Hydrophobized interaction

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

Both the structural and kinetic aspects of the protein-folding problem are complicated by the fact that folding takes place within a bath of water molecules. In fact, hydrophobic interactions are almost certainly crucial for both the relation of the sequence and the native structure, and the process by which a good sequence folds to its native structure. 

Hummer G, Garde S, Garcia A E, Pohorille A and Pratt L R 1996 An information theory model of hydrophobic interactions Proc. Natl Acad. Sc/. 93 8951... 

As with SCRF-PCM only macroscopic electrostatic contribntions to the Gibbs free energy of solvation are taken into account, short-range effects which are limited predominantly to the first solvation shell have to be considered by adding additional tenns. These correct for the neglect of effects caused by solnte-solvent electron correlation inclnding dispersion forces, hydrophobic interactions, dielectric saturation in the case of... 

D. E. Smith and A. D. J. Haymet. Free energy, entropy and internal energy of hydrophobic interactions computer simulations. J. Chem. P/iys., 98 6445-6454,... 

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... 

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. 

What distinguishes water from ordinary organic solvents and justifies the term hydrophobic interaction is the molecular origin of the effect, being entropy driven in pure water at room temperature and resulting primarily from the strong water-water interactions. 

Breslow immediately grasped the significance of his observation. He interpreted this discovery in terms of a hydrophobic effect Since in the Diels-Alder reaction. .. the transition state. .. brings together two nonpolar groups, one might expect that in water this reaction could be accelerated by hydrophobic interactions ". ... 

Tire importance of hydrophobic interactions in the aqueous acceleration is further demonstrated by a qualitative study described by Jenner on the effect of pressure on Diels-Alder reactions in water and a number of organic solvents. Invariably, the reactions in water were less accelerated by pressure than those in organic solvents, which is in line with the notion that pressure diminishes hydrophobic interactions. 

In conclusion, the special influence of water on the endo-exo selectivity seems to be a result of the fact that this solvent combines in it three characteristics that all favour formation of the endo adduct (1) water is a strong hydrogen bond donor, (2) water is polar and (3) water induces hydrophobic interactions. 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

What is the effect of water on the rate and selectivity of the Lewis-acid catalysed Diels-Alder reaction, when compared to oiganic solvents Do hydrogen bonding and hydrophobic interactions also influence the Lewis-acid catalysed process Answers to these questions will be provided in Chapter 2. 

The relative extents to which enforced hydrophobic interactions and hydrogen bonding influence the rate of the Diels-Alder reaction depends on the particular reaction under study". 

Appreciating the beneficial influences of water and Lewis acids on the Diels-Alder reaction and understanding their origin, one may ask what would be the result of a combination of these two effects. If they would be additive, huge accelerations can be envisaged. But may one really expect this How does water influence the Lewis-acid catalysed reaction, and what is the influence of the Lewis acid on the enforced hydrophobic interaction and the hydrogen bonding effect These are the questions that are addressed in this chapter. 

In summary, there are indications that neither hydrophobic interactions, nor donor- acceptor interactions are predominantly driving the arene - arene interaction. Osnsequently, we contend that these interactions are mainly governed by London - dispersion and electrostatic forces. 

The beneficial effect of water in the arene - arene interaction can be explained by the fact that this solvent is characterised by a low polarisability so that interactions of the aromatic rings with water are less efficient than with most organic solvents. Also the high polarity of water might lead to a polarisation of the aromatic rings, thereby enhancing electrostatic interactions. Finally, hydrophobic interactions may be expected to play a modest role. 

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