Enantiomer separation of drugs by capillary electrophoresis using proteins as chiral selectors

One-step preparation of amino-PEG modified poly(methyl methacrylate) microchips for electrophoretic separation of biomolecules

A simple method for a chemical surface modification of poly(methyl methacrylate) (PMMA) microchips with amino-poly(ethyleneglycol) (PEG-NH(2)) by nucleophilic addition-elimination reaction was developed to improve the separation efficiency and analytical reproducibility in a microchip electrophoresis (MCE) analysis of biomolecules such as proteins and enantiomers. In our procedure, the PEG chains were robustly immobilized only by introducing an aqueous solution of PEG-NH(2) into the PMMA microchannel. The electroosmotic mobilities on the modified chips remained almost constant during 35 days with 37 runs without any recoating. The PEG-NH(2) modified chip provided a fast, reproducible, efficient MCE separation of proteins with a wide variety of isoelectric points within 15s. Furthermore, the application of the modified chip to affinity electrophoresis using bovine serum albumin gave a good chiral separation of amino acids.

Ligand binding strategies of human serum albumin: how can the cargo be utilized?

Human serum albumin (HSA), being the most abundant carrier protein in blood and a modern day clinical tool for drug delivery, attracts high attention among biologists. Hence, its unfolding/refolding strategies and exogenous/endogenous ligand binding preference are of immense use in therapeutics and clinical biochemistry. Among its fellow proteins albumin is known to carry almost every small molecule. Thus, it is a potential contender for being a molecular cargo/or nanovehicle for clinical, biophysical and industrial purposes. Nonetheless, its structure and function are largely regulated by various chemical and physical factors to accommodate HSA to its functional purpose. This multifunctional protein also possesses enzymatic properties which may be used to convert prodrugs to active therapeutics. This review aims to highlight current overview on the binding strategies of protein to various ligands that may be expected to lead to significant clinical applications.

Advanced CE for chiral analysis of drugs, metabolites, and biomarkers in biological samples

An analysis of recent trends indicates that CE can show real advantages over chromatographic methods in ultratrace enantioselective determination of biologically active compounds in complex biological matrices. It is due to high separation efficiency and many applicable in-capillary electromigration effects in CE (countercurrent migration, stacking effects) enhancing significantly (enantio)separability and enabling effective sample preparation (preconcentration, purification, analyte derivatization). Other possible on-line combinations of CE, such as column coupled CE-CE techniques and implementation of nonelectrophoretic techniques (extraction, membrane filtration, flow injection) into CE, offer additional approaches for highly effective sample preparation and separation. CE matured to a highly flexible and compatible technique enabling its hyphenation with powerful detection systems allowing extremely sensitive detection (e.g. LIF) and/or structural characterization of analytes (e.g. MS). Within the last decade, more as well as less conventional analytical on-line approaches have been effectively utilized in this field and their practical potentialities are demonstrated on many new application examples in this article. Here, three basic areas of (enantioselective) drug bioanalysis are highlighted and supported by a brief theoretical description of each individual approach in a compact review structure (to create integrated view on the topic), including (i) progressive enantioseparation approaches and new enantioselective agents, (ii) in-capillary sample preparation (preconcentration, purification, derivatization), and (iii) detection possibilities related to enhanced sensitivity and structural characterization.

Enantiomeric separation of some flavanones using shinorhizobial linear octasaccharides in CE

Succinoglycan, a shinorhizobial exopolysaccharide produced by Shinorhizobium meliloti, is composed of an octasaccharide subunit. S. meliloti produces both high-molecular-weight and low-molecular-weight (M(r)<10 000) succinoglycans that consisted of monomer, dimer, or trimer of an octasaccharide unit. We isolated and purified the monomer among low-molecular-weight succinoglycans and used this microbial linear octasaccharide as a novel chiral additive for enantiomeric separation of some flavanones such as homoeriodictyol, hesperetin, naringenin, and isosakuranetin in CE. Throughout the present investigation, we firstly used noncyclic oligosaccharides for the chiral separation of flavanones. We also found that successful enantioseparation of four flavanones depends on the presence of succinate substituents of the linear monomeric octasaccharide in CE, suggesting that succinylation of succinoglycan monomer is decisive for the effective enantiomeric separation.

