Category Archives: solution-state DNP

Dynamic Nuclear Polarization of 13C Nuclei in the Liquid State over a 10 Tesla Field Range #DNPNMR

Orlando, Tomas, Rıza Dervişoğlu, Marcel Levien, Igor Tkach, Thomas F. Prisner, Loren B. Andreas, Vasyl P. Denysenkov, and Marina Bennati. “Dynamic Nuclear Polarization of 13C Nuclei in the Liquid State over a 10 Tesla Field Range.” Angewandte Chemie International Edition 58, no. 5 (January 28, 2019): 1402–6.

Nuclear magnetic resonance (NMR) techniques play an essential role in natural science and medicine. In spite of the tremendous utility associated with the small energies detected, the most severe limitation is the low signal-to-noise ratio. Dynamic nuclear polarization (DNP), a technique based on transfer of polarization from electron to nuclear spins, has emerged as a tool to enhance sensitivity of NMR. However, the approach in liquids is still facing several challenges. Here we report the observation of room temperature, liquid DNP 13C signal enhancements in organic small molecules as high as 600 at 9.4 Tesla and 800 at 1.2 Tesla. A mechanistic investigation of the 13CDNP field dependence shines light on parameters governing the underlying scalar DNP, indicating that DNP efficiency is raised by proper choice of the polarizing agent (paramagnetic center) and by halogen atoms as mediators of scalar hyperfine interaction. Observation of sizable DNP of 13CH2 and 13CH3 groups in organic molecules at 9.4 T opens perspective for a broader application of this method.

Large volume liquid state scalar Overhauser dynamic nuclear polarization at high magnetic field #DNPNMR

Dubroca, Thierry, Sungsool Wi, Johan van Tol, Lucio Frydman, and Stephen Hill. “Large Volume Liquid State Scalar Overhauser Dynamic Nuclear Polarization at High Magnetic Field.” Physical Chemistry Chemical Physics 21, no. 38 (2019): 21200–204.

Dynamic Nuclear Polarization (DNP) can increase the sensitivity of Nuclear Magnetic Resonance (NMR), but it is challenging in the liquid state at high magnetic fields. In this study we demonstrate significant enhancements of NMR signals (up to 70 on 13C) in the liquid state by scalar Overhauser DNP at 14.1 T, with high resolution (∼0.1 ppm) and relatively large sample volume (∼100 μL).

Overhauser DNP in Liquids on 13C Nuclei #DNPNMR

Bennati, Marina, and Tomas Orlando. “Overhauser DNP in Liquids on 13C Nuclei” 8 (2019): 8.

Dynamic nuclear polarization (DNP) in solution is known since the early days of magnetic resonance to deliver information about molecular motion and electron–nuclear spin relaxation. The method has emerged as a potentially powerful tool to increase sensitivity of high-field solution NMR. In this article, we summarize our recent insights into the mechanism of Overhauser DNP for 13C nuclei, which led to the observation of NMR signal enhancements in liquids up to three orders of magnitude and at magnetic fields of 3.4 T. In contrast to the well-studied case of Overhauser DNP with 1H nuclei, the 13C DNP mechanism can lead to signal enhancements increasing toward high magnetic fields. A key feature of these large high-field enhancements is the underlying scalar relaxation mechanism, which is responsive to correlation times on the subpicosecond time scale. Experiments using microwave (mw) cavities with exquisite control of mw irradiation fields have allowed for disentangling the counteracting effect of dipolar and scalar mechanisms as well as the impact of the 13C chemical environment. These mechanistic insights open up new perspectives for Overhauser liquid DNP as a general tool to increase sensitivity in high-field liquid NMR.

High-Field Liquid-State Dynamic Nuclear Polarization in Microliter Samples #DNPNMR

Yoon, Dongyoung, Alexandros I. Dimitriadis, Murari Soundararajan, Christian Caspers, Jeremy Genoud, Stefano Alberti, Emile de Rijk, and Jean-Philippe Ansermet. “High-Field Liquid-State Dynamic Nuclear Polarization in Microliter Samples.” Analytical Chemistry 90, no. 9 (May 2018): 5620–26.

