Assembly and performance of a 6.4 T cryogen‐free dynamic nuclear polarization system

We report on the assembly and performance evaluation of a 180‐GHz/6.4 T dynamic nuclear polarization (DNP) system based on a cryogen‐free superconducting magnet. The DNP system utilizes a variable‐field superconducting magnet that can be ramped up to 9 T and equipped with cryocoolers that can cool the sample space with the DNP assembly down to 1.8 K via the Joule–Thomson effect. A homebuilt DNP probe insert with top‐tuned nuclear magnetic resonance coil and microwave port was incorporated into the sample space in which the effective sample temperature is approximately 1.9 K when a 180‐GHz microwave source is on during DNP operation. ¹³C DNP of [1‐¹³C] acetate samples doped with trityl OX063 and 4‐oxo‐TEMPO in this system have resulted in solid‐state ¹³C polarization levels of 58 ± 3% and 18 ± 2%, respectively. The relatively high ¹³C polarization levels achieved in this work have demonstrated that the use of a cryogen‐free superconducting magnet for ¹³C DNP is feasible and in fact, relatively efficient—a major leap to offset the high cost of liquid helium consumption in DNP experiments.     

Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery

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 ¹³C 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 (T_1e) during magic angle spinning (MAS) NMR by observation of DNP‐enhanced NMR signals (T_1e=40±6 ms, 40 mM trityl, 4.0 kHz MAS, 4.3 K). With a 5 ms DNP period, electron decoupling results in a 195 % increase in signal intensity. MAS at 4.3 K, DNP, electron decoupling, and short recycle delays improve the sensitivity of ¹³C 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.     

A Method for Dynamic Nuclear Polarization Enhancement of Membrane Proteins

Dynamic nuclear polarization (DNP) magic‐angle spinning (MAS) solid‐state NMR (ssNMR) spectroscopy has the potential to enhance NMR signals by orders of magnitude and to enable NMR characterization of proteins which are inherently dilute, such as membrane proteins. In this work spin‐labeled lipid molecules (SL‐lipids), when used as polarizing agents, lead to large and relatively homogeneous DNP enhancements throughout the lipid bilayer and to an embedded lung surfactant mimetic peptide, KL₄. Specifically, DNP MAS ssNMR experiments at 600 MHz/395 GHz on KL₄ reconstituted in liposomes containing SL‐lipids reveal DNP enhancement values over two times larger for KL₄ compared to liposome suspensions containing the biradical TOTAPOL. These findings suggest an alternative sample preparation strategy for DNP MAS ssNMR studies of lipid membranes and integral membrane proteins.     

Complex Formation of the Tetracycline‐Binding Aptamer Investigated by Specific Cross‐Relaxation under DNP

While dynamic nuclear polarization (DNP) under magic‐angle spinning (MAS) is generally a powerful method capable of greatly enhancing the sensitivity of solid‐state NMR spectroscopy, hyperpolarization also gives rise to peculiar spin dynamics. Here, we elucidate how specific cross‐relaxation enhancement by active motions under DNP (SCREAM‐DNP) can be utilized to selectively obtain MAS‐NMR spectra of an RNA aptamer in a tightly bound complex with a methyl‐bearing ligand (tetracycline) due to the effective CH₃‐reorientation at an optimized sample temperature of approximately 160 K. SCREAM‐DNP can spectrally isolate the complex from non‐bound species in an RNA mixture. This selectivity allows for a competition assay between the aptamer and a mutant with compromised binding affinity. Variations in molecular structure and methyl dynamics, as observed by SCREAM‐DNP, between free tetracycline and RNA‐bound tetracycline are discussed.     

A table‐top PXI based low‐field spectrometer for solution dynamic nuclear polarization

We present the development of a portable dynamic nuclear polarization (DNP) instrument based on the PCI eXtensions for Instrumentation platform. The main purpose of the instrument is for study of ¹H polarization enhancements in solution through the Overhauser mechanism at low magnetic fields. A DNP probe set was constructed for use at 6.7 mT, using a modified Alderman–Grant resonator at 241 MHz for saturation of the electron transition. The solenoid for detection of the enhanced ¹H signal at 288 kHz was constructed with Litz wire. The largest observed ¹H enhancements (ε) at 6.7 mT for ¹⁴N‐CTPO radical in air saturated aqueous solution was ε~65. A concentration dependence of the enhancement is observed, with maximum ε at 5.5 mM. A low resonator efficiency for saturation of the electron paramagnetic resonance transition results in a decrease in ε for the 10.3 mM sample. At high incident powers (42 W) and long pump times, capacitor heating effects can also decrease the enhancement. The core unit and program described here could be easily adopted for multi‐frequency DNP work, depending on available main magnets and selection of the “plug and play” arbitrary waveform generator, digitizer, and radiofrequency synthesizer PCI eXtensions for Instrumentatione cards.     

