Influence of temperature on chiral high-performance liquid chromatographic separations of oxazepam and Prominal on chemically bonded β-cyclodextrin as stationary phase

Chiral recognition by enantioselective liquid chromatography: mechanisms and modern chiral stationary phases

An overview of the state-of-the-art in LC enantiomer separation is presented. This tutorial review is mainly focused on mechanisms of chiral recognition and enantiomer distinction of popular chiral selectors and corresponding chiral stationary phases including discussions of thermodynamics, additivity principle of binding increments, site-selective thermodynamics, extrathermodynamic approaches, methods employed for the investigation of dominating intermolecular interactions and complex structures such as spectroscopic methods (IR, NMR), X-ray diffraction and computational methods. Modern chiral stationary phases are discussed with particular focus on those that are commercially available and broadly used. It is attempted to provide the reader with vivid images of molecular recognition mechanisms of selected chiral selector-selectand pairs on basis of solid-state X-ray crystal structures and simulated computer models, respectively. Such snapshot images illustrated in this communication unfortunately cannot account for the molecular dynamics of the real world, but are supposed to be helpful for the understanding. The exploding number of papers about applications of various chiral stationary phases in numerous fields of enantiomer separations is not covered systematically.

High-performance liquid chromatographic enantioseparation of α-substituted glycine analogs on a quinine-based anion-exchanger chiral stationary phase under variable temperature conditions

The retention of enantiomers of chiral analytes, i.e. alpha-substituted glycine analogs, on a quinine-based anion-exchanger chiral stationary phase was studied in the temperature range of 5-70 degrees C and at different mobile phase compositions, using isocratic elution in the reversed-phase mode. By variation of both mobile phase composition and temperature, baseline separations could be achieved for these enantiomers. Separation could be optimized more quickly by adjusting the column temperature rather than the mobile phase composition. The dependence of the natural logarithms of retention and selectivity factors (lnk' and lnalpha) on the inverse of temperature, 1/T (van't Hoff plots) was used to determine thermodynamic data on the enantiomers. Calculated thermodynamic constants (Delta(DeltaH degrees ), Delta(DeltaS degrees ) and Delta(DeltaG degrees )) were applied to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system. The elution sequence of the enantiomers in most cases was determined.

Dynamic HPLC on chiral stationary phases: A powerful tool for the investigation of stereomutation processes

Dynamic HPLC on enantioselective stationary phases has become a well-established technique to investigate chiral molecules with internal motions that result in stereoinversion and occur on the time scale of the separation process. Kinetic parameters for the on-column interconversion phenomena can be extracted from experimental peak profiles by computer simulation or by direct calculation methods. The technique has been used in a wide range of temperatures and is complementary in scope to dynamic NMR spectroscopy.

Enantioseparation of chiralN-imidazole derivatives by electrokinetic chromatography using highly sulfated cyclodextrins: Mechanism of enantioselective recognition

Baseline separation of ten new substituted [1-(imidazo-1-yl)-1-phenylmethyl)] benzothiazolinone and benzoxazolinone derivatives, with one chiral center, was achieved by CD-EKC using highly sulfated CDs (alpha, beta, gamma highly S-CDs) as chiral selectors. The influence of the type and concentration of the chiral selectors on the enantioseparations was investigated. The highly S-CDs exhibit a very high enantioselectivity power since they allow excellent enantiomeric resolutions compared to those obtained with the neutral CDs. The enantiomers were resolved with analysis times inferior to 2.5 min and resolution factors R(s) of 3.73, 3.90, 1.40, and 4.35 for compounds 1, 2, 3, and 5, respectively, using 25 mM phosphate buffer at pH 2.5 containing either highly S-alpha-CD, highly S-beta-CD, and highly S-gamma-CD (3 or 4% w/v) at 298 K, with an applied field of 0.30 kV/cm. The determination of the enantiomer migration order for the various analytes and the study of the analyte structure-enantioseparation relationships display the high contribution of the interactions between the analytes phenyl ring and the CDs to the enantiorecognition process. The thermodynamic study of the analyte-CD affinities permits us to improve our knowledge about the enantioseparation mechanism.

