Of all the forces that operate to keep a grobular protein molecule in the globular shape, the hydrophobic force is considered to be the most important. Yet it is also the most poorly understood of all the forces. Last year, I obtained reliable values for the enthalpy of solvent reorganization by combining the experimental data and computer simulation data at room temperature. The surprising finding was that the enthalpy change upon reorganizing water molecules around an inert solute was (1) positive but (2) smaller than that for hydrocarbon liquids. These findings provided important clues to the origin of hydrophobicity. This year, the temperature dependence of the solvent reorganization process was studied using Baldwin's summary of experimental data and Gill's equation for the heat capacity change. Both the enthalpy and entropy changes of water reorganization is not at all small at higher temperatures but the.free energy change remains nearly constant with temperature. This observation completes the overall outline of the picture for the physical mechanism of hydrophobicity. It is now clear that the hydrophobicity arises from the small size of the water molecules. The """"""""iceberg"""""""" formation, which has long been considered to be the mechanism of hydrophobicity, turns out to be a neutral process, with a small hydrophilic contribution to the dissolution process.