13C dynamic nuclear polarization using isotopically enriched 4‐oxo‐TEMPO free radicals

The nitroxide‐based free radical 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) is a widely used polarizing agent in NMR signal amplification via dissolution dynamic nuclear polarization (DNP). In this study, we have thoroughly investigated the effects of ¹⁵N and/or ²H isotopic labeling of 4‐oxo‐TEMPO free radical on ¹³C DNP of 3 M [1‐¹³C] sodium acetate samples in 1 : 1 v/v glycerol : water at 3.35 T and 1.2 K. Four variants of this free radical were used for ¹³C DNP: 4‐oxo‐TEMPO, 4‐oxo‐TEMPO‐¹⁵N, 4‐oxo‐TEMPO‐d₁₆ and 4‐oxo‐TEMPO‐¹⁵N,d₁₆. Our results indicate that, despite the striking differences seen in the electron spin resonance (ESR) spectral features, the ¹³C DNP efficiency of these ¹⁵N and/or ²H‐enriched 4‐oxo‐TEMPO free radicals are relatively the same compared with ¹³C DNP performance of the regular 4‐oxo‐TEMPO. Furthermore, when fully deuterated glassing solvents were used, the ¹³C DNP signals of these samples all doubled in the same manner, and the ¹³C polarization buildup was faster by a factor of 2 for all samples. The data here suggest that the hyperfine coupling contributions of these isotopically enriched 4‐oxo‐TEMPO free radicals have negligible effects on the ¹³C DNP efficiency at 3.35 T and 1.2 K. These results are discussed in light of the spin temperature model of DNP. Copyright © 2016 John Wiley & Sons, Ltd.     

On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid‐State and Dissolution 13C NMR Spectroscopy

Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid‐state polarization enhancement at ambient conditions, and the maximization of ¹³C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's ¹³C behavior in nano‐ and micro‐particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid‐state experiments. It was found that ¹³C NMR signals could be boosted by orders of magnitude in either low‐ or room‐temperature solid‐state DNP experiments by utilizing naturally occurring paramagnetic P₁ substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin‐lattice relaxation times characteristic of diamond, coupled with a time‐independent cross‐effect‐like polarization transfer mechanism facilitated by a matching of the nitrogen‐related hyperfine coupling and the ¹³C Zeeman splitting. The efficiency of this solid‐state polarization process, however, is harder to exploit in dissolution DNP‐enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.     

19F Magic Angle Spinning Dynamic Nuclear Polarization Enhanced NMR Spectroscopy

The introduction of high‐frequency, high‐power microwave sources, tailored biradicals, and low‐temperature magic angle spinning (MAS) probes has led to a rapid development of hyperpolarization strategies for solids and frozen solutions, leading to large gains in NMR sensitivity. Here, we introduce a protocol for efficient hyperpolarization of ¹⁹F nuclei in MAS DNP enhanced NMR spectroscopy. We identified trifluoroethanol‐d₃ as a versatile glassy matrix and show that 12 mm AMUPol (with microcrystalline KBr) provides direct ¹⁹F DNP enhancements of over 100 at 9.4 T. We applied this protocol to obtain DNP‐enhanced ¹⁹F and ¹⁹F–¹³C cross‐polarization (CP) spectra for an active pharmaceutical ingredient and a fluorinated mesostructured hybrid material, using incipient wetness impregnation, with enhancements of approximately 25 and 10 in the bulk solid, respectively. This strategy is a general and straightforward method for obtaining enhanced ¹⁹F MAS spectra from fluorinated materials.     

Powder‐XRD and 14N magic angle‐spinning solid‐state NMR spectroscopy of some metal nitrides

Some metal nitrides (TiN, ZrN, InN, GaN, Ca₃N₂, Mg₃N₂, and Ge₃N₄) have been studied by powder X‐ray diffraction (XRD) and ¹⁴N magic angle‐spinning (MAS) solid‐state NMR spectroscopy. For Ca₃N₂, Mg₃N₂, and Ge₃N₄, no ¹⁴N NMR signal was observed. Low speed (ν_r = 2 kHz for TiN, ZrN, and GaN; ν_r = 1 kHz for InN) and ‘high speed’ (ν_r = 15 kHz for TiN; ν_r = 5 kHz for ZrN; ν_r = 10 kHz for InN and GaN) MAS NMR experiments were performed. For TiN, ZrN, InN, and GaN, powder‐XRD was used to identify the phases present in each sample. The number of peaks observed for each sample in their ¹⁴N MAS solid‐state NMR spectrum matches perfectly well with the number of nitrogen‐containing phases identified by powder‐XRD. The ¹⁴N MAS solid‐state NMR spectra are symmetric and dominated by the quadrupolar interaction. The envelopes of the spinning sidebands manifold are Lorentzian, and it is concluded that there is a distribution of the quadrupolar coupling constants Q_cc's arising from structural defects in the compounds studied. Copyright © 2015 John Wiley & Sons, Ltd.     

