Hydrophobic entropic contributions

The large difference in the hydrophobic binding of the cationic tetraphenyl-phosphonium and anionic tetraphenylborate is caused by the large changes in the enthalpy of the phase transfer. While the entropic contribution is similar for both ions, the AH is slightly positive (+ 14.6 kJmol-1) for tetraphenylpho-sphonium (TPP+), whereas it is negative (up to -15.1 kJmol-1) for tetraphenylborate (TPB ) [6]. 

The estimates for AGp, derived after accounting for hydrophobic and entropic contributions, depend critically on how these latter terms are estimated in particular, on whether entropic advantages of new vibrational modes in the complex are credited to AGp, or are accounted for by a reduction in the AGt+r term (Searle et al., 1992). 

These observations are difficult to explain if there were, as postulated by Hattori et al. (8), a smaller hydrophobic force contribution expected from the deuterated protein. Consequently, we suggest that the current results are consistent with our previous postulate that the entropic effect known as hydrophobic forces is sufficient to push the subunits together, and these aggregates are then stabilized by virtue of dispersion forces (16). Certainly, this postulate needs further experimental support however, it is entirely consistent with currently accepted ideas concerning hydrophobic forces. 

The stronger excitonic interaction in EB assemblies than that of ACR or AMAC is apparently due to a greater hydrophobic surface area of the former, as estimated from computer modeling studies (MacSpartan). Such increased hydrophobic surface is not expected from their structures (three six-membered ring systems) it also results in an enhanced entropic contribution to the binding energy when the probe is transferred from the aqueous phase to the interior of BAZrP, where there is little or no water. Therefore, the formation of these supramolecular assemblies may indeed involve a large entropic component, but this needs to be demonstrated experimentally. 

In the case of contact forces, the magnitude of the enthalpy term is relatively small. The entropic contribution, however, can be significant. When a nonpolar compound is in an aqueous solution, the water molecules form a highly ordered solvent shell around the nonpolar portions of the compound (Scheme 9.3). This phenomenon is called the hydrophobic effect. Once the compound buries itself into a binding site on a target, some solvating water molecules will... 

In both cases the reference state ( ) is pure unmixed components S = S + S and U =U +U, where S and U are the entropy and energy of n moles of pure solvent, and Sj and U refer to moles of polymer in pure amorphous polymer. The energy Uln (5.2.12) becomes a Helmholtz energy if x contains an entropic contribution. However, this Helmholtz energy only includes those entroplc contributions which are proportional to (for example, those originating from orientations of the solvent molecules, as in hydrophobic bonding), and not the configurational entropy of the chains as embodied in (5.2.111. 

Outliers in QSAR 1.92 included the 4-t-butyl and 4-OH analogs, whereas the 4-CONH, analog was an outlier in QSAR 1.93. These results were recently reanalyzed by Kim (217,218) with respect to the role of enthal-pic and entropic contributions to ligand binding with a-chymotrypsin. Use of the Fu-jiwara hydrophobic enthalpy parameter n and the hydrophobic entropy parameter irg led to the development of QSAR 1.94 and 1.95(219). 

Local hydrophobicityplays an important role in molecular recognition processes. It is generally accepted that the hydrophobic interaction between two molecules is related to both energetic and entropic contributions, but there is still no simple physical model available for hydrophobicity and hydrophobic interactions. However, there have been several attempts to define relative hydrophobicity values on the basis of empirical findings. 

Inferences can be drawn regarding the entropic contribution to the driving force ). The present author anticipated this effect to be the other way around.) In the same vein, the influence of urea as a co-adsorptive deserves attention this substance is a hydrophobic bond-breaker and its influence on c.m.c. s is well documented -S- ). 

Surface waters are displaced on formation of the protein-ligand complex and thus provide a favorable entropic contribution to the free energy of complex formation. In particular, displacement of the water found interacting on apolar surfaces makes a large contribution to the AG and provides the driving force for many interactions (the hydrophobic effect). 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

The X-ray crystal structure of plastocyanin has recently been established (10), which indicated that the core of the molecule is hydrophobic and notably aromatic because six of the seven phenylalanine residues are clustered there. Polar side chains are distributed on the exterior of plastocyanin molecule. Many hypotheses have been proposed to explain the electron-transfer pathway to and from the metal center of plastocyanin, such as a tunnelling mechanism along hydrophobic channels (11). High reactivity and entropic favorability have been reported for the electron-transfer reaction of plastocyanin with Fe(II) complex (12). The Cu complex bound to the amphiphilic block copolymer is interesting as a metal compound of plastocyanin, because both polymer and apoprotein environments are considered to produce a hydrophobic environment and a large effect on the electron-transfer reaction through its entropic contribution. 

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