A High-Conversion-Factor, Double-Resonance Structure for High-Field Dynamic Nuclear Polarization

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W^1/2 for the microwave conversion factor and 0.8 mT/W^1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state ¹H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about ?16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure.     

Low temperature probe for dynamic nuclear polarization and multiple-pulse solid-state NMR

Here, we describe the design and performance characteristics of a low temperature probe for dynamic nuclear polarization (DNP) experiments, which is compatible with demanding multiple-pulse experiments. The competing goals of a high-Q microwave cavity to achieve large DNP enhancements and a high efficiency NMR circuit for multiple-pulse control lead to inevitable engineering tradeoffs. We have designed two probes-one with a single-resonance RF circuit and a horn-mirror cavity configuration for the microwaves and a second with a double-resonance RF circuit and a double-horn cavity configuration. The advantage of the design is that the sample is in vacuum, the RF circuits are locally tuned, and the microwave resonator has a large internal volume that is compatible with the use of RF and gradient coils.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

1H DNP at 1.4 T of water doped with a triarylmethyl-based radical

Recently a triarylmethyl-based (TAM) radical has been developed for research in biological and other aqueous systems, and in low magnetic fields, 10 mT or less, large (1)H dynamic nuclear polarization (DNP) enhancements have been reported. In this paper the DNP properties of this radical have been investigated in a considerably larger field of 1.4 T, corresponding to proton and electron Larmor frequencies of 60 MHz and 40 GHz, respectively. To avoid excessive microwave heating of the sample, an existing DNP NMR probe was modified with a screening coil, wound around the sample capillary and with its axis perpendicular to the electric component of the microwave field. It was found that with this probe the temperature increase in the sample after 4 s of microwave irradiation with an incident power of 10 W was only 16 degrees C. For the investigations, 10 mM of the TAM radical was dissolved in deionized, but not degassed, water and put into a 1-mm i.d. and 6-mm long capillary tube. At 26 degrees C the following results were obtained: (I) The relaxivity of the radical is 0.07 (mMs)(-1), in accordance with the value extrapolated from low-field results; (II) The leakage factor is 0.63, the saturation factor at maximum power is 0.85, and the coupling factor is -0.0187. It is shown that these results agree very well with an analysis where the electron-dipolar interactions are the dominant DNP mechanism, and where the relaxation transitions resulting from these interactions are governed by translational diffusion of the water molecules. Finally, the possibilities of combining DNP with magnetic resonance microscopy (MRM) are discussed. It is shown that at 26 degrees C the overall DNP-enhanced proton polarization should become maximal in an external field of 0.3 T and become comparable to the thermal equilibrium polarization in a field of 30 T, considerably larger than the largest high-resolution magnet available to date. It is concluded that DNP MRM in this field, which corresponds to a standard microwave frequency of 9 GHz, has the potential to significantly increase the sensitivity in NMR and MRI experiments of small aqueous samples doped with the TAM radical.     

A tunable general purpose Q-band resonator for CW and pulse EPR/ENDOR experiments with large sample access and optical excitation

We describe a frequency tunable Q-band cavity (34 GHz) designed for CW and pulse Electron Paramagnetic Resonance (EPR) as well as Electron Nuclear Double Resonance (ENDOR) and Electron Electron Double Resonance (ELDOR) experiments. The TE(011) cylindrical resonator is machined either from brass or from graphite (which is subsequently gold plated), to improve the penetration of the 100 kHz field modulation signal. The (self-supporting) ENDOR coil consists of four 0.8mm silver posts at 2.67 mm distance from the cavity center axis, penetrating through the plunger heads. It is very robust and immune to mechanical vibrations. The coil is electrically shielded to enable CW ENDOR experiments with high RF power (500 W). The top plunger of the cavity is movable and allows a frequency tuning of ±2 GHz. In our setup the standard operation frequency is 34.0 GHz. The microwaves are coupled into the resonator through an iris in the cylinder wall and matching is accomplished by a sliding short in the coupling waveguide. Optical excitation of the sample is enabled through slits in the cavity wall (transmission ～60%). The resonator accepts 3mm o.d. sample tubes. This leads to a favorable sensitivity especially for pulse EPR experiments of low concentration biological samples. The probehead dimensions are compatible with that of Bruker flexline Q-band resonators and it fits perfectly into an Oxford CF935 Helium flow cryostat (4-300 K). It is demonstrated that, due to the relatively large active sample volume (20-30 μl), the described resonator has superior concentration sensitivity as compared to commercial pulse Q-band resonators. The quality factor (Q(L)) of the resonator can be varied between 2600 (critical coupling) and 1300 (over-coupling). The shortest achieved π/2-pulse durations are 20 ns using a 3 W microwave amplifier. ENDOR (RF) π-pulses of 20 μs ((1)H @ 51 MHz) were obtained for a 300 W amplifier and 7 μs using a 2500 W amplifier. Selected applications of the resonator are presented.     

