Column temperature programming in enantioseparation of dihydropyrimidinone compounds using derivatized cellulose and amylose chiral stationary phases

Utilization of the hysteresis phenomenon for chiral high-performance liquid chromatographic method selection in polar organic mode

Polysaccharide-based chiral stationary phases (CSPs) are outstandingly suitable to play a key role in chiral HPLC method selection strategies, since they provide high success rates. One reason for this ability is that they adopt a diversity of higher order structures in various eluents, resulting in versatile chiral environments. A potential to extend this versatility further was expected and examined in the present study, based on the recently discovered hysteretic behavior of a widely used chiral selector (CS), amylose tris(3,5-dimethylphenylcarbamate). The hindered transitions of its structure, which are behind the history dependence of its separation ability, were used as a tool to identify distinct states of the chiral selector in order to exploit an extended selectivity space. The identification was carried out using a single diagnostic compound, as opposed to the common approach where testing a library of compounds is required. Eluent mixtures consisting of 2-propanol and either methanol or ethanol were scrutinized in terms of stability and robustness of the observed retentions. The solvent mixtures that were eligible for practical application in these respects were used to construct a screening sequence, including identical compositions combined with different column pretreatment. The gain achievable by using the proposed sequence was then evaluated using 15 enantiomer pairs with focus on resolution, enantiomer elution order and chemoselectivity.

Chiral recognition by enantioselective liquid chromatography: mechanisms and modern chiral stationary phases

An overview of the state-of-the-art in LC enantiomer separation is presented. This tutorial review is mainly focused on mechanisms of chiral recognition and enantiomer distinction of popular chiral selectors and corresponding chiral stationary phases including discussions of thermodynamics, additivity principle of binding increments, site-selective thermodynamics, extrathermodynamic approaches, methods employed for the investigation of dominating intermolecular interactions and complex structures such as spectroscopic methods (IR, NMR), X-ray diffraction and computational methods. Modern chiral stationary phases are discussed with particular focus on those that are commercially available and broadly used. It is attempted to provide the reader with vivid images of molecular recognition mechanisms of selected chiral selector-selectand pairs on basis of solid-state X-ray crystal structures and simulated computer models, respectively. Such snapshot images illustrated in this communication unfortunately cannot account for the molecular dynamics of the real world, but are supposed to be helpful for the understanding. The exploding number of papers about applications of various chiral stationary phases in numerous fields of enantiomer separations is not covered systematically.