Retention mechanisms of dipeptides on superficially porous particle vancomycin- and teicoplanin-based chiral stationary phases

Chromatographic behavior of new chiral stationary phases (CSPs) Chiral-T and Chiral-V with teicoplanin and vancomycin antibiotics grafted onto superficially porous silica particles was studied in relation to dipeptide (DP) stereoisomers. The unbuffered water-methanol solutions were used as mobile phases (MPs). The effects of physical properties and molecular structure of analytes and selectors on retention and separation of DP stereoisomers are discussed herein. Chiral-T was evinced to exhibit high enantioselectivity, with highest α values attaining 16.5, 18.8 and 20.4 for Gly-Leu, dd/ll-Phe-Leu and ld/dl-Ala-Ala. At this point, Chiral-V did not exhibit enantioselectivity towards DP stereoisomers. The effect of MP composition on retention and enantioseparation of DPs was investigated. Lipophilicity of DPs was found to be an essential factor in the dependence of their retention vs. methanol concentration in МPs. Lipophobic DPs were eluted more quickly by water-rich solvents, with lipophilic DPs exhibiting an asymmetric U-shaped, or a descending dependence of retention factor vs. the methanol percentage on Chiral-T or Chiral-V, respectively. A theoretical model taking into account interaction of both solvents of a binary MP with both an analyte and adsorption sites was successfully applied so as to approximate and interpret the dependences of DP retention (monotonic and U-shaped) vs. a modifier content in MP. Water molecules were evinced to predominantly participate in competitive adsorption with DP molecules. The model predicted better solvation of lipophilic DPs by methanol and better solvation of lipophobic DPs by water. An attempt was made to verify the possibility of modeling by molecular docking the processes occurring during interaction between DP stereoisomers and CSPs, including consideration of the influence of competitive binding of eluent molecules in selector cavity.

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Chiral separation and molecular modeling study of decursinol and its derivatives using polysaccharide-based chiral stationary phases

Plant-based bioactive substances have long been used to treat inflammatory ailments, owing to their low toxicity and cost-effectiveness. To enhance plant treatment by eliminating undesirable isomers, optimizing the chiral separation techniques in pharmaceutical and clinical studies is important. This study reported a simple and effective method for chiral separation of decursinol and its derivatives, which are pyranocoumarin compounds with anti-cancer and anti-inflammatory properties. Baseline separation (Rs >1.5) was achieved using five different polysaccharide-based chiral stationary phases (CSPs) that differed in chiral origin, chiral selector chemistry, and preparation technique. To separate all six enantiomers simultaneously, n-hexane and three alcohol modifiers (ethanol, isopropanol, and n-butanol) were used as mobile phases in the normal-phase mode. The chiral separation ability of each column with various mobile phase compositions was compared and discussed. As a result, amylose-based CSPs with linear alcohol modifiers demonstrated superior resolution. Three cases of elution order reversal caused by modifications of CSPs and alcohol modifiers were observed and thoroughly analyzed. To elucidate the chiral recognition mechanism and enantiomeric elution order (EEO) reversal phenomenon, detailed molecular docking simulations were conducted. The R- and S-enantiomers of decursinol, epoxide, and CGK012 exhibited binding energies of -6.6, -6.3, -6.2, -6.3, -7.3, and -7.5 kcal/mol, respectively. The magnitude of the difference in binding energies was consistent with the elution order and enantioselectivity (α) of the analytes. The molecular simulation results demonstrated that hydrogen bonds, π-π interactions, and hydrophobic interactions have a significant impact on chiral recognition mechanisms. Overall, this study presented a novel and logical approach of optimizing chiral separation techniques in the pharmaceutical and clinical industries. Our findings could be further applied for screening and optimizing enantiomeric separation.

