Back-calculation of NMR observables. To compare simulated ensembles to solutionstate NMR measurements, 43 a total of 158 ensemble-averaged observables (13 NMR chemical shifts, 138 NOE distances, and 6 vicinal J-coupling constants) were back-calculated from the simulation data, by averaging predicted observables over 20 randomly selected structures for each microstate, then calculating observables as microstate population-weighted averages. SHIFTX2 (v1.11) was used to calculate predictions for six H N and seven H α chemical shifts. Predicted vs. experimental chemical shift observables for all force fields are shown in Figure ) and Figure ), respectively. Nuclear overhauser effect (NOE) distances were calculated using MDTraj. Predicted vs. experimental NOE distances for all force fields are shown in Figure . Six vicinal J-coupling constants for H N and H α were computed with the BICePs (v2.0) package 33 (Figure ), using Karplus equation parameters from Vögeli, Bax et al.