High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR

Dynamic nuclear polarization (DNP) transfers electron spin-polarization to nuclear spins in close proximity, increasing sensitivity by two-to-three orders of magnitude. This enables nuclear magnetic resonance (NMR) experiments on samples with low concentrations of analyte. The requirement of using cryogenic temperatures in DNP-enhanced solid-state NMR (ssNMR) experiments may impair the resolution and hence limit its broad application to biological systems. In this work, we introduce a “High-Temperature DNP” approach, which aims at increasing spectral resolution by performing experiments at temperatures of around 180?K instead of?~100?K. By utilizing the extraordinary enhancements obtained on deuterated proteins, still sufficiently large DNP enhancements of 11–18 are obtained for proton and carbon, respectively. We recorded high sensitivity 2D ¹³C–¹³C spectra in?~9?min with higher resolution than at 100?K, which has similar resolution to the one obtained at room temperature for some favorable residues.     

Dynamic Nuclear Polarization by Thermal Mixing Under Partial Saturation

We describe a low-temperature thermodynamic model for dynamic nuclear polarization (DNP) via continuous-wave partial saturation of electron spin resonance (ESR) lines that are both homogeneously and inhomogeneously broadened. It is a variant of a reasoning proposed by Borghini, which in turn used Redfield’s thermodynamic treatment of saturation. Our variant is furthermore based on Provotorov’s insight that under partial saturation of a coupled-spin system two distinct spin temperatures should appear in a thermodynamical theory. We apply our model to DNP results obtained at a temperature of 1.2?K and in magnetic fields of 3.35 and 5?T on 1-¹³C labeled sodium acetate dissolved in a frozen D₂O/ethanol-d₆ solution doped with the free radical TEMPO.     

Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2?<?T?<?4.2?K

Cross polarization can provide significant enhancements with respect to direct polarization of low-γ nuclei such as ¹³C. Substantial gains in sample throughput (shorter polarization times) can be achieved by exploiting shorter build-up times τ_DNP(¹H)?<?τ_DNP(¹³C). To polarize protons rather than low-γ nuclei, nitroxide radicals with broad ESR resonances such as TEMPO are more appropriate than Trityl and similar carbon-based radicals that have narrow lines. With TEMPO as polarizing agent, the main Dynamic Nuclear Polarization (DNP) mechanism is thermal mixing (TM). Cross polarization makes it possible to attain higher polarization levels at 2.2?K than one can obtain with direct DNP of low-γ nuclei with TEMPO at 1.2?K, thus avoiding complex cryogenic technology.     

The thermoluminescence (TL), phosphorescence and cryoluminescence of n-type hexagonal (6H) SiC crystals

The low temperature (down to liquid helium temperature) TL, phosphorescence and cryoluminescence of n-type 6H SiC crystals is described. The crystals contained nitrogen as the major impurity at concentrations of about 10¹⁶ cm^-3. The glow curves exhibited peaks at about 25, 45, 70 and 90°K (in addition to a peak at 250°K). Thermal activation energies for the above peaks ranged between 0.02 and 0.14 eV (0.30 eV for the 250°K peak). These are much lower than energies reported earlier for nitrogen donor levels in 6H SiC. The values obtained for the 70–90°K peaks (0.08–0.14 eV) fit quite well those obtained by electrical transport measurements and Raman scattering.The crystals exhibited strong phosphorescence even at liquid helium temperature. This was shown to be only partly due to thermal release from traps, the other components being due to pair-recombination and optical release from the shallow traps by the black body radiation (BBR) from the walls of the cryostat. This BBR was found to be responsible also for the observed cryoluminescence.     

Low-Field, Time-Resolved Dynamic Nuclear Polarization with Field Cycling and High-Resolution NMR Detection

Dynamic nuclear polarization (DNP) effects in aqueous solutions of stable 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) radicals were studied in a pulsed mode of pumping the electron paramagnetic resonance (EPR) transitions. Our fast field cycling setup allowed us to perform the EPR pumping at low magnetic fields and to detect the enhanced nuclear magnetic resonance signals at 7 T with high spectral resolution. Pumping was performed at two different frequencies, 300 MHz and 1.4 GHz, corresponding to magnetic fields around 10 and 48.6 mT, respectively. For both frequencies, the DNP enhancements were close to the limiting theoretical values of ?110 (¹⁴N TEMPOL) and ?165 (¹⁵N TEMPOL). Our pulsed experiments exploit coherent motion of the electronic spins in the radio-frequency field as seen by an oscillatory component in the dependence of the DNP effect on the radio-frequency pulse duration. The DNP enhancement was studied in detail as a function of the pulse length, duty cycle, delay between the pulses, and applied power. The analysis of the results shows that pulsed DNP experiments provide an opportunity to achieve enhancements of about ?110 with relatively low applied power as compared to the standard continuous-wave DNP experiments. An adequate theoretical approach to the problem under study is suggested.     

