A closer study of chiral retention mechanisms

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.

Correctness of the van 't Hoff analysis for homogeneous or heterogeneous retention

It is already known that the physical meaning of the numerical values of the calculated enthalpy (ΔH) and entropy (ΔS) change in chromatography via the van 't Hoff plot analysis is rather questionable. In the former work of these authors, it has been demonstrated that by serially coupling two reversed-phase columns of the same inner diameter but of different retention mechanism, the obtained thermodynamic values are not related to the numerical results obtained on the individual columns respectively. Since surface heterogeneity of the stationary phase is intrinsically present in chiral chromatography (enantioselective and non-selective sites), the calculation of ΔH and ΔS should be revisited in that field. In this study, more details of the pressure dependence were investigated. Using special POPLC columns, the effect of the column length on the calculated thermodynamic values was investigated. The calculated values differ by 30-400% when the column length is increased from 4 cm to 12 cm. The dependence of the calculated results on the applied flow rate was already highlighted earlier, and here the emphasis was put on the instrument on which the separation was performed. It turns out that there is a difference when using a Shimadzu HPLC system compared to using a Waters Acquity UPLC system. Because the Eyring method serves as an alternative route to calculate the thermodynamic values from chromatographic data, the differences obtained by the two methods were also investigated.

Heterogenous adsorption mechanisms for describing enantioselective retention in normal-phase liquid chromatography

In this study, the enantioselective retention behaviors of methyl mandelate (MM) and benzoin (B) were investigated using Chiralpak IB as a sorbent and ethanol, 1-propanol, and 1-butanol as solvent modifiers in the normal-phase mode. For both MM and B, similar chiral recognition mechanisms were observed, potentially involving at least two types of chiral adsorption sites. With a retention model describing local retention behaviors, an enantioselectivity model based on a three-site model was proposed to describe the data. Fitted parameters were also used to analyze the contributions of each type of adsorption site to the apparent retention behavior. Combining the local retention model with the three-site model provided a qualitative and quantitative explanation for the correlation between modifier concentration and enantioselectivity. Overall, our results indicated that heterogeneous adsorption mechanisms are a key aspect in understanding enantioselective retention behaviors. Distinct local adsorption sites contribute differently to apparent retention behaviors, with these contributions being influenced by the mobile phase composition to varying degrees. Hence, enantioselectivity changes with variations in modifier concentration.

Retention mechanisms of rasagiline and its analogues on superficially porous particle vancomycin- and teicoplanin-based chiral stationary phases

Retention and separation of enantiomers of amine derivatives of indane and tetralin (rasagiline and its analogues) on chiral stationary phases (CSPs) Chiral-T and Chiral-V with teicoplanin and vancomycin antibiotics grafted onto superficially porous silica particles under conditions of reversed-phase and polar organic chromatography were studied. The mobile phases (MP) were water-methanol and acetonitrile-methanol solvents modified with triethylamine-acetic acid buffer. The effects of molecular structure and physical properties of the analytes on enantioselective retention are discussed. The retention mechanism is hypothesized to involve the ion-ion attraction between the positively charged amino group of an analyte and the carboxylate anion of either antibiotic. The binding occurs outside of the antibiotic's aglycon basket that accounts for relatively low enantioselectivity observed. The presence of a large substitute at the analyte's amino group complicates enantiorecognition. The effect of the MP solvent composition on retention and enantioseparation was investigated. It is a complex phenomenon combined of different oppositely directed influences that resulted in different shapes, increasing, decreasing, or U-shaped, of the retention factor vs. composition dependences. A model taking into account the interaction of both solvents of a binary MP with both an analyte and an adsorption site was successfully applied to approximate a majority of the studied systems. Pros and cons of the model are discussed.

