Page 77 - Chiral Separation Techniques

2.5 Concluding Remarks 53
Table 2-7. Summary of optimization parameters on glycopeptide CSPs.

New polar organic mode a. Type of acid and base
b. Acid/base ratio
c. Concentration of acid and base
d. Flow rate

Reversed phase a. Type of organic modifier
b. Concentration of organic modifier
c. Type of aqueous buffer
d. Concentration of aqueous buffer
e. pH of aqueous buffer
f. Flow rate
g. Temperature
Normal phase a. Type of polar solvent
b. Concentration of polar solvent
c. Acid and base as modifiers
d. Temperature

2.5 Concluding Remarks

The macrocyclic glycopeptides vancomycin, teicoplanin and ristocetin A have
proved to be powerful chiral stationary phases. The unique structures and the vari-
ety of functional groups on the macrocycles utilize a large number of interactions
possible for chiral recognition. Covalently bonded to silica gel through multiple
linkages, these CSPs are multi-modal and it is possible to switch from one mobile
phase to another without deleterious effects. The enantiomeric separations are some-
times predictable for structure-related racemates. One important characteristic for
the glycopeptide CSPs is their complementary effect, which makes column coupling
an efficient and economical screening methodology. The method development with
these CSPs is discussed in detail in the new polar organic mode, and reversed-phase
and normal-phase modes. The enantiomeric separations with glycopeptide CSPs can
be optimized by controlling flow rate, temperature, the acid/base ratio and concen-
tration, the type and amount of organic modifier, the type, concentration and pH of
aqueous buffer. As a result of extensive research on these CSPs by scientists world-
wide, the summarized method development and optimization protocols may help to
stimulate further investigation and understanding of the chiral recognition mecha-
nism, as well as rapid and efficient resolution of greater numbers of chiral com-
pounds.

Acknowledgment

The authors would like to thank Vicki Sutter and Leslie Wrenn for their help with
the preparation of this manuscript.