Study of interaction between drug enantiomers and human serum albumin by flow injection-capillary electrophoresis frontal analysis

Flow injection (FI)-CE coupled with frontal analysis (FA) was applied to the study of stereoselectivity binding of amlodipine (AL) to HSA. Under protein-drug binding equilibrium, the unbound concentrations of drug enantiomers were measured by plateau height. The stereoselectivity of AL binding to HSA was proved by the different free fractions of two enantiomers. In physiological phosphate solution (pH 7.4, ionic strength 0.17) when 200 microM (+/-)AL was equilibrated with 300 microM HSA, the concentration of unbound R-AL was about 1.5 times higher than that of its antipode. The binding constants of two enantiomers, KR-AL and KS-AL, were 9910-11200 and 90200-104000 M(-1), respectively. The results obtained by the method were compared with those determined by conventional equilibrium dialysis (ED)-CE and fluorescence spectra. Hydroxypropyl-beta-CD (HP-beta-CD) (10 mM) was used as a chiral selector in pH 3.7 phosphate buffer. L-tryptophan (L-try) and ketoprofen (Ket) were used as displacement reagents to investigate the binding sites of AL to HSA. A binding synergism effect between hydrochlorothiazide (QL) and AL was observed and the results suggested that QL can destroy binding equilibrium of R-AL and S-AL toward HSA and they can occupy the same binding site of HSA (site I). The reproducibility was confirmed by RSD (RSD<1.5%) of the plateau height determined by FI-CE frontal analysis (FI-CE-FA). The FI-CE-FA was a good method to study protein-drug interaction.

The role of cyclodextrins in chiral capillary electrophoresis

The members of the enantiomeric pairs frequently show rather different biological effects, so their chiral selective synthesis, pharmacological studies and analysis are necessary. CE has unique advantages in chiral analysis. The most frequently used chiral selectors are CDs in this field. This paper gives a short view on the advantages on CE in direct chiral separations, emphasizing the role of CDs. The reason for the broad selectivity spectra of CDs is discussed in detail. The physical background of chiral selective separations is briefly shown in CE. Their interaction mechanisms are shortly defined. The general trend of their use is statistically evaluated. Most frequently used CDs and CD derivatives are characterized. Advantages of ionizable CDs and single-isomer derivatives are shown. The general trend of their use is established.

Capillary electrophoresis for pharmaceutical analysis

This chapter describes the application of capillary electrophoresis (CE) to pharmaceutical analysis. The areas of pharmaceutical analysis covered are enantiomer separation, analysis of small molecules such as amino acids or drug counter-ions, pharmaceutical assay, related substances determinations, and physiochemical measurements such as log P and pKa of compounds. The different electrophoretic modes available and their advantages for pharmaceutical analysis are described. Recent applications of CE for each subject area are tabulated with electrolyte details. Information on electrolyte choice and method optimization to obtain optimal separations is included.

Separation of flavanone enantiomers and flavanone glucoside diastereomers from Balanophora involucrata Hook. f. by capillary electrophoresis and reversed-phase high-performance liquid chromatography on a C18 column

A pair of flavanone glucoside diastereomers, (2R)- and (2S)-eriodictyol-5-O-beta-d-glucopyranoside (1a, 1b), was successfully separated by RP-C(18) high-performance liquid chromatography from Balanophora involucrata Hook. f. Some other compounds, including a pair of flavanone enantiomers, (2R)- and (2S)-eriodictyol (2a, 2b), and a pair of flavanone glucoside diastereomers, (2R)- and (2S)-eriodictyol-7-O-beta-d-glucopyranoside(3a, 3b), were separated by capillary electrophoresis from the same plant. The absolute configurations at C-2 of 1a and 1b were determined based on their circular dichroism spectra. Enzymatic hydrolysis of 1a and 1b by beta-d-glucosidase afforded (2R)- and (2S)-eriodictyol, respectively, which were used as the authentic standards for co-elution to determine the migration order of the enantiomers, 2a and 2b. We also report the first example of identifying the migration order of 2a and 2b and resolving the separation of 3a and 3b by capillary electrophoresis. In addition, 1a was unambiguously characterized for the first time by NMR spectra.