Boosting the enantioresolution of zwitterionic-teicoplanin chiral stationary phases by moving to wide-pore core-shell particles

A novel zwitterionic-teicoplanin chiral stationary phase (CSP), based on superficially porous particles (SPPs) of 2.7 µm particle diameter and 160 Å pore size, has been prepared and evaluated towards the enantioseparation of important classes of compounds, including chiral drugs, pesticides, and N-derivatized amino acids. The comparison with two analogous CSPs prepared on SPPs with 2.7 and 2.0 µm particle diameter and 90 Å pore size has revealed that the use of large-pore particles allows to dramatically improve both the enantioselectivity and the resolution-per-analysis-time, at the point that the column prepared with the new CSP outperformed the one packed with the finest particles. On the novel wide-pore CSP, the separation of fifteen racemates of pratical importance was significantly improved in terms of both enantioselectivity and resolution-per-analysis time-compared to the CSPs based on SPPs with smaller pores (90 Å). Such a CSP would be suitable for very fast enantioseparations allowing the saving of solvent for greener high-efficiency/high-throughput applications.

Enantioselective adsorption dynamics of leucyl-leucine in a Chirobiotic R column

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_ads) being of order of magnitude 20-60 s^-1. k_ads does not depend on the configuration of Leu-Leu stereoisomers, although their affinity toward the chiral selector depends on this factor. This supports the above hypothesis that eddy dispersion is mainly responsible for the observed peculiarities in the dynamic behavior of dipeptides, and adsorption kinetics has secondary importance in this phenomenon.

Enantioselective retention mechanisms of dipeptides on antibiotic-based chiral stationary phases. II. Effect of the methanol content in the mobile phase

Effect of mobile phase (water-methanol) composition on the enantioseparation of dipeptides on the chiral stationary phase Chirobiotic R was investigated using Ala-Ala, Leu-Leu, Gly-Leu, and Leu-Gly as case studies. The lipophilicity of dipeptides was found to be an essential factor in the dependence of their retention on the methanol percentage, the retention factor of lipophobic dipeptides increasing monotonously and that of lipophilic dipeptides changing according to an asymmetric U-shaped trajectory as methanol concentration increases. The behavior of enantioselectivity as a function of the methanol content also depends on the lipophilicity of dipeptide. For lipophilic Leu-Leu, the dependence has a dome-like shape, and for more lipophobic dipeptides, Ala-Ala and Gly-Leu, it is an increasing function of the methanol concentration. The importance of solvation equilibria in the bulk liquid and on the surface of the stationary phase for the total retention is discussed from the thermodynamic point of view. Special consideration is given to the adsorption of the water-methanol mixture on the surface of the Chirobiotic R stationary phase.

Chiral high-performance liquid and supercritical fluid chromatographic enantioseparations of limonene-based bicyclic aminoalcohols and aminodiols on polysaccharide-based chiral stationary phases

Enantioseparation of limonene-based bicyclic 1,3-aminoalcohols and 1,3,5- and 1,3,6-aminodiols was performed by normal-phase high-performance liquid chromatographic and supercritical fluid chromatographic (SFC) methods on polysaccharide-based chiral stationary phases. The effects of the composition of the mobile phase, the column temperature and the structures of the analytes and chiral selectors on retention and selectivity were investigated by normal-phase LC and SFC technique. Thermodynamic parameters derived from selectivity-temperature-dependence studies were found to be dependent on the chromatographic method applied, the nature of the chiral selector and the structural details of the analytes. Enantiorecognition in most cases was enthalpically driven but an unusual temperature behavior was also observed: decreased retention times were accompanied by improved separation factors with increasing temperature, i.e. some entropically driven separations were also observed. The elution sequence was determined in all cases. The separation of the stereoisomers was optimized in both chromatographic modalities.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.

