∆G protein (pH) = ∆∆G(pH) + ∆G(pH), (8) where ∆G(pH) corresponds to Equation 1 with a reference pK • a corresponding to residue A. Equation 8 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 ).