Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin–spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin-spin interactions for significant electron–nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in high-field DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (<100 K). For example, large DNP enhancements (∼300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at <20 K, nuclear polarization using lower microwave power (<100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.     

DYNAMIC NUCLEAR POLARIZATION OF ORGANIC COMPOUND DOPED WITH FREE RADICALS

For those organic compounds which lack free radicals, a doping approach named melting-liquid-nitrogen-quench was developed in order to perform dynamic nuclear polarization(DNP)-nuclear magnetic resonance experiments. By using this method, the ¹H, ¹³C, ¹⁵N DNP enhancements were observed with free-radical-doped dibenzofuran and benzamide. The enhancement mechanism and relation between the concentration of unpaired electrons and the maximum DNP enhancement are discussed.     

Understanding Overhauser Dynamic Nuclear Polarisation through NMR relaxometry

Overhauser dynamic nuclear polarisation (DNP) represents a potentially outstanding tool to increase the sensitivity of solution and solid state NMR experiments, as well as of magnetic resonance imaging. DNP signal enhancements are strongly linked to the spin relaxation properties of the system under investigation, which must contain a paramagnetic molecule used as DNP polariser. In turn, nuclear spin relaxation can be monitored through NMR relaxometry, which reports on the field dependence of the nuclear relaxation rates, opening a route to understand the physical processes at the origin of the Overhauser DNP in solution. The contributions of dipole–dipole and Fermi-contact interactions to paramagnetic relaxation are here described and shown to be responsible to both the relaxometry profiles and the DNP enhancements, so that the experimental access to the former can allow for predictions of the latter.     

掺磷的硅中电子自旋共振

本文叙述了在室温、液N₂和液H₂及其降压温度下,利用3厘米波段双重调制波谱仪对掺磷原子浓度为～10¹⁵—～10¹⁸/厘米³的硅样品进行的电子自旋共振研究,文中还简述了低温控制系统。实验观察到传导电子、表面缺陷中心和电子-核超精细结构谱线,以及有效电子-施子核对的互作用谱线,并获得相应的g因子。在14°K下,得到施主核上电子波函数幅度的平方|Ψ(0)|²和电子-核超精细互作用常数α_D,与G.Feher在1.25°K下利用电子-核双共振方法得到的结果相近。     

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.     

Dynamic nuclear polarization at 9T using a novel 250 GHz gyrotron microwave source. 2003

In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9 T (250 GHz for g = 2 electrons, 380 MHz for ¹H). In these experiments, ¹H enhancements of up to 170 ± 50 have been observed in 1-¹³C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20 K; in addition, we have observed significant enhancements in ¹⁵N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ∼17 have been obtained in two-dimensional ¹³C–¹³C chemical shift correlation spectra of the amino acid U–¹³C, ¹⁵N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.     

Comparison of Overhauser DNP at 0.34 and 3.4?T with Frémy’s Salt

Dynamic nuclear polarization (DNP) is investigated in the liquid state using a model system of Frémy's salt dissolved in water. Nuclear magnetic resonance signal enhancements at 0.34 and 3.4?T of the bulk water protons are recorded as a function of the irradiation time and the polarizer concentration. The build-up rates are consistent with the T(1n) of the observed water protons at room temperature (for 9?GHz/0.34?T) and for about 50?±?10?°C at 94?GHz/3.4?T. At 94?GHz/3.4?T, we observe in our setup a maximal enhancement of -50 at 25?mM polarizer concentration. The use of Frémy's salt allows the determination of the saturation factors at 94?GHz by pulsed ELDOR experiments. The results are well consistent with the Overhauser DNP mechanism and indicate that higher enhancements at this intermediate frequency require higher sample temperatures.     

Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy’s Salt Measured in a Magnetic Field of 9.2?T

The temperature dependence of the water-proton dynamic nuclear polarization (DNP) enhancement from Fremy’s salt nitroxide radicals was measured in a magnetic field of 9.2?T (corresponding to 260?GHz microwave (MW) and 392?MHz NMR frequencies) in the temperature range of 15–65?°C. The temperature could be determined directly from the proton NMR line shift of the sample. Very high DNP enhancements of ?38 (signal integral) or ?81 (peak intensity) could be achieved with a high-power gyrotron MW source. The experimental findings are compared with classical Overhauser theory for liquids, which is based on the translational and rotational motion of the molecules and with molecular dynamics calculations of the coupling factor.     

A spectrometer for EPR,DNP, and multinuclear high-resolution NMR

We describe a magnetic resonance spectrometer capable of EPR, dynamic nuclear polarization, and multinuclear high-resolution NMR. The operating field is 1.4 T, corresponding to Larmor frequencies of 40 GHz and 60 MHz for electrons and protons, respectively. The microwave side of the probe is based on a Fabry-Perot resonator (FPR ), an open structure that enhances power-to-field conversion for efficient saturation of the EPR for dynamic polarization, and further permits in situ detection for EPR. This allows the external field to be set at, rather than scanned for, the optimal DNP position. Moreover, we have found that adjustments necessary for maximizing DNP may be done via optimization of the EPR signal, a feature of particular significance for samples which exhibit NMR signals on the borderline of detectability, i.e., samples for which DNP is of special importance. 'H and '3C polarization enhancements achieved using the FPR are compared with devices used by others, in particular the horn /reflector system used by Wind and co-workers. Direct '3C enhancements large enough to detect 2.5 x 10'6 spins in (fluoranthenyl)2 PF6 after a single one-second polarization period have been obtained, and the first high-field 'Li DNP results are also presented.     

