Experimental and theoretical study of the adsorption behavior and mass transfer kinetics of propranolol enantiomers on cellulase protein as the selector

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Review of Formation and Gas Characteristics in Shale Gas Reservoirs

An accurate understanding of formation and gas properties is crucial to the efficient development of shale gas resources. As one kind of unconventional energy, shale gas shows significant differences from conventional energy ones in terms of gas accumulation processes, pore structure characteristics, gas storage forms, physical parameters, and reservoir production modes. Traditional experimental techniques could not satisfy the need to capture the microscopic characteristics of pores and throats in shale plays. In this review, the uniqueness of shale gas reservoirs is elaborated from the perspective of: (1) geological and pore structural characteristics, (2) adsorption/desorption laws, and (3) differences in properties between the adsorbed gas and free gas. As to the first aspect, the mineral composition and organic geochemical characteristics of shale samples from the Longmaxi Formation, Sichuan Basin, China were measured and analyzed based on the experimental results. Principles of different methods to test pore size distribution in shale formations are introduced, after which the results of pore size distribution of samples from the Longmaxi shale are given. Based on the geological understanding of shale formations, three different types of shale gas and respective modeling methods are reviewed. Afterwards, the conventional adsorption models, Gibbs excess adsorption behaviors, and supercritical adsorption characteristics, as well as their applicability to engineering problems, are introduced. Finally, six methods of calculating virtual saturated vapor pressure, seven methods of giving adsorbed gas density, and 12 methods of calculating gas viscosity in different pressure and temperature conditions are collected and compared, with the recommended methods given after a comparison.

Enantiomeric Profiling of Chiral Pharmacologically Active Compounds in the Environment with the Usage of Chiral Liquid Chromatography Coupled with Tandem Mass Spectrometry

The issue of drug chirality is attracting increasing attention among the scientific community. The phenomenon of chirality has been overlooked in environmental research (environmental occurrence, fate and toxicity) despite the great impact that chiral pharmacologically active compounds (cPACs) can provoke on ecosystems. The aim of this paper is to introduce the topic of chirality and its implications in environmental contamination. Special attention has been paid to the most recent advances in chiral analysis based on liquid chromatography coupled with mass spectrometry and the most popular protein based chiral stationary phases. Several groups of cPACs of environmental relevance, such as illicit drugs, human and veterinary medicines were discussed. The increase in the number of papers published in the area of chiral environmental analysis indicates that researchers are actively pursuing new opportunities to provide better understanding of environmental impacts resulting from the enantiomerism of cPACs.

Experimental and theoretical study of anion-exchange preparative chromatography for neptunium: the first application to thorium(IV) and its equilibrium and kinetics

In order to study equilibrium and kinetic parameters in anion-exchange chromatography for preparatory purpose, a quantitative model for nonlinear anion-exchange chromatography in porous media was constructed, by paying special attention to interstitial length along void structure (cm) distinguished from apparent length (cm*). Langmuir-type adsorption isotherm for thorium(IV), as a natural substitution for neptunium(IV), in 6 mol dm(-3) nitric acid to anion-exchanger MSA-1 (200-400 mesh) was investigated in batch-wise and chromatographic experiments. The equilibrium parameters determined by batch-wise experiments determined as k=2.4x10(2) mol(-1) dm3 s(-1) and s0=0.5 mol dm(-3) agrees very well with the values of k=222 mol(-1) dm3 s(-1) and s0=0.5 mol dm(-3) derived from fitting by the numerical calculation. Kinetic parameters of ks and D affect band profile similarly, thereby maximum value of each parameter was evaluated as ks=1.3 mol(-1) dm3 s(-1) and D=9x10(-4) cm2 s(-1) by the numerical calculations.

