Experiments were performed using an Avance III 500 MHz Bruker spectrometer equipped with a 5 mm CPQCI 1H/19F/13C/15N/D Z-GRD cryoprobe at 298 K as described previously . Spectra were recorded using 80 scans of a CPMG pulse sequence that attenuates broad resonances. A CPMG delay of 0.133 s was used, to maximize the difference between the signal intensity of spy molecule alone and in the presence of protein. The transmitter frequency was placed close to the resonance of O1 = -35451 Hz (-75.3 ppm). Protein was used at 5 μM, spy molecule was used at 100 μM, compounds were tested at 100 μM in buffer containing 20 mM HEPES pH 8, 50 mM NaCl, 1 mM TCEP, 20% D2O, 2% DMSO. All NMR data were processed and analysed using TopSpin (Bruker). The dissociation constant was calculated as described previously .