Fig. 3. ¹⁹F NMR supports the sampling of an excited conformational state by the α1-helix.

a, Close-up view of the α1-helix, where the mutation N12C was introduced to allow for BTFA labeling. Hydrophobic residues are shown in blue; charged and polar solvent-exposed residues are shown in yellow. b, Helical wheel projection for residues 3–12 in the α1-helix. Coloring as in a. c, ¹⁹F NMR spectrum of Xrn2 ΔZnF N12C^BTFA. d, CPMG and on-resonance R_1ρ dispersion profiles for Xrn2 ΔZnF N12C^BTFA samples recorded at 500 MHz and 600 MHz ¹H frequency at 313 K. Data points are shown with error bars derived from multiple measurements; lines show the best fit derived from the simultaneous analysis of all datasets from five temperatures with one global | Δω | . At 313 K, this yielded: p_GS = 0.50 ± 0.06; k_ex = 913 ± 108 s⁻¹; and | Δω | = 0.15 ± 0.01 p.p.m. (Supplementary Table 6). Data points are shown as mean ± s.d., derived from three duplicate NMR measurements. e, Correlation between the exchange rate (k_ex) and the temperature. Note that the recording of the ¹⁹F relaxation data is considerably faster than the recording of the ¹³C data, as the latter depends on a series of 2D NMR spectra. Data points are shown as mean ± s.d., derived from 500 Monte Carlo simulations (Extended Data Fig. 5, Supplementary Figs. 12 and 13 and Supplementary Table 6). f, CPMG dispersion profiles of Xrn2 ΔZnF N12C^TET at 11.7 T (500 MHz ¹H frequency) and 14.1 T (600 MHz ¹H frequency) at 313 K. The data were fit with the population p_GS = 50% and the exchange constant k_ex = 913 s⁻¹ obtained from analysis of the BTFA data. Data points are shown as mean ± s.d., derived from three duplicate NMR measurements. a.u., arbitrary unit.