Direct resolution of enantiomers by liquid affinity chromatography on albumin-agarose under isocratic conditions

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.

Liquid chromatographic enantioseparations on crown ether-based chiral stationary phases

Various liquid chromatographic chiral stationary phases (CSPs) have been developed for the resolution of racemic compounds. In particular, CSPs based on chiral crown ethers have been known to be useful in the resolution of racemic compounds containing a primary amino group. In this paper, the development of two types of CSPs based on the chiral crown ethers containing 1,1'-binaphthyl unit(s) or tartaric acid units as chiral barrier(s) and their applications to the resolution of various racemic compounds were reviewed. The structural characteristics of eleven CSPs and their chromatographic behaviors for the resolution of racemic primary amino and non-primary amino compounds were included.

Enantioseparation of (DL)-tryptophan by spiral tube assembly counter-current chromatography and evaluation of mass transfer rate for enantiomers

Spiral tube assembly counter-current chromatography was successfully applied in enantioseparation of dl-tryptophan using bovine serum albumin as chiral selector. An improved biphasic aqueous-aqueous solvent system 12.0% (w/w) polyethyleneglycol 8000-9.0% (w/w) dibasic potassium phosphate-0.1% ammonia-78.9% water was used as the solvent system for counter-current chromatography, in which bovine serum albumin was predominantly distributed in the lower phase of the two-phase aqueous system. The aqueous-aqueous solvent system gave a very high enantioselectivity for d- and l-tryptophan at α=2.605 along with distribution ratio DD=1.200 and DL=0.461. High peak resolution was obtained for enantioseparation of 2.0mg of dl-tryptophan by spiral tube assembly counter-current chromatography under room temperature. It was found that 0.1% ammonia added in the aqueous-aqueous solvent system greatly improved the enantioseparations. An unusual extremely broad peak for l-tryptophan was observed during enantioseparations. In order to give an explanation, mass transfer rates of d- and l-enantiomers through the interface between the two phases were measured. It was found that l-tryptophan showed lower mass transfer rate than d-tryptophan. Further discussions were proposed for possible reasons for mass transfer rate difference between the enantiomers.

Synthesis and chromatographic properties of a chiral stationary phase derived from bovine serum albumin immobilized on magnesia-zirconia using phosphonate spacers

A novel bovine serum albumin (BSA)-modified magnesia-zirconia stationary phase was prepared using the sodium salt of cis-(3-methyloxiranyl)phosphonic acid (fosfomycin) as spacer and glutaraldehyde as coupler. Baseline separation of six derivatized amino acids (DNB-Leu, Dansyl-Val, etc.) was achieved on this column using ammonium acetate buffer-isopropanol mobile phase at a flow rate of 1.0 mL/min. The effects of mobile phase composition, eluent pH value, column temperature, and flow rate on the retention and separation of chiral compounds were also investigated. The BSA chiral stationary phase (BSA-CSP) was relatively stable under experimental conditions. The coupling reaction in this method was mild, reliable, and reproducible; thus it was also suitable for the immobilization of various biopolymers with amino groups in the preparation of chromatography stationary phases.

Incorporation of the fluorescent amino acid 7-azatryptophan into the core domain 1-47 of hirudin as a probe of hirudin folding and thrombin recognition

7-Azatryptophan (AW), a noncoded isostere of tryptophan (W), possesses interesting spectral properties. In particular, the presence of a nitrogen atom at position 7 in the indolyl nucleus of AW results in a red shift of the absorption maximum and fluorescence emission by 10 and 46 nm, respectively, compared to W. In the present work, we report the chemical synthesis and the conformational and functional characterization of an analog (denoted as Y3AW) of the N-terminal domain 1-47 of hirudin, a highly potent thrombin inhibitor, in which Tyr 3 has been replaced by AW. The results obtained were compared with those of the corresponding Y3W analog. We found that the replacement W --> AW reduces affinity for thrombin by 10-fold, likely because of the lower hydrophobicity of AW compared with that of W. Measurements of the resonance energy transfer effect, which was observed between Tyr13 and the amino acid at position 3 upon disulfide-coupled folding, demonstrate that AW behaves as a better energy acceptor than W for studying protein renaturation. The interaction of Y3AW with thrombin was studied by exciting the sample at 320 nm and recording the change in fluorescence of Y3AW on binding to the enzyme. Our results indicate that the fluorescence of AW of hirudin 1-47 in the Y3AW-thrombin complex is strongly quenched, possibly because of the presence of two structural water molecules at the hirudin-thrombin interface that can promote the nonradiative decay of AW in the excited state. The data herein reported demonstrate that the incorporation of AW can be of broad applicability in the study of protein folding and protein-protein interaction.