A multiple chemical equilibria approach to modeling and interpreting the separation of amino acid enantiomers by chiral ligand-exchange chromatography

A model of chiral ligand-exchange chromatography (CLEC) is presented that combines the non-ideal equilibrium-dispersion equation for solute transport with equations describing all chemical equilibria within the column. The model connects elution band profiles to the time and space resolved formation of diastereomeric complexes in both the mobile and stationary phases, thereby providing insights into the overall separation mechanism. The stoichiometries and formation constants for all equilibrium complexes formed in the mobile phase are taken from standard thermodynamic databases and independent potentiometric titration experiments. Formation constants for complexes formed with the stationary phase ligand are determined from potentiometric titration data for a water-soluble analogue of the ligand. Together this set of pure thermodynamic parameters can be used to calculate the equilibrium composition of the system at any operating condition. The model includes a temperature-dependent pure-component parameter, determined by regression to a single elution band for the pure component, that corrects for subtle effects associated with immobilizing the ligand (i.e., the chiral selector) onto the stationary phase. Model performance is assessed through comparison with chromatograms for two hydrophobic amino acid racemates loaded on the Nucleosil Chiral-1 CLEC column. The model is also applied to a restricted optimization of column operating conditions to assess its predictive power. In both cases, model predictions compare well with experiment while also providing a molecular understanding of the separation process and its dependence on column operating conditions.

Discovery of invisible extra fronts in single-component frontal analysis in liquid chromatography

Frontal analysis (FA) is often used in the "staircase mode", in which the solute concentration in the eluent increases stepwise. We demonstrate here in the single-component case, that all eluted breakthrough curves (fronts) for the second and subsequent steps consist solely of displaced plateau molecules. The newly introduced molecules (i.e., the introduced mass) instead elute later, in a breakthrough front hidden from detection, i.e., the mass front. These effects were studied using experimentally verified numerical calculations, the mass fronts being visualized using an enantiomer pair in an achiral separation system. Notably, the mass front displays no self-sharpening effects, even under strongly nonlinear conditions. Instead, the front is sigmoidal in shape.

Theoretical investigations of the chromatographic separation of interacting enantiomers

Separation of a pair of enantiomers by liquid chromatography is modeled using the equilibrium dispersive (ED) model of chromatography. It is assumed that the chiral stationary phase used for the separation consists of two types of adsorption sites, including chiral selectors linked to the surface and nonselective centers belonging to the achiral matrix. Additionally, intermolecular interactions between adsorbed enantiomers are taken into account. The corresponding equilibrium adsorption isotherms of the enantiomers are derived by means of the mean field approximation (MFA) and used as input data for the ED model. Special attention is paid to the influence of the lateral interactions on the effectiveness of the enantiomer separation. In particular, we examine the effect of the interactions on the shape and relative position of the chromatographic peaks associated with the enantiomers. Furthermore, the influence of the spacer length, which modifies screening of the lateral interactions, on the adsorption process is studied. The obtained results suggest that the lateral interactions combined with the screening effect may cause serious changes in the separation, depending on the nature (attraction or repulsion) and strength of the interactions as well as on the spacer length.

Investigation of the adsorption behaviour of a chiral model compound on a tartardiamide-based network-polymeric chiral stationary phase

The adsorption behaviour of the enantiomers of 2-phenylbutyric acid on the chiral stationary phase (CSP) Kromasil CHI-TBB was studied using hexane/MTBE (90/10) as eluent. Adsorption isotherms were acquired at 40 different enantiomer concentrations in the interval between 7.6 microM and 305 mM, an approximately 40,000-fold dynamic range. The adsorption data fitted well to the bi-Langmuir model, indicating a heterogeneous surface with two different types of adsorption sites having different equilibrium constants and capacities; namely one chiral site and one non-chiral site. A comparison with earlier adsorption studies on modern CSPs revealed that the capacity value of the "true" chiral site of Kromasil CHI-TBB is the largest reported so far. The elution profiles simulated with these parameters show excellent agreement with the corresponding experimental profiles. Guidelines for comparisons of loading capacities of CSPs are presented.

