Hydration and protein folding in water and in reverse micelles: Compressibility and volume changes

The partial specific volume and adiabatic compressibility of proteins refle ct the hydration properties of the solvent-exposed protein surface, as well as changes in conformational states. Reverse micelles, or water-in-oil mic roemulsions, are protein-sized, optically-clear microassemblies in which hy dration can be experimentally controlled. We explore, by densimetry and ult rasound velocimetry, three basic proteins: cytochrome c, lysozyme, and myel in basic protein in reverse micelles made of sodium bis (2-ethylhexyl) sulf osuccinate, water, and isooctane and in aqueous solvents. For comparison, w e use beta -lactoglobulin (pl = 5.1) as a reference protein. We examine the partial specific volume and adiabatic compressibility of the proteins at i ncreasing levels of micellar hydration. For the lowest water content compat ible with complete solubilization, all proteins display their highest compr essibility values, independent of their amino acid sequence and charge. The se Values lie within the range of empirical intrinsic protein compressibili ty estimates. In addition, we obtain volumetric data for the transition of myelin basic protein from its initially unfolded state in water free of den aturants, to a folded, compact conformation within the water-controlled mic roenvironment of reverse micelles. These results disclose yet another aspec t of the protein structural properties observed in membrane-mimetic molecul ar assemblies.