pH-Dependent Stability and Cooperativity of Protein Unfolding from Differential Scanning Calorimetry Using FitFoldData

Protein folding is highly sensitive to environmental conditions such as pH, which can influence internal hydrogen bonding and interactions with the solvent. In this study, we use our FitFoldData online tool to analyze published differential scanning calorimetry (DSC) data on the unfolding of four proteins measured across different pH values. For each dataset, we examine the fitted thermodynamic parameters, including the intrapeptide and peptide–solvent hydrogen-bonding energies (h and hps) and the Zimm–Bragg cooperativity parameter (σ). In the analyzed datasets, under both acidic and alkaline conditions, h and hps range from 2 to 8.5 kJ/mol, while σ is on the order of 10−3 to 10−2. In addition, the two-state cooperativity parameter k2 is estimated from the DSC curves as the enthalpy ratio. By considering the size-dependent relative fluctuation, 1/N, we introduce a “two-state range” around the value k2=1 for each protein with a given number N of peptide units. We find that pH can shift proteins into or out of the two-state range of k2 values.