Fig. 3.

(A and B) Ion preferential interaction coefficient ${\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g},S}$ (Left) and free energy of Mg²⁺–pseudoknot interaction $\mathrm{\Delta }{G}_{\mathrm{M}\mathrm{g},S}$ (Right) for BWYV in 54 mM KCl (A) and 79 mM KCl (B), at $T=$ 25 °C. The calculations were performed for the folded, intermediate, and unfolded states (S = F, I, or U) (see main text and Fig. 4 for definition of the states). Experimental data for the F and I states are plotted as dashed lines (67). The results for the U state serve as a prediction of the simulations. The differences between the F and I states, $\mathrm{\Delta }{\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g}}={\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g},F}-{\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g},I}$ and $\mathrm{\Delta }\mathrm{\Delta }{G}_{I-F}=\mathrm{\Delta }{G}_{\mathrm{M}\mathrm{g},F}-\mathrm{\Delta }{G}_{\mathrm{M}\mathrm{g},I}$, are plotted in B, Insets. $\mathrm{\Delta }{\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g}}$ (Left Inset) represents the number of Mg²⁺ ions released when BWYV transitions from the F to the I state. $\mathrm{\Delta }\mathrm{\Delta }{G}_{I-F}$ (Right Inset) is the change in the free energy of the F state relative to the I state upon addition of Mg²⁺ ions. The error bars in Insets are relatively large. However, it is clear that $\mathrm{\Delta }{\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g}}$ and $\mathrm{\Delta }\mathrm{\Delta }{G}_{I-F}$ are not constant in the range of [Mg²⁺] here.