An alternative approach to the resolution of enantiomers is to use a chiral mobile
phase. In this technique a transient diastereomeric complex is formed between the
enantiomers and the chiral mobile phase agent. Examples of chiral mobile phase
agents include albumin,a 1 -acid glycoprotein,a,b- andg-cyclodextrins, camphor-
10-sulphonic acid andN-benzoxycarbonylglycyl-L-proline, all of which are used
with a reversed-phase chromatographic system. For example, this technique has been
used to show that cannabidiol, one of the main components of marijuana, consists of
(þ) and () forms only one of which is physiologically active.
The most successful approach to chiral chromatography, however, has been the use
of a chiral stationary phase. This is based upon the principle that the need for a three-
point interaction between the stationary phase (working as achiral discriminator)
and the enantiomer would allow the resolution of racemic mixtures due to the
different spatial arrangement of the functional groups at the chiral centre in the
enantiomers. The cyclodextrins are cyclic oligosaccharides that have an open trun-
cated conical structure 6 to 8 A ̊(0.6 to 0.8 nm) wide at their base. Their inner surface
is predominantly hydrophobic, but secondary hydroxyl groups are located around
the wide rim of the cone.b-Cyclodextrin has seven glucopyranose units and contains
35 chiral centres anda-cyclodextrin has six glucopyranose units, 30 chiral centres
and is smaller thanb-cyclodextrin. Collectively they are referred to aschiral cavity
phasesbecause they rely on the ability of the enantiomer to enter the three-dimensional
cyclodextrin cage while at the same time presenting functional groups and hence
the chiral centre for interaction with hydroxyl groups on the cone rim.11.6 Ion-exchange chromatography
11.6.1 Principle
This form of chromatography relies on the attraction between oppositely charged
stationary phase, known as anion exchanger, and analyte. It is frequently chosen for
the separation and purification of proteins, peptides, nucleic acids, polynucleotides
and other charged molecules, mainly because of its high resolving power and high
capacity. There are two types of ion exchanger, namelycationandanion exchangers.
Cation exchangers possess negatively charged groups and these will attract positively
charged cations. These exchangers are also calledacidic ion exchangersbecause
their negative charges result from the ionisation of acidic groups. Anion exchangers
have positively charged groups that will attract negatively charged anions. The term
basic ion exchangersis also used to describe these exchangers, as positive charges
generally result from the association of protons with basic groups.11.6.2 Materials and applications
Matrices used include polystyrene, cellulose and agarose. Functional ionic groups
include sulphonate (–SO– 3 ) and quaternary ammonium (–NþR 3 ), both of which are459 11.6 Ion-exchange chromatography