Abstract
Neutral cyclodextrins (CDs) and derivatives carrying an overall cationic or anionic charge are widely utilised as chiral additives in liquid chromatography (LC) and particularly in capillary electrophoresis (CE). Five neutral CDs (α-CD, β-CD, γ-CD, hydroxypropyl-β-CD (HP-β-CD) and hydroxyethyl-β-CD (HE-β-CD)) and the anionic sulphobutylether-β-CD (SBE-β-CD) and carboxymethyl-β-CD (CM-β-CD) have been examined for the resolution of amlodipine enantiomers both in LC and in CE. In LC, although the neutral CDs yielded no enantioselectivity for amlodipine, CM-β-CD did show enantioselectivity (Rs=1.1, tR=35 min), albeit with poor peak shape. It was found, however, that the anionic SBE-β-CD gives a robust separation (Rs=1.7, tR2=21.5 min) when using acetonitrile–20 mM NaH2PO4 (pH 3.95), containing 20 mM SBE-β-CD (35:100, v/v). A spectrophotometric continuous-variation plot method was used to determine that the amlodipine–SBE-β-CD complex has predominantly a 1:1 stoichiometry in solution. Stability constants have been calculated for this 1:1 complex in LC. For each amlodipine enantiomer with the anionic SBE-β-CD, these were 596 and 561 M−1, respectively. The enantiomers of amlodipine are also separated in CE using hydroxypropyl-β-CD (20 mM) and the anionic SBE-β-CD (1 mM) and CM-β-CD (2.5 mM) where the anionic CDs were shown to offer an enhanced enantioselectivity over the neutral CD. The amlodipine enantiomer migration order in CE is found to be the same for both anionic and neutral CDs. Further examination of the enantiomer migration order indicated that the binding constant for the S enantiomer to the SBE-β-CD was greater than that for the corresponding R enantiomer. It was then possible, by examining the enantiomer retention, to conclude that in LC the enantioselective interaction between each amlodipine enantiomer and the anionic SBE-β-CD occurs in the bulk mobile phase and not on a dynamically coated chiral stationary phase.
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