Aligned deuterochloroform in cross‐linked polystyrene as a new sample for adjusting the magic angle in HR‐MAS

A new and easily usable sample for adjusting the magic angle in a high resolution magic angle spinning (HR‐MAS) probe head is a swollen polymer stick in CDCl₃. The deuterium resonance shows a quadrupolar splitting if the rotor assembly is not at the magic angle, but a sharp singlet if the magic angle is correctly adjusted. Copyright © 2012 John Wiley & Sons, Ltd.     

High magnetic field solid‐state NMR analyses by combining MAS,MQ‐MAS,homo‐nuclear and hetero‐nuclear correlation experiments

A general strategy of structural analysis of alumina silicate by combining various solid‐state NMR measurements such as single pulse, multi‐quantum magic angle spinning, double‐quantum homo‐nuclear correlation under magic angle spinning (DQ‐MAS), and cross‐polarization hetero‐nuclear correlation (CP‐HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for ¹H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of ²⁷Al in single pulse, DQ‐MAS, and even in triple‐quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ‐MAS methods for homo‐nuclear correlations between both ²⁷Al–²⁷Al and ²⁹Si–²⁹Si and by CP‐HETCOR for hetero‐nuclear correlations between ²⁷Al–²⁹Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina‐silicate minerals. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel use of glow discharge optical emission spectroscopy (GDOES) to study corrosion of AA2024‐T3 in the presence and absence of inhibitors

Recent interest in environmentally friendly alternatives to chromate‐based corrosion inhibitors has led to the development of a range of novel coating formulations. The work described herein is aimed at developing a novel methodology to contribute to investigation of the self‐healing and active corrosion protection of the new coatings. An experimental procedure has been developed to model a defect in the coating by fixing coated specimens in close proximity to the uncoated AA2024‐T3, each separated by a narrow gap containing sodium chloride solution. After exposure to the corrosive environment, elemental depth profiles of the uncoated specimens were acquired by glow discharge optical emission spectroscopy (GDOES). The depth profiles of selected elements (notably aluminium, oxygen and copper) were shown to have characteristics which can be correlated with bulk surface roughening/intensity of corrosion, the thickness of the corroded layer and de‐alloying/re‐distribution of copper. An unanticipated inhibitory effect was noted in the case of a coating doped with γ‐Al₂O₃ (γ‐alumina or AluOx). Copyright © 2015 John Wiley & Sons, Ltd.     

High‐resolution magic angle spinning 1H NMR measurement of ligand concentration in solvent‐saturated chromatographic beads

A method based on ¹H high‐resolution magic angle spinning NMR has been developed for measuring concentration accurately in heterogeneous materials like that of ligands in chromatography media. Ligand concentration is obtained by relating the peak integrals for a butyl ligand in the spectrum of a water‐saturated chromatography medium to the integral of the added internal reference. The method is fast, with capacity of 10 min total sample preparation and analysis time per sample; precise, with a reproducibility expressed as 1.7% relative standard deviation; and accurate, as indicated by the excellent agreement of derived concentration with that obtained previously by ¹³C single‐pulse excitation MAS NMR. The effects of radiofrequency field inhomogeneity, spin rate, temperature increase due to spinning, and distribution and re‐distribution of medium and reference solvent both inside the rotor during spinning and between bulk solvent and pore space are discussed in detail. © 2016 The Authors Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.     

Efficient Dynamic Nuclear Polarization at 800 MHz/527 GHz with Trityl‐Nitroxide Biradicals

Cross‐effect (CE) dynamic nuclear polarization (DNP) is a rapidly developing technique that enhances the signal intensities in magic‐angle spinning (MAS) NMR spectra. We report CE DNP experiments at 211, 600, and 800 MHz using a new series of biradical polarizing agents referred to as TEMTriPols, in which a nitroxide (TEMPO) and a trityl radical are chemically tethered. The TEMTriPol molecule with the optimal performance yields a record ¹H NMR signal enhancement of 65 at 800 MHz at a concentration of 10 mM in a glycerol/water solvent matrix. The CE DNP enhancement for the TEMTriPol biradicals does not decrease as the magnetic field is increased in the manner usually observed for bis‐nitroxides. Instead, the relatively strong exchange interaction between the trityl and nitroxide moieties determines the magnetic field at which the optimum enhancement is observed.     

