Electrostatic attraction-repulsion model with Cinchona alkaloid-based zwitterionic chiral stationary phases...

In the present paper, we demonstrated that Cinchona alkaloid cyclohexyl sulfonic acid-based zwitterionic chiral selectors (SOs) and the respective chiral stationary phases (CSPs) can be successfully employed for the enantioseparation of underivatized thus zwitterionic amino acids (the selectands, SAs) even in the absence of ionic additives in the eluent, generally used as displacer counter-ions in ion exchange chromatography. Therefore, we provided evidence that cooperative "intramolecular and intermolecular counter-ion effects" of the zwitterionic SO moiety and the zwitterionic SAs can be sufficient to modulate alone the retention characteristics without a loss of stereoselectivity. Four fully constrained β-amino acids were used as target compounds for this study. The analyses were carried out with either neat methanol, acetonitrile, water or their binary hydro-organic mixtures. A U-shaped retention profile was observed both with methanol- and acetonitrile-based eluents. Except a few cases, enantioselectivity experienced a remarkable amelioration at the "balanced region" of a buffer free hydroorganic mobile phase composition. At "the bottom" of the U-shaped curve, high α- and resolution values could be reached with most of the screened mobile phases. An electrostatically driven "attraction-repulsion model" was postulated to explain the very favourable characteristic of the two studied zwitterion-type CSPs for the retention and enantiomer separation of zwitterionic analytes.