Processes Involving Hydrophobic Effects

The processes named in the title of this section represent a rather narrow but extremely important group of processes taking place in condensed media. In the realm of chemistry it is frequently possible to deal with a process under study either in the gas phase or in a solution. Only rarely does the chosen solvent mimic the conditions experienced by the reactants in a more or less ideal gas phase. In the majority of instances, the influence of the solvent is significant and causes changes in the equilibria and rate constants and modifies or changes the reaction mechanism. In some cases, the solvent-induced rate constant change amounts to several orders of magnitude. The subject we are dealing with is of a different nature, being closely connected with heterogeneous equilibria: two-phase liquid systems - immiscible liquids and the behavior of low-solubility species in water. The former systems frequently include small molecules, while the latter are mostly represented by large molecules or by macromolecules playing a role in biological systems. All these phenomena belong to a group of hydrophobic effects; their basic features are mentioned in section 3.1. For convenience, references on selected recent theoretical and experimental works dealing with various features of hydrophobic interactions are mentioned: interpretation of hydrophobic effects and calculations of these interaction [27-30], relation to adsorption [31], low solubility of hydrocarbon [32, 33], methane solvation and association [34, 35], entropy role [36, 37], protein structure and stabilization [38-42], interactions between biomolecules [43-45], and to host-guest interactions [46-48].

We do not recommend using the term hydrophobic bond. We believe that a bond can be considered to be formed when the approach of the subsystems is connected with a significant energy decrease. This is not the case of the class of processes under discussion. They are connected with a positive, zero, or small negative enthalpy change. The entropy change for the overall interaction including the solvent molecules is large and positive, while the total Gibbs energy change is negative. Clearly, the isolated association of partners is connected with an entropy decrease. Obviously, the above-mentioned entropy increase is due to structural reorganization of the solvent, usually water. Very numerous processes playing a fundamental role in biological systems belong to this class: protein folding, formation of micelles, enzyme-substrate interaction, antigen-antibody interaction and a vast number of related processes.

Hydrophobic effects play an essential role in both physical (e.g., transport) and chemical processes associated with biotransformations. They are specially important for the initial step (approach of. partners and their partial desolvation) and the final step (separation of the transformed subsystems).

Discussion of the nature of the hydrophobic effect and its relevance to the above-cited processes is proceeding and probably an ultimate decision will not be reached rapidly. The essential difficulty is that the hydrophobic interaction (i.e., solvent-induced interaction between two or a set of nonpolar molecules, small or large) is not at present amenable to experiment because of the extremely low solubility of these nonpolar solutes in water. The situation with hydrophobic hydration seems to be more favorable but, also not clear enough at the present.

The results of the hydrophobic hydration of small molecules in water, as treated by the efficient integral equation theory of Pratt and Chanlder [49] are in fair agreement with experiments [50]. The predictive efficiency of the Pratt-Chandler theory for solvent-induced solute-solute interaction is good.

In the realm of hydrophobic interactions, MD computer experiments provide great assistance in two respects. First, they offer valuable information on specific, new systems, and, second, they provide useful data for testing theoretical models. The first computer simulations of the hydrophobic interaction were not reliable enough. An MC scheme introduced by Pangali et al. [51] led to the potential of mean force for two Lennard-Jones spheres in water. It evolved that they passed between two minima, one with closely associated spheres and one with solvent-separated spheres; the latter was more populated. MD experiments supported this finding.

The importance of solvent-separated minima was demonstrated by several authors with systems such as a pair of methane molecules, or a pair of rare-gas atoms in water.

Papers were published where solute-dimer dissociation is more pronounced that dimer formation. However, Wallquist [52] demonstrated that, in a system of 18 methane-like species plus 107 H20 molecules, hydrophobic association takes place. Faced with the contradictory finding on the association and dissociation of nonpolar solutes, Wallquist concluded that it is the many-body part of the potential of mean force which is responsible for the association tendency between nonpolar solutes. It was Smith et al. [53] and Skipper [54] who showed that there is an attractive hydrophobic interaction between two methane molecules in water, which is entropy-driven. From the point of view of this contradictory feature of all the mentioned investigations, a study on association and dissociation of nonpolar and polar van der Waals pair in water should be mentioned [55]. A series of MD simulation (298 K, 1 atm) was performed for pairs of van der Waals spheres with radii of 200, 250, and 300 pm dissolved in 214 water molecules. These spheres were nonpolar or polar. In the latter case, they bear partial charges of the same size and opposite sign (+0.1 and -0.1; -0.3 and -0.3). For molecules of various sizes a smooth shift from associative to dissociative interaction was found with charging of the van der Waals pair. "Snapshots" from the course of the MD run indicate the presence of both tight and solvent-separated nonpolar van der Waals pairs. Finally, evidence was obtained that the hydrophobic interaction is an entropy-driven process [56].

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