Aug 21, 2019

Room temperature CO oxidation catalysed by supported Pt nanoparticles revealed by solid-state NMR and DNP spectroscopy #DNPNMR

Klimavicius, Vytautas, Sarah Neumann, Sebastian Kunz, Torsten Gutmann, and Gerd Buntkowsky. “Room Temperature CO Oxidation Catalysed by Supported Pt Nanoparticles Revealed by Solid-State NMR and DNP Spectroscopy.” Catalysis Science & Technology 9, no. 14 (2019): 3743–52.

A series of 1 and 2 nm sized platinum nanoparticles (Pt-NPs) deposited on different support materials, namely, γ-alumina (γ-Al2O3), titanium dioxide (TiO2), silicon dioxide (SiO2) and fumed silica are investigated by solid-state NMR and dynamic nuclear polarization enhanced NMR spectroscopy (DNP). DNP signal enhancement factors up to 170 enable gaining deeper insight into the surface chemistry of Pt-NPs. Carbon monoxide is used as a probe molecule to analyze the adsorption process and the surface chemistry on the supported Pt-NPs. The studied systems show significant catalytic activity in carbon monoxide oxidation on their surface at room temperature. The underlying catalytic mechanism is the water–gas shift reaction. In the case of alumina as the support the produced CO2 reacts with the surface to form carbonate, which is revealed by solid-state NMR. A similar carbonate formation is also observed when physical mixtures of neat alumina with silica, fumed silica and titania supported Pt-NPs are studied.

Aug 19, 2019

Transport of hyperpolarized samples in dissolution-DNP experiments #DNPNMR

Kiryutin, Alexey S., Bogdan A. Rodin, Alexandra V. Yurkovskaya, Konstantin L. Ivanov, Dennis Kurzbach, Sami Jannin, David Guarin, Daniel Abergel, and Geoffrey Bodenhausen. “Transport of Hyperpolarized Samples in Dissolution-DNP Experiments.” Physical Chemistry Chemical Physics 21, no. 25 (2019): 13696–705.

Dissolution dynamic nuclear polarization (D-DNP) experiments rely on the transfer of a sample between two high-field magnets. During this transfer, samples might experience passage through regions where the stray fields of the magnets are very weak, can approach zero, and even change their sign. This can lead to unexpected spectral features in spin systems that undergo transitions from weak- to strong-coupling regimes and vice versa, much like in field cycling nuclear magnetic resonance experiments. We herein demonstrate that the spectral features observed in D-DNP experiments can be rationalized, provided the time-dependence of the spin Hamiltonian upon field cycling is sufficiently adiabatic. Under such conditions, a passage through a weak static field can lead to the emergence of a long-lived state (LLS) based on an imbalance between the populations of singlet and triplet states in pairs of nuclei that are strongly coupled during the passage through low field. The LLS entails the appearance of anti-phase multiplet components upon transfer to a high-field magnet for observation of NMR signals.

Aug 16, 2019

Detecting acetylated aminoacids in blood serum using hyperpolarized 13C-1Η-2D-NMR

Katsikis, Sotirios, Ildefonso Marin-Montesinos, Christian Ludwig, and Ulrich L. Günther. “Detecting Acetylated Aminoacids in Blood Serum Using Hyperpolarized 13C-1Η-2D-NMR.” Journal of Magnetic Resonance 305 (August 2019): 175–79.

Dynamic Nuclear Polarization (DNP) can substantially enhance the sensitivity of NMR experiments. Among the implementations of DNP, ex-situ dissolution DNP (dDNP) achieves high signal enhancement levels owing to a combination of a large temperature factor between 1.4 and 300 K with the actual DNP effect in the solid state at 1.4 K. For sufficiently long T1 relaxation times much of the polarization can be preserved during dissolution with hot solvent, thus enabling fast experiments during the life time of the polarization. Unfortunately, for many metabolites found in biological samples such as blood, relaxation times are too short to achieve a significant enhancement. We have therefore introduced 13C-carbonyl labeled acetyl groups as probes into amino acid metabolites using a simple reaction protocol. The advantage of such tags is a sufficiently long T1 relaxation time, the possibility to enhance signal intensity by introducing 13C, and the possibility to identify tagged metabolites in NMR spectra. We demonstrate feasibility for mixtures of amino acids and for blood serum. In two-dimensional dDNP-enhanced HMQC experiments of these samples acquired in 8 s we can identify acetylated amino acids and other metabolites based on small differences in chemical shifts.

Aug 14, 2019

Sensitivity analysis of magic angle spinning dynamic nuclear polarization below 6 K #DNPNMR

Judge, Patrick T., Erika L. Sesti, Edward P. Saliba, Nicholas Alaniva, Thomas Halbritter, Snorri Th. Sigurdsson, and Alexander B. Barnes. “Sensitivity Analysis of Magic Angle Spinning Dynamic Nuclear Polarization below 6 K.” Journal of Magnetic Resonance 305 (August 2019): 51–57.

