Quantitative effects of antihydrophobic agents on binding constants and solubilities in water

The effects of urea and of guanidinium chloride on binding constants in water for 6-(4-tert-butylanilino)-naphthalene-2-sulfonate and of bis(p-tert-butylphenyl) phosphate binding to beta-cyclodextrin and to N,N'-bis(6-beta-cyclo-dextrinyl)imidazolium ion have been determined. Their effects on the water solubility of p-tert-butylbenzyl alcohol and p-methylbenzyl alcohol have also been examined. Quantitative correlations show that the effects of these additives, which diminish hydrophobic effects, are similar for release of a tert-butylphenyl group from a cyclodextrin cavity into water or for solubilizing such a group from a second phase. The effects of these agents on the binding constants for double-ended substrates binding to the bis(cyclodextrin) host are much larger than for a simple substrate binding to monomeric cyclodextrin, consistent with additivity of free-energy perturbations. Ethanol also decreases binding in these systems, and increases solubilities, but the quantitative correlations are less straightforward.