FIGURE 5.

(A) Nominal two-state picture for cooperative binding of metal ions (M) to an undocked state (U), enabling progression to a docked state (D(M)_n) with metal ion dissociation constant, K_D. (B) Mechanism adapted from Kim et al. (36) to describe docking of the GAAA tetraloop and receptor with and without Mg²⁺, where K_Mg and K′_Mg are Mg²⁺ -dissociation constants and the rate constants reflect docking and undocking resolved by FRET. (C) Simplified parallel model to describe [Na⁺]- and [Mg²⁺]-dependence for the observed fraction of docked molecules with Mg²⁺ and Na⁺ dissociation constants.