Nuclear hyperpolarization in liquid state by dynamic nuclear polarization (DNP) has been of great interest because of its potential use in NMR spectroscopy of small samples of biological and chemical compounds in aqueous media. Liquid state DNP generally requires microwave resonators in order to generate an alternating magnetic field strong enough to saturate electron spins in the solution. As a consequence, the sample size is limited to dimensions of the order of the wavelength, and this restricts the sample volume to less than 100 nL for DNP at 9 T (~ 260 GHz). We show here a new approach that overcomes this sample size limitation. Large saturation of electron spins was obtained with a high-power (~ 150 W) gyrotron without microwave resonators. Since high power microwaves can cause serious dielectric heating in polar solutions, we designed a planar probe which effectively alleviates dielectric heating. A thin liquid sample of 100 μm of thickness is placed on a block of high thermal conductivity aluminum nitride with a gold coating, that serves both as a ground plane and as a heat sink. A meander or a coil were used for NMR. We performed 1H DNP at 9.2 T (~ 260 GHz) and at room temperature with 10 μL of water, a volume that is more than 100 times larger than reported so far. The 1H NMR signal is enhanced by a factor of about -10 with 70 W of microwave power. We also demonstrated liquid state 31P DNP in fluorobenzene containing triphenylphosphine, and obtained an enhancement of ~200.

In-situ Overhauser-enhanced nuclear magnetic resonance at less than 1 μT using an atomic magnetometer #DNPNMR #ODNP

Lee, Hyun Joon, Seong-Joo Lee, Jeong Hyun Shim, Han Seb Moon, and Kiwoong Kim. “In-Situ Overhauser-Enhanced Nuclear Magnetic Resonance at Less than 1 μT Using an Atomic Magnetometer.” Journal of Magnetic Resonance 300 (March 1, 2019): 149–52.

The development of atomic magnetometers has led to nuclear magnetic resonance (NMR) in zero and ultralow magnetic fields without using cryogenic sensors. However, in-situ detection, meaning that a sample locates in the detection space beside a vapor cell, has been conducted only with parahydrogen-induced polarization. Other hyperpolarization techniques remain unexplored yet. In this work, we demonstrate that Overhauser dynamic nuclear polarization allows in-situ NMR detection with an atomic magnetometer at less than 1 μT. The 1H NMR signal of a nitroxide radical solution was observed at 13.83 Hz, which corresponds to 325 nT. Signal-to-noise ratio was 32 after sixteen averages. On the Larmor precession of 1H spins, a decaying oscillation was superimposed. We attribute it to a transient 87Rb spin precession in response to a non-adiabatic field variation. This work shows a new capability of zero- and ultralow-field NMR.

Multi-resonant photonic band-gap/saddle coil DNP probehead for static solid state NMR of microliter volume samples #DNPNMR

Nevzorov, Alexander A., Sergey Milikisiyants, Antonin N. Marek, and Alex I. Smirnov. “Multi-Resonant Photonic Band-Gap/Saddle Coil DNP Probehead for Static Solid State NMR of Microliter Volume Samples.” Journal of Magnetic Resonance 297 (December 2018): 113–23.

The most critical condition for performing Dynamic Nuclear Polarization (DNP) is achieving sufficiently high electronic B1e fields over the sample at the matched EPR frequencies, which for modern high-resolution NMR instruments fall into the millimeter wave (mmW) range. Typically, mmWs are generated by powerful gyrotrons and/or extended interaction klystrons (EIKs) sources and then focused onto the sample by dielectric lenses. However, further development of DNP methods including new DNP pulse sequences may require B1e fields higher than one could achieve with the current mmW technology. In order to address the challenge of significantly enhancing the mmW field at the sample, we have constructed and tested one-dimensional photonic band-gap (PBG) mmW resonator that was incorporated inside a double-tuned radiofrequency (rf) NMR saddle coil. The photonic crystal is formed by stacking ceramic discs with alternating high and low dielectric constants. The thicknesses of the discs are chosen to be λ/4 or 3λ/4, where λ is the wavelength of the incident mmW field in the corresponding dielectric material. When the mmW frequency is within the band gap of the photonic crystal, a defect created in the middle of the crystal confines the mmW energy, thus forming a resonant structure. An aluminum mirror in the middle of the defect has been used to split the structure in order to reduce its size and simplify the resonator tuning. The latter is achieved by adjusting the width of the defect by moving the aluminum mirror with respect to the dielectric stack using a gear mechanism. The 1D PBG resonator was the key element for constructing a multi-resonant integrated DNP/NMR probehead operating at 198 GHz EPR / 300 MHz 1H / 75.5 MHz 13C NMR frequencies. Initial tests of the multi-resonant DNP/NMR probehead were carried out using a quasioptical 200 GHz bridge and a Bruker Biospin Avance II spectrometer equipped with a standard Bruker 7 T wide-bore 89 mm magnet parked at 300.13 MHz 1H NMR frequency. The mmW bridge built with all solid-state active components allows for the frequency tuning between ca. 190 to ca. 198 GHz with the output power up to 27 dBm (0.5 W) at 192 GHz and up to 23 dBm (0.2 W) at 197.5 GHz. Room temperature DNP experiments with a synthetic single crystal high-pressure high-temperature (HPHT) diamond (0.3Å~0.3Å~3.0 mm3) demonstrated dramatic 1,500–fold enhancement of 13C natural abundance NMR signal at full incident mmW power. Significant 13C DNP enhancement (of about 90) have been obtained at incident mmW powers of as low as <100 μW. Further tests of the resonator performance have been carried out with a thin (ca. 100 μm thickness) composite polystyrene-microdiamond film by controlling the average mmW power at the optimal DNP conditions via a gated mode of operation. From these experiments, the PBG resonator with loaded Q≃250 and finesse provides up to 12-fold/11 db gain in the average mmW power vs. the non-resonant probehead configuration employing only a reflective mirror.

