Apolar entropy change

Now, if we assume that the active sites of these enzymes have a hydrophobic pocket at Sj as well as discrete subsites for substrate amino acids, we can explain these results by assigning different levels of importance to these different modes of interaction for the two enzymes. To account for the Pi specificity of FKBP, we not only assume a more prominent role for Pi-Si interactions but also that these interactions are characterized by dehydration of the Michaelis complex, E S, as it proceeds to the transition state, [E S]t. What we are suggesting here is that in E S, the Pi residue is not yet buried in Si and that the active site and the substrate are still at least partially solvated. As E S proceeds to [E S], the Pi residue becomes buried in the Si pocket and the residual water of solvation is expelled from the active site. This scenario can reasonably account for the large values of A/ft and ASt that we observe for reactions of FKBP, since the formation of hydrophobic contacts between apolar groups in aqueous solution is known to be accompanied by positive enthalpy and entropy changes (Nemethy, 1967). Likewise, to account for the lack of Pi specificity for CyP, we assume that subsite interactions play a more prominent role than do Pi—Si interactions. Thus, the Pi-Si hydrophobic interactions that dominate the thermodynamic parameters for FKBP have a smaller role for this enzyme. 

The negative entropy changes observed in all solvents are a result of an ordering of solvent molecules in the environment of the zwitterionic form. Since polar solvents are per se more structured than apolar solvents, proportionally less negative entropy changes are obtained in more polar solvents such as ethanol. The rate of the spiropyran/ merocyanine interconversion is also solvent-dependent, as is the position of the visible absorption band of (27a), which, as is typical for a merocyanine, exhibits a pronounced negative solvatochromism (see Section 6.2) [99c, 99d]. 

The main driving force for micellization is dehydration of apolar parts of the surfactant molecules. At 20°C, hydrophobic (de)hydration is mainly determined by an entropy change. 

From a thermodynamic point of view, self-aggregation of amphiphilic molecules in apolar solvents involves a favorable enthalpic term due to intermolecular bonding counteracted by an unfavorable entropic term due to a partial loss of molecular translational and rotational degrees of freedom. Using vapor pressure osmometry, for example, it has been found that the enthalpies of formation of molecular aggregates of dodecylammonium propionate in benzene and cyclohexane are 83.4 kJ/mol and -57.4 kJ/mol, whereas the entropy changes for the same processes are -0.23 kJ/mol and -0.14 kJ/mol, respectively [2]. 

In order to illustrate the role played by entropy S in adsorption processes, the entropy change AaS accompanying the adsorption in a very simple case was computed, by using the standard statistical mechanics rigid rotor/harmonic oscillator formula[94]. In Fig. 1.18 the adsorption of an Ar atom at the surface of an apolar solid is schematically illustrated. The Ar atom approaches the solid surface from the gas phase the translation entropy of the solid, which is fixed in the space, is taken as zero, whereas the free Ar atoms, before interacting with the solid surface, possess a translational entropy St which amounts to 150 and 170 J moP at T = 100 and 298K, respectively, at pAr = 100 Torr. 

In general, the outcome of polymer melt intercalation is determined by the interplay of entropic and enthalpic factors." The confinement of the polymer chains inside the silicate galleries results in a decrease in the overall entropy of the polymer chains, and the increased conformational freedom of the tethered ammonium cations compensates the entropy loss as the silicate layers separate with each other. However, the small increase in the gallery spacing does not affect the total entropy change rather, the total enthalpy will drive the intercalation. The enthalpy of mixing has been classified into two components the interaction between polymer and ammonium cations and the interaction between the layered polar silicates and the polymer chains. In most conventional organo-modified silicates, the tethered ammonium cations are apolar. The apolar interactions between the polymer and ammonium cations are unfavorable to the... 

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