PyrroTriPol: a Semi-rigid Trityl-Nitroxide for High Field Dynamic Nuclear Polarization

Abstract

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) has significantly broadened the scope of solid-state NMR to study biomolecular systems and materials. In recent years, the advent of very high field DNP combined with fast MAS has brought new challenges in the design of polarizing agents (PA) used to enhance nuclear spin polarization. Here, we present a trityl-nitroxide PA family based on a piperazine linker, named PyrroTriPol, for both aqueous and organic solutions. These new radicals have similar properties to that of TEMTriPol-I and can be readily synthesized, and purified in large quantities thereby ensuring widespread application. The family relies on a rigid bridge connecting the trityl and the nitroxide offering a better control of the electron spin-spin interactions thus providing improved performance across a broad range of magnetic fields and MAS frequencies while requiring reduced microwave power compared to bis-nitroxides. We demonstrate the efficiency of the PyrroTriPol family under a magnetic field of 9.4, 14.1 and 18.8 T with respect to TEMTriPol-I. In particular, the superiority of PyrroTriPol was demonstrated on γ-Al2O3 nanoparticles which enabled the acquisition of a high signal-to-noise surface-selective 27Al multiple-quantum MAS experiment at 18.8 T and 40 kHz MAS frequency

AsymPol-TEKs as efficient polarizing agents for MAS-DNP in glass matrices of non-aqueous solvents

“…Magic angle spinning dynamic nuclear polarization (MAS-DNP) has expanded the scope of solid-state Nuclear Magnetic Resonance (ssNMR) spectroscopy. [1][2][3][4][5][6] MAS-DNP boosts 1 H spin polarization by taking advantage of the higher polarization of unpaired electrons spins that are present in so-called Polarizing Agents (PAs). 7,8 This results in an improved sensitivity that enables applications to challenging materials and the study of complex biomolecular systems.…”

“…[9][10][11][12][13][14][15][16][17][18][19][20] Several DNP mechanisms have been reported at high magnetic fields, including the solid effect, the Overhauser effect, and the cross effect. 2 The cross-effect (CE) is currently the most efficient in terms of sensitivity 3,[21][22][23][24] and necessitates the use of PAs containing two coupled electron spins. 7 The electron spin polarization difference is transferred to the isotope of interest via a complex mechanism driven by the sample spinning and microwave (mw) irradiation 3,5,[25][26][27][28][29][30][31][32][33][34] at cryogenic temperature (typically B100 K 35,36 or lower 4,[37][38][39][40][41][42][43][44] ).…”

“…2 The cross-effect (CE) is currently the most efficient in terms of sensitivity 3,[21][22][23][24] and necessitates the use of PAs containing two coupled electron spins. 7 The electron spin polarization difference is transferred to the isotope of interest via a complex mechanism driven by the sample spinning and microwave (mw) irradiation 3,5,[25][26][27][28][29][30][31][32][33][34] at cryogenic temperature (typically B100 K 35,36 or lower 4,[37][38][39][40][41][42][43][44] ). However, further improvements remain mandatory to fully unlock the potential of MAS-DNP especially for challenging samples, and at high magnetic fields and fast MAS frequencies.…”

Efficient 18.8 T MAS-DNP NMR reveals hidden side chains in amyloid fibrils

“…An alternate strategy is to design a new class of DNP polarizing agents (or hetero-biradicals) that maximally exploit the cross-effect matching conditions at high magnetic fields. 38,43,[51][52][53] Among these novel biradicals, we will characterize the DNP performances of NATriPol-3 and SNAPol-1 radicals, which were demonstrated to yield efficient DNP performances (vide infra). 54,55 Molecular motions present in biological macromolecules average NMR interactions (such as chemical shifts) over different conformations and thus may lead to narrow lines.…”