Accelerated ¹⁹F biomolecular magic-angle spinning NMR with paramagnetic dopants

The advantageous characteristics attributed to the ${}^{19}F$ nucleus have made it a popular target for nuclear magnetic resonance (NMR) once again in recent years. Aside from solution NMR, an increasing number of studies have been conducted applying solid-state magic-angle spinning (MAS) NMR to fluorine-labelled samples. Here, the high chemical shift anisotropy and strong dipolar couplings can be utilised to get structural insights into proteins and measure long distances. Despite increasing popularity and promising benefits, the sensitivity of biomolecular ${}^{19}F$ MAS NMR often suffers from slow longitudinal $T_1$ relaxation and therefore long recycle delays. In this work, we expand paramagnetic doping, an approach commonly used to reduce proton $T_1$ relaxation times, to ${}^{19}F$ -labelled biological samples. We study the effect of Gd(DTPA) and Gd(DTPA-BMA) on ${}^{19}F$ $T_1$ and $T_2$ , and ${}^{13}C$ $T_1$ and $T_2$ relaxation in a [5- ${}^{19}{F}^{13}C$ ]-tryptophan-labelled protein via ${}^{19}F$ -detected MAS NMR experiments. The observed paramagnetic relaxation enhancement substantially reduces measurement times of ${}^{19}F$ MAS NMR experiments without compromising resolution. Additionally, we report the chemical shift assignments of all four fluorotryptophan signals in the $12\times 39$ $\mathrm{kDa}$ -large protein TET2 using a mutagenesis approach.