Chiral
Chiral is a term used in stereochemistry which is the study of the three-dimensional structure of chemical compounds. Isomers of the same substance that only differ in the spatial arrangement of their atoms are called stereoisomers. Certain sterioisomers that only differ in their capacity for rotating the plane of polarized light passed through them are termed optically active or chiral. Stereoisomers that exhibit chiral properties are called enantiomers. Thus, the adjective chiral indicates that the substance can rotate the plane of any polarized light that is passed though it. In fact, chiral, describes an optical property of a substance.
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Chiral-Stationary-Phases
analytes exhibiting strong chiral selectivity.
There are specific interactive sites that provide chiral selectivity, but there
are many more sites that only contribute to general retention. These other
sites can be deactivated by mobile phase additives (e.g. octylamine) which
reduces the overall retention and increases the chiral selectivity. The second
type consists of relatively small molecular weight chiral substances bonded to
silica 9 Pirkle (37). Each bonded group has a limited number of chiral centers
available but, due to their small size, there can be a large number of groups
bonded to the silica (as opposed to much larger complex chiral moieties). It
follows, that a relatively high probability is maintained of the solute
interacting with a chiral center. The advantage of the Pirkle chiral phases is
that, as the overall interacting molecule is small, the solutes are not
strongly retained and thus the chiral selectivity becomes the dominant factor.
The third type is based on
HPLC Chiral-Stationary-Phases
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chiral-Chromatography Chiral-Polysiloxane-Stationary-Phases
The second type of chiral stationary phase
consisted of relatively small molecular weight chiral substances bonded to
silica and were pioneered by Pirkle (20). Although each bonded group has a
limited number of chiral centers available, due to their small size, there are
a large number of them on the silica (as opposed to much larger complex chiral
moieties), so, a relatively high interaction probability with a chiral center
is maintained. The advantage of the Pirkle chiral phase is that the overall
interacting molecule is small, and. so, the extra interactive contributions to
retention are also small. It follows, that the chiral selectivity becomes the
dominant factor controlling retention. The third type are the polymers of
cellulose and amylose developed by Okamato (21) The polymers are derivatized to
link appropriate interactive groups to the cellulose polymer which is then
physically coated onto a silica support. The fourth type is based on the
Retention Chiral-Chromatography Chiral-Polysiloxane-Stationary-Phases
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Available-Stationary-Phase Chiral
. If the stationary phase
is also chiral in nature, it is likely that one enantiomer in the sample will
fit closely to the stationary phase surface whereas the other will be
stearically excluded and thus have less stationary phase with which to
interact. The first chiral separations in GC were reported by Gil-Av et al.
as in 1966 (7), but, surprisingly, the use of GC for the separation of
enantiomers has only recently been investigated and developed into a practical
system. The use of chiral stationary phases in GC has been dogged by
entantiomeric instability arising from the racemization of both the chiral
stationary phase and the chiral solutes at elevated temperatures. In addition,
at the elevated temperatures necessary to elute the solutes in a reasonable
time, the chiral selectivity of the stationary phase can also be
impaired
Principles Available-Stationary-Phase Chiral
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Available-Stationary-Phase Chiral
as the carrier gas.
The use of LC
for chiral separations is easier to carry out and generally more efficient. A
number of racemic mixtures can be easily separated using a reverse-phase column
and a mobile phase doped with a chiral reagent. In some cases, the reagent is
adsorbed strongly on to the stationary phase, under which circumstances, the
chiral selectivity resides in the stationary phase. Conversely, if the reagent
remains predominantly in the mobile phase, then the chiral selectivity will be
in the mobile phase. Camphor sulphonic acid and quinine are examples of mobile
phase additives. The most common method used to achieve chiral selectivity is
to bond chirally selective compounds to silica in a similar manner to a reverse
phase (e.g., example of which is afforded by the cyclodextrins
Principles Available-Stationary-Phase Chiral
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Applications Chiral-Separations
g-cyclodextrin is distinctly more selective than the b material. It has been employed in the separation of a very wide range of compound classes, and from very small to very large molecules. Yet another stationary phase has been synthesized by substituting the cyclodextrin hydroxyl groups with pure the 'S' hydroxypropyl groups followed by permethylation. As a result, the size selectivity of the material is reduced but more polar (hydrophilic) groups are introduced. The b material has a greater chiral selectivity than the a or g phases. This material provides a good general purpose column. It is clear that there are many possibilities for derivatizing the cyclodextrins to provide unique interactive character; there are a large number commercially available and many more are likely to be synthesized in the future.
It is obvious, that the properties of the different chiral stationary phases available will differ considerably. Moreover, as the nature of many of the synthetic
Capillary Applications Chiral-Separations
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Chiral-Stationary-Phases Cyclodextrin
of interactive possibilities ranging from weak
and strong dispersive interactions, to polar interactions that span from
induced dipole interaction, through dipole–dipole interaction, to strong
hydrogen bonding. In addition, at the right pK, basic and acidic ionic
interactions can also be invoked. More importantly, with 32 stereogenic centers
the probability of interaction between chiral centers of solute and stationary
phase is relatively high.
Cyclodextrin
The
cyclodextrin based chiral stationary phases are some of the more popular
materials used for contemporary chiral separations. One of their advantages
lies in their use with all types of solvent. They can be used very effectively
in the reversed phase mode and, as well as being usable as a normal phase. The
cyclodextrins and their derivatives have been widely used for all types of
chiral separations and can often be used for preparative separations.
Cyclodextrin-based phases are readily available, covalently bonded
HPLC Chiral-Stationary-Phases Cyclodextrin