Separation of drug enantiomers by liquid chromatography and capillary electrophoresis, using immobilized proteins as chiral selectors

Proteins display interesting chiral discrimination properties owing to multiple possibilities of intermolecular interactions with chiral compounds. This review deals with proteins which have been used as immobilized chiral selectors for the enantioseparation of drugs in liquid chromatography and capillary electrophoresis. The main procedures allowing the immobilization of proteins onto matrices, such as silica and zirconia particles, membranes and capillaries are first presented. Then the factors affecting the enantioseparation of drugs in liquid chromatography, using various protein-based chiral stationary phases (CSPs), are reviewed and discussed. Last, chiral separations already achieved using immobilized protein selectors in affinity capillary electrochromatography (ACEC) are presented and compared in terms of efficiency, stability and reproducibility.

Direct resolution of enantiomers in high-performance immunoaffinity chromatography under isocratic conditions

This paper describes the application of stereoselective antibodies as tailor-made chiral selectors for the separation of enantiomers in HPLC under isocratic conditions. Stereoselective monoclonal antibodies to D- and L-alpha-amino acids, raised against protein conjugates of p-amino-D- and L-phenylalanine, were immobilized on a synthetic high-flow-through support material and used for rapid enantiomer separation of a number of amino acids at flow rates between 0.1 and 10 mL/min. Since separations could be performed in a mild buffer, column lifetime considerably exceeded that of classical immunoaffinity systems. Using an anti-D-amino acid antibody as chiral selector, the L-enantiomers eluted with the void volume, while the D-enantiomers eluted second. Inverted elution orders were obtained on chiral stationary phases prepared from an anti-L-amino acid antibody. These results demonstrate, for the first time, that antibody-based chiral stationary phases are useful for routine enantiomer separation under true high-performance chromatographic conditions.

Modeling of the separation of the enantiomers of 1-phenyl-1-propanol on cellulose tribenzoate

The competitive adsorption isotherms of rac-1-phenyl-1-propanol on cellulose tribenzoate were measured by competitive frontal analysis. The experimental data were fitted to four different isotherm models: Langmuir, Bilangmuir, Langmuir-Freundlich, and Tóth. The fittings of the experimental data to all four models were satisfactory. It was excellent in the case of the Langmuir-Freundlich and the Tóth models. Overloaded elution profiles calculated with the Tóth isotherm were in good agreement with the experimental profiles in all the different experimental conditions investigated. This work extends to the case of binary mixtures the equivalence between the general rate and the lumped pore diffusion models already demonstrated for pure compounds when the ratio between the Stanton and the Biot numbers exceeds 5. The adsorption energy distribution for the Tóth isotherm was also calculated.

Comparison of L-tryptophan binding capacity of BSA captured by a polymer brush with that of BSA adsorbed onto a gel network

A polymer brush containing a diethylamino group as an anion-exchange group was appended onto a polymer substrate by radiation-induced graft polymerization and subsequent chemical modifications. Bovine serum albumin as a chiral ligand for L-tryptophan was bound to the polymer brush at a density ranging from 17 to 150 g BSA/l. For comparison, BSA was adsorbed onto the gel network containing a diethylaminoethyl group. The molar binding ratio of L-tryptophan to BSA on the polymer brush was 1.7-fold higher than that to BSA on the gel network.

Synthesis and characteristics of the human serum albumin-triazine chiral stationary phase

Human serum albumin (HSA) was successfully bonded to silica with s-triazine as activator. The coupling reaction by this method was rapid and effective. The triazine-activated silica is relatively stable and can be installed for at least 1 month without obvious loss of reactivity when stored below 30 degrees C, pH below 7. It was observed that the amount of bound HSA reached 120 mg/g silica calculated from the UV absorbance difference of the HSA solution. d, l-tryptophan was selected as the probe solute to characterize the properties of HSA bonded s-triazine chiral stationary phase, and separation factor of 9.4 was obtained for d,l-tryptophan. Furthermore, the amount of effective HSA on silica was measured by high-performance frontal analysis, and only 16.8 mg/g silica was responsible for the resolution of d,l-tryptophan. These results indicate that the amount of both the bound and effective HSA on silica with triazine as activator was much higher than those by the Schiff base coupling method. Different kinds of enantiomers were resolved successfully on the aminopropylsilica-bonded HSA s-triazine chiral stationary phase.