Determination of association constants between steroid compounds and albumins by partial-filling ACE

ACE is a popular technique for evaluating association constants between drugs and proteins. However, ACE has not previously been applied to study the association between electrically neutral biomolecules and plasma proteins. We studied the affinity between human and bovine serum albumins (HSA and BSA, respectively) and three neutral endogenous steroid hormones (testosterone, epitestosterone and androstenedione) and two synthetic analogues (methyltestosterone and fluoxymesterone) by applying the partial-filling technique in ACE (PF-ACE). From the endocrinological point of view, the distribution of endogenous steroids among plasma components is of great interest. Strong interactions with albumins suppress the biological activity of steroids. Notable differences in the association constants were observed. In the case of the endogenous steroids, the interactions between testosterone and the albumins were strongest, and those between androstenedione and the albumins were substantially weaker. The association constants, K(b), for testosterone, epitestosterone and androstenedione and HSA at 37 degrees C were 32 100 +/- 3600, 21 600 +/- 1500 and 13 300 +/- 1300 M(-1), respectively, while the corresponding values for the steroids and BSA were 18 800 +/- 1500, 14 000 +/- 400 and 7800 +/- 900 M(-1). Methyltestosterone was bound even more strongly than testosterone, while fluoxymesterone was only weakly bound by the albumins. Finally, the steroids were separated by PF-ACE with HSA and BSA used as resolving components.

Enantioseparations by using capillary electrophoretic techniques. The story of 20 and a few more years

This paper provides the author's insight on the past, present and future of performing enantioseparations using capillary electrophoretic (CE) techniques. These techniques are discussed from the historical point of view, as well as based on their potential as the separation techniques of today and the future. The overview covers mechanistic as well as practical aspects of CE techniques.

Recent accomplishments in the field of enantiomer separation by CEC

The present review intends to summarize recent developments in the field of enantioselective separations and analysis by CEC. It covers studies published in English language in common peer-reviewed journals within the period between 2003 and 2006. Both, methods making use of chiral mobile phase additives as well as chiral stationary phases for electrochromatographic enantiomer separations, are reviewed. Achievements that have been made on the various column technologies, such as open-tubular, particle-packed, inorganic, organic and particle-fixed (hybrid-type) monolithic as well as molecularly imprinted polymer phases, are discussed.

Immunoaffinity CE in clinical analysis of body fluids and tissues

This paper reviews immunoaffinity CE procedures developed since 1998 for drug, hormone, and disease marker analyses of body fluids and tissues. Immunoaffinity CE and related techniques are described. Examples of clinical applications are included.

pH dependency of ligand binding to cellobiohydrolase 1 (Cel7A)

The affinity and enantioselectivity have been determined for designed propranolol derivatives as ligands for Cel7A by capillary electrophoresis (CE) at pH 7.0. These results have been compared to measurements at pH 5.0. In agreement with previous studies, the affinity increased at the higher pH. However, the affinity was not as dependent of the ligand structure at pH 7.0 as at pH 5.0, and the selectivity was generally decreased. Instead, at pH 7.0, the changes in binding were mainly dependent on the presence of additional dihydroxyl groups, indicating an increased importance of the electrostatic interactions. To evaluate the pH dependent variations in binding, changes in both the ligand and in the enzyme had to be taken into account. To ensure that the ligands had the same charge in all measurements, pKa-values of all compounds were determined. The ligand-protein interaction has also been studied by inhibition experiments at both pHs to evaluate the specific binding to the active site when competing with the substrate p-nitrophenyl lactoside (pNPL). With support of docking computations we propose a hypothesis on the effect of the ligand structure and pH dependency of the binding and selectivity of amino alcohols to Cel7A.

