Page 101 - Chiral Separation Techniques

3.7 Combinatorial Libraries of Selectors for HPLC 77

analyte and the total concentration of enantiomers in the solution determined from
UV adsorption, the enantiomeric ratio of components in the supernatant liquid could
easily be calculated. The results are summarized in Fig. 3-10. Obviously, no selec-
tivity is expected for the glycine-based selectors. Thus the readings for these selec-
tors set the accuracy limits of the circular dichroism method. Comparison of ellip-
ticities measured with the same selector prepared on both different resins indicated
that the more polar and hydrophilic TG resin afforded lower selectivities. However,
the effect of the matrix did not change the fact that the highest ellipticity was found
for beads with the same selectors. This suggested that the support chemistry might
affect the screening, a fact, that was also demonstrated quite convincingly by our
research group [8, 10].

Fig. 3-10. Ellipticities measured at 260 nm for separations achieved with the members of the parallel
library of 16 dipeptide CSPs (Reprinted with permission from ref. [87]. Copyright 1999, American
Chemical Society.)

The two best selectors resulting from Li’s screening, DNB-L-ala and DNB-L-leu,
were then prepared on a larger scale, attached to silica beads modified with 3-amino-
propyl-triethoxysilane, and the CSPs were packed into columns. Respective separa-
tion factors of 4.7 and 12 were found for the separation of racemic naphthyl leucine
ester 17 using these CSPs.