Protein hydration and volumetric properties

Pressure effects on proteins stem from volumetric differences between their conformational states. These differences implicate rigid structure-based solvent excluded void volumes, although hydration and thermal expansivity differences between states may also play a role. Defining quantitatively the contributions of hydration and solvent excluded voids to protein volumetric properties and thermal expansivities remains a major challenge. Experimental information concerning thermal expansivity can be gained from pressure perturbation calorimetric studies (PPC). We review here recent results from PPC that suggest that while hydration plays a significant role in the volumetric properties of unfolded states of proteins, the volumetric properties of folded states are defined by structural and energetic properties of the folded chain.