Electrostatic entropy term

Table 4-3 also shows that the electrostatic entropy term (expressed as —7 A.S c e) has a destabilizing contribution, thus reflecting the decrease in the configurational flexibility of the water molecules arising from the electrostatic forces exerted by the polar solute. This term, however, is notably smaller than the non-electrostatic... 

The absence of any difference between the plasma and an ideal gas as regards the enthalpy means that the electrostatic part of the free energy is entirely an entropy term the electrostatic contribution to the entropy is therefore simply... 

The entropic effects of the creation of a cavity in the water to accommodate the ion and of the compression from the gas to the solution were taken care of by a neutral term A5 t =-3-600(r,/nm)J-K -mor [51]. The electrostatic entropy effects were calculated separately for the electrostricted hydration shell of width Arj and for the water near the ion but outside this shell. The permittivity and its temperature derivative in the former were assumed to have the infinitely large field values e = n =1.776 and (de /ffr)p=2idn /ffnp=-lxlO- K- (at 298.15K),... 

At the most complex level we use equation (24) to present the results in the form of Table 5. Table 5 shows the inhibitor-enzyme (column 2) and enzyme components (column 3) of the relative binding free energy. These are then divided into sub components. The sub components are the molecular mechanics terms (E, row 1 beneath inhibitor), solute entropy terms (-TS, ,, row 2 beneath inhibitor), electrostatic solvation free energy terms (AG, row 3 beneath inhibitor), nonpolar solvation terms (AG , row 4 beneath inhibitor), and the total solution phase free energy (G, row 5 beneath inhibitor). Column 4 of the table gives the total values for all sub components, these are obtained by summing columns 2 and 3 for a particular subcomponent. 

If we average the absolute values of the various sub components in column 4 across inhibitors we obtain, = 1.92 kcal/mol, < A[A(AG, )]> = 2.11 kcal/mol, < -TA(AS , J> = 0.27 kcal/mol, and = 0.13 kcal/mol. This analysis reveals that the molecular mechanics energy terms and the electrostatic solvation terms are by far the most important contributors to the relative binding free energy. Conformational entropy and solute-solvent van der Waals interaction play little role in the relative binding thermodynamics for these systems. 

Since n = R/ph, the second, electrostatic, term is negligible in comparison to the third, mixing entropy, term. As a result the micellar radius exhibits two interesting features (i) R is independent of p and (ii) it reflects exttemely strong stretching of the coronal chains... 

The concepts of destabilization of reactants and stabilization of products described for pyrophosphate also apply for ATP and other phosphoric anhydrides (Figure 3.11). ATP and ADP are destabilized relative to the hydrolysis products by electrostatic repulsion, competing resonance, and entropy. AMP, on the other hand, is a phosphate ester (not an anhydride) possessing only a single phosphoryl group and is not markedly different from the product inorganic phosphate in terms of electrostatic repulsion and resonance stabilization. Thus, the AG° for hydrolysis of AMP is much smaller than the corresponding values for ATP and ADP. 

T AS are obtained from (AF — AH). In the right-hand half of the diagram the magnitude of — T AS is greater than that of AF, in accordance with electrostatic theory compare (24). As pointed out in Sec. 98, although the molecular dipoles in methanol are less numerous than in water, they lose more entropy in fact they lose so much more entropy that the term —T AS is predominant in producing the e.m.f. of the cells (198) placed back to back. 

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