The Temperature-Dependence of Hydrophobic Association in Water. Pair versus Bulk Hydrophobic Interactions

Abstract

The temperature dependence of hydrophobic interactions of methane-like particles in water is analyzed in terms of free energy, entropy, internal energy, and the second osmotic virial coefficient. A large computational effort (approximately 15 ns cumulative trajectory length at each temperature) has been undertaken in order to guarantee reliable free energy and entropy data. At 300 K association is controlled by entropy, but as the temperature rises the internal energy takes over and dominates at 500 K. Both internal energy and entropy change sign within this temperature range. Our results correspond qualitatively with the experimentally observed temperature effect for transfer of gaseous hydrophobic substances into water: ΔA shows a weak temperature dependence, while ΔE and ΔS vary strongly with temperature. The second osmotic virial coefficients were calculated at different temperatures. Agreement with osmotic virial coefficients measured by solubility experiments at 300 K was found. Our results indicate that pairwise hydrophobic association studied by molecular dynamics simulation shows the key effects reported for bulk hydrophobic interactions. At present, there is no evidence for a qualitative difference between pair and bulk hydrophobic interactions. It is demonstrated that the comparison of the second osmotic virial coefficient of the solute particles in water, B 2,aq, with that in the pure gas phase, B 2,g, is not apppropriate for an assessment of the influence of water on pairwise hydrophobic interactions.

Temperature dependence of the hydrophobic hydration and interaction of simple solutes: An examination of five popular water models

“…All models show an increase of the first peak with rising temperature. This observation is well in accordance with the interpretation that the association of two hydrophobic particles is stabilised by minimising the solvation entropy penalty and has already been reported by a large number of publications [8,9,14,16,20,21,23,24,25,27]. The idea is schematically depicted in Figure 7: The enhanced ordering of the solvent molecules in the hydration shell results in a negative solvation entropy.…”

“…As a consequence, with increasing temperature the association of hydrophobic particles is found to be enhanced in order to minimise the solvation entropy penalty [8,9]. This entropy-driven association process is usually referred to as hydrophobic interaction and has been subject of numerous simulation studies [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. * dietmar.paschek@udo.edu; http://ganter.chemie.uni-dortmund.de/~pas/ Hydrophobic effects are considered to play an important role concerning protein stability and other self assembly phenomena [4,28,29].…”

The application of the integral equation theory to study the hydrophobic interaction

“…It has been established long ago that PMF between two solute particles exhibits two distinct minima corresponding to contact (CC) and solvent-separated configuration (SSC). [2][3][4] The effect of temperature, 5,6 pressure, [7][8][9] solute concentration, 10 solvent polarization, 11 and solventsolvent interactions 12 on the structure of PMF has been reported. A lot can be learned from PMFs accompanying functions, namely, enthalpy, entropy, and heat capacity change for bringing two solutes from infinity to a certain distance.…”

Low-temperature and high-pressure induced swelling of a hydrophobic polymer-chain in aqueous solution

“…In particular, they are seen as an important driving force with regard to the folding of proteins [4,5,6]. Consequently, a wealth of studies of hydrophobic interactions using molecular simulation techniques have been undertaken over the past three decades [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Simulation studies have revealed that the contact state of a pair of hydrophobic particles in aqueous solution is entropically stabilized at ambient conditions [10,11].…”