Stereoselective antibodies to free α-hydroxy acids

This work describes antibodies exhibiting high stereoselectivity and class-specificity towards the enantiomers of free alpha-hydroxy acids. Since the antibodies interact primarily with the carboxyl-hydroxyl-hydrogen triad about the stereogenic center, they are useful for enantiomer analysis of a variety of structurally different alpha-hydroxy acids including aromatic and aliphatic compounds, e.g. lactic acid. The utility of such antibodies for enantiomer separation in chromatography was demonstrated. Comparative studies of these and previously described anti-alpha-amino acid antibodies revealed that both types of antibodies bind only to analytes that possess both the corresponding target structure and the correct configuration. Thus, substitution of an amino group for the alpha-hydroxyl group results in a complete loss of binding activity with the anti-alpha-hydroxy acid antibodies, while an alpha-amino group is essential for the interaction between analytes and anti-alpha-amino acid antibodies.

Adsorption of the enantiomers of 2,2,2-trifluoro-1-(9-anthryl)-ethanol on silica-bonded chiral quinidine-carbamate

The adsorption isotherms of the enantiomers of 2,2,2-trifluoro-1-(9-anthryl)-ethanol from a toluene-acetonitrile solution onto a Chiris Chiral AX:QD1 column were measured using the pulse method. The isotherm data were modeled with a bi-Langmuir isotherm model, indicating the presence of two different types of adsorption sites on this stationary phase, nonselective and enantioselective sites. The latter are homogeneous but interact with both enantiomers, albeit with different energies. The thermodynamic characteristics of these two types of sites were characterized by their adsorption constants and saturation capacities and by the influence of the temperature on these different parameters.

Prediction on the chiral behaviors of drugs with amine moiety on the chiral cellobiohydrolase stationary phase using a partial least square method

Quantitative Structure-Resolution Relationship (QSRR) using the Comparative Molecular Field Analysis (CoMFA) software was applied to predict the chromatographic behaviors of chiral drugs with an amine moiety on the chiral cellobiohydrolase (CBH) columns. As a result of the Quantitative CoMFA-Resolution Relationship study, using the partial least square method, prediction of the behavior of drugs with amine moiety upon chiral separation became possible from their three dimensional molecular structures. When a mixed mobile phase of 10 mM aqueous phosphate buffer (pH 7.0) - isopropanol (95:5) was employed, the best Quantitative CoMFA-Resolution Relationship, derived from the study, provided a cross-validated q2 = 0.933, a normal r2 = 0.995, while the best Quantitative CoMFA-Separation Factor Relationship, also derived from the study, yielded a cross-validated q2 = 0.939, a normal r2 = 0.991. When all of these results are considered, this QSRR-CoMFA analysis appears to be a very useful tool for the preliminary prediction on the chromatographic behaviors of drugs with an amine moiety inside chiral CBH columns.

Study of mechanisms of chiral discrimination of amino acids and their derivatives on a teicoplanin-based chiral stationary phase

The behavior of a series of amino acids and some of their methyl ester hydrochloride, N-acetyl and N-tert-butyloxycarbonyl derivatives has been investigated on a teicoplanin-based chiral stationary phase by changing the chromatographic conditions, namely, the type and amount of mobile phase organic modifier and the ionic strength of the solutions. By using species with significantly different characteristics and chemical reactivity, some general conclusions regarding the chiral recognition process on this kind of stationary phase have been formulated. The importance of the carboxylic moiety for the formation of the complex between enantiomers and the aglycone basket of teicoplanin has been demonstrated via chromatography. Additionally, the increased possibility to make an hydrogen bond between the amidic hydrogen of the acetylated compounds and an amidic group on the stationary phase has been proposed to be pivotal for the stability of the complex aglycone D-enantiomer. Phenomena leading to the exclusion from the chiral stationary phase of one or both enantiomers have been rationalized by considering the ionic interactions between stationary phase, molecules to be separated and the surrounding medium and/or steric hindrance effects. The understanding of some of the observed phenomena may be important for optimizing the performance of the separation on aglycone-based media.

