Figure 6.

(a) Reduced-χ² comparing the experimental and first-principles isotropic chemical shifts for 30 geometry-optimized active-site models with varying protonation states (structures and labeling given in Scheme S1). Models with reduced-χ² greater than 1.75 can be excluded with over 95% confidence. (b) Reduced-χ² comparing the experimental and first-principles isotropic and anisotropic chemical shifts for the 10 best fast-exchange equilibrium models. For each optimized isotropic model (red), the corresponding population weighted CSA tensors are also compared to the experimental principal axis components for the substrate C′ (gray) and Schiff base N (blue) sites.