15N–13C δp0 PAIN-CP polarization transfer for pC = pN = 0.75 and pH = 1.8 (a) and pC = pN = 2.9, pH = 1.5 (b). 15N–13C δp0 DCP polarization transfer for pC = 3.5, pN = 2.5 and pH = 10 (c) and pC = 3.5, pN = 2.5, pH = 5 (d). Both sets of simulations were performed at ω0H/2π = 750 MHz and ωr/2π = 20 kHz. The spin system is extracted from the Crh x-ray structure (PDB ID: 1mu4) (Ref. 30) and is composed of L63N, three carbonyls (V61C′, T62C′, and L63C′), and the four protons mainly involved in the spin dynamics (T62H, T62Hα, L63H, and L63Hα). The chemical shifts are taken from the protein assignments (Ref. 31). The 13C carrier offset is set at 177 ppm, L63N, and the protons are irradiated on resonance. The simulation includes typical CSA tensor parameter for nitrogen (99 ppm, 0.19), carbonyls (−76 ppm, 0.9) and proton (5 ppm, 0.7).