Investigating the chiral selectivity of Leucine-Valine Surfactant in the presence of Ethylene and Propylene Diamine Counterions

In this study, the effects of alkyl diamine counterion selection and pH on the chiral separation of several binaphthyl derivatives with L,L- undecanoyl leucine-valinate (und-LV) was examined by use of Micellar Electrokinetic Chromatography (MEKC). The objective of this research seeks to expand an understanding of the parameters required for optimization of the chiral selectivity of the und-LV surfactant in order to advance its use in enantioseparation. The chiral separations of [1,1'-bi-2-naphthyl-2,2'-diyl hydrogen (BNP) 1,1′-bi-2-naphthyl-2,2′-diamine (BNA) and 1,1′-bi-2-naphthol (BOH)] by und-LV varied widely in the presence of three counterions, 1,2 diaminoethane, 1,2 diaminopropane, and 1,3 diaminopropane, at pH 8 and pH 11. The results of this study have shown that separation of BNP enantiomers at pH 8 produces similarly optimal resolutions with use of 1,2 diaminoethane (10.9) and 1,2 diaminopropane (10.12), while the remaining counterion confers poor chiral selectivity of und-LV, evidenced by a weak resolution of 1.6. Interestingly, use of 1,2 diaminopropane in the separation of BNP enantiomers resulted in the lowest (1.61) retention factor value of all separations conducted under the same conditions, while use of 1,3 diaminopropane resulted in a separation with the highest retention factor value (2.38). With all counterions, conducting separations of each binaphthyl derivative at pH 11 decreased the observed retention factor values. Resolution values of 1,2 dimaminoethane and 1,2 diaminopropane also decreased under this pH, though use of 1,3 diaminopropane instead demonstrated a slight increase in its resolution value. Similar trends are observed when separating enantiomers of BOH at pH 8, while separation of BNA enantiomers in an equitable pH instead demonstrated that use 1,3 diaminopropane provides a high resolution value (6.12) in comparison to its use with the other binaphthyl derivatives. Unique structural characteristics of each counterion, their varying protonation states under high and low pH conditions, and pH-dependent charges of each analyte likely contribute to these findings.