Polymeric alkenoxy amino acid surfactants: V. Comparison of carboxylate and sulfate head group polymeric surfactants for enantioseparation in MEKC

Abstract

In this work, six amino acid derived (L-leucinol, L-leucine, L-isoleucinol, L-isoleucine, L-valinol, and L-valine) polymeric chiral surfactants with carboxylate and sulfate head groups that were recently synthesized in our laboratory [30, 33, 35] are compared for the simultaneous enantioseparation of several groups of structurally similar analytes under neutral and basic pH conditions. The physicochemical properties of the monomers and polymers of both classes of sulfated and carboxylated surfactants are compared. In addition, cryogenic high-resolution electron microscopy showed tubular structures with distinct order of the tubes of 50-100 nm width. A Plackett-Burmann experimental design is used to study the factors that influence the chiral resolution and analysis time of ten structurally related phenylethylamines (PEAs). It is observed that increasing the number of hydroxy groups on the benzene ring of the PEAs resulted in deterioration of enantioseparation using any of the six polymeric surfactants. For all three classes of PEAs, polysodium N-undecenoxycarbonyl-L-amino acidate (poly-L-SUCAA)-type surfactants provided enhanced resolution compared to that of polysodium N-undecenoxycarbonyl-L-amino acid sulfates (poly-L-SUCAASS). Several classes of basic and neutral chiral compounds (e.g., beta-blockers benzoin derivatives, PTH-amino acids, and benzodiazepines) also provided improved chiral separations with poly-L-SUCAA. Among the poly-L-SUCAAs, polysodium N-undecenoxycarbonyl-L-isoleucine sulfate (poly--SUCL) exhibited overall the best enantioseparation capability for the investigated basic and neutral compounds, while among the poly-L-SUCAASs, polysodium N-undecenoxycarbonyl-L-isoleucine sulfate (poly-L-SUCILS), and polysodium N-undecenoxycarbonyl-L-valine sulfate (poly-L-SUCVS) proved to be equally effective for enantioseparation. This work clearly demonstrates that variation in the head group of polymeric alkenoxy amino acid surfactants has a significant effect on chiral separations.