Stereodifferentiating Drug−Biomolecule Interactions in the Triplet Excited State: Studies on Supramolecular Carprofen/Protein Systems and on Carprofen−Tryptophan Model Dyads

Stereoselective interaction between a chiral nonsteroidal antiinflammatory drug, namely carprofen (CP), and human serum albumin (HSA) was studied, and the results were compared with those obtained with model dyads. In the presence of albumin the same triplet-triplet transition was detected for both CP stereoisomers; however, time-resolved measurements revealed a remarkable stereodifferentiation in the CP/HSA interaction. For each stereoisomer, the decay dynamics evidenced the presence of two components with different lifetimes that can be correlated with complexation of CP to the two possible albumin binding sites (site I and site II). This assignment was confirmed by using ibuprofen, a site II displacer. Thus, the shorter lived components, for which stereodifferentiation was more important (tauR/tauS ca. 4), were ascribed to the CP triplet state in site I; the lifetime shortening can be attributed to electron-transfer quenching by the only tryptophan (Trp) of the protein. Laser flash photolysis of model dyads containing covalently linked CP and Trp revealed formation of the expected Trp radical cation, providing support for such a mechanism. Moreover, significant stereodifferentiation was observed between the (R)- and (S)-CP-Trp dyads. In the case of CP/HSA complexes, as well as in the model compounds, the stereodifferentiation detected in the decays is in good agreement with that observed in the formation of the only CP photoproduct, resulting from a photodehalogenation process. Moreover, stereodifferentiation was also found to occur for the photobinding of CP to the protein.

Chiral resolution of tryptophan derivatives by CE using canine serum albumin and bovine serum albumin as chiral selectors

This work deals with the application of BSA and canine serum albumin (CSA) for enantioseparation of tryptophan derivatives with CE. The aim of this work was the investigation of the influence of different functional groups of tryptophan derivatives on enantioseparation. CSA as a chiral selector was tested to compare its selector properties with those of BSA. The enantiomers of the tryptophan derivatives were separated by adding BSA or CSA to the BGE. The influence of pH, temperature, BSA and CSA concentration and organic modifiers was investigated. It was found that the stereoselectivity for the different tryptophan derivatives is dependent on the albumin species. It turned out that the different functional groups of the derivatives showed a significant influence on stereoselectivity.

Penicillin G acylase as chiral selector in CE using a pullulan-coated capillary

In the present study, penicillin G acylase (PGA), an enzyme belonging to the family of hydrolases, has been investigated as chiral selector in CE using the partial filling technique. Owing to the strong disposition of PGA to be adsorbed by the inner capillary wall, permanently coated capillaries were used to diminish both the protein-wall interactions and the EOF. In particular, the silica surface of the capillary was chemically coated by an antiadhesive and an hydrophilic layer of pullulan, a high-molecular-mass homopolysaccharide. The coating procedure consisted in the silanization with glycidoxypropyltrimethoxysilane and the subsequent coupling of the hydroxyl groups of pullulan onto the silanized capillary. Using this approach, a significant EOF suppression was obtained within a wide pH range (pH 3.0-9.0); this result was very important in order to find the suitable conditions for the application of partial filling technique. The optimization of partial filling was carried out by considering the effects of different experimental conditions (buffer pH, PGA concentration, and loading duration), on the migration time and enantioresolution of rac-ketoprofen. Under the selected conditions as: 100 mM sodium phosphate buffer (pH 5.5) containing 240 microM of PGA (partial filling of 120 s at a pressure of 50 mbar), a series of acidic compounds resulted to be enantioresolved in about 10 min. The long-term stability of the proposed coating was evaluated; more than 100 injections were performed without significant loss of reproducibility.