Enantioselective LC analysis of synephrine in natural products on a protein-based chiral stationary phase

An enantioselective LC method with photodiode array detection (PAD) was developed for the enantioseparation of (+/-)-synephrine from C. aurantium L. var. amara fruits and phytotherapic derivatives by using a protein-based chiral stationary phase with cellobiohydrolase as the chiral selector (Chiral-CBH). Analyses were carried out on a Chiral-CBH column (100x4.0 mm i.d., 5 microm), with a mobile phase consisting of 2-propanol (5%, w/w) in sodium phosphate buffer (pH 6.0; 10 mM) and disodium EDTA (50 microM). The flow rate was 0.8 mL/min. Detection was set at 225 nm. To identify the order of elution, the racemate was resolved by the preparation of suitable diastereoisomeric salts with antipodes of appropriate organic acids. Isolation of synephrine from C. aurantium fruits and phytoproducts was performed by solid-phase extraction (SPE) with a strong cation-exchange phase. The method developed was validated and was found to be linear in the 0.40-40.14 microg/mL range (r2=1.000, P<0.0001) for both synephrine enantiomers. The limit of detection (LOD) for each enantiomer was 0.04 microg/mL. The limit of quantification (LOQ) for each enantiomer was 0.13 microg/mL. Intra-day precision (calculated as %R.S.D.) ranged from 0.03 to 0.24% for (-)-synephrine and from 0.03 to 0.35% for (+)-synephrine. Inter-day precision (calculated as %R.S.D.) ranged from 0.07 to 1.45% for (-)-synephrine and from 0.06 to 1.26% for (+)-synephrine. Intra- and inter-day accuracies (calculated as %recovery) were in the ranges of 97.4-100.6 and 98.0-101.6% for (-)-synephrine, and in the ranges 97.0-101.5 and 98.1-102.8% for (+)-synephrine. The results of the application of the method to the analysis of C. aurantium samples showed that (-)-synephrine was the main component. (+)-Synephrine was not detected in C. aurantium fruits and was present in low concentration in the phytoproducts.

Stereolabile chiral compounds: analysis by dynamic chromatography and stopped-flow methods

Enantiomerization and diastereomerization reactions of chiral compounds play a major role in all aspects of chemistry spanning a wide bridge from drug development to supramolecular chemistry. Traditionally, these reactions are studied by variable-temperature NMR spectroscopy and chiroptical methods such as polarimetry. However, powerful complimentary methods based on chromatography and electrophoresis have been developed and applied to a variety of stereolabile chiral compounds. This tutorial review explains the principles, applications, and limitations of dynamic chromatography and chromatographic and electrophoretic stopped-flow analysis for the investigation of isomerization reactions of chiral compounds.

Study of the mechanism of enantioseparation Part VI: Thermodynamic study of HPLC separation of some enantiomers of phenylcarbamic acid derivatives on a (S,S) Whelk-O 1 column

The enantiomers of thirteen 2-, 3-, and 4-alkoxyphenylcarbamic acid 2-methoxy-1-[(4-methylpiperazino)methyl]ethyl ester were separated on a (S,S) Whelk-O 1 CSP column isothermally in the range of 0-50 degrees C at 10 degrees C increments, using methanol/ water (90/10, v/v) containing 17.5 mmol L(-1) acetic acid and 14.36 mmol L(-1) triethylamine as a mobile phase. The dependence of the natural logarithms of retention and selectivity factors (In k, In alpha, respectively) on the inverse of temperature, 1/T, was used to determine thermodynamic data of enantiomers of alkoxysubstituted phenylcarbamic acid 2-methoxy-1-[(4-methylpiperazino) methyl]ethyl esters. Enthalpyentropy compensation plots showed that all of the compounds in this study separate via the same enthalpy-driven chiral recognition mechanism.

Simultaneous enantioselective separation of azelastine and three of its metabolites for the investigation of the enantiomeric metabolism in rats. I. Liquid chromatography-ionspray tandem mass spectrometry and electrokinetic capillary chromatography

Enantioselective separation methods and the enantioselective determination of the anti-allergic drug azelastine and of three of its main phase I metabolites in a biological matrix underwent chromatographic and electrophoretic investigations. An enantioselective assay of a coupling of HPLC using a beta-cyclodextrin chiral stationary phase to ionspray tandem mass spectrometry is presented. Additionally, this assay is compared to another enantioselective assay using electrokinetic capillary chromatography with beta-cyclodextrin and carboxymethyl-beta-cyclodextrin in polyacrylamide-coated capillaries. For capillary electrophoresis (CE) the importance of polyacrylamide coating for the validation of this separation method is highlighted. Extracted rat plasma samples of enantioselective metabolism studies were measured by both validated assays. Differences in the pharmacokinetics and pharmacodynamics were evaluated for the main substance azelastine and its main metabolite demethylazelastine. So, a first hint about the enantioselectivity of biotransformation of azelastine in rats was seen after oral application of either enantiomer or the racemate to rats.