Degree of solute inclusion in native beta-cyclodextrin: chromatographic approach

The reversed-phase liquid chromatography retention and separation of a series of D,L dansyl amino acids were investigated over a wide range of salting-out agent (sucrose) concentrations using native beta-cyclodextrin as a chiral stationary phase. An original treatment was developed to determine the number of sucrose molecules (n) excluded from the solute-beta-cyclodextrin cavity interface when the analyte transfer occurred. Using the n values, the relative degrees of compound inclusion were calculated and correlated to the steric bulkiness of the solute. Thermodynamic parameter variations are discussed in relation to the inclusion degree of the dansyl amino acids. This numerical approach is a valuable tool to explore the steric effects implied in the host-guest complex formation.

Application of the restricted-access precolumn packing material alkyl-diol silica in a column-switching system for the determination of ketoprofen enantiomers in horse plasma

The group of LiChrospher ADS (alkyl-diol silica) sorbents that make part of a unique family of restricted-access materials, have been developed as special packings for precolumns used in the LC-integrated sample processing of biofluids. The advantage of these sorbents lies in the direct injection of untreated biological fluids, that is without sample clean-up, the elimination of the protein matrix with a quantitative recovery together with an on-column enrichment. The present method is based on previous work applying UV detection at 260 nm for ketoprofen determinations. Plasma samples introduced to the ADS precolumn using a 0.1 M phosphate buffer, pH 7.0. After washing with the buffer the ADS column was backflushed with the mobile phase 0.01 M phosphate buffer-6% (v/v) 2-propanol-5 mM octanoic acid at a pH of 5.5, thus transporting the analytes to the chiral-HSA (human serum albumin) (100x4.0 mm) column where the separation of the ketoprofen enantiomers was achieved with a resolution factor of 1.4. The developed column-switching method was fully applicable to plasma injections.

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.

Chiral separation of DL-tryptophan using porous membranes containing multilayered bovine serum albumin crosslinked with glutaraldehyde

Bovine serum albumin (BSA) as a chiral ligand was captured uniformly throughout a porous hollow-fiber membrane at a level of 160 mg/g by the polymer chains grafted onto the membrane. BSA was bound in three layers with an end-on orientation to diethylamino groups on the graft chains which expanded from the pore surface towards the pore interior due to mutual electrostatic repulsion. Subsequently, crosslinking of BSA with a 0.025% (w/w) of glutaraldehyde in a Tris-HCl buffer (pH 8) for 4 h was effective in stabilizing the amount of BSA immobilized at a level of 150 mg/g. A solution of DL-tryptophan in a Tris-HCl buffer as a mobile phase permeated the crosslinked-BSA multilayered membrane and produced a chromatogram with a separation factor of 12. BSA leakage was not detected in the mobile phases at various pH values and organic modifiers.

Enantioselectivity properties of human serum albumin immobilized on anion-exchangers based on polyvinylimidazole-coated silica. Effect of protein loading on separation properties

Chiral chromatographic supports were obtained by continuously applying solutions contained HSA to ion-exchange columns. The columns were packed with silica modified with polyvinylimidazole and a copolymer polyvinylpyrrolidone-polyvinylimidazole (75:25) respectively, quaternized and crosslinked. Small changes in the concentration of NaCl during immobilization of HSA lead to variations in the amount of HSA bound to the supports. These variations have consequences in terms of chromatographic retention (k'), selectivity (alpha) and resolution (Rs) of enantiomers. The effects of varying the pH and organic modifier of the mobile phase on the chromatographic properties were also examined.

Effects of organic modifiers on the chiral recognition by different types of silica-immobilized bovine serum albumin

We prepared three columns containing bovine serum albumin immobilized on silica by different means and the effects of organic modifiers in the eluent on chiral separation were studied using N-substituted amino acids. Adsorption on silica, covalent immobilization to diol-silica with carbonyldiimidazole (CSP-II) and covalent immobilization to amino-silica with glutaraldehyde were studied. CSP-II had the highest stereoselectivity and was the most affected by organic modifiers in the eluent. The hydrophobicity of amino acid moiety affected the chiral recognition of N-benzoylamino acids and the aromaticity of the N-substituted group was important.

Chiral separation mechanisms in protein-based HPLC columns. 2. Kinetic studies of (R)- and (S)-warfarin binding to immobilized human serum albumin

This work used plate height measurements to investigate the kinetics of (R)- and (S)-warfarin binding to an immobilized HSA column. The dissociation rate constants for (R)- and (S)-warfarin on this column increased from 0.06 to 1.9 s-1 and from 0.06 to 0.36 s-1 between 4 and 45 degrees C. The corresponding association rate constants increased from 2.4 x 10(4) to 3.2 x 10(5) M-1 s-1 for (R)-warfarin and from 4.4 x 10(4) to 7.2 x 10(4) M-1 s-1 for (S)-warfarin over the same temperature range. From the dissociation data, it was found that an increase in temperature led to a large decrease in the plate height due to stationary phase mass transfer for both enantiomers. Further studies indicated that (R)- and (S)-warfarin had similar activation energies for their binding to HSA. For (R)-warfarin, most of this energy requirement was due to the change in enthalpy of the system, while for (S)-warfarin, it was mainly due to the change in entropy. All of these results agree with an earlier model, in which (R)- and (S)-warfarin were proposed to interact with regions on the interior and exterior of HSA, respectively. In addition, these results offer a number of useful insights into the mechanisms of protein-based chiral separations.

