THERMODYNAMICS OF UNFOLDING OF RIBONUCLEASE-A UNDER HIGH-PRESSURE - A STUDY BY PROTON NMR

Thermodynamic stability of ribonuclease A (6.2 mM pH 1.0, 0.15 M KCl, in (H2O)-H-2) has been studied in the pressure range of 1 to 2000 atm and in the temperature range of 7.5 to 40 degrees C with a high pressure H-1 NMR technique at 400 MHz. His epsilon proton resonances were used as reporter groups to measure fractions of folded and unfolded species. Gibbs energy differences between folded and unfolded species were obtained as functions of pressure for different temperatures and as functions of temperature for different pressures. The volume increase upon unfolding, Delta V, was negative and temperature-dependent, decreasing from -10 ml/mol at 7.5 degrees C to -30 ml/mol at 37 degrees C. From the least squares-fitting of experimental Gibbs energy differences to a theoretical expression holding pressure and Delta C-p constant, we determined best-fit values of Delta G, Delta H, Delta S and Delta C-p for different values of pressure in the temperature range 7.5 to 40 degrees C. We found that Delta C-p is dependent on pressure, decreasing from 1.79 kcal/mol K at 1 arm to 1.08 kcal/mol;K at 2000 atm. These findings appear to be consistent with a notion that the state of hydration of non-polar side-chains upon unfolding of the protein is a major factor that determines the pressure dependence of the conformational stability of ribonuclease A under the chosen experimental condition. (C) 1995 Academic Press Limited