<Superscript>13</Superscript>C Dynamic Nuclear Polarization Using Derivatives of TEMPO Free Radical

The nitroxide-based 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical is widely used in ¹³C dynamic nuclear polarization (DNP) due to its relatively low cost, commercial availability, and effectiveness as polarizing agent. While a large number of TEMPO derivatives are available commercially, so far, only few have been tested for use in ¹³C DNP. In this study, we have tested and evaluated the ¹³C hyperpolarization efficiency of eight derivatives of TEMPO free radical with different side arms in the 4-position. In general, these TEMPO derivatives were found to have slight variations in efficiency as polarizing agents for DNP of 3 M [1-¹³C] acetate in 1:1 v/v ethanol:water at 3.35 T and 1.2 K. X-band electron paramagnetic resonance (EPR) spectroscopy revealed no significant differences in the spectral features among these TEMPO derivatives. ²H enrichment of the ethanol:water glassing matrix resulted in further improvement of the solid-state ¹³C DNP signals by factor of 2 to 2.5-fold with respect to the ¹³C DNP signal of non-deuterated DNP samples. These results suggest an interaction between the nuclear Zeeman reservoirs and the electron dipolar system via the thermal mixing mechanism.     

Stimulation von ZnS-Phosphoren mit langwelligem Infrarot

Stimulation experiments with ZnS phosphors, using IR wavelengths from 2 to 15μ, were performed at the temperature of liquid helium. For this purpose a cryostat was constructed which allowed to keep the samples and the screening device at the temperature of liquid helium. Moreover glow curves after different decay times at 6 °K were taken. After excitation of ZnS phosphors a strong release of carriers from relatively deep traps is taking place although the phosphor is kept at the temperature of liquid helium. This emptying of traps is accompanied by a luminous recombination of the released electrons with the activator levels producing an intense afterglow which can be observed over a long time. This phenomenon cannot be explained by thermal release of trapped electrons into the conduction band, followed by recombination with the activator levels because of the depth of the emptied traps. The rate of thermal carrier release was calculated to be about 10^?29 sec^?1 for 0,05 eV deep traps, but the observed rate was of the order of 10^?4 sec^?1. An emptying of traps by IR-stimulation can be excluded because the phosphor was surrounded by a concentric screening device kept at 4.2 °K. The effect can be explained by luminous tunneling of trapped electrons to the activator levels. An estimate of the tunneling rate gives a value agreeing with the experimental results. If such a “tunnel afterglow” does exist a spectral shift to longer wavelengths compared to the usual fluorescence and phosphorescence is to be expected. This could be actually observed. Additional experiments included IR-stimulated emission after various decay times, glow curves taken after such stimulation, and studies of the influence of temperature. The results rule out the possibility that the carriers were released from the traps by IR light. Apparently, IR radiation affects the potential barrier between the traps and the activators so that the rate of recombination by tunneling increases.     

Transfer of1H and13C dynamic nuclear polarization from immobilized nitroxide radicals to flowing liquids

In this study,¹H and¹³C dynamic nuclear polarization (DNP) was generated at a magnetic field strength of 0.33 T utilizing silica phase immobilized nitroxide (SPIN) samples. The polarization was subsequently transferred to flowing liquids and monitored at a magnetic field strength of 4.7 T. These solid/liquid intermolecular transfer (SLIT) experiments provide efficient polarization transfer without the necessity of the free radical system present in the monitoring fluid. Specifically, ultimate¹H SLIT DNP Overhauser enhancements of ?56 and ?110 have been observed for benzene and chloroform in the presence of SPIN system 2, respectively. The¹³C SLIT DNP enhancement for benzene is dominated by three-spin effects and poor leakage factors (f _c). However, a particularly favorable case is the chloroform/SPIN 2 system which exhibits a scalar dominated enhancement. For this case, positive enhancements 40–60 times the¹³C thermal Boltzmann magnetization at 4.7 T have been observed. The large scalar dominated¹³C DNP enhancement for this system represents one of the largest experimental enhancements reported to date. The¹³C DNP spectra for other samples which exhibit favorable scalar¹³C dominated enhancements (e.g., Freon 113) are also presented. Three different SPIN systems were also prepared and characterized in the present study.     

