Chromatographic enantioseparation and adsorption thermodynamics of hydroxy acids and their derivatives on antibiotic-bas
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ORIGINAL
Chromatographic enantioseparation and adsorption thermodynamics of hydroxy acids and their derivatives on antibiotic‑based chiral stationary phases as affected by eluent pH Otar Sh. Gogolishvili1 · Elena N. Reshetova1 Received: 23 May 2020 / Revised: 22 September 2020 / Accepted: 12 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The study was dedicated to retention regularities and adsorption thermodynamics of enantiomers of aromatic hydroxy acids and their derivatives on chiral stationary phases (CSPs) Nautilus-E, Nautilus-R and Chirobiotic T with antibiotics eremomycin, ristomycin and teicoplanin from water–ethanol eluents under conditions of linear liquid chromatography. The enantioselective properties of the chiral adsorbents under study were compared. The interrelationship between the structure of the adsorbates under study, the retention and separation selectivity of their enantiomers was evinced. Dissociation constants of hydroxy acids under study in aqueous ethanol solutions were determined. The dependences of retention, separation, and thermodynamic values’ characteristics vs. eluent pH were revealed. The retention mechanism of the hydroxy acid enantiomers was found out to vary with varying eluent pH. Ion–ion interactions were evinced to determine the retention and separation selectivity of hydroxy acid enantiomers on Nautilus-E and Nautilus-R CSPs, whereas nonionic interactions on Chirobiotic T CSP can play a key role. The enantioselectivity of Chirobiotic T CSP is first of all regulated by the anionic form of hydroxy acids. The retention and enantiorecognition mechanism on the Nautilus-E and Nautilus-R CSPs were revealed to be different. The retention mechanisms for S- and R-enantiomers were found out to be similar on both CSPs. On the Nautilus-E and Nautilus-R CSPs, a distinction between the adsorption mechanisms of the enantiomers of hydroxy acids and their esters was found out. On Nautilus-E, a distinction between the adsorption mechanisms of the enantiomers hydroxy acids with different lengths of the n-alkyl substituent in the side chain containing the stereogenic center was found out. Keywords Macrocyclic antibiotics · Enantiomers · Hydroxy acids · Enthalpy-entropy compensation · Enantioselectivity Abbreviations CSP Chiral stationary phase CS Chiral selectors HA Hydroxy acids MP Mobile phase MA Mandelic acid CC Chiral center CE Compensation effect
* Elena N. Reshetova [email protected] 1
Institute of Technical Chemistry of the Ural Branch of the Russian Academy of Sciences, 3 Academician Korolev Str., Perm 614013, Russia
Introduction The introduction of macrocyclic glycopeptide antibiotics as chiral selectors (CS) into chromatographic practice was suggested by Armstrong and his colleagues in 1994 [1, 2]. An antibiotic structure contains 3 or 4 macrocycles (which form the “aglycon basket”), 16–38 stereogenic centers, up to six sugar units and a variety of functional groups of different ionogenic basic and acidic nature [3]. These selectors
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