Dynamic nuclear polarization (DNP) improves signal-to-noise in nuclear magnetic resonance (NMR) spectroscopy. Signal-to-noise in NMR can be further improved with cryogenic sample cooling. Whereas MAS DNP is commonly performed between 25 and 110 K, sample temperatures below 6 K lead to further improvements in sensitivity. Here, we demonstrate that solid effect MAS DNP experiments at 6 K, using trityl, yield 3.2Â more sensitivity compared to 90 K. Trityl with solid effect DNP at 6 K yields substantially more signal to noise than biradicals and cross effect DNP. We also characterize cross effect DNP with AMUPol and TEMTriPol-1 biradicals for DNP magic angle spinning at temperatures below 6 K and 7 Tesla. DNP enhancements determined from microwave on/off intensities are 253 from AMUPol and 49 from TEMTriPol-1. The higher thermal Boltzmann polarization at 6 K compared to 298 K, combined with these enhancements, should result in 10,000Â signal gain for AMUPol and 2000Â gain for TEMTriPol-1. However, we show that AMUPol reduces signal in the absence of microwaves by 90% compared to 41% by TEMTriPol-1 at 7 Tesla as the result of depolarization and other detrimental paramagnetic effects. AMUPol still yields the highest signal-to-noise improvement per unit time between the cross effect radicals due to faster polarization buildup (T1DNP = 4.3 s and 36 s for AMUPol and TEMTriPol-1, respectively). Overall, AMUPol results in 2.5Â better sensitivity compared to TEMTriPol-1 in MAS DNP experiments performed below 6 K at 7 T. Trityl provides 6.0Â more sensitivity than TEMTriPol-1 and 1.9Â more than AMUPol at 6 K, thus yielding the greatest signal-to-noise per unit time among all three radicals. A DNP enhancement profile of TEMTriPol-1 recorded with a frequency-tunable custom-built gyrotron oscillator operating at 198 GHz is also included. It is determined that at 7 T below 6 K a microwave power level of 0.6 W incident on the sample is sufficient to saturate the cross effect mechanism using TEMTriPol-1, yet increasing the power level up to 5 W results in higher improvements in DNP sensitivity with AMUPol. These results indicate MAS DNP below 6 K will play a prominent role in ultra-sensitive NMR spectroscopy in the future.

Aug 12, 2019

Application and methodology of dissolution dynamic nuclear polarization in physical, chemical and biological contexts #DNPNMR

Jannin, Sami, Jean-Nicolas Dumez, Patrick Giraudeau, and Dennis Kurzbach. “Application and Methodology of Dissolution Dynamic Nuclear Polarization in Physical, Chemical and Biological Contexts.” Journal of Magnetic Resonance 305 (August 2019): 41–50.

Dissolution dynamic nuclear polarization (d-DNP) is a versatile method to enhance nuclear magnetic resonance (NMR) spectroscopy. It boosts signal intensities by four to five orders of magnitude thereby providing the potential to improve and enable a plethora of applications ranging from the real-time monitoring of chemical or biological processes to metabolomics and in-cell investigations. This perspectives article highlights possible avenues for developments and applications of d-DNP in biochemical and physicochemical studies. It outlines how chemists, biologists and physicists with various fields of interest can transform and employ d-DNP as a powerful characterization method for their research.

Aug 9, 2019

Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

Hilschenz, Ingo, Sangwon Oh, Seong-Joo Lee, Kwon Kyu Yu, Seong-min Hwang, Kiwoong Kim, and Jeong Hyun Shim. “Dynamic Nuclear Polarisation of Liquids at One Microtesla Using Circularly Polarised RF with Application to Millimetre Resolution MRI.” Journal of Magnetic Resonance 305 (August 2019): 138–45.

Magnetic resonance imaging in ultra-low fields is often limited by mediocre signal-to-noise ratio hindering a higher resolution. Overhauser dynamic nuclear polarisation (O-DNP) using nitroxide radicals has been an efficient solution for enhancing the thermal nuclear polarisation. However, the concurrence of positive and negative polarisation enhancements arises in ultra-low fields resulting in a significantly reduced net enhancement, making O-DNP far less attractive. Here, we address this issue by applying circularly polarised RF. O-DNP with circularly polarised RF renders a considerably improved enhancement factor of around 150,000 at 1.2 lT. A birdcage coil was adopted into an ultra-low field MRI system to generate the circularly polarised RF field homogeneously over a large volume. We acquired an MR image of a nitroxide radical solution with an average in-plane resolution of 1 mm. De-noising through compressive sensing further improved the image quality.

Aug 7, 2019

Overhauser DNP FFC study of block copolymer diluted solution #DNPNMR

Gizatullin, Bulat, Carlos Mattea, and Siegfried Stapf. “Overhauser DNP FFC Study of Block Copolymer Diluted Solution.” Magnetic Resonance Imaging 56 (February 2019): 96–102.

Overhauser dynamic nuclear polarization (DNP) is the dominating hyperpolarization technique to increasing the nuclear magnetic resonance signal in liquids and diluted systems. The enhancement obtained depends on the overall mobility of the radical-carrying molecule but also on its specific interaction with the host molecules. Information about the nature of molecular and radical dynamics can be identified from determining the nuclear T1 as a function of Larmor frequency by Fast Field Cycling (FFC) relaxometry. In this work, DNP and FFC methods were combined for a detailed study of 1H Overhauser DNP enhancements at 340 mT (X-band) and 73 mT (S-band) for diluted solutions of a block-copolymer with and without the addition of TEMPO radicals. NMR relaxation dispersions of these solutions are measured at thermal polarization and DNP conditions in the Xband, and the obtained DNP data were analyzed by a model of electron-nucleus interactions modulated by translational diffusion. The coupling factors for the two different blocks of the copolymer are obtained independently from DNP and NMRD experiments. An additional contribution from scalar interactions was found for polystyrene blocks.