Stereoselective Separations of Free and N-tagged alpha-, beta-, gamma-amino acids, and Peptides Using
Chiral Zwitterionic and Anionic Exchangers: Reflections on a Successful Concept
Wolfgang Lindner, Institute of Analytical Chemistry, University of Vienna, Austria
By now it is widely believed that chromatographic separations of enantiomers shifted from art to routine. For medium polar and non-polar compounds this perception is true but it may not be the case for very polar analytes like free proteinogenic and unusual non-proteinogenic amino acids, peptides, and other compounds. Some systems are available to achieve the resolution of alpha-amino acids such as the macrocyclic antibiotics-based chiral columns. In contrast, we approached the challenge with a different concept developing chiral zwitterionic ion exchangers (ZWIX) based on the molecular fusion of both chiral cationic and anionic structural elements to create a single chiral but now ampholytic selector moiety. Mechanistically, retention is controlled by long range electrostatic interactions in concert with additional intermolecular interaction forces associated with hydrogen, p-p, and hydrophobic bonding, whereby a simultaneous double ion pairing process applies for zwitterionic analytes. Ion exchange operates in all kinds of solvents, including water, methanol, acetonitrile, DMSO, and DMF, but also under subFC conditions. Retention follows the stoichiometric displacement model. The spectrum of stereoselectivity (enantio- and diastereoselectivity) of this new class of chiral stationary phases and columns is broad and allows the resolution of a large number of very different alpha-, beta-, and gamma-amino acids, diverse peptides and peptomimetics. Mobile phase conditions and the volatile buffer components can be selected and optimized to enable LC-MS/MS applications with high sensitivity and analyte specificity. Preparative LC applications can also be envisioned as a result of the high loading capacity of these novel ZWIX type chiral stationary phases. The lecture will focus on the discussion of mechanistic and practical aspects supported by real world applications.