Chiral separations using dipeptide polymerized surfactants: effect of amino acid order

Chiral separations using various polymerized dipeptide surfactants in electrokinetic capillary chromatography (EKC) are investigated. The two main dipeptide surfactants used in this study were sodium N-undecylenyl-L-valine-L-leucine (L-SUVL), and sodium N-undecylenyl-L-leucine-L-valine (L-SULV). These studies were performed in order to determine if the order of amino acids in dipeptide surfactants is important in terms of chiral recognition and separations. Both the monomer and the polymer of these two surfactants were compared for the separation of two model atropisomers, (+/-)-1,1-bi-2-naphtol (BOH) and (+/-)-1,1'-bi-2-naphthyl-2,2'-diyl hydrogen phosphate (BNP). Some advantages and disadvantages of the polymer relative to the monomer are discussed. Four other surfactants, the polymers of sodium N-undecylenyl-L-leucine-L-leucine (L-SULL), sodium N-undecylenyl-L-valine-L-valine (L-SUVV), sodium N-undecylenyl-L-valine (L-SUV), and sodium N-undecylenyl-L-leucine (L-SUL), were also used in this study, and their performance was compared to that of poly(L-SULV). These data show conclusively that the order of amino acids in dipeptide surfactants has a dramatic effect on chiral recognition. Our investigations indicate that poly-(L-SULV) provides the best enantioselectivity among the four dipeptide and two single amino acid surfactants for the separation of BNP and BOH. The advantages of poly-(L-SULV) are demonstrated via the ultrafast separation of the enantiomers of BNP and BOH in less than 1 min.