where r j are predicted observables indexed by j, r expt j are experimental observables, and σ j is the expected uncertainty of forward model predictions for each observable. Thus, a value of χ 2 /N ≈ 1 represents the best expected agreement. Estimates of σ j for chemical shift predictions come from the reported assessments of the accuracy of the SHIFTX2 algorithm: 74? σ H N = 0.2351 ppm and σ H α = 0.1081 ppm. Vicinal J-coupling constant predictions, 3 J(H N H α ), are assumed to have an uncertainty of σ = 0.33 Hz. Uncertainties for NOE distances were set to 1.0 Å, typical of the range between lower and upper limits used in NMR refinement.