基于直接数字合成技术的电子自旋-核自旋双共振实验射频信号源设计

为了满足脉冲式电子顺磁共振谱仪中电子自旋-核自旋双共振模块的需要,利用直接数字合成器设计并制作了射频信号源. 该部件产生的射频脉冲具备对频率、幅度和相位快速精确调制的能力,对原子核自旋有较强操控能力.     

Portable X-band system for solution state dynamic nuclear polarization

This paper concerns instrumental approaches to obtain large dynamic nuclear polarization (DNP) enhancements in a completely portable system. We show that at fields of 0.35 T under ambient conditions and at X-band frequencies, 1H enhancements of >100-fold can be achieved using nitroxide radical systems, which is near the theoretical maximum for 1H polarization using the Overhauser effect at this field. These large enhancements were obtained using a custom built microwave transmitter and a commercial TE102 X-band resonant cavity. The custom built microwave transmitter is compact, so when combined with a permanent magnet it is readily transportable. Our commercial X-band resonator was modified to be tunable over a range of approximately 9.5-10 GHz, giving added versatility to our fixed field portable DNP system. In addition, a field adjustable Halbach permanent magnet has also been employed as another means for matching the electron spin resonance condition. Both portable setups provide large signal enhancements and with improvements in design and engineering, greater than 100-fold 1H enhancements are feasible.     

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.     

Study of natural diamonds by dynamic nuclear polarization-enhanced C nuclear magnetic resonance spectroscopy

The results of a study of two types of natural-diamond crystals by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state ¹³C nuclear magnetic resonance (NMR) are reported. The home-built DNP magic-angle spinning (MAS) ¹³C NMR spectrometer operates at 54 GHz for electrons and 20.2 MHz for carbons. The power of the microwave source was about 30 W and the highest DNP enhancement factor came near to 10³. It was shown that in the MAS spectra the ¹³C NMR linewidths of the I_b-type diamond were broader than those of I_aB3-type diamond. From the hyperfine structure of the DNP enhancement as a function of frequency, four kinds of nitrogen-centred and one kind of carbon-centred free radicals could be identified in the I_b-type diamond. The hyperfine structures of the DNP enhancement curve that originated from the anisotropic hyperfine interaction between electron and nuclei could be partially averaged out by MAS. The ¹³C polarization time of DNP was rather long, i.e. 1500 s, and the spin—lattice relaxation time (without microwave irradiation) was about 300 s, which was somewhat shorter than anticipated. Discussions on these experimental results have been made in this report.     

Cryogenic 35 GHz pulse ENDOR probehead accommodating large sample sizes: Performance and applications

The construction and performance of a cryogenic 35 GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE₁₀₂ cavity in which the radio frequency (rf) B₂-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B₂-field strength per square-root of the transmitted rf power over the frequency range 2–180 MHz. The distributions of the microwave B₁- and E₁-field, and the rf B₂-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions.     

