- . This is the same database used for the OpenEye application pK a Prospector. To begin, we filtered database entries for experimental measurements which were aqueous (including removing measurements in D 2 O), taken between 20 and 25 degrees Celsius, and not tagged as very uncertain. This resulted in a set of 9,890 molecules with 26,519 experimental measurements. The large number of experimental results compared to number of molecules is not solely due to molecules with multiple pK a s; rather, it is primarily due to replicate measurements for certain molecules. In such cases we performed a weighted average, propagating the estimated uncertainties. As we are most interested in biologically relevant ionization, we also removed molecules where the experimental pK a s were outside a range of 0 to 14.