The standard enthalpy or entropy change upon transfer of a small nonpolar molecule from a nonaqueous phase into water at a given temperature is generally different for different solute species. However, if the heat capacity change is independent of temperature, there exists a temperature at which the enthalpy or the entropy change becomes the same for all solute species within a given class. Similarly, the enthalpy or the entropy change of protein denaturation, when extrapolated to high temperature assuming a temperatureindependent heat capacity change, shows a temperature at which its value becomes the same for many different globular proteins on a per weight basis. It is shown that the existence of these temperatures can be explained from a common formalism based on a linear relationship between the thermodynamic quantity and a temperature-independent molecular property that characterizes the solute or the protein. For the small nonpolar molecule transfer processes, this property is the surface area or the number of groups that are brought in contact with water. For protein denaturation, it is suggested that this property measures the polar/nonpolar mix of the internal interaction within the protein interior. Under a certain set of assumptions, this model leads to the conclusion that the nonpolar and the polar groups of the protein contribute roughly equally to the stability of the folded state of the molecule and that the solvent-accessible surface area of the denatured form of a protein is no more than about two-thirds that of the fully extended form.

The extent ofvolume fluctuation is calculated for two simple geometrical models of globular protein molecules subjected to a potential that is proportional to the surface area freshly generated by the thermal breathing motion. The proportionality constant, y, has the unit of surface tension. The calculated values are compared with estimates made from the compressibility measurements. After an approximate correction for the hydration effect, the experimental values are found to be between those calculated by using y values of 25 and 46 cal/mol/A2 (1 cal = 4.184 J). These values bracket previously reported independent estimates of interfacial tension that presumably operates at the interface between a nonpolar molecule and water. This result appears to indicate that the solvent water plays a significant role in determining the extent of volume fluctuation of globular proteins and that the concept, and the actual value of the estimate, of the interfacial tension around a nonpolar molecule in water may, in fact, be useful in some applications.