Dynamic nuclear polarization enhancement of protons and vanadium‐51 in the presence of pH‐dependent vanadyl radicals

We report applications of dynamic nuclear polarization to enhance proton and vanadium‐51 polarization of vanadyl sulfate samples doped with TOTAPOL under magic angle spinning conditions. The electron paramagnetic resonance response stemming from the paramagnetic ⁵¹V species was monitored as a function of pH, which can be adjusted to improve the enhancement of the proton polarization. By means of cross‐polarization from the proton bath, ⁵¹V spins could be hyperpolarized. Enhancement factors, build‐up times, and longitudinal relaxation times T₁(¹H) and T₁(⁵¹V) were investigated as a function of pH. Copyright © 2014 John Wiley & Sons, Ltd.     

A high performance nano-structure conductive coating on a Crofer22APU alloy fabricated by a novel spinel powder reduction coating technique

A nano-structure conductive coating was fabricated on a Crofer22APU alloy interconnect by an original coating strategy using Mn_0.9Y_0.1Co₂O₄ (MYC) novel spinel nanocrystalline powder. A unique treatment method by which the spinel powder was reduced was used to prepare the green coating. The resulting coating was about 12 μm in thickness, and was composed of MYC nanocrystalline with an average particle size of about 100 nm. The coating was well adhered with the substrate alloy. Less than 4 mΩ cm² of the area specific resistance (ASR) was obtained, and no obvious degradation was observed for a coated alloy (whose coating thickness was about 30 μm) after operated at 800 °C for 538 h under seven thermal cyclings. The coated alloy exhibited excellently electrical performance and long-term stability compared with the uncoated one. The exploration of the novel spinel powder reduction coating technique for alloy interconnect to obtain cheap coatings with excellent microstructure and performance showed a promising prospect for the practical application of solid oxide fuel cells (SOFCs).     

Construction and 13C hyperpolarization efficiency of a 180 GHz dissolution dynamic nuclear polarization system

Dynamic nuclear polarization (DNP) via the dissolution method has become one of the rapidly emerging techniques to alleviate the low signal sensitivity in nuclear magnetic resonance (NMR) spectroscopy and imaging. In this paper, we report on the development and ¹³C hyperpolarization efficiency of a homebuilt DNP system operating at 6.423 T and 1.4 K. The DNP hyperpolarizer system was assembled on a wide‐bore superconducting magnet, equipped with a standard continuous‐flow cryostat, and a 180 GHz microwave source with 120 mW power output and wide 4 GHz frequency tuning range. At 6.423 T and 1.4 K, solid‐state ¹³C polarization P levels of 64% and 31% were achieved for 3 M [1‐¹³C] sodium acetate samples in 1 : 1 v/v glycerol:water glassing matrix doped with 15 mM trityl OX063 and 40 mM 4‐oxo‐TEMPO, respectively. Upon dissolution, which takes about 15 s to complete, liquid‐state ¹³C NMR signal enhancements as high as 240 000‐fold (P=21%) were recorded in a nearby high resolution ¹³C NMR spectrometer at 1 T and 297 K. Considering the relatively lower cost of our homebuilt DNP system and the relative simplicity of its design, the dissolution DNP setup reported here could be feasibly adapted for in vitro or in vivo hyperpolarized ¹³C NMR or magnetic resonance imaging at least in the pre‐clinical setting. Copyright © 2017 John Wiley & Sons, Ltd.     

Highly stable magnetically separable copper nanocatalyst as an efficient catalyst for C(sp2)–C(sp) and C(sp2)–C(sp2) cross‐coupling reactions

A copper catalyst has been explored as an efficient and recyclable catalyst to effect Sonogashira and Suzuki cross‐coupling reactions. After modification of 2‐(((piperazin‐1‐ylmethyl)imino)methyl)phenol (PP) on the surface of amorphous silica‐coated iron oxide (Fe₃O₄@SiO₂@Cl) magnetic core–shell nanocomposite, copper(II) chloride was employed to synthesize the Fe₃O₄@SiO₂@PP‐Cu catalyst, affording a copper loading of 1.52 mmol g⁻¹. High yield, low reaction times, non‐toxicity and recyclability of the catalyst are the main merits of this protocol. The catalyst was characterized using Fourier transform infrared, X‐ray photoelectron, energy‐dispersive X‐ray and inductively coupled plasma optical emission spectroscopies, X‐ray diffraction, scanning and transmission electron microscopies, and vibrating sample magnetometry.