Sorbitol-Based Glass Matrices Enable Dynamic Nuclear Polarization beyond 200 K

In magic angle spinning dynamic nuclear polarization (MAS-DNP) experiments, paramagnetic species are often dispersed in rigid glass-forming matrices such as glycerol/water mixtures, but their modest glass-transition temperature (<i>T</i>_<i>g</i>) restricts the viable temperature range for MAS-DNP. To expand applications of DNP at higher temperatures, new matrices and physical insights are required. Here we demonstrate that sorbitol, <i>T</i>_<i>g</i> ≈ 267 K, advantageously replaces glycerol, <i>T</i>_<i>g</i> ≈ 190 K, to carry out DNP at higher temperature while maintaining an identical ¹³C NMR spectrum footprint and thus minimizing spectral overlap. DNP stops being effective in glycerol/water at ∼180 K, but sorbitol/DMSO gives a significant enhancement at 230 K with AsymPol-POK biradicals at 600 MHz/395 GHz. For the first time, a simple analytical model is proposed that provides physical insights and explains the effect of biradical concentration, the temperature dependence of the enhancement, the signal buildup times, and the enhanced signal-to-noise ratio. The model reveals that electron spin relaxation is the limiting factor for high-temperature DNP in the case of AsymPol-POK. We showcase the efficacy of this new DNP formulation on an intriguing chitin sample extracted from cicada exoskeleton which allowed for the recording of rapid heteronuclear correlation spectra at 100 and 225 K.