Fig. 1.

(A) The 2D ¹³C–¹³C DARR spectra (100 ms mixing time) of Pf1 illustrating linewidth differences between DNP conditions at 106 K (purple) and the same sample at 273 K (green). (B) Expansions around several assigned peaks with unique chemical shifts are shown. Although some well-resolved peaks at 273 K remained well resolved at 106 K, peaks in more crowded regions can become unrecognizable. Even for well-resolved peaks, linewidths broadened approximately twofold, from <100 Hz (0.5 ppm) to >200 Hz (1.3 ppm), making assignments more challenging. See SI Appendix for additional experimental details, including demonstration of 35-fold enhancement in 3.2-mm rotor and 60-fold enhancement in 1.9-mm rotor (SI Appendix, Fig. S1).