Nonpolar hydration

Gallicchio E, Linda Yu Zhang, Levy RM (2002) The SGB/NP Hydration Free Energy Model Based on the Surface Generalized Bom Solvent Reaction Field and Novel Nonpolar Hydration Free Energy Estimators. J. Comput. Chem. 23 517-529... 

Despite the fact that nonpolar hydration forces dominate whenever hydrophobic interactions [46] are important, the general availability of accurate models for the nonpolar component of the hydration-free energy is lacking. The structure and properties of proteins in water is highly influenced by hydrophobic interactions [47-50]. Hydrophobic interactions also play a key role in the mechanism of ligand binding to proteins [30,51-53], Empirical surface area models [54] for the nonpolar component of the solvation free energy are widely used [28,37,55-62]. Surface area models are useful as a first... 

We developed the Analytical Generalized Born plus Non-Polar (AGBNP) model, an implicit solvent model based on the Generalized Born model [37-40,44, 66] for the electrostatic component and on the decomposition of the nonpolar hydration-free energy into a cavity component based on the solute surface area and a solute-solvent van der Waals interaction free energy component modeled using an estimator based on the Born radius of each atom. 

E. Gallicchio, L. Y. Zhang, and R. M. Levy. The SGB/NP hydration free energy model based on the surface generalized Born solvent reaction field and novel nonpolar hydration free energy estimators. / Comput. Chem., 23(5] 517-529, 2002. 

Hydrophobic effects include two distinct processes hydrophobic hydration and hydrophobic interaction. Hydrophobic hydration denotes the way in which nonpolar solutes affect the organisation of the water molecules in their immediate vicinity. The hydrophobic interaction describes the tendency of nonpolar molecules or parts thereof to stick together in aqueous media " . A related frequently encountered term is hydrophobicity . This term is essentially not correct since overall attractive interactions exist between water and compounds commonly referred to as... 

As is suggested frequently , this term might well result from the restriction of the hydrogen bonding possibilities experienced by the water molecules in the first hydration shell. For each individual water molecule this is probably a relatively small effect, but due to the small size of the water molecules, a large number of them are entangled in the first hydration shell, so that the overall effect is appreciable. This theory is in perfect agreement with the observation that the entropy of hydration of a nonpolar molecule depends linearly on the number of water molecules in the first hydration shell ". ... 

In the traditional view hydrophobic interactions are assumed to be driven by the release of water molecules from the hydrophobic hydration shells upon the approach of one nonpolar solute to another. Although the ideas about the structure of the hydrophobic hydration shell have changed, this view is essentially unaltered... 

The distinction between pairwise and bulk hydrophobic interactions is often made, although some authors doubt the existence of an intrinsic difference between the two ". Pairwise hydrophobic interactions denote the interactions behveen two isolated nonpolar solutes in aqueous solution. They occur in the regime where no aggregation takes place, hence below the critical aggregation concentration or solubility limit of the particular solute. If any breakdown of the hydrophobic hydration shell occurs, it will be only transient. 

If one would ask a chemist not burdened with any knowledge about the peculiar thermodynamics that characterise hydrophobic hydration, what would happen upon transfer of a nonpolar molecule from the gas phase to water, he or she would probably predict that this process is entropy driven and enthalpically highly unfavourable. This opinion, he or she wo ild support with the suggestion that in order to create room for the nonpolar solute in the aqueous solution, hydrogen bonds between water molecules would have to be sacrificed. 

Finally, also size and shape of the nonpolar solute seem to influence the formation of hydrophobic hydration shells. Particularly the curvature of the nonpolar surface has been suggested to be... 

The observation that in the activated complex the reaction centre has lost its hydrophobic character, can have important consequences. The retro Diels-Alder reaction, for instance, will also benefit from the breakdown of the hydrophobic hydration shell during the activation process. The initial state of this reaction has a nonpolar character. Due to the principle of microscopic reversibility, the activated complex of the retro Diels-Alder reaction is identical to that of the bimoleciilar Diels-Alder reaction which means this complex has a negligible nonpolar character near the reaction centre. O nsequently, also in the activation process of the retro Diels-Alder reaction a significant breakdown of hydrophobic hydration takes placed Note that for this process the volume of activation is small, which implies that the number of water molecules involved in hydration of the reacting system does not change significantly in the activation process. 

Chemists and biochemists And it convenient to divide the principal organic substances present m cells into four mam groups carbohydrates proteins nucleic acids and lipids Structural differences separate carbo hydrates from proteins and both of these are structurally distinct from nucleic acids Lipids on the other hand are characterized by a physical property their solubility m nonpolar solvents rather than by their structure In this chapter we have examined lipid molecules that share a common biosynthetic origin m that all their carbons are derived from acetic acid (acetate) The form m which acetate occurs m many of these processes is a thioester called acetyl coenzyme A... 

Fig. 10 shows the radial particle densities, electrolyte solutions in nonpolar pores. Fig. 11 the corresponding data for electrolyte solutions in functionalized pores with immobile point charges on the cylinder surface. All ion density profiles in the nonpolar pores show a clear preference for the interior of the pore. The ions avoid the pore surface, a consequence of the tendency to form complete hydration shells. The ionic distribution is analogous to the one of electrolytes near planar nonpolar surfaces or near the liquid/gas interface (vide supra). 

We are applying the principles of enzyme mechanism to organometallic catalysis of the reactions of nonpolar and polar molecules for our early work using heterocyclic phosphines, please see ref. 1.(1) Here we report that whereas uncatalyzed alkyne hydration by water has a half-life measured in thousands of years, we have created improved catalysts which reduce the half-life to minutes, even at neutral pH. These data correspond to enzyme-like rate accelerations of >3.4 x 109, which is 12.8 times faster than our previously reported catalyst and 1170 times faster than the best catalyst known in the literature without a heterocyclic phosphine. In some cases, practical hydration can now be conducted at room temperature. Moreover, our improved catalysts favor anti-Markovnikov hydration over traditional Markovnikov hydration in ratios of over 1000 to 1, with aldehyde yields above 99% in many cases. In addition, we find that very active hydration catalysts can be created in situ by adding heterocyclic phosphines to otherwise inactive catalysts. The scope, limitations, and development of these reactions will be described in detail. 

Okazaki, S. Nakanishi, K. Touhara, H., Monte Carlo studies on the hydrophobic hydration in dilute aqueous solutions on nonpolar molecules, J. Theor. Biol. 1979, 71, 2421-2429... 

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