where ⟨x i ⟩ is the probability that residue i is protonated. However, such a curve, when computed from experiment, may be flatter or irregularly shaped, often necessitating the application of specialized fitting procedures. In such cases, not only does the curve suggest a non-linear dependence, an analysis of the complete pH-dependent behaviour is often more insightful than defining the residue by a single pK a value.