Monolayers of Chiral Calix[4]Resorcinarenes: Surface Pressure and Surface Potential Studies

Abstract

Chiral amphiphilic C-undecylcalix[4]resorcinarenes substituted with phenylethyl group or L(-)nore-phedrine were found to form well-organized mono-layers at the aqueous solution-air interface. The substituents, L(-)norephedrine and phenylethyl group, determined the area occupied by the molecule on the water subphase. Introduction of these substituents lead also to perpendicular dipole moments of the molecules in the monolayers ca. 6 times larger than those of the parent amphiphilic calixresorcinarene, CAL11. Interactions of the compounds with K⁺ were detected by the increase of the surface potential values measured at maximum packing of the monolayer. Addition of amino acids to the subphase lead to conformational changes in the monolayers evidenced by increased surface mean molecular area of the unmodified C-undecyl-calix[4]resorcinarene. These changes were explained by the formation of hydrogen bonds with the amino acids at the expense of hydrogen bonding between the calixarene molecules in the monolayer. In contrast to unsubstituted calixresorcinarenes, interactions of the L(-)norephedrine-and phenylethyl-substituted molecules with amino acids could be easily recognized by the decrease of surface potential and dipole moment in monolayers formed by these calixarenes on subphases containing amino acids. A significant drop in the surface potential and an increased area per molecule demonstrated more specific interactions with selected amino acids: L(-)norephedrine-substituted calixarene interacted with D-valine and the phenylethyl-substituted, with D-tryptophan.