Energy of desolvation

One of the most remarkable results from the molecular simulation studies of aqueous electrolyte solutions was that no additional molecular forces needed to be introduced to prevent the much smaller ions (Na has a molecular diameter of less than 0.2 nm) from permeating the membrane, while permitting the larger water molecules (about 0.3 nm in diameter) to permeate the membrane. This appeared to be due to the large ionic clusters formed. The ions were surrounded by water molecules, thus increasing their effective size quite considerably to almost 1 nm. A typical cluster formed due to the interaction between the ions and a polar solvent is shown in Fig. 7. These clusters were found to be quite stable, with a fairly high energy of desolvation. The inability of the ions to permeate the membrane is also shown... 

Pearlman, R. S. (1986) Molecular surface area and volume Their calculation and use in predicting solubilities and free energies of desolvation. In Partition coefficient, Determination and Estimation. Dunn, III, W. J., Block, J. H., Pearlman R. S., Eds., pp. 3-20, Pergamon Press, New York. 

This is because the energy of desolvation of a charged group is huge. It is so large that the structure will rearrange so that water can penetrate the protein to... 

An important application for the crown ethers in synthetic work is for solubilization of salts such as KCN in nonpolar solvents for use in SN2 displacements. If the solvent has a low anion-solvating capability, then the reactivity of the anion is enhanced greatly. Consequently many displacement reactions that proceed slowly at elevated temperatures will proceed at useful rates at room temperatures, because the energy of desolvating the anion before it undergoes SN2 displacement is low (Section 8-7F). For example, potassium fluoride becomes a potent nucleophilic reagent in nonpolar solvents when complexed with 18-crown-6 ... 

X/4 will be constant since Xj /4 will be small for an oxygen or nitrogen base and X2/4 will be constant for a single substrate HA. If the major contribution to WR (or WH) consists of the free energy of desolvation of B (or BH+) these solvation changes, and therefore WR and WP, may remain roughly constant for a series of similar bases. 

Figure 13.20 Energetic basis of ion selectivity. The energy cost of dehydrating a potassium ion is compensated by favorable interactions with the selectivity filter. Because a sodium ion is too small to interact favorably with the selectivity filter, the free energy of desolvation cannot be compensated and the sodium ion does not pass through the channel.

This theoretical examination of carbonyl addition reactions serves to emphasize the enormous role that solvation effects play. As indicated in Figure 3.22 and other studies, gas phase addition of hydroxide ion to esters is calculated to be exothermic and to encounter only a very small barrier at TSla and TS2a. The major contribution to the activation barrier (18.5kcal/mol) that is observed in solution is the energy of desolvation of the hydroxide ion. We return to a discussion of solvation effects on carbonyl additions is Section 3.8. 

R. S. Pearlman, in Partition Coefficient Determination and Estimation, W. J. Dunn, J. H. Block, and R. S. Pearlman, Eds., Pergamon Press, New York, 1986. Molecular Surface Area and Volume Their Calculation and Use in Predicting Solubilities and Free Energies of Desolvation. J. S. Murray, P. Lane, T. Brinck, K. Paulsen, M. E. Grice, and P. Politzer, J. Phys. Chem., 97, 9369 (1993). Relationships of Critical Constants and Boiling Points to Computed Molecular Surface Properties. 

Figure 2 Two thermodynamic models that link perturbations in enzyme-inhibitor binding equilibria to values for interaction energies. E, enzyme I, inhibitor E l, enzyme-inhibitor complex AAGinteract energetic contribution of the perturbed interaction to complex formation AGso1v,e energy of desolvation for the enzyme AGsoiv, energy of desolvation for the inhibitor AGresoiv, energy of resolvation of the E l complex.

The correlation was unexpectedly high as seen in Figure 13.4.3.2, implying that the change of substrate specificity of enzyme in organic solvent stems to a large extent from the energy of desolvation of the substrate. 

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