Heterogeneous Adsorption of 1-Indanol on Cellulose Tribenzoate and Adsorption Energy Distribution of the Two Enantiomers

The distributions of the adsorption energies (AED) of two enantiomers, (R)-1- indanol and (S)-1-indanol, on a chiral stationary phase were measured and the results are discussed. The chiral phase used is made of cellulose tribenzoate coated on porous silica. The AEDs were determined using the expectation maximization method, a numerical method that uses directly the raw experimental isotherm data, inverts this set of data into an AED, and introduces no arbitrary information in the calculation. However, it uses the Langmuir equation as the local isotherm. The experimental data fit very well to the bi-Langmuir isotherm model for the more retained enantiomer. Our results show that the AEDs of these two enantiomers have no energy modes that would be identical (same mean energy, mode profile, and mode area), in contrast to numerous cases previously studied, e.g., that of the beta-blockers on a Cel7A column. This indicates a significantly different retention mechanism.

Kinetics and equilibrium of multicomponent adsorption on chiraly templated surfaces

In this contribution we propose a simple model of adsorption of a binary (racemic) mixture on a chiraly templated surface. As an example, the adsorption of a liquid mixture of enantiomers on a chiral stationary phase (CSP) is considered. In particular, we study the effect of the lateral interactions in the adsorbed phase on the kinetic and equilibrium isotherms of the enantiomers. Additionally, we investigate the influence of the composition of the surface on the performance of the CSP in the presence of the lateral interactions. To that end, the adsorption of the mixture is modeled by using Monte Carlo simulations as well as by applying an analytical approach involving rate equations coupled with the Mean Field Approximation (MFA). The predictions of the theory are found to be in good agreement with the results of the simulations.

Probability rule for chiral recognition

Molecular Chirality is of central interest in biological studies because enantiomeric compounds, while indistinguishable by most inanimate systems, show profoundly different properties in biochemical environments. Enantioselective separation methods, based on the differential recognition of two optical isomers by a chiral selector, have been amply documented. Also, great effort has been directed towards a theoretical understanding of the fundamental mechanisms underlying the chiral recognition process. Here we report a comprehensive data examination of enantio separation measurements for over 72000 chiral selector-select and pairs from the chiral selection compendium CHIRBASE. The distribution of alpha = k'(D)/k'(L) values was found to follow a power law, equivalent to an exponential decay for chiral differential free energies. This observation is experimentally relevant in terms of the number of different individual or combinatorial selectors that need to be screened in order to observe alpha values higher than a preset minimum. A string model for enantiorecognition (SMED) formalism is proposed to account for this observation on the basis of an extended Ogston three-point interaction model. Partially overlapping molecular interaction domains are analyzed in terms of a string complementarity model for ligand-receptor complementarity. The results suggest that chiral selection statistics may be interpreted in terms of more general concepts related to biomolecular recognition.

Modeling of preparative chromatography processes with slow intraparticle mass transport kinetics

Mathematical modeling of the preparative chromatography process accompanied with complex intraparticle mass transport mechanism involving surface diffusion is discussed. As an experimental base for the analysis two steroid compounds, methyl esters of hydroxycholanic acids (bile acids), deoxycholic and cholic acids were selected. For these compounds surface diffusion kinetics were found to have a marked influence on the band broadening. The isocratic chromatography process was performed in a normal-phase preparative system with ternary mixture of solvents containing hexane, ethyl acetate and methanol as a modifier under different operating conditions, e.g., at various mobile phase compositions and inlet concentrations. The efficiency of the system was found to be dependent on the mass of sample injected as well as on the contents of the modifier. Such a phenomenon was suggested to originate from the contribution of the surface diffusion kinetics to the overall mass transport mechanism. For identifying the general trends and concentration dependencies of the surface diffusion coefficient the simplified approach was proposed. The set of chromatographic band profiles registered at different inlet concentration and mobile phase composition were used for determining the influence of the local solid-phase concentration on the mass transport mechanism. For the simulations the transport-dispersive model was used, in which all sources of mass transport resistances were lumped in the properly adjusted mass transport coefficient. The accuracy of this model was verified by comparing its predictions to the solutions of the general rate model.

