Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR.

Open Access Article. Published on 02 October 2017. Downloaded on 21/03/2018 13:51:47. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Chemical Science View Article Online EDGE ARTICLE Cite this: Chem. Sci., 2017, 8, 8150 View Journal | View Issue Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR† Frédéric Mentink-Vigier, a Guinevere Mathies,b Yangping Liu, c Anne-Laure Barra,d Marc A. Caporini,e Daniel Lee,a Sabine Hediger,a Robert G. Griffinb and Gaël De Paëpe *a Dynamic nuclear polarization (DNP) has the potential to enhance the sensitivity of magic-angle spinning (MAS) NMR by many orders of magnitude and therefore to revolutionize atomic resolution structural analysis. Currently, the most widely used approach to DNP for studies of chemical, material, and biological systems involves the cross-effect (CE) mechanism, which relies on biradicals as polarizing agents. However, at high magnetic fields ($5 T), the best biradicals used for CE MAS-DNP are still far from optimal, primarily because of the nuclear depolarization effects they induce. In the presence of bisnitroxide biradicals, magic-angle rotation results in a reverse CE that can deplete the initial proton Boltzmann polarization by more than a factor of 2. In this paper we show that these depolarization losses can be avoided by using a polarizing agent composed of a narrow-line trityl radical tethered to a broad-line TEMPO. Consequently, we show that a biocompatible trityl-nitroxide biradical, TEMTriPol-1, provides the highest MAS NMR sensitivity at $10 T, and its relative efficiency increases with the magnetic field strength. We use numerical simulations to explain the absence of Received 16th May 2017 Accepted 1st October 2017 depolarization for TEMTriPol-1 and its high efficiency, paving the way for the next generation of polarizing agents for DNP. We demonstrate the superior sensitivity enhancement using TEMTriPol-1 by DOI: 10.1039/c7sc02199b recording the first solid-state 2D 13C–13C correlation spectrum at natural isotopic abundance at rsc.li/chemical-science a magnetic field of 18.8 T. Introduction Magic-angle angle spinning (MAS) NMR is a powerful method to obtain atomic resolution structures of many systems of chemical, biological, and physical interest. Unfortunately, the inherent low sensitivity of the method places limitations on the a Univ. Grenoble Alpes, CEA, CNRS, INAC-MEM, F-38000 Grenoble, France. E-mail: b Francis Bitter Magnet Laboratory, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA c Tianjin Key Laboratory on Technologies Enabling Development of Clinical Therapeutics and Diagnostics, School of Pharmacy, Tianjin Medical University, Tianjin 300070, China d Laboratoire National des Champs Magnétiques Intenses – CNRS, Univ. Grenoble Alpes, F-38042 Grenoble, France e Bruker BioSpin Corporation, 15 Fortune Drive, Billerica, MA 01821, USA † Electronic supplementary 10.1039/c7sc02199b information (ESI) available. See DOI: ‡ Current address: CIMAR-National High Magnetic Field Laboratory, 1800 E Paul Dirac Dr, Tallahassee, FL, 32310, USA. § Current address: Department of Chemistry, Universitätsstraße 10, 78464 Konstanz, Germany. University of Konstanz, { Current address: Amgen Inc., 360 Binney Street, Cambridge, MA 02142, USA. 8150 | Chem. Sci., 2017, 8, 8150–8163 size and complexity of the structural studies for which it can be used.1 Dynamic nuclear polarization (DNP) represents an approach to address this sensitivity problem2–6 and in recent years DNP was successfully employed to answer structural questions in complex chemical and biological systems,7–15 and in materials science.16–23 MAS-DNP NMR spectrometers typically rely on NMR probes compatible with operation under cryogenic conditions (80–100 K)24–26 and dedicated gyrotrons for the required highfrequency, high-power continuous microwave irradiation at frequencies up to 527 GHz.27–31 Paramagnetic polarizing agents are introduced into the NMR sample, usually in the form of stable free radicals such as bisnitroxides.32 Continuous microwave (mw) irradiation at or near the electron Larmor frequency (u0S) induces the transfer of the large electron spin polarization to surrounding nuclear spins, generating 1H NMR signal enhancements up to a factor of 658. The cross-effect (CE) mechanism33–37 is currently the most efficient way to enhance this nuclear polarization and as it requires two interacting electrons, biradicals are the polarizing agents of choice.38–43 The combination of CE and MAS generates the polarization transfer through a series of energy level crossings/anti-crossings (rotor events).44–46 Specically, during This journal is © The Royal Society of Chemistry 2017 View Article Online Open Access Article. Published on 02 October 2017. Downloaded on 21/03/2018 13:51:47. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Edge Article the sample rotation the mw irradiation periodically generates a large polarization difference between the two electron spins and this polarization difference is periodically transferred to the nuclei during CE rotor events (where |u0,Sa  u0,Sb|  |un|).44–46 However, recent contributions47–49 have also shown that the Boltzmann equilibrium nuclear polarization can be substantially depleted by the presence of bisnitroxide polarizing agents and sample spinning alone. For instance, 1H NMR signals at 9.4 T and 100 K from samples doped with bisnitroxides TOTAPOL39 and AMUPol42 show a decrease of the equilibrium intensity in the absence of mw irradiation and during MAS of 20 and 60% compared to an undoped sample, respectively.48 This phenomenon, called depolarization,47 corresponds to the CE mechanism in absence of mw irradiation and it has been shown to limit the DNP efficiency.48,49 Additionally, like most continuous-wave DNP mechanisms (with the recent exception of the Overhauser effect50), the CE efficiency decreases at higher magnetic elds.2,28,31,48,50,51 Although current bisnitroxide polarizing agents have provided success in a variety of challenging applications at moderate magnetic eld (up to 10 T), the actual sensitivity gain at higher magnetic elds (14.1 and 18.8 T, corresponding to 600 and 800 MHz for 1H, respectively) is drastically decreased and thus directly compromises the study of more complex systems that require higher resolution.6,27–30,48,51 In this paper we show that the current sensitivity limitation at high magnetic eld can be overcome through the use of heteroradical polarizing agents, e.g. trityl-nitroxide biradicals. Notably, we report unprecedented sensitivity gains that translate into one order of magnitude of additional time-savings compared to the best polarizing agents currently in use at 18.8 T (the highest magnetic eld available to date for DNP). The improved NMR sensit (...truncated)