Recall that for an isolated protonatable group (e.g., capped peptide) with a known reference pK • a , the free energy of deprotonation is not fixed, but is linearly related to the pH via ∆G(pH) = RT log Equation 25 provides a family of solutions that depend on the pH value. To determine the pK a , we find the point where ∆G protein (pH) = 0. This pH corresponds to the pK a that would be observed in a titration experiment and follows from the Henderson-Hasselbalch equation (Equation , main text).