Electrostatic interaction relative strengths

General details about denatured states were discussed in Chapter 17, section B. Specific details are obtained from advanced NMR methods. But any residual native structure in CI2 under denaturing conditions is below the limits of direct structural detection by NMR. The pKa values of the 10 acidic residues in the native state of CI2 have been measured by NMR to give indirect information.28 Most of them are 2-3 units lower than they are in model compounds. The pKa values in the acid-denatured state are, on average, 0.3 unit lower than the model compound values in pure water, but the difference disappears as ionic strength increases. This indicates some compactness in the denatured state, but it could be induced by electrostatic interactions. By all criteria, the denatured state of CI2 is a relatively expanded state. 

The solubilisation of proteins and amino acids in organic solvents by reversed micelles provides a new method for the selective recovery, separation and concentration of bioproducts using liquid->liquid extraction techniques. Selectivity is affected by electrostatic interactions between the charged residues or moieties of the solute and the surfactant headgroups. These interactions are mediated by electrostatic screening as affected by solution ionic strength. The more hydrophobic the amino acid residue, the more favourable is the solubilisation of this residue in the partially structured water pool of the reversed micelle relative to the bulk, unstructured water phase. 

Eq. (4-10) can be used only for solvents of equal acid and base strength, because only the effect of the solvent relative permittivity on the degree of ionization is considered. Under these conditions, Eq. (4-10) predicts that the logarithm of the ionization constant of HA should be inversely proportional to the relative permittivity of the solvent in which HA is dissolved. However, one has to take into account the fact that the relative permittivities near solute ions can differ considerably because of the effect of dielectric saturation, which hinders the precise calculation of electrostatic interactions. Because of these restrictions, Eq. (4-10) can be expected to yield only semiquantitative results. Nevertheless, it allows us to predict qualitatively how the charge type of an acid affects the ionization constant in solvents of different relative permittivities. 

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