A multi-sample 94 GHz dissolution dynamic-nuclear-polarization system

We describe the design and initial performance results of a multi-sample dissolution dynamic-nuclear-polarization (DNP) polarizer based on a Helium-temperature NMR cryostat for use in a wide-bore NMR magnet with a room-temperature bore. The system is designed to accommodate up to six samples in a revolver-style sample changer that allows changing samples at liquid-Helium temperature and at pressures ranging from ambient pressure down to 1 mbar. The multi-sample setup is motivated by the desire to do repetitive in vivo measurements and to characterize the DNP process by investigating samples of different chemical composition. The system can be loaded with up to six samples simultaneously to reduce sample loading and unloading. Therefore, series of experiments can be carried out faster and more reliably. The DNP probe contains an oversized microwave cavity and includes EPR and NMR capabilities for monitoring the DNP process. In the solid state, DNP enhancements corresponding to ～45% polarization for [1-(13)C]pyruvic acid with a trityl radical have been measured. In the initial liquid-state acquisition experiments described here, the polarization was found to be ～13%, corresponding to an enhancement factor exceeding 16,000 relative to thermal polarization at 9.4 T and ambient temperature.     

Elastic constants of galena down to liquid helium temperatures

The elastic constants of single crystal galena have been determined from the measured ultrasonic velocities down to liquid helium temperature. A cryostat incorporating an arrangement to inject the liquid bonding material at low temperature is described. At 5 K, the values of elastic constants are C₁₁=14.9₀, C₁₂=3.5₁ and C₄₄=2.9₂×10¹⁰ N/m².     

Estimation of the ultracold neutron production by a source designed for the WWR-M reactor

The results of calculations related to designing an ultracold neutron source with superfluid helium for the WWR-M reactor have been presented. The ultracold neutron production rate has been estimated for different types of premoderator chamber filling. The dependence of this rate on the temperature of helium has been determined. If the premoderator chamber is filled with liquid orthodeuterium, the ultracold neutron production rate remains almost constant in the range of helium temperatures of 1.0–1.5 K and is as high as 3.1 × 10³ cm^–3 s^–1.     

Liquid State DNP on Metabolites at 260?GHz EPR/400?MHz NMR Frequency

We have performed liquid state (“Overhauser”) dynamic nuclear polarization (DNP) experiments at high magnetic field (9.2?T, corresponding to 260?GHz EPR and 400?MHz ¹H-NMR resonance frequency) on solutions of pyruvate, lactate and alanine in water with TEMPOL nitroxide radicals as polarizing agent. We present experimental results showing DNP enhancement on metabolite methyl protons, varying for the different target metabolites. It is shown that the enhancements are achieved through direct coupling between the radicals and the target metabolites in solution, i.e., the effect is not mediated by the solvent water protons. The coupling factors between the TEMPOL radicals and the metabolites observed are a factor of 3–5 smaller compared to direct polarization transfer from TEMPOL to water protons.     

Supraleitung des Galliums bei hohem Druck

A high-pressure modification of Gallium was formed at liquid helium temperature by the application of a pressure in excess of 35000 atm. It was superconducting with a transition temperature of 6·38°K. The high-pressure form remained after the pressure was removed at helium temperature. This resulted, however in a shift in transition temperature to 7·5°K. The average derivativedT _c/dp was ?3·10^?5°K/atm.     

Dynamic nuclear polarization system for the SANS-J-II spectrometer at JAEA

We have developed a dynamic nuclear polarization (DNP) system for the SANS-J-II spectrometer at the JRR-3 atomic research reactor of Japan Atomic Energy Agency (JAEA). The DNP system is composed of a split-type horizontal superconducting magnet (3.3 T), a Gunn oscillator as a microwave source (94 GHz), and a cryostat (1.2 K). In particular, a sample cell with 40 in inner diameter and the magnet with a field homogeneity of 5×10⁻⁵ in a volume of 25 mm×8 mm were employed to polarize samples with a diameter of 20 mm for the ultra small-angle scattering experiment using the magnetic lens installed at the SANS-J-II spectrometer [S. Koizumi, H. Iwase, J. Suzuki, T. Oku, R. Motokawa, H. Sasao, H. Tanaka, D. Yamaguchi, H.M. Shimizu, T. Hashimoto, J. Appl. Crystallogr. 40 (2007) s474]. We obtained the proton polarization |P|=32% in the polyethylene doped with 2,2,6,6-tetra-methyl-piperidine-1-oxyl (TEMPO).     

