Principles of Chiral Separations by Free-Solution Electrophoresis - Chiral Separations by Capillary Electrophoresis - page 42

medium molecular size that ranges approximately from 700 to 2000 Daltons. The
compounds were put on the market as drugs. A bit later, their tendency to bind to
carboxylic groups of amino acids inspired the use of these compounds and their
derivatives as chiral selectors. Rifamycin is a compound of one macrocyclic ring,
the glycopeptidic antibiotics of the molecular weight above 1400 Daltons consist of
up to four rings. The BGEs with the macrocyclic antibiotics embody high separa-
tion efi ciency and a higher selectivity for the discrimination of anions than the
BGEs with cyclodextrins. Crown ethers are synthetic macrocyclic oligoethers
usually substituted with four carboxylic groups or with some other substituents.
The inclusion in their ring is always one of the supposed interactions. With these
selectors, the selectivity of the reported separations is close to that of cyclodex-
trins. Considering other tested selectors types, two of them are worth mentioning
here. The ligand-exchange selectors are based on the complexes of divalent copper,
zinc, nickel, or cadmium with amino acids or hydroxycarboxylic acids. These selec-
tors were highly popular in the early years of the separation science. Recently, their
practical importance decreased due to several competing selector types. Bile salts
are used almost exclusively in the micellar separation systems. All the listed selec-
tors are of a low or medium molecular weight. With the exception of cyclodextrins,
the stoichiometric stability constants for the complexation of these selectors with
chiral analytes occur seldom in the literature.
The afi nity chiral selectors may be considered as macromolecular relatives of
the macrocyclic antibiotics. Chemically, these biopolymers are usually proteins and
glycoproteins. In contrast to the other selectors, the steric structure (conformation) of
the afi nity chiral selectors strongly depends on the pH and chemical composition of
the BGE, see, e.g., Ref. [31]. The conformation markedly affects the capability of the
afi nity selectors for the chiral discrimination of particular chiral species, see, e.g.,
Refs. [32,33]. One afi nity chiral selector may therefore mimic the enantioselective
capabilities of an array of various structurally rigid chiral selectors provided that its
conformation is manipulated properly. The relative molecular mass of the afi nity
selectors is usually of the order of 10 4 Daltons, the stability constants for the compl-
exation with low-molecular-weight analytes range from 10 4 to 10 6 L/mol. Such high
stabilities indicate a potential for highly specii c separations; unfortunately, they
usually indicate decreased separation efi ciency, too. The widths and shapes of the
detected zones of low-molecular-weight analytes separated with the afi nity selectors
rather resemble liquid chromatographic than electrophoretic separations.
2.5 CLOSINGCOMMENT
Several separation techniques dealt with the analytical properties of chiral selectors
and with their capability to discriminate sterically different forms of chiral species
belonging to different classes of compounds. Basic pieces of knowledge extracted
from these studies can be applied in all the separation techniques, if specii c fea-
tures of particular techniques are taken into account [32,34-36]. The i eld of chiral
separations is not completely covered yet. Despite this, chiral separations extensively
applied in practice for many years proved to be one of the indispensable means
contributing to the present boom in the investigation of life that is chiral from its