Properties of hydrophobic free energy found by gas–liquid transfer

Abstract

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.folding energetics | reference solvent | osmolytes | cavity work T he current method of measuring hydrophobic free energy is based on liquid-liquid transfer of alkane solutes and uses the solubility of liquid alkanes in water. The method was criticized by Ben-Naim and Marcus (1) because the solvation free energy of a nonpolar solute is found from gas-liquid transfer results. The original suggestion of Kauzmann (2) was to quantify hydrophobic free energy with data for the transfer of nonpolar solutes between water and a reference solvent. He drew an analogy between solvent transfer of a nonpolar solute and folding, which transfers nonpolar side chains out of water into the protein interior. It was pointed out recently (3) that, when the liquid-liquid transfer of an alkane solute is divided into two successive gas-liquid transfers, less than half of the overall transfer free energy occurs in the hydrophobic transfer from water into vapor, indicating that liquid-liquid transfer gives seriously incorrect values of hydrophobic free energy. Wolfenden and Lewis (4) showed earlier that the reason for a major free energy change when an alkane solute is transferred from vapor into liquid alkane is the presence of a strong interaction among alkane molecules in a liquid alkane.The conclusion drawn here is that hydrophobic free energy should be defined and measured by the hydrophobic transfer of alkane solutes from water into vapor. The consequences of making this change are examined. A pressing issue is whether hydrophobic free energy, whose apparent value is reduced more than twofold by changing from liquid-liquid to gas-liquid transfer,...

The new view of hydrophobic free energy

“…The gas-liquid transfer free energy, DG GL , which corresponds to the new DG h [7], is the sum of DG c (the cavity work) and E a (the solute-solvent interaction energy), according to independent derivations of Eq. (1) (below) by Lee [10] and Pollack [11].…”

“…and the second transfer, from vapor to liquid alkane, is not obviously related to the protein folding process. For this and the following reasons, I argued recently [7] that hydrophobic free energy should be defined and measured by the ''hydrophobic'' transfer of alkane solutes from water into vapor.…”

“…and the second transfer, from vapor to liquid alkane, is not obviously related to the protein folding process. For this and the following reasons, I argued recently [7] that hydrophobic free energy should be defined and measured by the ''hydrophobic'' transfer of alkane solutes from water into vapor.The new definition is consistent with the criticism of Ben-Naim and Marcus [4] that the solvation free energy of a non-polar solute must be measured by gas-liquid transfer. The new definition is also consistent with work by Makhatadze and Privalov [8] and Wu and Prausnitz [9], who used gas-liquid transfer data to solve folding problems involving the hydrophobic factor.…”

“…Kauzmann [1] and Tanford [2] expected that removing the hydrocarbon solute from water would account for most of the free energy change in liquid-liquid transfer but this expectation was not correct: data are available for the DG values of both gas-liquid transfers, which show that the ''hydrophobic'' transfer from water to vapor accounts for less than half of the total DG (5)(6)(7). This disconcerting fact was realized as early as 1976 when Wolfenden and Lewis [6] studied why water is a poor solvent for liquid hydrocarbons.…”

“…The second half of the analogy, which involves solute transfer from vapor into liquid alkane, is omitted because the transfer of a non-polar side chain into the protein interior has different properties [7] from those of the transfer of an alkane solute from vapor into liquid alkane. These other properties include the topological connection of the non-polar side chain to the polypeptide chain, the presence of semipolar, H-bonded-NHCO-groups in the protein interior, and close packing of the groups inside a folded protein.…”