Pressure effects on folded proteins in solution. Hydrogen exchange at elevated pressures.

The observed rate constants for base-catalyzed hydrogen exchange reactions between solvent water and peptide nitrogen in lysozyme, ribonculease A, oxidized ribonuclease A, and poly(DL-lysine) are all enhanced by an increase in pressure. Activation volumes have been calculated from the pressure effect on these rate constants. For the folded proteins lysozyme and ribonuclease A, deltaV for base-catalyzed exchange changes from about +9 ml/mol at atmospheric pressure -3 ml/mol at 2500 kg/cm2. The same quantity, determined for the random coil polypeptides oxidized ribonuclease A and poly(DL-lysine), does not show this dependence upon pressure. These effects can be understood either in terms of solvent penetration on the folded proteins or the onset of a small degree of pressure induced unfolding. Possible mechanisms by which such penetration could occur are discussed.