Biomolecular solid state NMR with magic-angle spinning at 25K

A magic-angle spinning (MAS) probe has been constructed which allows the sample to be cooled with helium, while the MAS bearing and drive gases are nitrogen. The sample can be cooled to 25 K using roughly 3 L/h of liquid helium, while the 4-mm diameter rotor spins at 6.7 kHz with good stability (±5 Hz) for many hours. Proton decoupling fields up to at least 130 kHz can be applied. This helium-cooled MAS probe enables a variety of one-dimensional and two-dimensional NMR experiments on biomolecular solids and other materials at low temperatures, with signal-to-noise proportional to 1/T. We show examples of low-temperature ¹³C NMR data for two biomolecular samples, namely the peptide Aβ_14–23 in the form of amyloid fibrils and the protein HP35 in frozen glycerol/water solution. Issues related to temperature calibration, spin–lattice relaxation at low temperatures, paramagnetic doping of frozen solutions, and ¹³C MAS NMR linewidths are discussed.     

A Mössbauer cryostat equipped with a Gifford McMahon cryocooler

A new cryostat for Mössbauer spectroscopic measurements has been developed around a cryogenerator of the Gifford McMahon type operating with helium gas in a closed circuit. Both source and absorber can be cooled to about 30K and the unwanted vibrations caused by the helium compression/expansion cycle have been limited to approximately 0.05–0.06 mms^?1.     

纳米氦液滴-超低温化学研究装置

成功研制了新一代纳米氦液滴实验装置．氦液滴是由高压的高纯氦气通过超低温的脉冲阀绝热膨胀形成的,通过调节连接在超低温氦冷头上的脉冲阀的温度(10～30 K)及脉冲阀内氦气的背景压力(10～40 atm),氦液滴的大小在包含103到105个氦原子间连续可调,和传统的连续氦液滴束源比较起来,脉冲束源的强度提高了一个量级以上,提供了一个和商品化脉冲激光器结合使用,研究超低温条件下超流体氦中的化学反应动力学的机会．通过研究氦液滴中包裹的CH₃I分子的光解动力学来对仪器的性能进行测试,利用离子速度影像技术研究了超低温纳米氦液滴中包裹的CH₃I分子在252 nm下的光解动力学,光解产物甲基通过(2+1)共振增强多光子电离并运用离子影像进行检测,结果表明光解产物的平动能及角分布被氦原子环境有效的弛豫．也证实了运用脉冲氦液滴束源研究衰减光谱的可行性,通过对氦液滴中掺杂苯的衰减光谱的研究,还发现小于3%的衰减信号都可以被检测到,表明所研制的脉冲氦液滴束的稳定性及检测器的灵敏度都是很高的．     

Thermodynamic studies of successive phase transitions in BaZnGeO4 and a new solid phase below 186.1 K

The successive phase transitions of BaZnGeO₄ have been studied on meltsolidified samples. A new solid phase (named phase VI) has been found below 186.1 K in samples of large particle size (diameter:D0.1 mm). The higher temperature crystalline phase V can be supercooled easily down to liquid helium temperature. On heating, however, it transforms into phase VI above 95 K in a slow exothermic process. Heat capacities have been measured by adiabatic calorimetry between 14 and 300 K. The enthalpy and entropy of the V–VI phase transition are 187.1 Jmol^–1 and 0.971 J K^–1 mol^–1, respectively. The corresponding data for the IV–V phase transition at 199.8 K are 229.3 J mol^–1 and 1.168 JK^–1 mol^–1. The phase VI does not appear in samples of smaller particle size (D0.1 mm).     

Snowballs of radioactive ions — nuclear spin polarization of core ions

Short-lived ions¹²B (beta-radioactive, T_1/2=20.3 ms) sustaining nuclear spin polarization were introduced into superfluid helium at 1.7 K. It was found that the¹²B ions were transported as charged entities under a static electric field and that the nuclear polarization was maintained throughout the lifetime of¹²B nuclei. Polarization of¹²B was determined through beta-NMR method. Snowball, a singly charged microcluster of helium atoms formed around an impurity ion, is responsible for the behaviour and thus constitutes a suitable environment for preserving nuclear polarization of the core ions¹²B. In a separate experiment snowballs were produced by implanting⁸Li (T_1/2=830 ms) into liquid helium and detected by means of alpha particles from the core ions to guarantee that the snowballs survive longer than the lifetime of¹²B.     

基于散射实验极化靶的2.5T/1.3K的动态核极化(DNP)系统的开发

动态核极化法(Dynamic Nuclear Polarization, DNP)是利用热平衡下的电子在磁场中的高自旋极化率转移到原子核自旋的技术,从而极大的提高原子核自旋极化率。多种动态极化靶材料已广泛的用于自旋物理散射实验。本文介绍一种简单实用,共同开发的日本山形大学DNP系统,包括超导磁场,氦4蒸发恒冷器,微波系统以及NMR核磁共振检测系统,测得中子靶材料氘带丁醇(D-butanol)中氘核的极化率在2.5T/1.3K达到+6.5%。