The hydrophobic effect: is water afraid, or just not that interested?
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The hydrophobic effect: is water afraid, or just not that interested? Todd P. Silverstein1 Received: 16 July 2020 / Accepted: 10 September 2020 © Springer Nature Switzerland AG 2020
Abstract Our understanding of the hydrophobic effect has advanced greatly since 1990, with the help of experimental, theoretical, and computer simulation results. The key hydrophobic signature of positive ∆C°P and negative ∆S° at room temperature has been interpreted in light of the importance of solvent cavity creation, solvent-excluded volume, and solute–water intermolecular forces, along with some unusual thermodynamic properties of pure water. Application of the hydrophobic effect to the hydration of small nonpolar solutes, protein folding stability, and protein–ligand binding is discussed in detail in this review, with an emphasis on thermodynamic analyses and interpretations. Keywords Hydrophobic effect · Protein unfolding · Protein stability · Thermophilicproteins
Introduction The fact that oil and water do not mix is a commonly observed outcome of the hydrophobic effect. It is hard to overestimate the importance of this effect in real-world chemistry and biochemistry: The cleaning action of soaps and detergents, the influence of surfactants on surface tension, chromatographic separation (reverse-phase), the formation of lipid membranes and micelles, partitioning across membranes (e.g., the blood–brain barrier) and the bioavailability of drugs and toxins, the folding and stability of proteins and their ability to bind hydrophobic ligands are all based in large part on the tendency of nonpolar groups to aggregate in aqueous solution. It is thus not surprising that this has been an area of intense study for more than a century. In 1998 I published a paper in the Journal of Chemical Education entitled “The real reason why oil and water don’t mix” [1]. In this paper I pointed out that all undergraduate general and organic chemistry textbooks got it wrong when they attempted to explain why oil and water don’t mix. All of these texts claimed that the process of transferring a nonpolar solute into water is endothermic, due to the necessity of breaking water–water hydrogen bonds in order to * Todd P. Silverstein [email protected] 1
Department of Chemistry (Emeritus), Willamette University, Salem, OR 97301, USA
create a cavity for the solute. In fact, it has been known since at least the 1940s that this process is exothermic at room temperature, hence the reason why oil and water don’t mix is because of the negative entropy change upon hydration of the nonpolar solute. One simply cannot attempt to explain this phenomenon without addressing water’s loss of motional freedom. In preparing to write this updated review of the hydrophobic effect for ChemTexts, I consulted a large number of general and organic chemistry textbooks published since 2005; imagine my disappointment that with one notable exception [2], all of the textbooks still purveyed the same error. One book even attempted to somehow mitigate the erron
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