title: A practical method for the quantitative assessment of non-enantioselective versus enantioselective interactions encountered in liquid chromatography on brush-type chiral stationary phase.
abstract: A convenient experimental method for the quantitative assessment of enantioselective versus non-enantioselective interactions in liquid chromatography on brush-type chiral stationary phases (CSPs) is described. This procedure involves the systematic evaluation of the retention characteristics of resolved enantiomers as chiral selectands, SAs, on a set of CSPs containing chiral selectors, SOs, attached to an achiral support S with well-defined but different surface SO loading levels. The emerging body of retention data can be dissected into non-enantioselective and enantioselective increments by equations accounting separately for the two co-existing equilibrium processes, namely the "unproductive" adsorption of the enantiomers on the achiral domains of the CSP and their "productive" enantioselective association with the surface-anchored chiral SOs. The general applicability of this approach was demonstrated using a set of CSPs loaded with different densities of a quinine carbamate anion exchange-type SO, and three acidic model SAs, i.e., N-Fmoc-phenylalanine, N-[(3,5-dipropoxybenzyloxy)carbonyl]leucine and 2-(2,4-dichlorophenoxy)propanoic acid, employing nonpolar, polar organic and reversed phase mobile phase conditions. In all tested mobile phase environments, the global retention characteristics of the studied SO was found to be dominated by enantioselective SA/SO binding rather than by non-enantioselective SA/support interactions, providing evidence for chiral recognition processes primarily capitalizing on strong electrostatic intermolecular binding forces. For the investigated compounds, the polar organic mode was identified as the most favorable mobile phase scenario, producing the highest apparent enantioseparation factors, α(app), in combination with the lowest degree of non-enantioselective adsorption. It is anticipated that the described approach toward the deconvolution of non-enantioselective versus enantioselective interactions will also be applicable to other types of brush-type CSPs. Most important, the quantitative information on the inherent thermodynamics of SO-SA interactions emerging from these studies will permit an unambiguous interpretation of the changes of the chiral recognition characteristics occurring as a consequence of mobile phase modifications. We are confident that this knowledge will be helpful in the design and evaluation of new non-invasive surface and immobilization chemistries, and will also provide valuable guidance for the optimization of operation conditions for large scale preparative enantiomer separations. Finally, it should be stressed that the advanced strategy for distinguishing enantioselective from non-enantioselective contributions has general applicability to all liquid phase-based enantioseparation techniques, well beyond the realm of chromatography, such as liquid-liquid extraction and batch-binding processes.
authors.affiliation: Department of Organic Chemistry, University of Tübingen, Germany. levkin@kit.edu
authors.forename: Pavel
authors.initials: P
authors.lastname: Levkin
authors.forename: Norbert M
authors.initials: NM
authors.lastname: Maier
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lindner
authors.forename: Volker
authors.initials: V
authors.lastname: Schurig
mesh_terms: D002851:Chromatography, High Pressure Liquid / Q000295:instrumentation / Q000379:methods*; D013237:Stereoisomerism