Optimization of CZE for analysis of phytochemical bioactive compounds

Advantages of CZE such as high efficiency, low cost, short analysis time, and easy implementation result in its wide applications for analysis of phytochemical bioactive compounds (e.g. flavonoids, alkaloids, terpenoids, phenolic acid, saponins, anthraquinones and coumarins). However, several aspects, including sample preparation, separation, and detection have significant effects on CZE analysis. Therefore, optimization of these procedures is necessary for development of the method. In this review, sample preparation such as extraction method and preconcentration, separation factors including buffer type, concentration and pH, additives, voltage and temperature, as well as detection, e.g. direct and indirect UV detection, LIF and MS were discussed for optimization of CZE analysis on phytochemical bioactive compounds. The optimized strategies were also reviewed.

Potential of human serum albumin as chiral selector of basic drugs in affinity electrokinetic chromatography-partial filling technique

The enantiomeric resolution of compounds using HSA by means of affinity EKC (AEKC)-partial filling technique is the result of a delicate balance between different experimental variables such as protein concentration, running pH (background electrophoretic buffer (BGE), protein, and compound solutions), and plug length. In this paper, the possibility of using HSA as chiral selector for enantioseparation of 28 basic drugs using this methodology is studied. The effect of the physicochemical parameters, the structural properties of compounds, and compound-HSA protein binding percentages over their chiral resolution with HSA is outlined. Based on the results obtained, a decision tree is proposed for the "a priori" prediction of the capability of HSA for enantioseparation of basic drugs in AEKC. The results obtained indicated that enantioresolution of basic compounds with HSA depends on the hydrophobicity, polarity, and molar volume of compounds.

Enantioseparation of phenotiazines by affinity electrokinetic chromatography using human serum albumin as chiral selector: application to enantiomeric quality control in pharmaceutical formulations

Nowadays, there is a special interest within the pharmaceutical laboratories to develop single enantiomer formulations and consequently a need for analytical methods to determine the enantiomeric purity of drugs. The present paper deals with the enantiomeric separation of promethazine and trimeprazine enantiomers by affinity electrokinetic chromatography (AEKC)-partial filling technique using human serum albumin (HSA) as chiral selector. A multivariate optimization of the most critical experimental variables in enantioresolution, running pH, HSA concentration and plug length, is carried out to obtain enantioresolution of promethazine and trimeprazine. The estimated maximum and optimum resolution of trimeprazine and prometazine enantiomers (Rs=1.74 and 2.01, respectively) corresponded to the following experimental conditions: pH 7.5; [HSA] 170 microM and plug length 190 s and pH 7.6; [HSA] 170 microM and plug length 170 s, for trimeprazine and prometazine, respectively. The developed methodologies were applied for the enantiomeric quality control of promethazine and trimeprazine enantiomers in commercially available pharmaceutical formulations. Resolution, accuracy, reproducibility, cost and sample throughput of the proposed methodologies make it suitable for quality control of the enantiomeric composition of promethazine and trimeprazine in pharmaceutical preparations.

On-chip chiral separation based on bovine serum albumin-conjugated carbon nanotubes as stationary phase in a microchannel

A novel method of chiral separation based on protein-stationary phase immobilized in a poly(methyl methacrylate) microfluidic chip was developed. BSA conjugated with the shortened carboxylic single-walled carbon nanotubes (SWNTs) was employed as the chiral selector. Successful separation of tryptophan enantiomers was achieved in less than 70 s with a resolution factor of 1.35 utilizing a separation length of 32 mm. This is the first example of chiral separation based on SWNTs-BSA conjugates as stationary phase immobilized in microchip channel. The stability of the stationary phase in the channel was examined by microchip electrophoresis with laser-induced fluorescence detection. Factors that influenced the chiral separation resolution were examined. Under the optimized conditions, the proposed modified chip revealed adequate repeatability concerning run-to-run. These results show that the use of SWNTs-BSA conjugates within microfluidic channels hold great promise for a variety of analytical schemes.