Perspective of Overhauser dynamic nuclear polarization for the study of soft materials #DNPNMR

Biller, Joshua R., Ryan Barnes, and Songi Han. “Perspective of Overhauser Dynamic Nuclear Polarization for the Study of Soft Materials.” Current Opinion in Colloid & Interface Science 33 (January 1, 2018): 72–85.

Solution state Overhauser dynamic nuclear polarization (ODNP) has been studied for 60years, but only in recent years has found applications of broad interest to biophysical sciences of hydration dynamics (HD-ODNP) around biomolecules and surfaces. In this review we describe state-of-the-art HD-ODNP methods and experiments, and identify technological and conceptual advances necessary to broadly disseminate HD-ODNP, as well as broaden its scope. Specifically, incomplete treatment of the saturation factor leads to the use of high microwave powers that induce temperature-dependent effects in HD-ODNP that can be detrimental to the stability and property of the sample and/or data interpretation, and thus must be corrected for. Furthermore, direct measurements of the electron spin relaxation times for the nitroxide radical-based spin labels used in HD-ODNP have recently caught up with the ambient solution conditions of relevance to HD-ODNP experiment, allowing us to envision an explicit treatment of the saturation factor. This would enable “single-shot” HD-ODNP at one or two concentrations and power levels, cutting down experimental times from the typical hours to minutes. With the development of a user-friendly and robust operation, the application of HD-ODNP experiments can be broadened for the study of biomolecules, biomaterials, soft polymer materials (i.e. hydrogels) and surfaces. In fact, any hydrated materials that can be viably spin labeled can yield information on local water dynamics and interfaces, and so guide the design of soft materials for medical and pharmaceutical uses. A brief introduction to spin-labeling, and exemplary applications to soft materials is discussed to serve as inspiration for future studies.

Perspectives on hyperpolarised solution-state magnetic resonance in chemistry #DNPNMR

Dumez, Jean-Nicolas. “Perspectives on Hyperpolarised Solution-State Magnetic Resonance in Chemistry.” Magnetic Resonance in Chemistry 55, no. 1 (2016): 38–46.

This perspective article reviews some of the recent developments in the field of hyperpolarisation, with a focus on solution-state NMR spectroscopy of small molecules. Two techniques are considered in more detail, dissolution dynamic nuclear polarisation (D-DNP) and signal amplification by reversible exchange (SABRE). Some of the opportunities and challenges for applications of hyperpolarised solution-state magnetic resonance in chemistry are discussed. 

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla #DNPNMR #ODNP

Schöps, Spindler Philipp, and Prisner Thomas, “Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla.”

Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.

Chemical-shift-resolved (1)(9)F NMR spectroscopy between 13.5 and 135 MHz: Overhauser-DNP-enhanced diagonal suppressed correlation spectroscopy

George, C. and N. Chandrakumar, Chemical-shift-resolved (1)(9)F NMR spectroscopy between 13.5 and 135 MHz: Overhauser-DNP-enhanced diagonal suppressed correlation spectroscopy. Angew Chem Int Ed Engl, 2014. 53(32): p. 8441-4.

Overhauser-DNP-enhanced homonuclear 2D (19)F correlation spectroscopy with diagonal suppression is presented for small molecules in the solution state at moderate fields. Multi-frequency, multi-radical studies demonstrate that these relatively low-field experiments may be operated with sensitivity rivalling that of standard 200-1000 MHz NMR spectroscopy. Structural information is accessible without a sensitivity penalty, and diagonal suppressed 2D NMR correlations emerge despite the general lack of multiplet resolution in the 1D ODNP spectra. This powerful general approach avoids the rather stiff excitation, detection, and other special requirements of high-field (19)F NMR spectroscopy.

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