13C CP MAS NMR spectra of (A) 1M U-[13C-15N] urea and (B) 0.5 M U-[13C-15N] proline at 80 K with and without microwave irradiation. The DNP enhancements are ε=181 and ε=134, respectively. Both samples contained 10 mM TOTAPOL in a 60/30/10 ratio of d8-glycerol/D2O/H2O. Experimental parameters are: 4 scans, recycle delay 4 s, microwave power ~12.5 W, γB1(1H)=83 kHz, γB1(13C)=71 kHz, ωr/2π= 5 kHz.