Understanding Nonlinear Composition Dependency of Enantioselectivity in Chiral Separation Using Mixed Micelle

Mixtures of chiral and achiral building blocks of supramolecules exhibit interesting cooperative phenomena, indicated by the nonlinear relationship between chiral properties and the mole fraction of chiral building blocks. However, the nonlinear com-position dependence of the chiral recognition capability of spherical mixed micelles is not well understood. In this work, achiral (N-dodecanoyl-glycine or N-dodecanoyl-2-methylalanine) and chiral (N-dodecanoyl-L-alanine) surfactants were studied in detail. Micellar electrokinetic chromatography found nonlinear composition dependencies of enantioselectivity. Capillary electrophoresis, circular dichroism (CD) spectroscopy, surface tension measurement, and fluorescence spectroscopy were used to investigate the following four possible mechanisms: (i) synergistic and antagonistic interactions between two different kinds of surfactants; (ii) the chiral transfer from chiral to achiral surfactant; (iii) differences in the capacity factor of surfac-tants; and (iv) enhanced cooperative chiral recognition of the chiral and achiral surfactant boundaries in the mixed micelle. Based on derived equations, the effect of (i) and (ii) was negligibly small, while (iii) and (iv) were dominant for the observed nonlinear enantioselectivity.