Figure 1. Y101 dynamics is kinetically independent from the inactive to active transition.

(A-C) ¹⁵N-CPMG relaxation dispersion profiles and R₂₀β values for residues undergoing exchange between the inactive and active states (k_ex) are compared to the true exchange-free transverse relaxation rates determined independently (R₂₀β)³¹ for A) S85D, B) Y101F, and C) BeF₃⁻ -activated NtrC^R. The CPMG curves refer to residues 78 (cyan), 79 (yellow), 82 (magenta), 88 (orange), 90 (light brown), 100 (purple), 101 (red), 102 (blue), and the independently determined (R₂₀β) are shown as R₂^eff at ν_CPMG→∞. The difference between R₂₀β and R₂β_1000Hz is plotted for all amides in panels (D-F), showing that many residues sense a second fast exchange process that cannot be suppressed by the applied CPMG field strength. G) The residues with exchange from both processes, the inactive/active interconversion and this second faster process (orange) are clustered around Y101 (blue) in both the inactive (pdbID 1DC7) and active (pdbID 1DC8) state structures.