Figure 6: Intrinsic relaxation rate R_1,A* determines the range of accessible exchange rates.

a, Simulated R₁ relaxation curves of spin B (R_1,B* = 3.0 s⁻¹) in exchange with A with R_1,A* = 10 s⁻¹ (top) or 100 s⁻¹ (bottom). The fraction of spins in state B was set to 0.15 and the rate of transition from state B to state A, k_BA, varied from 0 to 200 s⁻¹ (black 0, red 1, blue 2, green 5, cyan 10, magenta 20, brown 50, orange 100, purple 200). b, 1D ¹⁹F NMR spectra of dHis/A381C-TET GltPh in the absence (top) and presence (bottom) of 3 molar equivalents of Ni²⁺ ions. Raw data are black, fits are magenta and deconvoluted peaks are blue. c, Paramagnetic R₁ relaxation curve of the S0 (black circles), S1 (red squares), S2 (blues triangles) and S3 (green reverse triangles) peaks of dHis/A381C-TET GltPh in the presence of Ni²⁺ ions. All measurements were performed in the presence of 200 mM NaCl and 10 μM L-asp. Solid lines represent bi-exponential fits for the S0 and S1 peaks and mono-exponential fits for the S2 and S3 peaks. The fitted parameters for S0 are: k_fast = 124.7 ± 32.0 s⁻¹, k_slow = 2.2 ± 1.7 s⁻¹; S1: k_fast = 97.5 ± 5.4 s⁻¹, k_slow = 2.8 ± 0.5 s⁻¹; S2: k = 5.4 ± 0.8 s⁻¹; S3: k = 4.5 ± 0.5 s⁻¹. The bi-exponential nature of R₁ relaxations of peaks S0 and S1 may reflect the presence of small but significant chemical exchange with the low-PRE peaks. Two independently prepared samples yielded similar results.