Use of ribozyme cleavage kinetics to measure salt-induced changes in solution pH

Several ribozymes are active in molar concentrations of monovalent salts, and pH rate curves under such conditions are consistent with mechanisms involving general acid-base catalysis. Interpreting the apparent pK(a) values requires an accurate estimation of solution pH, which can be difficult to obtain using a typical glass pH electrode in the presence of high salt concentrations. In the current work we have used the VS ribozyme as a "biological pH meter" to evaluate the effects of molar concentrations of monovalent salts on changes in solution pH. We find that almost all of the measured change in pH observed in high concentrations of LiCl is due to a real change in solution pH. In contrast, high concentrations of NaCl cause errors in pH measurement in addition to changes in actual pH. Different buffer systems affect both the direction and the magnitude of pH changes: we observed changes in measured pH of up to 1 pH unit, which require proper interpretation to avoid adverse effects on the conclusions drawn from pH rate and other experiments that utilize very high salt concentrations.