Figure 3.

Kinetic titration curve of k_obs for the cleavage of the HH16 ribozyme with increasing concentrations of LaCl₃ in a solution containing 8 mM MgCl₂ (•). At low concentrations of LaCl₃ both metal ion binding sites are occupied with Mg²⁺, as illustrated in structure a. At intermediate concentrations (1–8 μM LaCl₃), the two sites are heterogeneously complexed with one La³⁺ ion and one Mg²⁺ ion (structure b). At high concentrations of LaCl₃ both sites are occupied by La³⁺ (structure c). The combination of 3 μM LaCl₃ and 8 mM MgCl₂ in the reaction solution yields a higher value of k_obs than is observed in the presence of either 3 μM LaCl₃ alone (⋄) or 8 mM MgCl₂ alone (• at [LaCl₃] = 0 μM).