Molecular Theory and the Effects of Solute Attractive Forces on Hydrophobic Interactions

Abstract

The role of solute attractive forces on hydrophobic interactions is studied by coordinated development of theory and simulation results for Ar atoms in water. We present a concise derivation of the local molecular field (LMF) theory for the effects of solute attractive forces on hydrophobic interactions, a derivation that clarifies the close relation of LMF theory to the EXP approximation applied to this problem long ago. The simulation results show that change from purely repulsive atomic solute interactions to include realistic attractive interactions diminishes the strength of hydrophobic bonds. For the Ar-Ar rdfs considered pointwise, the numerical results for the effects of solute attractive forces on hydrophobic interactions are of opposite sign and larger in magnitude than predicted by LMF theory. That comparison is discussed from the point of view of quasi-chemical theory, and it is suggested that the first reason for this difference is the incomplete evaluation within LMF theory of the hydration energy of the Ar pair. With a recent suggestion for the system-size extrapolation of the required correlation function integrals, the Ar-Ar rdfs permit evaluation of osmotic second virial coefficients B2. Those B2 also show that incorporation of attractive interactions leads to more positive (repulsive) values. With attractive interactions in play, B2 can change from positive to negative values with increasing temperatures. This is consistent with the historical work of Watanabe, et al., that B2 ≈ 0 for intermediate cases. In all cases here, B2 becomes more attractive with increasing temperature.

Short solvent model for ion correlations and hydrophobic association

“…To provide a more stringent test of LMF theory for very strong hydrophobic interactions we also considered the association of repulsive-core Weeks-Chandler-Andersen (WCA) versions of both the Ar and fullerene solutes, where both solute-water and solute-solute VDW attractions are turned off. Understanding the changes in hydrophobic interactions resulting from VDW solute-solvent and solvent-solvent attractions has been the focus of much recent work (72)(73)(74)(75).…”

Intramolecular Interactions Overcome Hydration to Drive the Collapse Transition of Gly15

“…Further, an emerging body of work on prototypical hydrophobes such as methane, 20 argon, 48 and larger alkanes 44,49–51 shows that solute–solvent attractive interactions can temper hydrophobic association. This suggests that “hydrophilic hydration and the intramolecular interactions are as important as, if not more important than, the hydrophobic effects” 17 in determining the structure of peptides and proteins.…”

Electrostatic and induction effects in the solubility of water in alkanes

“…In the alkane phase, to lowest order in solute density the contribution of solutesolute pairing to the excess chemical potential is 2ρ (a) wB2 [43,44], whereB 2 is the osmotic second virial coefficient. Quantum chemical calculations using B3LYP/6-311+G(d,p) and the SMD continuum solvation model [45] shows that association is weaker in the solvent than in the gas-phase at 298 K. (The weakening of interaction in the alkane is analogous to the weakening of association between hydrophobes in water due to solute-water attractive interactions [32,46,47].) On this basis we can infer that |B 2 | < |B 2 | and thus dimerization ought to be inconsequential even in the alkane phase.…”