FIGURE 7.

(A) Least-squares fits of fractional docked population (N_docked/(N_docked + N_undocked)) versus [Na⁺] to Eq. 7, resulting in f_max = 0.55 ± 0.05, a Hill coefficient 1.3 ± 0.3, and K_D = 180 ± 30 mM. The asymptotic value (f_max) is consistent with Mg²⁺ studies in Fig. 5, suggesting a ≈32–34% nondocking RNA subpopulation. (B) f_free as a function of [Mg²⁺] at low [Na⁺] (25 mM) with a fit to Eq. 10 that also allows for a [Na⁺] docking pathway (Fig. 5 C), yielding f_max = 0.55 ± 0.04, n = 8 ± 2, and and demonstrating high cooperativity with respect to Mg²⁺ observed in the presence of minimal Na⁺.