Fig. 2.

Concentration-dependent exchange dynamics of htt^NTQ₇ observed by NMR. (A) ¹³Cα exchange line-broadening measured by CPMG relaxation dispersion $\left({}^{\mathrm{13}}\mathrm{C}\mathrm{\alpha }-R_{\mathrm{ex}}=R_{2,\mathrm{eff}}^{25\mathrm{Hz}}-R_{2,\mathrm{eff}}^{\mathrm{2,000}\mathrm{Hz}}\right)$ at 600 MHz and (B) ¹³Cα exchange induced shifts (¹³Cα-δ_ex = δ_obs − δ_ref) for htt^NTQ₇ (800 MHz) at three concentrations (0.4, 0.75, and 1 mM) and htt^NT (900 MHz) at one concentration (0.75 mM) and 5 °C. For htt^NTQ₇, the reference shifts, δ_ref, in the absence of exchange were determined by fitting the concentration dependence of the observed shifts, δ_obs, for each residue to a second-order polynomial (SI Appendix, SI Materials and Methods); for htt^NT, the reference shifts are obtained from the spectrum recorded on a 20 μM sample. (C) ${}^{15}\mathrm{N}-R_2^{1\mathrm{kHz}}$ transverse relaxation rate profiles obtained from ¹⁵N-R_1ρ measurements at 600 MHz and 10 °C recorded with a spin-lock radiofrequency field strength of 1 kHz for htt^NTQ₇ (10 μM and 1 mM), (Met⁷O)-htt^NTQ₇ (1 mM), and htt^NT (10 μM and 1 mM). (D) Correlation of ${}^{15}\mathrm{N}-R_2^{1\mathrm{kHz}}$ and (¹⁵N-δ_ex)² for htt^NTQ₇ at 800 MHz and 5 °C. (E) Expansion of selected regions of the 900-MHz ¹H–¹³C constant time HSQC spectra of 0.8 mM (blue) and 20 μM (red) samples of htt^NTQ₇ (Left) and htt^NT (Right) at 5 °C. Error bars represent 1 SD (when not shown, they are within the circles representing the experimental data).