A new example of reversal of the order of migration of enantiomers, as a function of cyclodextrin concentration and pH, by cyclodextrin-modified capillary zone electrophoresis: enantioseparation of 6,6'-dibromo-1,1'-binaphthyl-2,2'-diol.

Enantioseparation of 6,6'-dibromo-1,1'-binaphthyl-2,2'-diol (DBBD) by cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was studied using the three native alpha, beta, and gamma cyclodextrins, the three hydroxypropylated cyclodextrins (2-hydroxypropyl-alpha, beta, and gamma), heptakis-2,6-di-O-methyl-beta-CD (DM-beta-CD), and heptakis-2,3,6-tri-O-methyl-beta-cyclodextrin (TM-beta-CD). First, the acidity constants of DBBD were determined using capillary electrophoresis, before performing enantioseparation. The influence of the concentrations of the studied cyclodextrins on the enantioseparation was explored and the experimental optimal concentrations were determined and compared to the theoretical optimal concentrations. Moreover, the apparent complexation constants between each studied cyclodextrin and the two DBBD enantiomers were evaluated using a non-linear curve fitting method and three linear plotting methods (x-reciprocal, y-reciprocal and double reciprocal). For TM-beta-CD, the order of migration of the enantiomers of DBBD reversed as a function of TM-beta-CD concentration. The influence of the nature of methylated cyclodextrin derivatives (methyl-beta-CD (M-beta-CD) and DM-beta-CD) was then studied. Inversion of the order of migration of the enantiomers of DBBD was observed for DM-beta-CD, whereas the S enantiomer of DBBD always migrated first for M-beta-CD.

Enantioseparation of binaphthol and its monoderivatives by cyclodextrin-modified capillary zone electrophoresis: a mathematical approach.

A simple model for the separation of atropisomers of binaphthol and its monoderivatives by means of cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was used to describe the migration behavior of poly charged enantiomers in a chiral separation system. This mathematical approach allowed for the determination of the optimal cyclodextrin concentrations for the enantioseparation of binaphthols by taking into account the influence of the formed complex mobilities. Moreover, using this theoretical approach, the reversal of the enantiomers' migration order as a function of cyclodextrin concentration was predicated. The apparent complexation constants between the cyclodextrins and the binaphthol and its monoderivatives could be calculated using a non-linear curve fitting method and three linear plotting methods (x-reciprocal, y-reciprocal and double reciprocal). Good agreements between the theoretical and experimental cyclodextrin concentrations were obtained.

New ionic liquid for inorganic cations analysis by capillary electrophoresis: 2-hydroxy-N,N,N-trimethyl-1-phenylethanaminium bis(trifluoromethylsulfonyl)imide (phenylcholine NTf2).

The potentialities of new ionic liquids (ILs) based on choline were evaluated as an electrophoretic medium in capillary electrophoresis for the analysis of alkaline and alkaline earth cations (Li(+), K(+), Na(+), Cs(+), Mg(2+), Ba(2+), Ca(2+), and Sr(2+)) with indirect UV detection. Two types of capillaries were tested: an untreated fused silica and fused silica coated with a film of polyvinylalcohol. The coated capillary proved to be the best adapted for the metal ions studied. Moreover, it appeared that the nature of the ionic liquid anion influenced the baseline stability, and the bis(trifluoromethylsulfonyl) imide (NTf(2) (-)) anion seemed to be the most efficient. These preliminary studies led us to synthesize a new ionic liquid, 2-hydroxy-N,N,N-trimethyl-1-phenylethanaminium NTf(2) (phenylcholine NTf(2)). This liquid was able to act as the running electrolyte and probe, generating the background signal in indirect UV light and consequently simplifying the electrophoretic medium. Excellent baseline stability, good reproducibility, as well as good sensitivity of detection were obtained with this new ionic liquid. Thus, 510,000 plates/meter for Li(+) with 40 mM IL were successfully obtained. The optimal concentration of IL was 20 mM with a detection limit ranging from 28 microg L(-1) for Li(+) to 1,000 microg L(-1) for Cs(+). This method (phenylcholine NTf(2) with polyvinylalcohol capillary) was applied to analyze different commercial source and mineral waters. Finally, the potentiality of this ionic liquid in nonaqueous capillary electrophoresis was explored. The use of phenylcholine NTf(2) with a fused silica capillary, in pure methanol medium and in the presence of acetic acid, made it possible to obtain separation selectivity different from that obtained in aqueous medium.

Enantioseparation of binaphthol and its mono derivatives by cyclodextrin-modified capillary zone electrophoresis.

Chiral binaphthols belong to the group of most effective ligands for asymmetrical catalysis. In this context, various binaphthols presenting original substituents have been synthesized. Their study through capillary electrophoresis is the object of this work. The literature dedicated to the separation of atropisomers by capillary electrophoresis, corresponding only to binaphthol, reveals that its enantioseparation is always delicate because of the influence of many factors and the resolutions obtained are weak. Therefore, for a structured optimization, we first successfully evaluated the acidity constants of different binaphthols by means of capillary electrophoresis. With these known physicochemical characteristics, we could successfully carry out enantiomeric separations of the different binaphthols at pH 11.5, practically in completely ionized form, in phosphate medium, and in the presence of cyclodextrin (CD), with analysis times lower than 8min. The nature of CDs (alpha-CD, beta-CD, gamma-CD, hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD), HP-beta-CD, HP-gamma-CD and trimethyl-beta-CD (TM-beta-CD)) and other factors in relation to enantiomeric resolution (applied voltage, nature and concentration of the electrolyte, and concentration of cyclodextrin) were optimized. These studies allowed us to determine the optimal conditions of separation (concentration and nature of CD) for each of the studied binaphthols. It is necessary to mention that, for the 1,1'-binaphthyl-2,2'-diol (Binol) at pH 11.5, the S atropisomer always migrated first, regardless of the nature and concentration of the cyclodextrin used. Moreover, an inversion in elution order of the two atropisomers as a function of pH was observed with gamma-CD (pH range: 10-11.5). The R atropisomer migrated first at pH 10. At pH 10.8 the migration order of the two atropisomers of Binol was reversed as a function of gamma-CD concentration. Finally, the addition of chiral ionic liquids (R(-)-1-hydroxy-N,N,N-trimethylbutan-2-aminium bis(trifluoromethylsulfonyl)imide and S(+)-tetrabutylammonium camphorsulfonate) was conducted. In the case of S(+)-tetrabutylammonium camphorsulfonate, a weak antagonistic effect was observed with modeling the evolution of enantiomeric resolution by means of the experimental design, while in the case of R(-)-1-hydroxy-N,N,N-trimethylbutan-2-aminium bis(trifluoromethylsulfonyl)imide the effect was neutral.