The ionic environment determines ribozyme cleavage rate by modulation of nucleobase pK_a

Determining the concentration of cation required to obtain maximal cleavage rate over the pH range examined. Cleavage rate constants were determined in 1× reaction solutions at the indicated pH containing the indicated concentrations of (A) MgCl₂, (B) LiCl, (C) NaCl, (D) NH₄Cl, (E) CaCl₂, and (F) MnCl₂. Data are plotted as a fraction of maximal k _obs at the indicated pH. Data in B were fit to the Hill equation (solid lines); other lines simply connect the data points. The dashed line in B illustrates inhibition, by an unknown mechanism, at LiCl concentrations above 2.5 M at pH 9.1. High concentrations of Ca²⁺ at high pH also inhibit ribozyme cleavage by an unknown mechanism.