Determination of the interconversion energy barrier of enantiomers by separation methods

Separation methods have become versatile tools for the determination of kinetic activation parameters and energy barriers to interconversion of isomers and enantiomers in the last 20 years. New computer-aided evaluation systems allow the on-line determination of these data after separating minute amount of pure compounds or mixture of isomers or enantiomers, respectively. Both dynamic interconversion during the separation process as well as static stopped-flow techniques have been applied to determine the kinetic activation parameters and interconversion energy barriers by separation methods. The use of (1) combinations of batchwise kinetic studies with enantioselective separations, (2) a continuous flow model, (3) a comparison of real chromatograms with simulated ones, (4) stopped-flow techniques, (5) stochastic methods, (6) approximation functions and (7) deconvolution methods, for the determination of interconversion energy barriers by separation methods is summarized in detail.

Direct separation and quantitative determination of clenbuterol enantiomers by high performance liquid chromatography using an amide type chiral stationary phase

Enantiomers of clenbuterol were directly separated by a new high performance chromatographic method on Chirex 3005 column. Several parameters such as mobile phase composition, column temperature and flow rate were studied. Baseline enantioseparation was achieved, using the optimized mobile phase of n-hexane-1,2-dicholoethane-methanol (54:38:8, v/v/v) at 17 degrees C and 1.0 ml/min, with the separation factor (alpha) 1.43 and the resolution factor (R(S)) 1.81. The mechanism of separation was also discussed. Standard linear calibration cures were established for the R- and S-enantiomers, over the range of 26.1-1,045.8 and 5.7-229.6 nmol/ml, with the correlation coefficient of 0.9999 for both. The limits of detection were 0.47 and 1.04 nmol/ml for R- and S-enantiomers, respectively. Recovery and precision of the method were also evaluated, which had been successfully used to monitor and identify quantitatively the profile of the clenbuterol enantiomers in human serum.

Direct calculation and computer simulation of the enantiomerization barrier of oxazepam in dynamic HPLC experiments--a comparative study

Dynamic chromatographic methods constitute a versatile approach to the rapid and precise determination of enantiomerization barriers of stereolabile drugs. In the present study enantioselective dynamic high-performance liquid chromatography (DHPLC) was employed to determine the enantiomerization barrier of oxazepam. Dynamic elution profiles, exhibiting plateau formation and/or peak broadening between 20 and 60 degrees C at pH 2.6 and pH 8 were obtained in the presence of the chiral stationary phase (CSP) Nucleodex-beta-PM (permethylated beta-cyclodextrin chemically bonded to silica) using a 6:4 mixture of phosphate buffer and methanol as mobile phase. Evaluation of the experimental chromatograms was performed by the novel approximation function (AF) (without computer simulation), and by the stochastic model implemented in the ChromWin simulation software (with computer simulation) furnishing the respective apparent forward rate constants, k(1)(app)(T). From the rate constants, k(1)(app)(T), measured at variable temperatures, the kinetic Eyring activation parameters, deltaG(T)(#), deltaH(#) and deltaS(#), of the enantiomerization of oxazepam were obtained. By variation of the flow rate of the mobile phase, the expected independence of the enantiomerization barrier from the chromatographic time scale was demonstrated for the first time.

Enantioseparation of various amino acid derivatives on a quinine based chiral anion-exchange selector at variable temperature conditions. Influence of structural parameters of the analytes on the apparent retention and enantioseparation characteristics

The influence of temperature on the performance of an enantioselective anion-exchange type chiral selector (SO) was systematically investigated. The resolution of the enantiomers of 23 N-acylated amino acids (selectands, SAs) on a covalently immobilized quinine tert.-butylcarbamate chiral stationary phase (CSP) was studied under linear chromatographic conditions over a temperature range of 0-85 degrees C with hydro-organic buffers (pHa 6.0) as mobile phases. The apparent enantioseparation factors increased considerably at low column temperatures, indicating that enthalpic contributions are the dominating thermodynamic driving force for chiral recognition for all investigated SAs. Retention factors gave non-linear van't Hoff plots, while the corresponding apparent enantioseparation factors showed linear van't Hoff behavior. Correlations between magnitude and sign of the relative thermodynamic parameters of enantioselective adsorption (deltadeltaG, deltadeltaH and deltadeltaS) and specific structural features of the analytes, i.e., steric and electronic nature of the various side chains and the N-acyl groups, are discussed with the aim to rationalize their possible contributions to the overall chiral recognition.

