNP magic angle spinning at temperatures below 6â¯K and 7 Tesla. DNP enhancements determined from microwave on/off intensities are 253 from AMUPol and 49 from TEMTriPol-1. The higher thermal Boltzmann polarization at 6â¯K compared to 298â¯K, combined with these enhancements, should result in 10,000× signal gain for AMUPol and 2000× gain for TEMTriPol-1. However, we show that AMUPol reduces signal in the absence of microwaves by 90% compared to 41% by TEMTriPol-1 at 7 Tesla as the result of depolarization and other detrimental paramagnetic effects. AMUPol still yields the highest signal-to-noise improvement per unit time between the cross effect radicals due to faster polarization buildup (T1DNPâ¯=â¯4.3â¯s and 36â¯s for AMUPol and TEMTriPol-1, respectively). Overall, AMUPol results in 2.5× better sensitivity compared to TEMTriPol-1 in MAS DNP experiments performed below 6â¯K at 7â¯T. Trityl provides 6.0× more sensitivity than TEMTriPol-1 and 1.9× more than AMUPol at 6â¯K, thus yielding the greatest signal-to-noise per unit time among all three radicals. A DNP enhancement profile of TEMTriPol-1 recorded with a frequency-tunable custom-built gyrotron oscillator operating at 198â¯GHz is also included. It is determined that at 7â¯T below 6â¯K a microwave power level of 0.6â¯W incident on the sample is sufficient to saturate the cross effect mechanism using TEMTriPol-1, yet increasing the power level up to 5â¯W results in higher improvements in DNP sensitivity with AMUPol. These results indicate MAS DNP below 6â¯K will play a prominent role in ultra-sensitive NMR spectroscopy in the future.
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Radicais Livres/química , Compostos de Tritil/química , Algoritmos , Isótopos de Carbono , Temperatura Baixa , Espectroscopia de Ressonância Magnética/métodos , Isótopos de Nitrogênio , Sensibilidade e Especificidade , Razão Sinal-Ruído , Ureia/químicaRESUMO
Spherical rotors in magic angle spinning (MAS) experiments have significant advantages over traditional cylindrical rotors including simplified spinning implementation, easy sample exchange, more efficient microwave coupling for dynamic nuclear polarization (DNP), and feasibility of downscaling to access higher spinning frequencies. Here, we implement spherical rotors with 4â¯mm outside diameter (o.d.) and demonstrate spinning >28â¯kHz using a single aperture for spinning gas. We show a modified stator geometry to improve fiber optic detection, increase NMR filling factor, and improve alignment for sample exchange and microwave irradiation. Higher NMR Rabi frequencies were obtained using smaller radiofrequency (RF) coils on small-diameter spherical rotors, compared to our previous implementation of MAS spheres with an o.d. of 9.5â¯mm. We report nutation fields of 110â¯kHz on 13C with 820â¯W of input power and 100â¯kHz on 1H with 800â¯W of input power. Proton decoupling fields of 78â¯kHz were applied over 20â¯ms of signal acquisition without any sign of arcing. Compared to our initial demonstration of a split coil for 9.5â¯mm spheres, this current implementation of a double-saddle coil inductor for 4â¯mm spheres not only intensifies the RF fields, but also improves RF homogeneity. We achieve an 810°/90° nutation intensity ratio of 0.84 at 300.197â¯MHz (1H). We also show electromagnetic simulations predicting a nearly 3-fold improvement in electron Rabi frequency of 0.99â¯MHz (with 4â¯mm spheres) compared to 0.38â¯MHz (with 3.2â¯mm cylinders), with 5â¯W of incident microwave power. Further improvements in magnetic resonance spin control are expected as RF inductors and microwave coupling are optimized for spherical rotors and scaled down to the micron scale.
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Espectroscopia de Ressonância Magnética/instrumentação , Algoritmos , Simulação por Computador , Campos Eletromagnéticos , Desenho de Equipamento , Tecnologia de Fibra Óptica , Gases/química , Micro-Ondas , Ondas de RádioRESUMO
Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins. Herein, we demonstrate that electron decoupling with chirped microwave pulses enables improved observation of DNP-enhanced 13 C spins in direct dipolar contact with electron spins, thereby leading to an optimal delay between transients largely governed by relatively fast electron relaxation. We report the first measurement of electron longitudinal relaxation time (T1e ) during magic angle spinning (MAS) NMR by observation of DNP-enhanced NMR signals (T1e =40±6â ms, 40â mM trityl, 4.0â kHz MAS, 4.3â K). With a 5â ms DNP period, electron decoupling results in a 195 % increase in signal intensity. MAS at 4.3â K, DNP, electron decoupling, and short recycle delays improve the sensitivity of 13 C in the vicinity of the polarizing agent. This is the first demonstration of recovery times between MAS-NMR transients being governed by short electron T1 and fast DNP transfer.
