Hydrophobic effect entropy changes

Water-soluble globular proteins usually have an interior composed almost entirely of non polar, hydrophobic amino acids such as phenylalanine, tryptophan, valine and leucine witl polar and charged amino acids such as lysine and arginine located on the surface of thi molecule. This packing of hydrophobic residues is a consequence of the hydrophobic effeci which is the most important factor that contributes to protein stability. The molecula basis for the hydrophobic effect continues to be the subject of some debate but is general considered to be entropic in origin. Moreover, it is the entropy change of the solvent that i... 

In cases where the solvation energies are large, as for example when ionic compounds dissolve in water, these hydrophobic effects, based on adverse changes in entropy, are swamped. Dissolving such compounds can be readily accomplished due to the very large energies released when the ions become hydrated. 

Cantor and SchimmeP provide a lucid description of the thermodynamics of the hydrophobic effect, and they stress the importance of considering both the unitary and cratic contributions to the partial molal entropy of solute-solvent interactions. Briefly, the partial molal entropy (5a) is the sum of the unitary contribution (5a ) which takes into account the characteristics of solute A and its interactions with water) and the cratic term (-R In Ca, where R is the universal gas constant and ( a is the mole fraction of component A) which is a statistical term resulting from the mixing of component A with solvent molecules. The unitary change in entropy 5a ... 

Several evidences, reported in the literature and briefly reviewed in the present article, demonstrate that the carbohydrate recognition at the surface of organized systems is somewhat different from that observed in isotropic media. These differences lie in (1) the conformation of carbohydrate which is affected by hydrophobization and by the nature of the surrounding lipids, (2) cluster effects from which can result in high energies of binding and which are affected by the fluidity of the lipid system, (3) entropy changes at the surface of a supra-molecular structure. 

Following this, the thermodynamic arguments needed for determining CMC are discussed (Section 8.5). Here, we describe two approaches, namely, the mass action model (based on treating micellization as a chemical reaction ) and the phase equilibrium model (which treats micellization as a phase separation phenomenon). The entropy change due to micellization and the concept of hydrophobic effect are also described, along with the definition of thermodynamic standard states. 

Since AG° = AH0- TAS° (see Chapter 6), it follows that the negative value of AG° for hydrophobic interactions must result from a positive entropy change, which may arise from the restricted mobility of water molecules that surround dissolved hydrophobic groups. When two hydrophobic groups come together to form a "hydrophobic bond," water molecules are freed from the structured region around the hydrophobic surfaces and the entropy increases. The AS° for Eq. 2-9 is about 12 J deg 1 mol-1. Attempts have been made to relate this value directly to the increased number of orientations possible for a water molecule when it is freed from the structured region.64 However, interpretation of the hydrophobic effect is complex and controversial.65-713... 

Enfolding a substrate in this way can serve to maximize the favorable entropy change associated with removing a hydrophobic substrate molecule from water. It also allows the enzyme to control the electrostatic effects that promote formation of the transition state. The substrate is forced to respond to the directed electrostatic fields from the enzyme s functional groups, instead of the disordered fields from the solvent. 

One must have accurate values for the magnitude of the fundamental forces that characterize the interactions in (1). These forces include primarily the hydrophobic effect, hydrogen bonding and electrostatic effects, and configurational entropy changes. Additionally, interactions with ligands, such as metal ions, urea, and protons, must be characterized for those cases in which these additional interactions are present. 

As mentioned earlier, proteins are subject to cold denaturation because they exhibit maximal stability at temperatures greater than 0°C. The basis of this effect is the reduction in the stabilizing influence of hydrophobic interactions as temperature is reduced. Recall that the burial of hydrophobic side-chains in the folded protein is favored by entropy considerations (AS is positive), but that the enthalpy change associated with these burials is unfavorable (AH, too, is positive). Thus, as temperature decreases, there is less energy available to remove water from around hydrophobic groups in contact with the solvent. Furthermore, as temperature is reduced, the term [— TAS] takes on a smaller absolute value. For these reasons, the contribution of the hydrophobic effect to the net free energy of stabilization of a protein is reduced at low temperatures, and cold-induced unfolding of proteins (cold denaturation) may occur. 

The hydrophobic effect can be measured in terms of transfer of a molecule from gaseous phase or dissolved in nonpolar solvent to water. Since the change in Gibbs free energy AG is associated with changes in ethalpy JH, and entropy JS according to... 

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