Page 234 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

214 polymer matrix under the influence of an electric field. Gel electrophoresis, a very
important bioanalytical technique, is done in a polyacrylamide gel and depends on
CHAPTER 2 molecular size and charge differences to achieve differential migration. This method
Stereochemistry, is primarily applied to macromolecules, such as polypeptides and oligonucleotides.
Conformation,
and Stereoselectivity Capillary electrophoresis (CE) has been developed as a technique for analysis of small
chiral molecules, especially drugs. The most widely applied method of analysis is
capillary zone electrophoresis, in which a chiral selector is added to the electrolyte
buffer. The difference in binding between the chiral selector and the two enantiomers
being analyzed then determines the rate of migration. Among the most commonly used
chiral selectors are cyclodextrins, which are cyclic oligosaccharides. The cyclodextrins
can be modified to incorporate polar, anionic, or cationic groups.

OR
RO O
O
O
OR RO OR O OR
O RO RO
O OR RO O
RO O
OR
OR
O Or
OR OR RO
OR R = CH , CH CH OH, (CH ) OH
RO O O 3 2 2 2 3
O – – –
2
2 4
2
2
2 2
O CH CO , (CH ) CO , (CH ) SO 3
–
O SO , PO H –
3
3
OR
OR
Macrocyclic antibiotics such as vancomycin and rifamycin B are also used a chiral
selectors.
OH
H N CH OH
2
CH 3 3 O
HO O
CH OH
O O 2 CH 3 CH
HO 3
O O
HO CH CO 2 CH 3
3
OH OH O
Cl CH 3 CH
O O H Cl O CH(CH ) CH O OH OH 3
3 2
3
N O CH3
N NH NH
HN H N
CO 2 H O H NHCH
O CONH 2 3
O O
OCH CO H
2
2
OH CH 3 O
HO OH vancomycin Rifamycin B
Another approach to chiral electrophoresis involves covalent attachment of the
chiral selector to either the capillary wall or the packing material. 203 For open columns,
where the packing material is attached to the capillary wall, a completely methylated
-cyclodextrin is linked to the surface through a polysiloxane (Chirasil-Dex 1). The
packed columns use silica bound to the methylated cyclodextrin (Chirasil-Dex 2).
203
V. Schurig and D. Wistuba, Electrophoresis, 20, 2313 (1999).