Dynamic nuclear polarization at 9T using a novel 250GHz gyrotron microwave source

In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9T (250 GHz for g=2 electrons, 380 MHz for 1H). In these experiments, 1H enhancements of up to 170+/-50 have been observed in 1-13C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20K; in addition, we have observed significant enhancements in 15N spectra of unoriented pf1-bacteriophage. Finally, enhancements of approximately 17 have been obtained in two-dimensional 13C-13C chemical shift correlation spectra of the amino acid U-13C, 15N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.     

Overhauser dynamic nuclear polarization and molecular dynamics simulations using pyrroline and piperidine ring nitroxide radicals

The efficiency of Overhauser dynamic nuclear polarization (DNP) depends on the local dynamics modulating the dipolar coupling between the two interacting spins. By attaching nitroxide based spin labels to molecules and by measuring the ¹H DNP response of solvent water, information about the local hydration dynamics near the spin label can be obtained. However, there are two commonly used types of nitroxide ring structures; a pyrroline based and a piperidine based molecule. It is important to know when comparing different experiments, whether changes in DNP enhancements are due to changes in local hydration dynamics or because of the different spin label structures. In this study we investigate the key parameters affecting DNP signal enhancements for 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl, a 5-membered ring nitroxide radical, and for 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy, a 6-membered ring nitroxide radical. Using X-Band DNP, field cycling relaxometry, and molecular dynamics simulations, we conclude that the key parameters affecting the DNP amplitude of the ¹H signal of water to be equal when using either nitroxide. Thus, experiments measuring hydration dynamics using either type of spin labels may be compared.     

1H dynamic nuclear polarization in supercritical ethylene at 1.4 T

(1)H dynamic nuclear polarization (DNP) has been measured in supercritical ethylene in the pressure range 60-300 bar in an external field of 1.4 T. A single-cell sapphire tube was used as a high-pressure cell, and powdered 1,3-bisdiphenylene-2-phenyl allyl (BDPA) free radicals were added and distributed at the wall of the cell. At all pressures the dominant DNP mechanism was a positive Overhauser enhancement, caused by proton-electron contact interactions at the fluid/solid radical interface. The observed enhancements varied from 12 at 60 bar to 17 at 300 bar. Besides the Overhauser enhancement, small solid state and thermal mixing enhancements also were observed, indicating that part of the ethylene was adsorbed at the radical surface for a prolonged time. The impacts of the experimental conditions on the Overhauser enhancement factors are discussed, and enhancements of at least 40-60 are estimated when the EPR saturation factor and the leakage factor become maximal. These data indicate that DNP-enhanced NMR has the potential of extending the impact of NMR in research areas involving supercritical fluids.     

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms

Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40?mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.     

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.     

Combined RHEED-AES study of the thermal treatment of (001) GaAs surface prior to MBE growth

Auger analysis and reflection high energy electron diffraction (RHEED) have been used to study the UHV thermal cleaning procedure of different chemically treated (001) GaAs surfaces when heated in ultra high vacuum. It is shown that the ultimate surface composition of the substrate critically depends on the nature and the thickness of the oxide layer formed during chemical treatment. The oxygen removal mechanism has been studied and a comparative analysis of AES and RHEED observations has been drawn. A low residual carbon coverage cleaning procedure is fully investigated and it results that a carbon coverage as low as ∼6×10⁻² monolayer induces surface faceting by heating the GaAs substrate at temperatures higher than 570°C. A (001) GaAs surface heated in an arsenic flux up to 570°C is As-stabilized and (411) facets appear at a temperature ranged between 575 and 585°C.     

Pulsed EPR studies of polycrystalline cesium hexamethyl hexacyclen sodide

The electron spin echo envelope modulation (ESEEM) technique of pulsed EPR spectroscopy has been used to measure weak¹³³Cs hyperfine couplings to trapped electrons in polycrystalline cesium hexamethyl hexacyclen sodide. Two magnetically distinct groups of weakly coupled¹³³Cs nuclei were found — one with an isotropic electron-nuclear hyperfine (Fermi contact) coupling of 0.34 MHz and the other with a contact coupling less than 0.1 MHz. Analysis of these data, by computer simulation, shows that the number of Cs nuclei that give rise to these interactions and their distance from the paramagnetic center is consistent with the hypothesis that the trapped electrons occupy vacant anion sites. The results indicate that the spread of unpaired electron spin density, along the axis formed by the “contact ion pair,” may be greater than that in the plane perpendicular to this axis.     

ZrO2 films deposited by photo-CVD at low temperatures

Zirconium oxide layers have been successfully deposited by photo-CVD at low temperatures. ZrO₂ growth was observed at temperatures as low as 100 °C. When deposited at 250 °C and above, these films exhibited a polycrystalline structure with a mixture of different crystal phases. Deposition at 300 °C was found to form moisture-free ZrO₂ films with a high refractive index of 2.1, a very low effective density of trapped electrons of ∼8.8×10⁸ cm^-2 and an interface trap density of 6.6×10⁹ cm^-2 eV^-1 being readily obtained. Received: 17 December 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002     

Electron stimulated desorption of carbon monoxide from a (111) niobium surface

Electron stimulated desorption of CO from the (111) face of a Nb single crystal produced both CO⁺ and O⁺ ions after adsorption at 150°K on a clean surface. When the surface was heated to above 250 °K only O⁺ ions were observed, and this current disappeared as the temperature was increased to 700 °K. Readsorption (at 150 °K) was inhibited following the 700 °K heating. These data indicate the formation on heating of a tightly bound surface phase with very low ionic desorption cross section. Threshold energies for CO⁺ and O⁺ ion production were 10.0 ± 0.5 eV and 19.0 ± 0.5 eV, respectively. The cross section for electron stimulated depopulation of the O⁺ producing phase was (4 ± 1) × 10^?18 cm² for 100 eV electrons.