Separation mechanisms for palonosetron stereoisomers at different chiral selector concentrations in MEKC.

The separation mechanisms for palonosetron (PALO) stereoisomers in MEKC using sodium cholate (SC) as surfactant and chiral selector have been studied, in a wide range of concentrations below and above the CMC. It was found that SC micelles only provide chirally selective recognition for 3a carbon chiral center in PALO molecules. The resolution of the configurations of 2 carbon chiral center is achieved by the difference of mobility in continuous phase. A schematic diagram depicting the separation mechanisms and the corresponding migration orders among all of four stereoisomers was proposed based on the measured separation parameters. A MEKC method to achieve the complete separation of four stereoisomers in very short time using a very low chiral selector concentration, instead of high concentrations generally considered, was developed based on the understanding of the mechanisms.

Solvent-modified MEKC for the enantioseparation of palonosetron hydrochloride and related enantiomeric impurities.

The effect of ten water-soluble organic solvents on MEKC separation of palonosetron hydrochloride (PALO) stereoisomers using sodium cholate (SC) as chiral selector has been studied. The first chiral CE method fit for the analysis of unwanted PALO distomers (enantiomeric impurities) of low concentrations in the presence of high concentration of the main eutomer has been developed, based on solvent-modified MEKC mode. It was found that methanol provides the best separation among the solvents tested. And an SC concentration of 30 mM is proper to provide good resolutions in shorter time and adequate sample capacity, instead of 70 mM as previously reported. The developed method can be used to analyze unwanted PALO distomers of a few micrograms per milliliter in the presence of the main eutomer with a concentration as high as 1.0 mg/mL.

Separation of palonosetron stereoisomers by electrokinetic chromatography using sodium cholate as chiral selector: comparison of separation modes and elucidation of migration orders.

Based on sodium cholate as chiral selector, four stereoisomers of palonosetron hydrochloride, i.e. PALO (3aS, 2S), PALO (3aR, 2R), PALO (3aS, 2R), and PALO (3aR, 2S), have been separated by five EKC modes, i.e. MEKC, solvent-modified MEKC, cosurfactant-modified MEKC, MEEKC, and MEEKC without cosurfactant. The performances of different modes were compared. The migration order and its change with experimental conditions were elucidated. In every mode studied, the migration orders in each enantiomeric pair were (3aS, 2S), (3aR, 2R) and (3aS, 2R), (3aR, 2S), respectively, determined by the selectivity of chiral selector (chromatographic mechanism). Enantiomeric pair (3aS, 2S), (3aR, 2R) was eluted before enantiomeric pair (3aS, 2R), (3aR, 2S) due to mobility difference (electrophoretic mechanism). For the separation between (3aR, 2R) and (3aS, 2R), the second enantiomer of the first pair and the first enantiomer of the second pair, two mechanisms gave opposite migration orders according to the measured selectivity and mobility data. Therefore, three different migration orders were observed at different conditions, depending on the relative strength of two effects.

Effect of molecular structure of tartrates on chiral recognition of tartrate-boric acid complex chiral selectors in chiral microemulsion electrokinetic chromatography.

Eight l-tartrates and a d-tartrate with different alcohol moieties were used as chiral oils to prepare chiral microemulsions, which were utilized in conjunction with borate buffer to separate the enantiomers of beta-blockers or structurally related compounds by the chiral microemulsion electrokinetic chromatography (MEEKC) method. Among them, six were found to have a relatively good chiral separation performance and their chiral recognition effect in terms of both enantioselectivity and resolution increases linearly with the number of carbon atoms in the alkyl group of alcohol moiety. The tartrates containing alkyl groups of different structures but the same number of carbon atoms, i.e. one of straight chain and one of branched chain, provide similar enantioseparations. The trend was elucidated according to the changes in the difference of the steric matching between the molecules of two enantiomers and chiral selector. Furthermore, it was demonstrated for the first time that a water insoluble solid compound, di-i-butyl l-tartrate (mp. 73.5 degrees C), can be used as an oil to prepare a stable microemulsion to be used in the chiral MEEKC successfully. And a critical effect of the microemulsion for chiral separation, which has never been reported before, was found in this experiment, namely providing a hydrophobic environment to strengthen the interactions between the chiral selector and enantiomers.