Chiral separation of racemic closantel and ultratrace detection of its enantiomers in bacteria by enhanced liquid chromatography-tandem mass spectrometry combined with postcolumn infusion of ammonia

Reliable analysis of ultratrace antibiotics in bacterial cells may become a new means to elucidate the antibacterial mechanism, drug resistance and environmental fate. In this work, an ultrahigh-sensitive, accurate and enhanced liquid chromatography-tandem mass spectrometric method was first developed for chiral separation and detection of racemic closantel, as an antibacterial adjuvant. Optimizing acetonitrile-water-formic acid system that is compatible with mass spectrometry as a mobile phase, the baseline separation of two enantiomers was achieved by using EnantioPak® Y1-R chiral column, and the resolution of the two analytes was more than 1.95. Further adopt the strategy of postcolumn infusion of ammonia, the mobile phase pH was reversed from acidic condition suitable for the optimal chromatographic separation of R- and S-closantel to alkaline, so that closantel could realize efficient electrospray ionization under the preferred negative ion mode. The bacterial cells were subjected to be frozen-cracked, and the analytes were extracted with acetonitrile after clipping the pointed bottom of the Eppendorf tube into a new tube. The method was linear over concentration ranges of 0.5-50 pg/mL (r²≥0.99) for R- and S-closantel. The detection limits of target analytes were all 0.15 pg/mL in bacterial cells. The average recoveries of two enantiomers ranged from 81.2% to 107.8% with relative standard deviations below 15%. The method proposed might be important support for the deep research of the stereoselectivity of biological activity, toxicity and metabolism of closantel enantiomers.

Study of Different Chiral Columns for the Enantiomeric Separation of Azoles Using Supercritical Fluid Chromatography

The enantiomeric separation of antifungal compounds is an arduous task in pharmaceutical and biomedical fields due to the different properties that each diastereoisomer presents. The enantioseparation of a group of fungicides (sulconazole, bifonazole, triadimefon and triadimenol) using supercritical fluid chromatography was achieved in this work. For this goal, four different chiral columns based on polysaccharide derivatives, as well as the effect of different chromatographic parameters such as temperature, type and percentage of organic modifier (methanol, ethanol and isopropanol), were thoroughly investigated. The inversion of the elution order of enantiomers as a result of a change in the stationary phase or organic modifier was also evaluated by employing a circular dichroism detector. The best separation conditions, in terms of the enantioresolution and analysis time, were obtained with the Lux® Cellulose-2 column using isopropanol as the organic modifier.

Development of a novel HPLC-ESI-MS/MS method to analyze the stereoselective pharmacokinetics and tissue distribution of isoconazole enantiomers in rats

Isoconazole with an asymmetrical carbon is a broad-spectrum antimicrobial imidazole, but there is still lack of relevant report about the potential enantioselectivity in biological samples. The object of this research was to develop and validate a sensitive and effective high performance liquid chromatography-electrospray ionization coupled with tandem mass spectrometry (HPLC-ESI-MS/MS) method for stereoselective separation and determination of isoconazole enantiomers in Sprague-Dawley (SD) rat plasma and tissues. The greater enantioseparation of isoconazole enantiomers was obtained on a Chiralpak IC column with a mobile phase consisted of acetonitrile-10 mM aqueous ammonium acetate (90:10, v/v) under the reversed-phase mode. Subsequently, the studied compounds and internal standard (IS) were detected on a multiple reaction monitoring (MRM) mode with positive electrospray ionization source. The experimental and theoretical Electronic Circular Dichroism (ECD) spectra were employed to confirm the absolute configuration of isoconazole enantiomers. Eventually, after full method validation, the newly developed method was successfully applied to the study of enantioselectivity in plasma and tissues in SD rats. Results illustrated that the enantioselective differences in plasma were observed for the evidence that the concentrations of S-(-)-isoconazole were always higher than R-(+)-isomer. In terms of tissue distribution, liver, kidney, lung, spleen, and small intestine were the mainly distributed tissues and then followed by heart and muscle. This is the first study to reveal the stereoselective behavior of isoconazole enantiomers in vivo, which also provides reliable and valuable reference for further elucidating the enantioselective metabolisms of isoconazole enantiomers.

An overview of chiral separations of pharmaceutically active substances by HPLC (2018-2020)

This review provides a brief survey of chiral separation of pharmaceutically active substances published over the last 3 years (2018-2020). Chiral separation of drugs is an important area of research. The control of enantiomeric purity and determination of individual enantiomeric drug molecules is a necessity especially for clinical, analytical, and regulatory purposes. Among chromatographic resolution methods, high-performance liquid chromatography based on chiral stationary phases remains the most popular and effective method used for chiral separation of various drugs. In this review, attention is paid to several classes of chiral stationary phases that have been the most frequently used for drug enantioseparation during this period.

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