Determination of enantiomerization barriers by dynamic and stopped-flow chromatographic methods

In recent years, dynamic chromatography and stopped-flow chromatographic techniques have become versatile tools for the determination of enantiomerization and isomerization barriers. Increasing demands for the stereochemical safety of chiral drugs contributed to the rapid development of new techniques. New computer-aided evaluation systems allow the on-line determination of interconversion barriers from the experimental chromatograms. Both dynamic chromatography and stopped-flow chromatography have been applied to the entire range of chromatographic methods (GC, SFC, HPLC, CE).

Unusual effects of separation conditions on chiral separations

Unusual effects in liquid chromatographic separations of enantiomers on chiral stationary phases are reviewed with emphasis on polysaccharide phases. On protein phases and Pirkle phases reversal of the elution order between enantiomers due to variation of temperature and mobile phase composition has been reported. Most of the nonanticipated observations have dealt with the widely used polysaccharide phases. Reversed retention order and other stereoselective effects have been observed by variation of temperature, organic modifier and water content in nonpolar organic mobile phases.

Separation of drugs by packed-column supercritical fluid chromatography

Packed-column supercritical fluid chromatography (pSFC) has been expected to analyze various kinds of compounds. Many researchers have expected a new chromatographic technique that overcomes the limitations of other techniques, HPLC and GC. In pharmaceutical development, chromatography plays an important role in the evaluation of safety and efficacy of a new compound. This article provides an overview of the separation of drugs by pSFC. The effects of the chromatographic parameters were studied for the separation of steroids. In chiral separation, the successful results were shown and compared with HPLC.

Temperature-induced inversion of elution order in the enantioseparation of sotalol on a cellobiohydrolase I-based stationary phase

The effect of temperature on the resolution of (RS)-sotalol by immobilized cellobiohydrolase I (CBH I) was studied between 5 and 40 degrees C and Van 't Hoff plots of ln k versus 1/T were acquired at different pH values of the aqueous mobile phase and in the presence of varying organic cosolvents. The elution order of the enantiomers reverses in the range between 17 and 28 degrees C. Beyond this range, enantioseparations with comparatively high resolution factors are achieved either by decreasing or by increasing the temperature. The composition of the mobile phase influences the "crossover" temperature as well as the character of the global adsorption process of the (R)-(-)-enantiomer. Under certain conditions, (R)-(-)-sotalol exhibits an unusual endothermic adsorption behavior. Its retention time increases with increasing temperature. At room temperature (23 degrees C) the enantiomeric elution order can also be regulated by the solvent additive.

Direct enantiomeric separations by high performance liquid chromatography using cyclodextrins

Due to their identical chemical and physical properties, the separation of enantiomers has been considered one of the most difficult challenges in chemistry from both an analytical or a preparative viewpoint. With the development and commercialization of many new or improved chiral stationary phases and chiral additives, interest in enantiomeric separation by HPLC has grown tremendously in the last two decades. Cyclodextrins and modified cyclodextrins are widely used chiral selectors. They are used as either the chiral stationary phases, as chiral mobile phase additives, or as chiral counter-ions. This review describes the historical development of derivatized and underivatized cyclodextrins in HPLC and their various applications.

Effect of temperature on separation of estradiol stereoisomers and equilin by liquid chromatography using mobile phases modified with beta-cyclodextrin

The unusual temperature effect on the retention of 17 alpha-estradiol, 17 beta-estradiol and equilin in reversed-phase liquid chromatography has been observed. Capacity factors (k') of the steroids were measured using mobile phase with different concentrations of beta-cyclodextrin (from 0 to 16 mM) in a fixed mobile phase composition (acetonitrile-water) and wide range of column temperatures (from 5 degrees to 80 degrees C). The plots of capacity factors vs reciprocal of absolute temperature are nonlinear in each case. At subambient temperatures the capacity factors decreased with temperature decrease. This effect is more evident for the natural 17 beta-estradiol than for its 17 alpha-isomer.