Published:Journal of Chromatographic Science,
ISSN 0021-9665Volume
39, Number 10, October 2001, pp. 117-122
Chromatographic Behavior of Ion Pair Enantiomers of Dansyl Leucine
Cyclohexylammonium Salt on a b-Cyclodextrin Stationary Phase and the Effect
of a Competitive-Binding Mobile Phase Additive
P.J. Skrdla[1], R.T. Robertson[2], V. Antonucci[1], and C.
Lindemann[1]
[1]Merck & Co., Inc., P.O. Box 2000, Rahway, NJ 07065 and
[2]Novartis Institute
for Biomedical Research, 1 Health Plaza, East Hanover, NJ 07936
The separation of dansyl leucine
enantiomers on a b-cyclodextrin stationary phase is significantly complicated
by the association
of the amino acid with its cyclohexylammonium counter ion, in a mobile phase
of 80:20 (v/v) methanol–water. This produces very unusual chromatography,
with two partially superimposed peaks observed for each enantiomer at lower column
temperatures. The peak shape is attributed to the irreversible, oncolumn conversion
of the ion pair (I) to the free, protonated (neutral) dansyl amino acid (II+H).
Increasing the ionic strength of the mobile phase greatly improves the chromatography
by transforming the solute species to enantiomers of II (the anionic, free amino
acid). Van’t Hoff plots are constructed for both species I and II (under
different mobile phase conditions) to provide thermodynamic insight into the
major enantioselective driving forces of separation. The chiral discrimination
of the stationary phase is found to be primarily enthalpically driven for both
solutes. Finally, 1-adamantanecarboxylic acid (ACA) is investigated as a solute-competitive
mobile phase additive to intentionally block the hydrophobic cyclodextrin cavities
on the stationary phase. By varying the concentration of ACA additive in the
mobile phase, control over the retention and chiral recognition of the stationary
phase is demonstrated.
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