Monte Carlo model of nonlinear chromatography: correspondence between the microscopic stochastic model and the macroscopic Thomas kinetic model

The Monte Carlo model of chromatography is a description of the chromatographic process from a molecular (microscopic) point of view and it is intrinsically based on the stochastic theory of chromatography originally proposed by Giddings and Eyring. The program was previously validated at infinite dilution (i.e., in linear conditions) by some of the authors of the present paper. In this work, it has been further validated under nonlinear conditions. The correspondence between the Monte Carlo model and the well-known Thomas kinetic model (macroscopic model), for which closed-form solutions are available, is demonstrated by comparing Monte Carlo simulations, performed at different loading factors, with the numerical solutions of the Thomas model calculated under the same conditions. In all the cases investigated, the agreement between Monte Carlo simulations and Thomas model results is very satisfactory. Additionally, the exact correspondence between the Thomas kinetic model and Giddings model, when near-infinite dilution conditions are approached, has been demonstrated by calculating the limit of the Thomas model when the loading factor goes to zero. The model was also validated under limit conditions, corresponding to cases of very slow adsorption-desorption kinetics or very short columns. Different hypotheses about the statistical distributions of the random variables "residence time spent by the molecule in mobile and stationary phase' are investigated with the aim to explain their effect on the peak shape and on the efficiency of the separation.

Investigation of the heterogeneous adsorption behavior of selected enantiomers on immobilized alpha1-acid glycoprotein

A complete census was made of the interactions between enantiomeric solutes and the chiral protein column CHIRAL-AGP with the theory of nonlinear LC as tool. The surface is heterogeneous, having a small number of strong enantioselective adsorption sites and a large number of weak nonselective ones. When the eluent pH was increased, the "linear" retention of (i) the amines increased strongly as a result of a strong increase in the enantioselective binding strength, whereas (ii) the retention of the aprot increased slightly as a result of an increase in both the enantioselective binding strength and its capacity. The retention of (iii) the acid has a maximum originating solely from the enantioselective binding energy, whereas the nonselective equilibria decreased steadily. For all compounds, the enantioselective equilibrium constants increase relatively more than the nonselective ones with increasing pH.

Chromatographic and electrophoretic studies of protein binding to chiral solutes

Protein interactions are important in determining the transport, metabolism and/or activity of many chiral compounds within the body. This review examines data that have been obtained on these interactions by various chromatographic and electrophoretic methods, especially those based on either high-performance liquid chromatography or capillary electrophoresis. Zonal elution, frontal analysis and vacancy methods are each considered, as are approaches that employ either soluble or immobilized proteins. There are a variety of different items that can be learned about a solute-protein system through these techniques. This includes information on the binding constants and number of binding sites for a solute-protein system, as well as the thermodynamic parameters, rate constants, interaction forces and binding site structure for the protein and solute. Numerous examples are provided throughout this review, as taken from the literature and from work performed within the author's laboratory.

Influence of the solute hydrophobicity on the enantioselective adsorption of beta-blockers on a cellulase protein used as the chiral selector

Adsorption isotherm data were acquired at different eluent pH values for the enantiomers of several beta-blockers on cellobiohydrolase I on silica gel. They fit well to the biLangmuir model, allowing the determination of the equilibrium constants and the monolayer capacities for chiral and nonselective adsorption. The adsorption of the S-enantiomers (eluted second) is exothermic at low pH, endothermic at high pH, and athermal in a narrow pH range depending on the beta-blocker. This transition pH range is lower for S-alprenolol than for the more hydrophobic S-propranolol, although their endothermic adsorption originates from hydrophobic interactions. This surprising observation is explained by the relative values of the isotherm coefficients. S-Alprenolol seems to have a more pronounced endothermic behavior than S-propranolol because the nonselective interactions of both compounds with the stationary phase are exothermic but their contribution to retention, relative to that of the endothermic chiral interactions, is less important for alprenolol. The order of increasing energy of the chiral interactions is the same as that of hydrophobicity, propranolol>alprenolol>metoprolol.

