Highly Efficient Polarizing Agents for MAS‐DNP of Proton‐Dense Molecular Solids

Abstract

Efficiently hyperpolarizing proton‐dense molecular solids through dynamic nuclear polarization (DNP) solid‐state NMR is still an unmet challenge. Polarizing agents (PAs) developed so far do not perform well on proton‐rich systems, such as organic microcrystals and biomolecular assemblies. Herein we introduce a new PA, cAsymPol‐POK, and report outstanding hyperpolarization efficiency on 12.76 kDa U‐13C,15N‐labeled LecA protein and pharmaceutical drugs at high magnetic fields (up to 18.8 T) and fast magic angle spinning (MAS) frequencies (up to 40 kHz). The performance of cAsymPol‐POK is rationalized by MAS‐DNP simulations combined with electron paramagnetic resonance (EPR), density functional theory (DFT) and molecular dynamics (MD). This work shows that this new biradical is compatible with challenging biomolecular applications and unlocks the rapid acquisition of 13C–13C and 15N–13C correlations of pharmaceutical drugs at natural isotopic abundance, which are key experiments for structure determination.

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

“…Bis-nitroxides versus hetero-biradicals for MAS-DNP Bis-nitroxides have been actively developed in recent years to provide better performance than the first generation of biradicals. [28][29][30][31][32][33][34][51][52][53][54] Some of these PAs are attractive solutions since they can also be synthetized in large scale enabling a broader access to the community. In addition, recent structures with larger e-e couplings such as, AsymPol-POK/cAsymPol-POK and m-TinyPol [32][33][34] have partly mitigated the magnetic field and MAS dependency.…”

“…33 These radicals can very efficiently polarize proton-dense systems, including at high fields and fast MAS. 33,34 Yet, a performance drop in larger diameter MAS rotors (e.g. 3.2 mm, sapphire) is still observed above 14.1 T. 33,34 In recent years, two organic radicals with relatively narrow EPR lines have been included in new biradical PAs for DNP, first the Finland trityl 35,43 and second l,3-bis(diphenylene)-2-phenylallyl (BDPA).…”

“…7 The biradical structures were subsequently optimized and nowadays efficient PAs include AMUPol, 28 TEKPol, 29 bCTol(-M), 30,31 and more recently TinyPol 32 and (c)AsymPol-POK. 33,34 Hetero-biradicals containing overlapping narrow and broad EPR lines, such as TEMTriPol-I 35,36 and HyTEK, 37 were introduced to overcome the drawbacks of bis-nitroxides, such as AMUPol and TEKPol, 28,29 that exhibit a decreased CE-DNP efficiency at very high field (> 10 T) and fast MAS (> 15 kHz). [35][36][37][38] These heterobiradicals require lower µw field to saturate one of the electron spin transitions as compared to nitroxides, which possess broader EPR lines that are harder to saturate.…”

“…33,34 Yet, a performance drop in larger diameter MAS rotors (e.g. 3.2 mm, sapphire) is still observed above 14.1 T. 33,34 In recent years, two organic radicals with relatively narrow EPR lines have been included in new biradical PAs for DNP, first the Finland trityl 35,43 and second l,3-bis(diphenylene)-2-phenylallyl (BDPA). 37 For water-based applications, the trityl-nitroxide TEMTriPol-I, 35 and the hydrophilic NATriPol derivatives 43 have been shown to exhibit good performance at high magnetic fields.…”

Solid-state NMR analysis of unlabeled fungal cell walls from Aspergillus and Candida species

“…Unlabeled fungal materials were mixed with the stock solution containing the biradicals needed for MAS-DNP. The stock solution contains 30 mM of AsymPol-POK biradicals ( Mentink-Vigier, 2018 , Harrabi, 2022 ) in 40 µL of d6-DMSO-D 2 O-H 2 O (1:8:1 vol%) that was used to avoid 13 C signal contribution from the solvents (e.g., from glycerol). The volume percentages of different solvents used here deviate from conventional recipes used for MAS-DNP of biomolecular samples.…”