Fig. 8.

Importance of treating both inner- and outer-sphere coordination. (A) Modified effective potential for Mg²⁺ with softened outer-sphere coordination interaction, while keeping the inner-sphere coordination interaction intact. (B and C) Comparison of (B) preferential interaction coefficient ${\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g}}$ for BWYV PK at 54 mM KCl and (C) ${\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g}}$ and $R_{\mathrm{\text{g}}}$ for 58-nt rRNA at 150 mM KCl using the modified potential. Despite a tiny difference in the outer-sphere coordination ($\sim$0.2 kcal/mol), the values of ${\mathrm{\Gamma }}_{\mathrm{M}\mathrm{g}}$ decrease significantly from those of the original model. In rRNA, the modified potential even cannot fold the rRNA at relevant Mg²⁺ concentrations, as can be seen in the high values of $R_{\mathrm{\text{g}}}$.