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. 2024 Dec 4;121(50):e2416682121. doi: 10.1073/pnas.2416682121

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Copyright © 2024 the Author(s). Published by PNAS.

This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY).

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Fig. 5. — m-values describe how the compaction of A17G FF evolves along the folding trajectory. (A) Representative amide ¹⁵N CEST profiles from I43 and S56 in A17G FF (16.4 T; T_EX = 400 ms; 2.5 °C) recorded with varying amounts of urea. Experimental data are shown with pink circles while the continuous brown line in each panel is calculated from the best-fit parameters. The vertical black, blue, green, and red lines correspond to the fitted $ϖ_{F}$ , $ϖ_{I 1}$ , $ϖ_{I 2}$ , and $ϖ_{U}$ values, respectively. (B) Variation of $Δ G_{F I 1}$ , $Δ G_{F I 2}$ , and $Δ G_{FU}$ as a function of urea concentration. The m-value for state K (m_K) is obtained from the slope of the urea concentration dependence of $Δ G_{FK} = - R T l n (p_{K} / p_{F})$ , with $p_{K}$ and $p_{F}$ obtained from the analysis of the amide ¹⁵N CEST profiles recorded at different urea concentrations, as illustrated in (A). (C) Variation of $Δ G_{F T S I 1 F}$ , $Δ G_{F T S I 2 F}$ , $Δ G_{F T S I 1 I 2}$ , and $Δ G_{F T S U I 1}$ as a function of urea concentration, where $Δ G_{FTSKJ}$ is the free energy difference between TS_KJ and the F state. The m-value for transition state TS_KL (m_TSKL) is obtained from the slope of the urea concentration dependence of $Δ G_{FTSKL} = - R T l n (p_{K} / p_{F}) - R T l n (k_{KL} / C)$ , where $p_{K}$ , $p_{F}$ and $k_{KL}$ are derived from the analysis of ¹⁵N CEST profiles recorded as a function of urea concentration. The constant C is set to be $10^{7}$ s⁻¹ and does not affect the value of m_TSKL. Best-fit four-state exchange parameters obtained at various urea concentrations are listed in SI Appendix, Table S4. (D) Urea m-values obtained from (B) and (C) at various points along the folding landscape of A17G FF. Ten structures (W11–K71) of the F state (WT FF), the I1 state (WT FF), and the I2 state (A17G FF) are shown. In the case of the I1 state, residues corresponding to the disordered H4 helix have been added for the purpose of illustration.