H DNP at 1.4 T of Water Doped with a Triarylmethyl-Based Radical

Recently a triarylmethyl-based (TAM) radical has been developed for research in biological and other aqueous systems, and in low magnetic fields, 10 mT or less, large ¹H dynamic nuclear polarization (DNP) enhancements have been reported. In this paper the DNP properties of this radical have been investigated in a considerably larger field of 1.4 T, corresponding to proton and electron Larmor frequencies of 60 MHz and 40 GHz, respectively. To avoid excessive microwave heating of the sample, an existing DNP NMR probe was modified with a screening coil, wound around the sample capillary and with its axis perpendicular to the electric component of the microwave field. It was found that with this probe the temperature increase in the sample after 4 s of microwave irradiation with an incident power of 10 W was only 16°C. For the investigations, 10 mM of the TAM radical was dissolved in deionized, but not degassed, water and put into a 1-mm i.d. and 6-mm long capillary tube. At 26°C the following results were obtained: (I) The relaxivity of the radical is 0.07 (mMs)⁻¹, in accordance with the value extrapolated from low-field results; (II) The leakage factor is 0.63, the saturation factor at maximum power is 0.85, and the coupling factor is −0.0187. It is shown that these results agree very well with an analysis where the electron–dipolar interactions are the dominant DNP mechanism, and where the relaxation transitions resulting from these interactions are governed by translational diffusion of the water molecules. Finally, the possibilities of combining DNP with magnetic resonance microscopy (MRM) are discussed. It is shown that at 26°C the overall DNP-enhanced proton polarization should become maximal in an external field of 0.3 T and become comparable to the thermal equilibrium polarization in a field of 30 T, considerably larger than the largest high-resolution magnet available to date. It is concluded that DNP MRM in this field, which corresponds to a standard microwave frequency of 9 GHz, has the potential to significantly increase the sensitivity in NMR and MRI experiments of small aqueous samples doped with the TAM radical.     

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.     

Theoretical treatment of pulsed Overhauser dynamic nuclear polarization: Consideration of a general periodic pulse sequence

A general theoretical approach to pulsed Overhauser-type dynamic nuclear polarization (DNP) is presented. Dynamic nuclear polarization is a powerful method to create non-thermal polarization of nuclear spins, thereby enhancing their nuclear magnetic resonance signals. The theory presented can treat pulsed microwave irradiation of electron paramagnetic resonance transitions for periodic pulse sequences of general composition. Dynamic nuclear polarization enhancement is analyzed in detail as a function of the microwave pulse length for rectangular pulses and pulses with finite rise time. Characteristic oscillations of the DNP enhancement are found when the pulse-length is stepwise increased, originating from coherent motion of the electron spins driven by the pulses. Experimental low-field DNP data are in very good agreement with this theoretical approach.     

ENDOR spectroscopic and molecular orbital study of the dynamical properties of the side chain in radical anions of ubiquinones Q-1, Q-2, Q-6, and Q-10

The dynamics of the side chain of the radical anions of ubiquinones Q-1 (2,3-dimethoxy-5-methyl-6-[3-methyl-2-butenyl]-1,4-benzoquinone), Q-2, Q-6, and Q-10 have been investigated using electron nuclear double-resonance (ENDOR) spectroscopy. When radicals are produced in the liquid phase, secondary radicals are also formed. The EPR spectra of these additional radicals overlap with the radical of interest. ENDOR spectroscopy was found to be capable for studying the dynamical properties of such conditions. The temperature dependence of the isotropic hyperfine coupling constants of the beta- and gamma-protons of the side chain was measured. The activation energy of the rotation and other dynamical properties of the side chain were calculated assuming that rotation can be modeled by the classical two-jump model. The rotation energy barrier for Q-1 was also determined by the hybrid Hartree-Fock/density functional method UB3LYP with the 6-31G(d) basis set. Calculated results were in good agreement with the experimental results. Despite the numerous parameters affecting the ENDOR linewidth ENDOR spectroscopy was shown to be a potential method for studying the dynamical properties of the mixtures of the radicals. Prominent forbidden transitions appear in the ENDOR spectra when alkali ions are present in the sample. From these transitions measured ENDOR-induced EPR spectra showed an additional doublet and phase transition in electron Zeeman frequency.     

A Dynamic Nuclear Polarization spectrometer at 95 GHz/144 MHz with EPR and NMR excitation and detection capabilities

A spectrometer specifically designed for systematic studies of the spin dynamics underlying Dynamic Nuclear Polarization (DNP) in solids at low temperatures is described. The spectrometer functions as a fully operational NMR spectrometer (144 MHz) and pulse EPR spectrometer (95 GHz) with a microwave (MW) power of up to 300 mW at the sample position, generating a MW B(1) field as high as 800 KHz. The combined NMR/EPR probe comprises of an open-structure horn-reflector configuration that functions as a low Q EPR cavity and an RF coil that can accommodate a 30-50 μl sample tube. The performance of the spectrometer is demonstrated through some basic pulsed EPR experiments, such as echo-detected EPR, saturation recovery and nutation measurements, that enable quantification of the actual intensity of MW irradiation at the position of the sample. In addition, DNP enhanced NMR signals of samples containing TEMPO and trityl are followed as a function of the MW frequency. Buildup curves of the nuclear polarization are recorded as a function of the microwave irradiation time period at different temperatures and for different MW powers.     

