Enantiodifferentiation of N-benzyloxycarbonylaminophosphonic and phosphinic acids and their esters using cyclodextrins by means of capillary electrophoresis

Applications of nuclear magnetic resonance spectroscopy for the understanding of enantiomer separation mechanisms in capillary electrophoresis

This review deals with the applications of nuclear magnetic resonance (NMR) spectroscopy to understand the mechanisms of chiral separation in capillary electrophoresis (CE). It is accepted that changes observed in the separation process, including the reversal of enantiomer migration order (EMO), can be caused by subtle modifications in the molecular recognition mechanisms between enantiomer and chiral selector. These modifications may imply minor structural differences in those selector-selectand complexes that arise from the above mentioned interactions. Therefore, it is mandatory to understand the fine intermolecular interactions between analytes and chiral selectors. In other words, it is necessary to know in detail the structures of the complexes formed by the enantiomer (selectand) and the selector. Any differences in the structures of these complexes arising from either enantiomer should be detected, so that enantiomeric bias in the separation process could be explained. As to the nature of these interactions, those have been extensively reviewed, and it is not intended to be discussed here. These interactions contemplate ionic, ion-dipole and dipole-dipole interactions, hydrogen bonding, van der Waals forces, π-π stacking, steric and hydrophobic interactions. The main subject of this review is to describe how NMR spectroscopy helps to gain insight into the non-covalent intermolecular interactions between selector and selectand that lead to enantiomer separation by CE. Examples in which diastereomeric species are created by covalent (irreversible) derivatization will not be considered here. This review is structured upon the different structural classes of chiral selectors employed in CE, in which NMR spectroscopy has made substantial contributions to rationalize the observed enantioseparations. Cases in which other techniques complement NMR spectroscopic data are also mentioned.

Cyclodextrins in capillary electrophoresis enantioseparations--recent developments and applications

Capillary EKC has been established as a versatile and robust CE method for the separation of enantiomers. Within the chiral selectors added to the BGE CDs continue as the most widely used selectors due to their structural variety and commercial availability. This is reflected in the large number of practical applications of CDs to analytical enantioseparations that have been reported between January 2006 and January 2008, the period of time covered by this review. Most of these applications cover aspects of life sciences such as drug analysis, bioanalysis, environmental analysis, or food analysis. Moreover, new CD derivatives have been developed in an attempt to achieve altered enantioselectivities and to further broaden the application range. Finally, efforts will be summarized that aim at an understanding of the molecular level of the chiral recognition between CDs and the analytes.

The role of cyclodextrins in chiral capillary electrophoresis

The members of the enantiomeric pairs frequently show rather different biological effects, so their chiral selective synthesis, pharmacological studies and analysis are necessary. CE has unique advantages in chiral analysis. The most frequently used chiral selectors are CDs in this field. This paper gives a short view on the advantages on CE in direct chiral separations, emphasizing the role of CDs. The reason for the broad selectivity spectra of CDs is discussed in detail. The physical background of chiral selective separations is briefly shown in CE. Their interaction mechanisms are shortly defined. The general trend of their use is statistically evaluated. Most frequently used CDs and CD derivatives are characterized. Advantages of ionizable CDs and single-isomer derivatives are shown. The general trend of their use is established.

Analysis of pD-Dependent complexation ofN-benzyloxycarbonylaminophosphonic acids by α-cyclodextrin. Enantiodifferentiation of phosphonic acid pKa values

The influence of aqueous solution pD on stereoselective complexation of N-benzyloxycarbonylaminophosphonic acids with alpha-cyclodextrin was investigated by means of nuclear magnetic resonance spectroscopy. The highest enantiodiscrimination was achieved at pD close to the pKa of less acidic hydroxyl group of the phosphonic moiety of analytes (6.5-7.5). This effect results from the stereoselective differentiation of pKa (up to 0.28 pD unit) upon complexation with applied chemical shift reagent. Moreover, analysis of 2D-ROESY spectra proved that the host-guest inclusion mode is strongly influenced by pD.