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Elétrons , Espectroscopia de Ressonância Magnética/instrumentação , Espectroscopia de Ressonância Magnética/métodos , Micro-Ondas , Animais , HumanosRESUMO
Dynamic nuclear polarization (DNP) with cryogenic magic angle spinning (MAS) provides significant improvements in NMR sensitivity, yet presents unique technical challenges. Here we describe a custom cryostat and suite of NMR probes capable of manipulating nuclear spins with multi-resonant radiofrequency circuits, cryogenic spinning below 6â¯K, sample exchange, and microwave coupling for DNP. The corrugated waveguide and six transfer lines needed for DNP and cryogenic spinning functionality are coupled to the probe from the top of the magnet. Transfer lines are vacuum-jacketed and provide bearing and drive gas, variable temperature fluid, two exhaust pathways, and a sample ejection port. The cryostat thermally isolates the magnet bore, thereby protecting the magnet and increasing cryogen efficiency. This novel design supports cryogenic MAS-DNP performance over an array of probes without altering DNP functionality. We present three MAS probes (two supporting 3.2â¯mm rotors and one supporting 9.5â¯mm rotors) interfacing with the single cryostat. Mechanical details, transmission line radio frequency design, and performance of the cryostat and three probes are described.
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Espectroscopia de Ressonância Magnética/instrumentação , Temperatura Baixa , Desenho de Equipamento , Espectroscopia de Ressonância Magnética/métodos , Imãs , Micro-Ondas , Ondas de RádioRESUMO
Magic angle spinning (MAS) is commonly used in nuclear magnetic resonance of solids to improve spectral resolution. Rather than using cylindrical rotors for MAS, we demonstrate that spherical rotors can be spun stably at the magic angle. Spherical rotors conserve valuable space in the probe head and simplify sample exchange and microwave coupling for dynamic nuclear polarization. In this current implementation of spherical rotors, a single gas stream provides bearing gas to reduce friction, drive propulsion to generate and maintain angular momentum, and variable temperature control for thermostating. Grooves are machined directly into zirconia spheres, thereby converting the rotor body into a robust turbine with high torque. We demonstrate that 9.5-mm-outside diameter spherical rotors can be spun at frequencies up to 4.6 kHz with N2(g) and 10.6 kHz with He(g). Angular stability of the spinning axis is demonstrated by observation of 79Br rotational echoes out to 10 ms from KBr packed within spherical rotors. Spinning frequency stability of ±1 Hz is achieved with resistive heating feedback control. A sample size of 36 µl can be accommodated in 9.5-mm-diameter spheres with a cylindrical hole machined along the spinning axis. We further show that spheres can be more extensively hollowed out to accommodate 161 µl of the sample, which provides superior signal-to-noise ratio compared to traditional 3.2-mm-diameter cylindrical rotors.
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Espectroscopia de Ressonância Magnética/instrumentação , Brometos , Desenho de Equipamento , Hélio , Espectroscopia de Ressonância Magnética/métodos , Compostos de Potássio , ZircônioRESUMO
Magic angle spinning (MAS) dynamic nuclear polarization (DNP) is widely used to increase nuclear magnetic resonance (NMR) signal intensity. Frequency-chirped microwaves yield superior control of electron spins and are expected to play a central role in the development of DNP MAS experiments. Time domain electron control with MAS has considerable promise to improve DNP performance at higher fields and temperatures. We have recently demonstrated that pulsed electron decoupling using frequency-chirped microwaves improves MAS DNP experiments by partially attenuating detrimental hyperfine interactions. The continued development of pulsed electron decoupling will enable a new suite of MAS DNP experiments that transfer polarization directly to observed spins. Time domain DNP transfers to nuclear spins in conjunction with pulsed electron decoupling is described as a viable avenue toward DNP-enhanced, high-resolution NMR spectroscopy over a range of temperatures from <6 to 320 K.