Apparent and true enantioselectivity in enantioseparations

The separation factor of two compounds in chromatography is the ratio of their equilibrium constants or retention factors. This parameter is universally employed to investigate their resolution and to optimize the experimental conditions of their analysis. In enantioseparations, the situation is more complex because there is a mixed retention mechanism. The retention factor is the sum of two contributions, one enantioselective, the other nonselective. Although both contribute to retention, the latter being identical for the two enantiomers and does not contribute to their separation. We show how these two contributions can be measured and how it becomes necessary to distinguish between the apparent, alpha(app), and the true, alpha(true), separation factors. The existence of nonselective sites is responsible for alpha(app) being less than alpha(true). Depending on the difference between these two factors, the more effective approach to improve a separation is either to increase the enantioselectivity or to reduce the nonselective interactions. Practical applications to separations of different beta-blockers on cellobiohydrolase are discussed. The apparent enantioselectivity of alprenolol is larger and increases faster with increasing pH than that of the more hydrophobic propranolol, in spite of the importance of hydrophobic interactions in the enantioselective mechanism. These two unexpected properties are discussed and explained.

Unexpected difference in enantioselective retention on cellulase (CHB I) silica stationary phase caused by exchange of potassium for sodium ion in the mobile phase

An increase in both retention and enantioselectivity for some beta-blocking agents was observed when exchanging potassium to sodium ion in the buffer used as mobile phase. A large effect of ionic strength on retention was observed, while the enantioselectivity was constant.

Study of the adsorption behavior of the enantiomers of 1-phenyl-1-propanol on a cellulose-based chiral stationary phase

Using single-step frontal analysis, we measured single-component and competitive adsorption isotherm data for the two enantiomers of 1-phenyl-1-propanol (PP). These experimental data were fitted to several competitive bi-Langmuir models (with 8, 6, 5 and 4 parameters) and to the competitive Langmuir model. The latter model accounted well for the behavior of both PP enantiomers on Chiracel OB (cellulose tribenzoate coated on silica gel). The parameters obtained were used in numerical calculations to predict the band profiles of the two single components and of their mixtures under overloaded conditions. The equilibrium-dispersive model provides satisfactory results, with minor differences between the calculated and the experimental profiles. These differences became negligible when a more complex kinetic model was used, with a concentration-dependent rate coefficient.

Determination of mobile phase effect on single-component adsorption isotherm by use of numerical estimation

Numerical estimation was used to determine adsorption isotherm parameters of a single-component in a normal-phase system. The distribution isotherm of methyl deoxycholate was described between the mobile phase containing hexane, ethyl acetate, methanol with varied concentration and a silica gel adsorbent. The effect of the mobile phase composition on the isotherm parameters and the band profiles was investigated. The results obtained were used to simulate the overload gradient elution. The validity of the method proposed was verified by comparison of the computer simulations with the experimental band profiles.

Determination of the thermodynamic contribution to peak asymmetry of basic solutes in reversed-phase liquid chromatography

To this day packing materials manufacturers are still trying to develop reversed-phase stationary phases that have silica more completely reacted with bonding ligands to afford more homogeneous particle surfaces. Incomplete bonding causes inhomogeneous effects that are readily observed when separating basic solutes because of the acidic silanols that are unreacted. However, it is still not understood exactly what types of silanol sites are unreacted or if metal impurities are contributing to the resulting peak asymmetry observed. A method is presented which utilizes (1) the frontal analysis method of chromatography to obtain adsorption/partition isotherms, (2) a heterogeneous Langmuir distribution model for the resulting isotherm, (3) an expectation-maximization numerical procedure to solve the mathematical problem to yield the most probable distribution of adsorption parameters, and (4) the equilibrium-dispersive model of chromatography incorporating the fitted isotherm model to check the validity of the sorption model with experimental observations. Correlation of packing materials characterization parameters with results obtained by this procedure will indicate what type of silanols or other sites are responsible for observed tailing behavior. Developers and manufacturers will then be able to more efficiently target their synthetic designs for "base-deactivated" reversed-phase silicas.

