Solid-state NMR experiments

All ssNMR experiments were conducted with an 18.8T Bruker Advance III HD spectrometer equipped with a 1.9 mm HCN magic angle spinning (MAS) probe. The sample temperature was maintained at 11–13 °C, which was calibrated externally with ethylene-glycol⁶². The ¹³C chemical shift was referenced at the DSS scale using adamantine with the downfield signal set at 40.48 ppm. The ¹H and ¹⁵N chemical shifts were indirectly calculated based on the ¹³C reference ⁶³. All experiments were performed with a MAS frequency of 17857 Hz. The 90° pulse lengths of ¹H, ¹³C, and ¹⁵N were 2.4 μs, 3.2 μs, and 4.15 μs, respectively. In the 2D J-coupled ¹H-¹³C correlation experiments, the spectral width and acquisition time were 266.25 ppm and 19.1 ms in the direct dimension, and 11.16 ppm and 15.2 ms in the indirect dimension. In the 2D J-coupled ¹H-¹⁵N correlation experiments, the spectral width and acquisition time were 220.22 ppm and 19.7 ms in the direct dimension, and 3.72 ppm and 16.1 ms in the indirect dimension. The spectral width and acquisition time used in the 2D ¹³C–¹³C DARR experiments were 266.23 ppm and 15.5 ms in the direct dimension and 266.23 ppm and 8.4 ms in the indirect dimension. In the 2D NCA experiments, the spectral width and acquisition time were 266.25 ppm and 14.4 ms in the direct dimension, and 44.0 ppm and 12.3 ms in the indirect dimension. In the 2D NCO experiments, the spectral width and acquisition time were 88.74 ppm and 14.4 ms in the direct dimension and 36.7 ppm and 12.1 ms in the indirect dimension. The NCA and NCO transfer was achieved by SPECIFIC-CP⁶⁴. In all experiments, the recycle delay was set as 1.5 s. All ssNMR data was processed using nmrPipe⁶⁵. and analyzed with Sparky (T. D. Goddard and D. G. Kneller, University of California, San Francisco).