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Dynamic nuclear polarization (DNP) can improve nuclear magnetic resonance (NMR) sensitivity by orders of magnitude. Polarizing agents containing unpaired electrons required for DNP can broaden nuclear resonances in the presence of appreciable hyperfine couplings. Here we present the first cross polarization experiments implemented with electron decoupling, which attenuates detrimental hyperfine couplings. We also demonstrate magic angle spinning (MAS) DNP experiments below 6â¯K, producing unprecedented nuclear spin polarization in rotating solids. 13C correlation spectra were collected with MAS DNP below 6â¯K for the first time. Polarization build-up times with MAS DNP (T1DNP, 1H) of urea in a frozen glassy matrix below 6â¯K were measured for both the solid effect and the cross effect. Trityl radicals exhibit a T1DNP (1H) of 18.7â¯s and the T1DNP (1H) of samples doped with 20â¯mM AMUPol is only 1.3â¯s. MAS below 6â¯K with DNP and electron decoupling is an effective strategy to increase NMR signal-to-noise ratios per transient while retaining short polarization periods.
Assuntos
Espectroscopia de Ressonância Magnética/métodos , Algoritmos , Isótopos de Carbono , Difusão , Elétrons , Razão Sinal-Ruído , Trítio , Ureia/químicaRESUMO
Solid state nuclear magnetic resonance (NMR) enables atomic-resolution characterization of the molecular structure and dynamics within complex heterogeneous samples, but it is typically insensitive. Dynamic nuclear polarization (DNP) increases the NMR signal intensity by orders of magnitude and can be performed in combination with magic angle spinning (MAS) for sensitive, high-resolution spectroscopy. Here we report MAS DNP experiments, for the first time, within intact human cells with >40-fold DNP enhancement and a sample temperature of <6 K. In addition to cryogenic MAS results at <6 K, we also show in-cell DNP enhancements of 57-fold at 90 K. In-cell DNP is demonstrated using biradicals and sterically shielded monoradicals as polarizing agents. A novel trimodal polarizing agent is introduced for DNP, which contains a nitroxide biradical, a targeting peptide for cell penetration, and a fluorophore for subcellular localization with confocal microscopy. The fluorescent polarizing agent provides in-cell DNP enhancements of 63-fold at a concentration of 2.7 mM. These experiments pave the way for structural characterization of biomolecules in an endogenous cellular context.
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Corantes Fluorescentes/química , Espectroscopia de Ressonância Magnética/métodos , Humanos , Microscopia Confocal , Estrutura Molecular , Ressonância Magnética Nuclear BiomolecularRESUMO
We describe a frequency-agile gyrotron which can generate frequency-chirped microwave pulses. An arbitrary waveform generator (AWG) within the NMR spectrometer controls the microwave frequency, enabling synchronized pulsed control of both electron and nuclear spins. We demonstrate that the acceleration of emitted electrons, and thus the microwave frequency, can be quickly changed by varying the anode voltage. This strategy results in much faster frequency response than can be achieved by changing the potential of the electron emitter, and does not require a custom triode electron gun. The gyrotron frequency can be swept with a rate of 20â¯MHz/µs over a 670â¯MHz bandwidth in a static magnetic field. We have already implemented time-domain electron decoupling with dynamic nuclear polarization (DNP) magic angle spinning (MAS) with this device. In this contribution, we show frequency-swept DNP enhancement profiles recorded without changing the NMR magnet or probe. The profile of endofullerenes exhibits a DNP profile with a <10â¯MHz linewidth, indicating that the device also has sufficient frequency stability, and therefore phase stability, to implement pulsed DNP mechanisms such as the frequency-swept solid effect. We describe schematics of the mechanical and vacuum construction of the device which includes a novel flanged sapphire window assembly. Finally, we discuss how commercially available continuous-wave gyrotrons can potentially be converted into similar frequency-agile high-power microwave sources.
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We report magic angle spinning (MAS) up to 8.5â¯kHz with a sample temperature below 6â¯K using liquid helium as a variable temperature fluid. Cross polarization 13C NMR spectra exhibit exquisite sensitivity with a single transient. Remarkably, 1H saturation recovery experiments show a 1H T1 of 21â¯s with MAS below 6â¯K in the presence of trityl radicals in a glassy matrix. Leveraging the thermal spin polarization available at 4.2â¯K versus 298â¯K should result in 71 times higher signal intensity. Taking the 1H longitudinal relaxation into account, signal averaging times are therefore predicted to be expedited by a factor of >500. Computer assisted design (CAD) and finite element analysis were employed in both the design and diagnostic stages of this cryogenic MAS technology development. Computational fluid dynamics (CFD) models describing temperature gradients and fluid flow are presented. The CFD models bearing and drive gas maintained at 100â¯K, while a colder helium variable temperature fluid stream cools the center of a zirconia rotor. Results from the CFD were used to optimize the helium exhaust path and determine the sample temperature. This novel cryogenic experimental platform will be integrated with pulsed dynamic nuclear polarization and electron decoupling to interrogate biomolecular structure within intact human cells.