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Pressure Perturbation Calorimetic Studies of the Solvation Properties and the Thermal Unfolding of Proteins in Solution

Abstract In this study, pressure perturbation calorimetry (PPC), a relatively new and efficient technique, was used to study the solvation and volumetric properties of the enzyme ribonuclease A (RNase A) in its native and unfolded state. In PPC, the coefficient of thermal expansion of the partial volume of the protein is deduced from the heat consumed or produced after small isothermal pressure jumps (±5bar), which strongly depends on the interaction of the protein with the solvent or co-solvent at the protein-solvent interface. The effects of various chaotropic and kosmotropic co-solvents (glycerol, sorbitol, urea, guanidinium hydrochloride, K2SO4) on the solvation and unfolding behaviour of RNase A was investigated as well, and the observed volume and expansivity changes are correlated with further thermodynamic properties obtained from differential scanning calorimetry (DSC). Depending on the type of co-solvent and its concentration, specific differences are found for the solvation properties of the protein, and the volume change upon unfolding may even change sign. Taken together, the data obtained lead to a deeper understanding of the solvation process of proteins in different co-solvents in their native and unfolded states.