Enantioselective retention mechanisms of dipeptides on antibiotic-based chiral stationary phases: Leucyl-leucine, glycyl-leucine, and leucyl-glycine as case studies

• Published in: Journal of Chromatography A 2019-09, Vol.1602, p.368-377
• Main Authors: Reshetova, Elena N., Kopchenova, Margarita V., Vozisov, Stepan E., Vasyanin, Alexander N., Asnin, Leonid D.
• Format: Article
• Language: English
• Subjects: Chiral chromatography; Dipeptides; Enthalpy-entropy compensation; Macrocyclic antibiotics; Retention mechanisms
• ISSN: 0021-9673
• DOI: 10.1016/j.chroma.2019.06.025

•Enantioselective retention of dipeptides on Chirobiotics T and R was studied.•Mobile phase pH controls retention via the ionization of solutes.•Different ionic forms of dipeptides adsorb differently.•Adsorption thermodynamics was analyzed via enthalpy-entropy compensation effect.•Differences in enantioseparation mechanisms on Chirobiotics T and R are discussed. Chromatographic behaviors of dipeptides consisting of leucine and glycine were studied on two antibiotic-based chiral stationary phases (CSPs) with teicoplanin (Chirobiotic T) or ristocetin A (Chirobiotic R) as chiral selectors under reversed-phase conditions. The effect of mobile phase pH on the retention of stereoisomers of dipeptides was investigated and thermodynamic characteristic of adsorption were measured at different pH values. It was shown that the retention of dipeptides depends on the ionization of their molecules in the mobile phase, as different ionic forms have different affinity towards antibiotic selectors. Enantioselectivity of the bound antibiotics with respect to Leu-Leu stereoisomers was achieved via steric modulation of ion-ion interactions between the solute and the selector, while in the case of Gly-Leu enantiomers non-ionic interactions such as hydrogen bonding might play the key role. In both cases, the dipeptides terminating in D-Leu were retained stronger than their optical antipodes, whereas the enantiomers of Leu-Gly were hardly separated. The regression analysis of the retention data applying the Horvath-Melander-Molnar model revealed that different types of enantioselectivity resides in particular ionic forms of the compounds: cations are responsible for the separation of diastereomeric pairs and the anionic and zwitterionic forms have a universal enantioselectivity on the Chirobiotic T CSP, and the anions and zwitterions are the enantioselective forms for the Chirobiotic R CSP.