Chiral separation of amino acid esters by micellar electrokinetic chromatography

Abstract

Micellar electrokinetic chromatography (MEKC) was used for the chiral separation of uncharged analytes (C- and N-protected amino acids). Sodium dodecyl sulfate (SDS) was the micelle forming agent, and different cyclodextrin (CD) derivatives were added as chiral selectors. Suitable conditions for the enantioseparation were found by variation of the separation conditions. The influence of addition of organic solvents like acetonitrile or methanol, and other chiral additives (camphor-10-sulfonic acid, malic acid) was examined. The addition of an organic modifier resulted in different effects on micelle formation, and thereby on the separation. The used chiral additives did not improve the selectivity. Furthermore, dependence of the electroosmotic flow (EOF), and the capacity factors on the concentration of CDs was investigated. Increasing the CD concentration, both the EOF to a smaller extent as well as the capacity factors decrease. Nevertheless, the enantioseparation is improved with a CD-concentration up to 30 mM. Higher CD-concentrations reduce the separation of the analytes.

Advances in chiral separations of small peptides by capillary electrophoresis and chromatography

“…Salami et al. used (+)‐18‐crown‐6‐tetracarbonic acid for resolving dipeptides. The addition of acetonitrile and tetra‐ n ‐butylammonium bromide resulted in improved chiral separation.…”

Review of aqueous chiral electrokinetic chromatography (EKC) with an emphasis on chiral microemulsion EKC

“…The mechanism of recognition is hydrogen bonding between oxygens on the planar chiral agent and the hydrogens of the amine group of the analyte [148]. The planar geometry of the selectand allows the formation of two diastereomeric complexes based on interactions with the faces of the crown ether that have different binding constants [149]. The structure of the main chiral crown ether employed in chiral EKC analyses, (1)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C 6 H 4 ), is shown in Fig.…”

“…8. Amino acids and some amino acid derivatives were enantioseparated by Salami et al [149] using 18C 6 H 4 . They found that larger groups at the analyte chiral center allowed for chiral discrimination (i.e., benzyl), whereas smaller functionalities precluded resolution (i.e., methyl).…”

Recent progress in capillary electrophoretic analysis of amino acid enantiomers

“…In CD-MEKC, a neutral or basic buffer (pH 7-10) containing both sodium dodecyl sulfate (SDS) and ␤-CD is mainly used as the BGS [63][64][65][66][67][68][69][70][71][72][73][74][75][76]. Modified CDs such as HP-␤-CD [76][77][78], DM-␤-CD [78] and HP-␥-CD [79] have been also utilized in the CD-MEKC mode. Because the incorporation by the SDS micelle compete with the inclusion into the cavity of CDs, the CD-MEKC separation of racemic amino acids can be easily optimized by tuning the concentrations of SDS and CDs.…”