Properties of hydrophobic free energy found by gas–liquid transfer
The hydrophobic free energy in current use is based on transfer of alkane solutes from liquid alkanes to water, and it has been argued recently that these values are incorrect and should be based instead on gas–liquid transfer data. Hydrophobic free energy is measured here by gas–liquid transfer of hydrocarbon gases from vapor to water. The new definition reduces more than twofold the values of the apparent hydrophobic free energy. Nevertheless, the newly defined hydrophobic free energy is still the dominant factor that drives protein folding as judged by ΔCp, the change in heat capacity, found from the free energy change for heat-induced protein unfolding. The ΔCp for protein unfolding agrees with ΔCp values for solvating hydrocarbon gases and disagrees with ΔCp for breaking peptide hydrogen bonds, which has the opposite sign. The ΔCp values for the enthalpy of liquid–liquid and gas–liquid transfer are similar. The plot of free energy against the apparent solvent-exposed surface area is given for linear alkanes, but only for a single conformation, the extended conformation, of these flexible-chain molecules. The ability of the gas–liquid hydrophobic factor to predict protein stability is tested and reasonable agreement is found, using published data for the dependences on temperature of the unfolding enthalpy of ribonuclease T1 and the solvation enthalpies of the nonpolar and polar groups.