MAS solid-state NMR experiments. All the NMR measurements were performed on a 16.4 T magnet with the JEOL HCN 1.0 mm MAS probe and a JEOL ECA-II spectrometer. The MAS rate was kept at 70 kHz throughout. For the GB1 measurements, the cooling gas at -40 °C was used to maintain the sample temperature ambient, and it was estimated to be ~20 °C from the T1 relaxation time of K 79 Br on the same MAS and sample cooling condition . The 13 C chemical shifts were referenced using the L-alanine C' signal at 179.778 ppm (DSS scale). 1 H and N chemical shifts were indirectly referenced to DSS and liquid ammonia, respectively . The carrier for C were set to 55, 175 ppm for Cα and C', respectively. The 15 N carrier was set to 120 ppm. The 1 H carrier was set to 5 ppm for the amino acid mixture measurements, and on a bulk water signal (~5 ppm) for the GB1 measurements. The 90° pulse width of 1 H, C, and 15 N was 1.3, 1.2, and 3.2 µs, respectively. For 1 H to 13 C / 15 N CP and the inverse CP were carried out by DOTA-CP 31 with single quantum condition, whose effective 1 H RF field strength was ~110 kHz and 13 C / 15 N RF field was ~40 kHz. C to N CP were carried out by double quantum (DQ) CP whose Hartman-Hahn matching condition of ~50 kHz for 15 N and ~20 kHz for C. In both CP magnetization transfers, linear-ramp gradient was used. In DOTA-CP, the gradient was applied to the 13 C or 15 N channels. In DQ CP, that was applied to the 13 C channel. The matching condition and gradient were experimentally optimized. The Ca-C' magnetization transfer was caried out by an INEPT-like manner . The spin echo durations 𝛿 ! and 𝛿 " in Figure for the transfer were experimentally optimized. The solvent suppression was performed by MISSISSIPPI without the homospoil gradient pulse . The RF strength of MISSIS-SIPPI was ~10 kHz. 1 H and 15 N decouplings were performed by ~10 KHz of rCW ApA33 and ~20 kHz of WALTZ-16 , respectively. The recycle delay was set to 0.8 s. The data sampling parameters were summarized in Table .