Chiral Separations by Micellar Electrokinetic Chromatography - Chiral Separations by Capillary Electrophoresis - page 202

The enantiomers are separated when their binding constants with the surfactant and/
or the mobilities of their diastereomeric complexes are different. However, it should
be remembered that diastereomeric interactions are dependent upon the partition
coefi cient of the analyte between the micelles and the bulk aqueous phase. In chiral
MEKC, the chiral moiety of the surfactant is located close to the surfactant polar
headgroup, which constitutes the Stern layer of the micelle. Thus, chiral recognition
takes place near the surface of the micelle. In other words, the chiral interface of the
surfactant micelles promotes chiral resolution. It is evident by the fact that enantio-
meric separations are not obtained when the surfactant concentration is below the
CAC value. For further understanding of the mechanism of chiral recognition and
for improvement of MEKC separations of enantiomers, an analysis of the separation
data for each chiral selector-analyte combination is essential.

8.5 SURFACTANT-BASED CHIRAL SELECTORS
Soon after the introduction of MEKC, Zare's group in 1985 i rst reported enantio-
meric separations of dansyl-amino acids by CE using l-histidine/Cu(II) complex
[95]. Since then much effort has been made to develop and characterize chiral PSPs
for MEKC with enhanced performance and/or unique selectivity. A summary is
given in Table 8.1. The chemical structures of some representative chiral surfactants
are shown in Figure 8.4.

8.5.1 A NIONIC S URFACTANTS
8.5.1.1 Bile Salts
Potential applications of MEKC in chiral analysis using various chiral anionic sur-
factants, for example, bile salts and sodium N -dodecanoyl-l-valinate (SDV) were
i rst shown by Terabe's group [96]. Bile salts, such as sodium cholate (SC), were
among the i rst chiral surfactants used in MEKC [7]. Bile salts are natural chiral
amphiphiles with a helical structure and are able to interact with relatively planar
and rigid analytes through electrostatic, hydrogen bonding, and hydrophobic interac-
tions. SC micelles were also shown to effectively separate l avanone-7- O -glycoside
epimers of neohesperidin and naringin [97]. Sodium taurocholate (STC) micelles in
acetate buffer were used for the resolution of metyrosine enantiomers [98]. Tian et al.
[99] also used SC micelles for the analysis of palonosetron hydrochloride stereoiso-
mers. Commercially available anionic surfactants derived from bile acids, such as
sodium deoxycholate (SDC) and sodium taurodeoxycholate (STDC), are the most
popular micelle-forming chiral selectors [17,100]. They have been used to separate
enantiomers of dansyl (DNS) derivatives of amino acids [7], 1-naphthylethylamine [101],
various anionic and neutral compounds [102], as well as drugs, e.g., diltiazem and
trimetoquinol [101,103-105], mephenytoin [106], and 3-hydroxy-1,4-benzodiazepins
[107]. However, diltiazem and trimetoquinol could only be enantioseparated by STDC.
In fact, STDC is considered to be the most effective chiral selector among bile salts
[98,108,109]. Recently, Hebling et al. [110] have demonstrated that chiral resolution of
( R , S )-1,1

-dihydrogen phosphate (BNP) is very sensitive to the aggre-
gation state of the SC surfactant. It was shown that the NMR spectral resolution of the
enantiomers strongly correlates with the chiral separation in MEKC. The H5-H7 edge

′ -binaphthyl-2,2 ′