Chromatographic evaluation of structure selective and enantioselective retention of amines and acids on cellobiohydrolase I wild type and its mutant D214N

The mechanisms of structure selective and enantioselective retentions of amines and acids on two chiral stationary phases based on wild type cellobiohydrolase I (CBH I) and its mutant D214N have been investigated. All the amino alcohols tested had an enantioselective site that overlaps with the catalytically active site of CBH I, whereas the enantioselectivity of prilocaine was not affected by the mutation. The hydroxyl group of the amino alcohols did not seem to be an important contributor to the total binding strength whereas a bromo substituent in the aromatic ring promotes a high enantioselectivity (alpha=7.05). Interestingly, the chiral recognition site of the acid warfarin overlaps with the binding site of the amino alcohols. Di-p-toluoyltartaric acid and dibenzoyltartaric acid were strongly retained probably due to electrostatic attraction, but no enantioselectivity was observed. The difference in retention characteristics for the amino alcohols on the two stationary phases was strongly pH-dependent. A change in elution order of different amino alcohols occurred when changing the pH from 5.0 to 7.0. The difference between the two phases was lower at low pH. The retention times could also be affected by ionic strength and by use of cellobiose as a mobile phase additive but no indication of ion-pair retention of the amines was observed, when adding hexanesulphonate as counter ion to the mobile phase. The temperature dependence of the retention of the enantiomers of propranolol at pH 7.0 on the mutant D214N was similar to what was earlier observed on the wild type CBH I at lower pH.

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.

Discrimination between enantioselective and non-selective binding sites on cellobiohydrolase-based stationary phases by site specific competing ligands

A systematic study was performed to investigate the influence of cellobiose or lactose on the enantioselective retention behaviour of some beta-blockers in liquid chromatography using Cellobiohydrolase (CHB) I from Trichoderma reesei or Cellobiohydrolase 58 from Phanerochaete chrysosporium immobilized on silica as stationary phases. The results revealed that the retention could be described by the function [equation; see text] where the observed capacity factor corresponds to the sum of an enantioselective mode being influenced by a site specific competing ligand (competitor) and a non-selective mode unaffected by the competitor. A non-constrained non-linear least-square regression gave in all cases virtually identical nondisplacable capacity factors (k'ns) for both enantiomers of the same drug. The experimental capacity factors (k'(x,C)) of the enantiomers all show a close fit to the adapted function. The Kd values calculated for the competitor were also virtually identical for each pair of enantiomers and were in accordance with Ki data determined for the competitors in classical enzyme kinetics experiments, demonstrating that one unique site; namely, the catalytic site, was responsible for the enantioselective binding. Similar results were obtained with the resolution of rac-alprenolol and rac-metoprolol on CBH I phase.

Recent chromatographic and electrophoretic enantioseparations of cardiovascular drugs

The chromatographic and electrophoretic enantiomeric separation and analysis of several clinically used cardiovascular drugs have been reviewed. Several examples of recently reported applications of enantioselective analysis and various cardiovascular agents are presented.

Affinity Chromatography: A Review of Clinical Applications

Affinity chromatography is a type of liquid chromatography that makes use of biological-like interactions for the separation and specific analysis of sample components. This review describes the basic principles of affinity chromatography and examines its use in the testing of clinical samples, with an emphasis on HPLC-based methods. Some traditional applications of this approach include the use of boronate, lectin, protein A or protein G, and immunoaffinity supports for the direct quantification of solutes. Newer techniques that use antibody-based columns for on- or off-line sample extraction are examined in detail, as are methods that use affinity chromatography in combination with other analytical methods, such as reversed-phase liquid chromatography, gas chromatography, and capillary electrophoresis. Indirect analyte detection methods are also described in which immunoaffinity chromatography is used to perform flow-based immunoassays. Other applications that are reviewed include affinity-based chiral separations and the use of affinity chromatography for the study of drug or hormone interactions with binding proteins. Some areas of possible future developments are then considered, such as tandem affinity methods and the use of synthetic dyes, immobilized metal ions, molecular imprints, or aptamers as affinity ligands for clinical analytes.