Fig. 6.

Stabilization free energies, $\mathrm{\Delta }\mathrm{\Delta }G$ (defined in Eq. 8), of the intermediate state (dashed lines) or the folded state (solid lines) relative to the unfolded state upon addition of Mg²⁺ ions for rRNA. $\mathrm{\Delta }\mathrm{\Delta }{G}_{I-F}$ is the difference between these 2 free energies $\mathrm{\Delta }\mathrm{\Delta }{G}_{I-F}=\mathrm{\Delta }\mathrm{\Delta }{G}_{U-F}-\mathrm{\Delta }\mathrm{\Delta }{G}_{U-I}$. In the I state, secondary structures are formed. Therefore, $\mathrm{\Delta }\mathrm{\Delta }{G}_{U-I}$ and $\mathrm{\Delta }\mathrm{\Delta }{G}_{I-F}$ are, respectively, the stabilization free energies of Mg²⁺ on secondary and tertiary structure formation.