ristocetin and leucylleucine

The dynamics of adsorption of the Leu-Leu stereoisomers in a chromatographic column packed with the Chirobiotic R chiral stationary phase bearing grafted antibiotic ristocetin A was studied by means of measurement and analysis of van Deemter plots. Similar measurements were carried out with weakly retained Gly-Gly for the sake of comparison. The bulk diffusion coefficients of the investigated dipeptides were also determined. It is found that the van Deemter plots of both the Leu-Leu stereoisomers and Gly-Gly have an uncommon convex-upward shape. Besides, the van Deemter B coefficients for the Leu-Leu stereoisomers, but not for Gly-Gly, have unusually high values. It is suggested that a high transcolumn contribution to eddy dispersion, which turned out to be enantioselective, accounts for these findings. Adsorption kinetics of all the dipeptides considered is relatively slow, the adsorption rate constant (k

Topics: Adsorption; Chromatography; Dipeptides; Kinetics; Ristocetin; Stereoisomerism

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.

Topics: Adsorption; Anti-Bacterial Agents; Chromatography, High Pressure Liquid; Dipeptides; Glycopeptides; Hydrogen Bonding; Hydrogen-Ion Concentration; Kinetics; Macrocyclic Compounds; Ristocetin; Stereoisomerism; Teicoplanin; Thermodynamics