Recognition by a solubilized receptor: Hydrogen bonding, solvophobic interactions, and solvent engineering inside micelles

Article abstract of DOI:10.1021/ja00155a004

This paper shows how a hydrophobic porphyrin-based receptor capable of multipoint recognition in organic solvents can be solubilized directly in water by incorporation inside micelles. Binding of ligands from the aqueous phase is analyzed in sodium dodecylsulphate (SDS) micelles using a simple model in which ligands first partition into the micelle and are then complexed by the receptor. Subtraction of ligand-micelle partitioning terms from observed binding energies gives energies for bimolecular association inside SDS micelles, which range from +1 to -22 kJ/mol (equilibrium constants from 0.7 to 7900 M^-1). Comparison of two receptors binding the same set of ligands in SDS and organic solvents then provides insight into the roles of ligand and receptor solvation in micellar recognition. Binding inside SDS micelles is found to be energetically similar to binding in methanol, in that hydrogen bonding in the SDS pseudophase is reduced relative to CH₂Cl₂, and association of ligands capable of nonpolar contacts with the receptor is enhanced. Micellar recognition is most effective when both hydrogen bonding and solvophobic forces act together, leading to increased chiral discrimination of hydrophobic amino acid derivatives. It is finally shown how receptor solvation inside micelles can be tuned by the addition of organic cosolvents, reducing solvophobic association and restoring some of the hydrogen bonding energy.