Overhauser DNP and EPR in a Mobile Setup: Influence of Magnetic Field Inhomogeneity

Power-dependent Overhauser dynamic nuclear polarization (DNP) enhancements and continuous-wave electron paramagnetic resonance (EPR) spectra of nitroxide radicals were measured in the magnetic field of a mobile Halbach-array permanent magnet and compared with results from a commercially available electromagnet. DNP saturation factors for varying microwave power were obtained from both measurement series and used to investigate how the increased magnetic field inhomogeneity present in the Halbach magnet affects the saturation efficiency. An EPR detection system was designed to allow continuous-wave EPR measurements at microwave power up to 20?W. Our results show that despite the lower magnetic field homogeneity, a Halbach-array magnet can be used for EPR and DNP-enhanced nuclear magnetic resonance of high quality providing almost the same performance as a more homogeneous electromagnet.     

A Q-band pulse EPR/ENDOR spectrometer and the implementation of advanced one- and two-dimensional pulse EPR methodology

A versatile high-power pulse Q-band EPR spectrometer operating at 34.5--35.5 GHz and in a temperature range of 4--300 K is described. The spectrometer allows one to perform one- and two-dimensional multifrequency pulse EPR and pulse ENDOR experiments, as well as continuous wave experiments. It is equipped with two microwave sources and four microwave channels to generate pulse sequences with different amplitudes, phases, and carrier frequencies. A microwave pulse power of up to 100 W is available. Two channels form radiofrequency pulses with adjustable phases for ENDOR experiments. The spectrometer performance is demonstrated by single crystal pulse ENDOR experiments on a copper complex. A HYSCORE experiment demonstrates that the advantages of high-field EPR and correlation spectroscopy can be combined and exploited at Q-band. Furthermore, we illustrate how this combination can be used in cases where the HYSCORE experiment is no longer effective at 35 GHz because of the shallow modulation depth. Even in cases where the echo modulation is virtually absent in the HYSCORE experiment at Q-band, matched microwave pulses allow one to get HYSCORE spectra with a signal-to-noise ratio as good as at X-band. Finally, it is shown that the high microwave power, the short pulses, and the broad resonator bandwidth make the spectrometer well suited to Fourier transform EPR experiments.     

Microwave field distribution in a magic angle spinning dynamic nuclear polarization NMR probe

We present a calculation of the microwave field distribution in a magic angle spinning (MAS) probe utilized in dynamic nuclear polarization (DNP) experiments. The microwave magnetic field (B(1S)) profile was obtained from simulations performed with the High Frequency Structure Simulator (HFSS) software suite, using a model that includes the launching antenna, the outer Kel-F stator housing coated with Ag, the RF coil, and the 4mm diameter sapphire rotor containing the sample. The predicted average B(1S) field is 13μT/W(1/2), where S denotes the electron spin. For a routinely achievable input power of 5W the corresponding value is γ(S)B(1S)=0.84MHz. The calculations provide insights into the coupling of the microwave power to the sample, including reflections from the RF coil and diffraction of the power transmitted through the coil. The variation of enhancement with rotor wall thickness was also successfully simulated. A second, simplified calculation was performed using a single pass model based on Gaussian beam propagation and Fresnel diffraction. This model provided additional physical insight and was in good agreement with the full HFSS simulation. These calculations indicate approaches to increasing the coupling of the microwave power to the sample, including the use of a converging lens and fine adjustment of the spacing of the windings of the RF coil. The present results should prove useful in optimizing the coupling of microwave power to the sample in future DNP experiments. Finally, the results of the simulation were used to predict the cross effect DNP enhancement (?) vs. ω(1S)/(2π) for a sample of (13)C-urea dissolved in a 60:40 glycerol/water mixture containing the polarizing agent TOTAPOL; very good agreement was obtained between theory and experiment.     

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.