Enthalpy change hydrophobic interaction

The most fundamental thermodynamic approach of Rudakov (6) applies to all condensed systems. The actual linear relationship is argued to exist between enthalpy (AH) and entropy (AS) of intermolecular interaction, as reflected in an approximately linear relationship between the total enthalpy and entropy. Special attention has been given to hydrophobic interaction (89, 90) in water solutions, which makes the isokinetic behavior more pronounced and markedly changes its slope. 

HYDROPHOBIC INTERACTIONS. These bonding interactions arise from the tendency of nonpolar side chains of amino acids (or lipids) to reside in the interior, nonaqueous environment of a protein (or membrane/ micelle/vesicle). This process is accompanied by the release of tightly bound water molecules from these apolar side-chain moieties. The hydrophobic effect is thermodynamically driven by the increased disorder i.e., A5 > 0) of the system, thereby overcoming the unfavorable enthalpy change i.e., AH < 0) for water release from the apolar groups. 

Where this factor plays a role, the hydrophobic interaction between the hydrocarbon chains of the surfactant and the non-polar parts of protein functional groups are predominant. An example of this effect is the marked endothermic character of the interactions between the anionic CITREM and sodium caseinate at pH = 7.2 (Semenova et al., 2006), and also between sodium dodecyl sulfate (SDS) and soy protein at pH values of 7.0 and 8.2 (Nakai et al., 1980). It is important here to note that, when the character of the protein-surfactant interactions is endothermic (/.< ., involving a positive contribution from the enthalpy to the change in the overall free energy of the system), the main thermodynamic driving force is considered to be an increase in the entropy of the system due to release into bulk solution of a great number of water molecules. This entropy... 

The thermodynamic parameters were calculated and are summarized in Table III. Both enthalpy and entropy decrease considerably with complex formation. Such a large decrease in enthalpy and entropy has not been found in other complex formation systems. For example, the change in enthalpy with the complex formation because of hydrophobic interactions generally is not so large and change in entropy is positive (23,24) and change in enthalpy with the complex formation in enzymatic hydrolysis of cellulose is slightly positive (25). These unusual decreases in enthalpy and entropy are inferred to be characteristics of the present complex formation system. 

How then can we account for the high degree of internal order routinely found within globular proteins We believe that combinations of the wide variety of electrostatic interactions reviewed above determine the precise three-dimensional structure of the interior of a protein. We argue that the sum of these interactions produces, at least in part, the enthalpy change on protein folding that is independent of the hydrophobic effect. Crystal structures of small organic compounds provide a useful model of protein interiors, and we now discuss some recent theoretical studies of these systems. 

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. 

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. 

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