Hydrophobic bonding concept

If apolar hydration is characterized by the conditions that AG° > 0, TAS < 0 and AH < 0, then a process which minimizes exposure of apolar groups to water should be a thermodynamically favoured process. Then if two apolar groups of either the same or different molecules come together in water, AS for this process will be positive because some of the structured water is released into the bulk solvent. Such association is called hydrophobic, hydrophobic bonding or hydrophobic interaction (Kauzmann, 1959). The term bond is probably inappropriate because the association is due to entropy rather than to enthalpy effects, a consequence of the disruption of the clathrate structure around the apolar solute (Jolicoeur and Friedman, 1974). Despite the general acceptance of the concept of hydrophobic association, there are different approaches to the problem of understanding this phenomenon. 

This paradox, of physics, is more apparent than real, and the chemists have persisted with the fiction that objects exist. The concept of a chemical bond, ionic, van der Waals, covalent, is taken for granted and is essential to chemistry. The first two make no sense except in the context of an infinite crystal. (An "ion pair" in solution, or a "hydrophobic bond" in water between two methane molecules is due to complex statistical mechanical solvent mediated association behaviom , to be discussed below.)... 

In eq 6, K is the Michaelis constant. Good correlation with a was also obtained with V. The best linear relations with a have been found using more or less pure enzjraies. The lack of success with a in biochemical systems has generally been attributed to steric interactions of substituents with the enzyme or lipoprotein membranes. Recent work would Indicate that while steric interactions are extremely important, the concept of lock-and-key fit of enzyme and substrate has been over-emphasized at the expense of hydrophobic bonding. The importance to the medicinal. chemist of the more flexible character of enzymes which is emerging from the work of Koshland and others has been analyzed by Belleau . ... 

That the concept of hydrophobic bonding does not square with the facts, is pointed out by Hildebrand (1979) who maintained that the very concept of a hvdrophobic effect is unreal. No hydrophobic substance has yet been discovered every substance is hydrophilic, although hydrocarbons are very little so. In support of the hydrophilicity of hydrocarbons, Hildebrand instanced that the energy required to evaporate a mole of butane from its aqueous solution (at 1 atm and 25°C) is 0.65 kcal greater than from its own pure liquid. He also pointed out that if you pour some octane on ice, you can see that the ice is instantly wet by it (Hildebrand, 1979). 

Finally, we note that there exists some controversy regarding the term HI, or hydrophobic bond (Hildebrand, 1968 Nemethy et aL, 1968 Ben-Naim, 1971a). In the next section, we shall define precisely the concept to which we assign the term HI. Although we will be mainly concerned with aqueous solutions, we will use the same notation when discussing non-aqueous solutions. 

The Pfeiffer effect is not adequately explained by the concept of an equilibrium shift or configurational activity. There is strong evidence for interaction between the resolvable complex and the optically active species by a bonding force that is proposed herein to be hydrophobic in character. Ion pair interaction (when oppositely charged species exist), equilibrium shift, differential association and the hydrophobic bonding are suggested as contributing to the Pfeiffer activity. 

The concept of hydrophobic bonding is a convenient myth (especially useful for physical chemists when talking to biologists). It pictures attractive forces between oily groups such as alkyl chains, which cause them to cluster together in the interiors of micelles and membranes. In reality, however, these are not the dominant interactions. The major interactions are those between water molecules. It is so thermodynamically unfavorable for a water molecule not to be in contact with other water molecules that hydrophobic species are excluded from the aqueous part of the phase. They are squeezed out and the process of micelle formation is said to be solvent-driven. Hydrophobicity is the absence of hydro-philicity [35]. 

Extensive molecular dynamic simulations of proline-catalysed asymmetric aldol condensation of propionaldehyde in water have revealed that the stereoselectivity can be attributed to differences in transition-state solvation pattems. " The hydrogen bond concept has been applied to design new proline-based organocatalysts. " 4-Hydroxyproline derivatives bearing hydrophobic groups in well-defined orientations have been explored as catalysts in water an advantage of aromatic substituents syn to the carboxylic acid moiety has been attributed to a stabilizing transition-state hydrophobic interaction and this is supported by quantum mechanics (QM) calculations. " Catalysts and solvents were screened for reaction between cyclohexanone andp-nitrobenzaldehyde. 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

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