Thermal stability of lysozyme as a function of ion concentration: A reappraisal of the relationship between the Hofmeister series and protein stability

Anion and cation effects on the structural stability of lysozyme were investigated using differential scanning calorimetry. At low concentrations (<5 mM) anions and cations alter the stability of lysozyme but they do not follow the Hofmeister (or inverse Hofmeister) series. At higher concentrations protein stabilization follows the well-established Hofmeister series. Our hypothesis is that there are three mechanisms at work. At low concentrations the anions interact with charged side chains where the presence of the ion can alter the structural stability of the protein. At higher concentrations the low charge density anions perchlorate and iodide interact weakly with the protein. Their presence however reduces the Gibbs free energy required to hydrate the core of the protein that is exposed during unfolding therefore destabilizing the structure. At higher concentrations the high charge density anions phosphate and sulfate compete for water with the protein as it unfolds increasing the Gibbs free energy required to hydrate the newly exposed core of the protein therefore stabilizing the structure.