Liquid chromatographic enantioseparation of carbocyclic β-amino acids possessing limonene skeleton on macrocyclic glycopeptide-based chiral stationary phases

Polar-ionic and reversed-phase high-performance liquid chromatographic separations of limonene-based cyclic β-amino acid enantiomers were carried out by using macrocyclic glycopeptide-based chiral selectors applying Chirobiotic T, TAG and R columns. The effects of additives, concentration of the co- and counter-ions and the temperature in polar-ionic mobile phase systems were studied. The influence of pH, MeOH content and alcohol additives were investigated in the reversed-phase mode. The difference in the change in standard enthalpy Δ(ΔH°), entropy Δ(ΔS°), and free energy Δ(ΔG°) was calculated from the linear van't Hoff plots derived from the ln α vs 1/T curves in the temperature range 5-40°C. Unusual temperature behavior was observed on Chirobiotic TAG for most of the analytes: decreased retention times were accompanied with increased separation factors with increasing temperature, and separation was entropically-driven. For two of the studied analytes enthalpically-driven enantioseparations were observed. The elution sequence was determined in all cases, but no general rule could be established.

A Comparative Study of Enantioseparations of Nα-Fmoc Proteinogenic Amino Acids on Quinine-Based Zwitterionic and Anion Exchanger-Type Chiral Stationary Phases under Hydro-Organic Liquid and Subcritical Fluid Chromatographic Conditions

The focus of this contribution is a comparative investigation of enantioseparations of 19 N^α-Fmoc proteinogenic amino acids on Quinine-based zwitterionic and anion-exchanger type chiral stationary phases employing hydro-organic and polar-ionic liquid and subcritical fluid chromatographic conditions. Effects of mobile phase composition (including additives, e.g., water, basis and acids) and nature of chiral selectors on the chromatographic performances were studied at different chromatographic modes. Thermodynamic parameters of the temperature dependent enantioseparation results were calculated in the temperature range 5–50 °C applying plots of lnα versus 1/T. The differences in standard enthalpy and standard entropy for a given pair of enantiomers were calculated and served as a basis for comparisons. Elution sequence in all cases was determined, where a general rule could be observed, both in liquid and subcritical fluid chromatographic mode the d-enantiomers eluted before the L ones. In both modes, the principles of ion exchange chromatography apply.

High-performance liquid chromatographic separation of paclitaxel intermediate phenylisoserine derivatives on macrocyclic glycopeptide and cyclofructan-based chiral stationary phases

High-performance liquid chromatographic methods were developed for the separation of enantiomers of four unnatural paclitaxel precursor phenylisoserine analogs on chiral stationary phases containing macrocyclic glycopeptides and cyclofructans as chiral selectors. The effects of the mobile phase composition, the nature and concentration of different mobile phase additives (alcohols, amines and acids) in different chromatographic modes, temperature and the structures of the analytes on the separations were investigated. Separations were carried out at constant mobile phase compositions in the temperature range 10-50°C on macrocyclic antibiotic-based and 5-35°C on cyclofructan-based columns and the changes in enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated. The elution sequence was determined in most cases; no general rule could be observed.

High-performance liquid chromatographic enantioseparation of Betti base analogs on a newly developed isopropyl carbamate-cyclofructan6-based chiral stationary phase

The direct separation of the enantiomers of 1-(α-aminoarylmethyl)-2-naphthol, 1-(α-aminoalkyl)-2-naphthol, 2-(α-aminoarylmethyl)-1-naphthol analogs, and 2-(1-amino-2-methylpropyl)-1-naphthol) was performed on a newly developed chiral stationary phase containing isopropyl carbamate-cyclofructan6 as chiral selector, with n-heptane/alcohol/trifluoroacetic acid as mobile phase. The effects of the mobile-phase composition, the nature and concentration of the alcoholic and acidic modifiers, and the structures of the analytes on the retention and resolution were investigated. In some cases, separations were carried out at constant mobile-phase compositions in the temperature range 5-40°C. Thermodynamic parameters and T(iso) values were calculated from plots of ln k' or ln α versus 1/T. -Δ(ΔH°) ranged from 2.8 to 3.2 kJ mol(-1) , -Δ(ΔS°) from 7.7 to 10.1 J mol(-1) K(-1) , and -Δ(ΔG°) from 0.2 to 0.5 kJ mol(-1) . It was found that the enantioseparations were enthalpy driven. The sequence of elution of the stereoisomers determined in some cases was (R) < (S).