Dynamic Nuclear Polarization Enhancement of 200 at 21.15 T Enabled by 65 kHz Magic Angle Spinning

Solid-state nuclear magnetic resonance under magic angle spinning (MAS) enhanced with dynamic nuclear polarization (DNP) is a powerful approach to characterize many important classes of materials, allowing access to previously inaccessible structural and dynamic parameters. Here, we present the first DNP MAS experiments using a 0.7 mm MAS probe, which allows us to reach spinning frequencies of 65 kHz, with microwave irradiation, at 100 K. At the highest magnetic field available for DNP today (21.1 T), we find that the polarizing agent HyTEK2 provides DNP enhancements as high as 200 at a spinning rate of 65 kHz at 100 K, and BDPA yields an enhancement of 106 under the same conditions. Fast spinning rates enable excellent DNP performance, but they also yield unprecedented 1H resolution under DNP conditions. We report well-resolved 1H-detected 1H–13C and 1H–15N correlation spectra of microcrystalline histidine·HCl·H2O.

Dynamic Nuclear Polarization and Other Magnetic Ideas at EPFL

Geoffrey Bodenhausen, Aurélien Bornet, Roberto Buratto, Marc Anthony Caporini, Diego Carnevale, Srinivas Chinthalapalli, Sami Jannin, Daniele Mammoli, Pascal Miéville, Jonas Milani, Angel Joaquin Perez Linde, Martial Rey, Nicola Salvi, Simone Ulzega, Shutao Wang

Although nuclear magnetic resonance (NMR) can provide a wealth of information, it often suffers from a lack of sensitivity. Dynamic Nuclear Polarization (DNP) provides a way to increase the polarization and hence the signal intensities in NMR spectra by transferring the favourable electron spin polarization of paramagnetic centres to the surrounding nuclear spins through appropriate microwave irradiation. In our group at EPFL, two complementary DNP techniques are under investigation: the combination of DNP with magic angle spinning at temperatures near 100 K ('MAS-DNP'), and the combination of DNP at 1.2 K with rapid heating followed by the transfer of the sample to a high-resolution magnet ('dissolution DNP'). Recent applications of MAS-DNP to surfaces, as well as new developments of magnetization transfer of H-1 to C-13 at 1.2 K prior to dissolution will illustrate the work performed in our group. A second part of the paper will give an overview of some 'non-enhanced' activities of our laboratory in liquid- and solid-state NMR.

Schweizerische Chemische Gesellschaft2012

Chemical Developments in DNP-NMR

Pascal Miéville

The Nuclear Magnetic Resonance (NMR) is an extraordinary tool to understand molecular structures. Its main limitation resides in its relatively low sensitivity when compared with other physico-chemical methods such as Mass Spectrometry (MS) or Optical Spectroscopy. Dynamic Nuclear Polarization (DNP), invented in the 50's but only widely applied in the last ten years, allows, by transferring the high polarization of paramagnetic centers to surrounding nuclei at low temperature, to enhance NMR signals by several orders of magnitude. This technique can be applied to solids at 100 K using Magic Angle Spinning (DNP-MAS) or in liquid phase through Dissolution-DNP as described by Ardenkjaer-Larsen in 2003. The efficiency of the polarization transfer is strongly dependent on the nature of the paramagnetic centers and on the way they are mixed with the target molecules. Part of this work will consist in the study of new ways to integrate paramagnetic centers into the matrix and also to limit their negative effects on NMR signals in liquid phase after dissolution. Because DNP-NMR has reached a stage where it should become a standard chemical method, a second objective of this thesis is to propose new chemical applications such as kinetic measurements that may be of interest in other domain of chemistry.

EPFL2011

De l'usage des protons hyperpolarisés pour augmenter la sensibilité de la RMN

Aurélien Bornet

Nuclear Magnetic Resonance (NMR) has become an inescapable technique for spectroscopic identification. Its main advantage comes from the sensitivity of NMR active nuclei embedded in a molecule to their chemical environment. NMR is also used daily in medical imaging. Magnetic Resonance Imaging (MRI) is not only remarkably versatile, but has the precious advantage of being non-invasive; moreover, the range of radiofrequency used implies that MRI deposit a limited amount of energy in tissues under investigation. Nevertheless, compared to other spectroscopic methods, NMR suffers from a relative lack of sensitivity. Indeed as the NMR transitions are low in energy, the difference of populations between the levels involved, known as polarization, is extremely low. The NMR signal, which is directly proportional to this polarization, is thus many orders of magnitude inferior to the theoretical maximum at full polarization. Dynamic Nuclear Polarization (DNP) allows one to circumvent this disadvantage by transferring the high electron spin polarization to the nuclear spins. This transfer happens via microwave irradiation under optimized conditions. The method has been constantly developed since the âfifties. A substantial breakthrough was achieved in 2003 by Golman, Ardenkjaer-Larsen and their collaborators. They proposed to dissolve a sample that has been hyperpolarized at low temperature (at about 1 K) in order to inject it into an animal, or, in fine, into a human patient, and to follow its bio-distribution and eventual metabolic conversion by MRI. This technique, called Dissolution-DNP (D-DNP), allows a signal increase on the order of 10'000, opening the way to many new experimental possibilities. Research in Dissolution-DNP was largely oriented toward the optimization of 13C polarization because of its long lifetimes. In the course of this Thesis, an alternative way that takes advantage of the proton (1H) polarization will be explored. Protons have the advantage that they can be polarized to a higher levels in a shorter time compared to 13C. Unfortunately, once ejected from the polarizer, in solution and at room temperature, the high proton magnetization will be short-lived compared to 13C. Along the chapters of this Thesis, different approaches will be proposed to maximize the advantages of 1H polarization, while minimizing its inconveniences. It is possible to use a standard NMR technique, known as Cross-Polarization (CP), to transfer the abundant magnetization of hyperpolarized protons to other nuclei like 13C, using suitable radiofrequency pulse sequences. The advantages of the 1H polarization are exploited inside the polarizer, while the interesting properties of 13C are put to use after dissolution. Still, it is also possible to observe directly the proton signal after dissolution. This can be extremely interesting, especially in the context of drug screening for pharmaceutical research. Two examples of such methods will be described. Finally, the use of hyperpolarized 1H signals after dissolution can be greatly improved if their relaxation rates could be attenuated. A first way of doing this, consisting in removing the paramagnetic species by filtration, will be explored. The use of Long-Lived States (LLS) will also be presented.

EPFL2015

De l'usage des protons hyperpolarisés pour augmenter la sensibilité de la RMN

Aurélien Bornet

EPFL2015