Principles of Chiral Separations by Free-Solution Electrophoresis - Chiral Separations by Capillary Electrophoresis - page 26

systems if a chirally impure chiral selector is utilized in the derivatization reaction.
Consequently, both qualitative and quantitative correctness of the analysis is lost.
With a chirally impure chiral selector ( R , S )C, a correct analysis can be obtained
only if the BGE is enriched with a chiral selector or with a mixture of chiral selec-
tors effective in direct chiral separations. However, in this case the indirect separa-
tion loses its main advantage and the derivatization reaction becomes a senseless
and complicating step. Obviously, general difi culties and disadvantages of achiral
derivatization reactions occur in the derivatization reactions with chiral selectors,
too. An irremovable drawback of the indirect chiral separations is their inherent
incapability to fully discriminate sterically different forms of compounds that con-
tain two and more chiral centers.
The indirect separations were very valuable for chiral separations of enantiomeric
pairs in the past century when the set of accessible chiral selectors was very lim-
ited and the electrophoretic chiral separations did not exist. At that time, the only
alternatives to achiral separations of derivatized enantiomers were their direct chiral
separations requiring very expensive chiral stationary phases and time-consuming
procedures. Therefore, the family of the derivatization agents for the indirect chiral
separations in chromatography is much more abundant than the family of deriva-
tization agents recommended in the electrophoretic literature, see, e.g., Ref. [5].
The development of the electrophoretic direct chiral separations started in the last
decade of the past century. The development, which is summarized in one mono-
graph [2] and in numerous reviews mostly cited in Refs. [2,5], led to a full dominance
of the direct chiral separations. As a consequence, the practical attractiveness of the
indirect chiral separations quickly decreased. Nowadays, their use is only justii ed
if the diastereoisomers provided by the derivatization step offer an additional proi t,
e.g., an easier detection of the separated zones or increased separation selectivity.

2.3 DIRECT SEPARATIONS

2.3.1 M ODELS OF D IRECT C HIRAL D ISCRIMINATION
Direct discrimination of enantiomers and other sterically different forms of chiral
species is conditioned by a special constituent of the separation system that interacts
differently with the sterically different forms of chiral species. In separation sci-
ence, for such a constituent we use the term chiral selector. There are two models
representing the mechanism of the direct chiral discrimination. A common feature
of these models is a sterically rigid arrangement of the whole chiral selector or at
least of the chiral selector part that induces the discrimination. Due to this rigidity,
the strength of the interaction of a chiral selector with sterically different forms of a
chiral species can be different.
Dalgliesh introduced the concept of a three-point interaction between the bind-
ing site present in a chiral selector and a chiral analyte (Figure 2.1) [7]. The revised
version of the concept [8] is applicable to chiral selectors of any type, size, charge,
aggregation state, and chemical form, in which the selectors interact with chiral ana-
lytes. The interactions 1-1 participating in chiral discrimination may

be of any kind (inclusion in a cavity, coulombic interaction, hydrogen bonding,

′ , 2-2 ′ , and 3-3 ′

π - π