Role of binding capacity versus binding strength in the separation of chiral compounds on protein-based high-performance liquid chromatography columns. Interactions of D- and L-tryptophan with human serum albumin

Frontal analysis was used to examine changes in the association constant (Ka) and moles of binding sites (mL) for D- and L-tryptophan on an immobilized HSA column under various elution conditions. Both enantiomers had single-site interactions under all conditions tested. At pH 7.0 and 25 degrees C, the strength of L-tryptophan/HSA binding was determined mostly by the change in enthalpy of the system, while D-tryptophan/HSA binding was dominated by the change in entropy. The interactions of L-tryptophan with HSA showed a large change when varying the temperature, pH, ionic strength or 1-propanol content of the mobile phase. In each case, changes in Ka accounted for most of the shifts in retention that were seen for L-tryptophan during zonal elution studies. However, mL for this compound was also affected when varying the pH and 1-propanol levels. Changes in Ka were responsible for most of the shifts in D-tryptophan retention that were seen when adjusting the mobile phase pH or ionic strength. In addition, the value of mL for D-tryptophan was affected by pH, temperature and 1-propanol levels. It was concluded that varying such chromatographic conditions can alter either the binding strength or number of binding sites for solutes injected onto immobilized protein columns.

Quantitative structure-retention relationships in the examination of the topography of the binding site of antihistamine drugs on alpha 1-acid glycoprotein

Quantitative relationships between the structure of antihistamine drugs (AHD) and their retention on an alpha 1-acid glycoprotein (AGP) HPLC column (QSRR) were studied in order to identify characteristic structural features of the binding site for AHD on AGP. The hydrophobicity of AHD was determined by HPLC on an immobilized artificial membrane (IAM) column. A highly significant QSRR equation was obtained which describes the retention of AHD on AGP in terms of the chromatographically determined hydrophobicity parameter, electron excess charge on the aliphatic nitrogen and a molecular size descriptor. The topography of the AHD-binding site on AGP was suggested to be a conical pocket with lipophilic regions at the mouth of the receptor and an anionic region close to the spike of the cone. Protonated aliphatic nitrogen is supposed to guide a drug molecule towards the anionic region of the binding site. Hydrophobic aryl moieties provide anchoring of the molecule in the lipophilic regions of the binding site. Steric hindrance prevents the molecule from plunging into the binding site.

Quantitative relationships between the structure of beta-adrenolytic and antihistamine drugs and their retention on an alpha 1-acid glycoprotein HPLC column

Chromatographic retention parameters of a series of 7 beta-adrenolytics and of 12 antihistamine drugs were determined employing an alpha 1-acid glycoprotein (AGP) high-performance liquid chromatographic (HPLC) column. For the group of antihistamines capillary electrophoretic (CE) retention was additionally measured in the presence of either AGP or human serum albumin (HSA). Two series of solutes hydrophobicity parameters were obtained by reversed-phase HPLC on an immobilized artificial membrane (IAM) column. The solutes studied were subjected to molecular modelling and the structural descriptors obtained were applied in studies of quantitative structure-retention (protein binding) relationships (QSRR). It was found that retention on AGP correlates well with the literature on physiological protein binding data. This retention was demonstrated to depend on hydrophobicity: to a lesser extent in the case of beta-adrenolytics and strongly in the case of antihistamines. Hydrophobicity, along with molecular width and electron excess charge on aliphatic nitrogen was demonstrated to describe retention of antihistamines on AGP. The AGP column is recommended as a convenient reactor for studies of drug-protein interactions. Preliminary CE data do not correlate with the HPLC data.

Methods for the analysis of enantiomers of racemic drugs application to pharmacological and pharmacokinetic studies

Although the existence and differences in biological behavior of optical isomers have long been appreciated, there has been an apparent reluctance to address these differences in pharmacology and the pharmaceutical sciences. At least part of this reluctance arises from the belief that the separation of enantiomers requires highly specialized analytical equipment and expertise. The purpose of this review is to present general principles that allow the separation of stereoisomers and demonstrate that these procedures can be accomplished using available and convenient chromatography techniques.