Entropic Contributions to AG

In the following section we will only consider the contribution to z0 from the configurations which are within the solvent cage region (the remaining contributions are evaluated in Exercise 5.1). Thus we will be focusing on entropic contributions to Ag at rather than AG. ... 

The determination of as function of temperature permits the separation of the enthalpic and entropic contribution to AG. Binding constants are considered high if they are in the range between 10 to corresponding to a AG of -25 to... 

Work from Sturtevant s laboratory detailed the kinetics and thermodynamics of zinc binding to apocarbonic anhydrase (carbonate dehydratase) selected data are recorded in Table II (Henkens and Sturtevant, 1968 Henkens etal., 1969). The thermodynamic entropy term A5 at pH 7.0 is 88 e.u. (1 e.u. = 1 cal/mol-K), and this is essentially matched by the binding of zinc to the hexadentate ligand cyclohexylenediamine tetraacetate where AS = 82 e.u. At pH 7.0 the enthalpy of zinc-protein association is 9.8 kcal/mol, but this unfavorable term is overwhelmed by the favorable entropic contribution to the free energy (AG = AH - T AS), where —TAS = -26.2 kcal/mol at 298 K (25°C). Hence, the kinetics and thermodynamics of protein-zinc interaction in this example are dominated by very favorable entropy effects. 

Because of the short timescale for the optical transition, solvent dipole orientations in the initially formed excited state are the same as in the ground state and there is no entropic change. For a self-exchange reaction, the contribution to AG is A0/4 as noted above. 

After learning to estimate AG and AS, we might ask how AAS is affected by the steric restriction of the protein environment. As is clear from eq. (9.7), we need the differences between the entropic contributions to A G rather than the individual AS. This requires the examination of the difference between the potential surfaces of the protein and solution reaction. Here we exploit the fact that the electrostatic potential changes rather slowly and use the approximation... 

The 5 1 rule may be justified on thermodynamic grounds. Thus, Doig and Williams [12] addressed the inconsistencies in I lory s treatment of the entropic contribution to protein denaturation, calculating AAG for denaturation for a cross-linked protein versus its non-cross-linked counterpart. At physiological temperature of 300 K, they estimated A AG 4.4 kcal/mol. This value is essentially independent of protein length and loop size and best represents the insensitivity of experimental values to loop size-dependent configurational entropies [13]. 

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). 

FIGURE 3.4 Assumed relation between the free energy of formation of hydrophobic bonds (—AG) and temperature. Also the entropic (T AS) and enthalpic (AH) contributions to AG are given. The relations greatly depend on the chemical constitution of the apolar groups involved. 

Based on the value of K in part (a), what would you conclude about this reaction What concept is demonstrated (b) Determine the magnitudes of the enthalpic (AH°) and the entropic (—TAS ) contributions to AG for the ligand exchange reaction. Explain the relative magnitudes, (c) Based on information in this exercise and in the A Closer Look box on the chelate effect, predict the sign of AH for the following hypothetical reaction ... 

Table 1 summarizes some of the most relevant results from thermodynamic studies on platinum single-crystal surfaces. In order to facilitate the comparison of thermodynamic data corresponding to different adsorption reactions, values of the thermodynamic properties at the standard state ( AG , A/T and AS ) are provided. Values of the lateral interaction parameter (a>), and its temperature dependence (dm/dT) are also given in Table 1. While the lateral interaction parameter measures the magnitude of the lateral interactions, its temperature dependence reflects the entropic contributions to the lateral interactions. Therefore, the enthalpic contribution to the lateral interaction energy, can be obtained from - T da/dT.

Since AGind = AHj d - TASjj,d, there are both enthalpic and entropic possible contributions to the stability of the host-guest complex, which is favored by a negative enthalpy of inclusion and by a positive entropy of inclusion [64]. The various contributions to AG are often referred to as the driving forces for the inclusion process. 

Miscible polymer blends are characterized by homogeneity up to the molecular level, with negative values for AG,n and positive values for T AS. At the same time, AGm always depends on the value of AH the pair of polymers tends to form a single phase when the entropic contribution to the free energy exceeds that of enthalpy and AH , < T AS. 

Another mode of chromatographic behavior was predicted and verified [3] when enthalpic and entropic contributions to the distribution coefficient balance out, that is, when the change in free energy disappears (AG = 0). This mode is called liquid chromatography at the critical adsorption point (LCCC). The polymeric nature of the sample (i.e., the repeating units) does not contribute to the retention of the species. Only structural defects like end-groups, comonomers, or branching points contribute to the separation. 

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