Capillary electrophoresis investigation on the structure-enantioselectivity relationship in synthetic cyclopeptides as chiral selectors

The Reciprocal Principle of Selectand-Selector-Systems in Supramolecular Chromatography †

In selective chromatography and electromigration methods, supramolecular recognition of selectands and selectors is due to the fast and reversible formation of association complexes governed by thermodynamics. Whereas the selectand molecules to be separated are always present in the mobile phase, the selector employed for the separation of the selectands is either part of the stationary phase or is added to the mobile phase. By the reciprocal principle, the roles of selector and selectand can be reversed. In this contribution in honor of Professor Stig Allenmark, the evolution of the reciprocal principle in chromatography is reviewed and its advantages and limitations are outlined. Various reciprocal scenarios, including library approaches, are discussed in efforts to optimize selectivity in separation science.

Chiral separations by capillary electrophoresis: Recent developments and applications

This paper provides an overview of the different classes of chiral selectors that are used in CE. The main properties of every class are described, together with the mechanism of enantioseparation. Newly introduced selectors are also discussed. Pharmaceutical and biomedical applications published from January 2004 till March 2005 are summarized.

Recent advances in peptide chiral selectors for electrophoresis and liquid chromatography

The application of peptides in chiral separations using techniques such as capillary electrophoresis (CE), electrokinetic capillary chromatography (EKC) and liquid chromatography is the focus of this review. Methods for finding peptide selectors using combinatorial library approaches are discussed, as well as recent advances in the use of peptides as general chiral selectors for electrophoresis and liquid chromatography. One example shows the effectiveness of polymeric dipeptide surfactants as general chiral selectors for electrophoresis. Another example shows the versatility of oligoproline chiral stationary phases, exhibiting resolution for a number of racemic analytes comparable to other well-established chiral stationary phases.

Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

This review summarizes recent developments in chiral separation in capillary zone electrophoresis (CZE), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC) covering literature published since the year 2000. New chiral selectors and innovative approaches for CE and CEC are introduced. Recent progress in column technology for CEC is highlighted and the development of new chiral stationary phases is discussed. This review is not dedicated to list applications but will focus on new developments.

Chiral Recognition of Peptide Enantiomers by Cinchona Alkaloid Derived Chiral Selectors: Mechanistic Investigations by Liquid Chromatography, NMR Spectroscopy, and Molecular Modeling

The chiral recognition mechanism of a cinchona alkaloid based chiral selector for N-protected peptide enantiomers was investigated. A chiral stationary phase derived from this selector was employed for liquid chromatographic enantiomer separations. It showed exceptionally high enantiomer discrimination for the (all-R)- and (all-S)-enantiomers of dialanine (alpha = 20), while a pronounced loss of chiral recognition occurred upon the insertion of an additional alanine residue into the peptide backbone. This reduction of enantioselectivity was investigated in great detail by NMR spectroscopy of complexes of the chiral selector and the analyte enantiomers accompanied by molecular modeling studies. Investigation of intramolecular NOEs provided the conformational states of the free and complexed forms of the selector. The analysis of complexation-induced shifts yielded information on intermolecular interactions and allowed us to propose binding models, which were further supported by the observation of intermolecular NOEs, indicating the relative arrangements of selector and analytes. Stochastic molecular dynamics simulations were able to reproduce the chromatographic retention orders and energy differences, as well as the intermolecular NOEs. The computational data were used to evaluate the intermolecular forces responsible for analyte binding. In addition, the relative contributions of the fragments of the chiral selector to the enantioselective binding event were assessed. A spatial arrangement of the chiral selector and the analyte allowing the primary ionic interaction as well as hydrogen bonding and pi-pi-stacking to take place simultaneously was found to be essential to obtain very high enantioselectivities.