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 (T_g) restricts the viable temperature range for MAS- DNP. To expand applications of DNP at higher temperature, new matrices and physical insights are required. Here we demonstrate that sorbitol, T_g≈267 K, advantageously replaces glycerol to carry out DNP at higher temperature while maintaining an identical 13C NMR spectrum footprint and thus minimizes spectral overlap. DNP stops being effective in glycerol/water at ~180 K, but DMSO/sorbitol gives significant enhancement at 230 K, with AsymPol-POK biradicals at 600 MHz / 395 GHz. The effect of biradical concentration, the temperature dependence of the enhancement, the buildup times and signal-to-noise ratio are rationalized for the first time with a simple analytical model. 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 shells which allowed for the recording of rapid heteronuclear correlation spectra at 100 and 225 K.