Volumetric properties of the molten globule state of cytochrome c in the thermal three-state transition evaluated by pressure perturbation calorimetry.

Thermodynamic properties, including volumetric properties, provide important information on protein stability and folding. Pressure perturbation calorimetry (PPC) is an effective technique for evaluating the temperature dependence of the thermal expansion coefficient, α(p), of biomaterials. Recently, the thermal N-to-D transition of cytochrome c at nearly pH 4 was found to be a three-state transition including a molten globulelike intermediate state. In this study, the thermal N-to-D transition of cytochrome c was examined by PPC measurements with a three-state model. As far as we know, this is the first report of a three-state analysis of PPC, since two-state analyses are traditionally applied. The V(p) of the MG1-like intermediate state in the N-to-D transition at pH 4 was found to be the value between the partial volumes of the N and D states, suggesting an increase of the hydrophobic hydration in this intermediate state.