Volume and Compressibility of Proteins.

The partial specific (or molar) volume, expansibility, and compressibility of a protein are fundamental thermodynamic quantities for characterizing its structure in solution. We review the definitions, measurements, and implications of these volumetric quantities in relation to protein structural biology. The partial specific volumes under constant molality (isomolal) and chemical potential (isopotential) conditions of the cosolvent (multicomponent systems) are explained in terms of preferential solvent interactions relevant to the solubility and stability of proteins. The partial expansibility is briefly discussed in terms of the effects of temperature on protein-solvent interactions (hydration) and internal packing defects (cavities). We discuss the compressibility-structure-function relationships of proteins based on analyses of the correlations between the partial adiabatic compressibilities and the structures or functions of various globular proteins (including mutants), focusing on the roles of the internal cavities in structural fluctuations. The volume and compressibility changes associated with various conformational transitions are also discussed in terms of the changes in hydration and cavities in order to elucidate the nonnative structures and the transition mechanisms, especially those associated with pressure denaturation.