Electrophoretic analysis of proteins and enantiomers using capillaries modified by a successive multiple ionic-polymer layer (SMIL) coating technique

The applicability of three-layer coatings consisting of three different polymers (A(+)-B(-)-C(+) coating) prepared by a successive multiple ionic-polymer layer (SMIL) coating technique to the immobilization of polypeptides and/or proteins onto the inner surface of the capillaries was investigated to provide a high-performance separation medium for proteins and enantiomers in capillary electrophoresis (CE). To obtain a stable protein-coated capillary, high molecular mass poly(ethyleneimine) (PEI) was employed as the first layer in the A(+)-B(-)-C(+) coating, and then a cationic protein was immobilized as the third layer. Comparisons of analytical performances between the A(+)-B(-)-C(+) coating and the conventional SMIL-coated (A(+)-B(-)-A(+) coating) capillary were conducted. The CE separation of cationic proteins was successfully achieved with the prepared capillaries. In addition, the polypeptide- and protein-coated capillaries were applied to the chiral separation of a binaphthyl compound. It should be noted that the chiral separation efficiency was strongly dependent on the second anionic polymer layer of the coating. Effects of the interaction between oppositely charged ionic polymer layers on the separation efficiency are discussed.

Chiral separation of acidic drug components by open tubular electrochromatography using avidin immobilized capillaries

Avidin was immobilized covalently onto the inner surface of fused silica capillaries as a stationary phase in an open tubular capillary electrochromatography (OT-CEC) for chiral separations. The modification was attained by a combination of glutaraldehyde with both an amino-silylated fused silica surface and avidin using a Schiff base formation reaction. This method couples the advantage of high efficiency and small consumption of a chiral selector with the possibility of UV detection without limitations of protein absorption. In addition, the prepared capillary was stable for 50 days with over 100 runs. To evaluate the electrochromatographic performance of the prepared capillaries, the chiral separation of abscisic acid and arylpropionic acids were investigated. Effects of the modification condition of protein, pH of running buffer, and an organic modifier on the enantioseparation were also investigated. In addition, the avidin immobilized capillary was employed for the selective chiral analysis with an electrospray ionization mass spectrometry (ESI-MS) detection scheme.

Immobilization of phospholipid-avidin on fused-silica capillaries for chiral separation in open-tubular capillary electrochromatography

Phospholipid-coated fused-silica capillaries with immobilized avidin were applied in the chiral separation of D,L-tryptophan, D,L-PTH-serine, and D,L-PTH-threonine at pH 7.4 by open-tubular CEC. Liposomes prepared from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(Cap biotinyl), or 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(Biotinyl) with different amounts of phosphatidylserine were assessed as phospholipid coating materials. The stability of the coating and the success of the coating procedure were evaluated in terms of the repeatability of the enantiomer migration times and the resolution of enantiomers. The coating procedure itself significantly affected the migration times and resolution of the enantiomers. Reliable chiral separations with high separation efficiencies were achieved through careful choice of the coating method.

Capillary zone electrophoretic chiral discrimination using 6-O-(2-hydroxy-3-trimethylammoniopropyl)-β-cyclodextrin as a chiral selector

A charged highly water-soluble CD derivative, 6-O-(2-hydroxy-3-trimethylammoniopropyl)-beta-CD (herein noted as 6-HPTMA-beta-CD) was synthesized and successfully used as a chiral selector for enantiomeric separation of some acidic compounds by CZE in an uncoated capillary. Substitution with 2-hydroxy-3-trimethylammoniopropyl groups at the primary hydroxyl group of the CD was aimed at influencing the magnitude and selectivity of analyte-CD interactions. The behavior of 6-HPTMA-beta-CD was compared with that of the commercially available quaternary ammonium-beta-CD (QA-beta-CD) under the same separating conditions. The experiments were carried out using a BGE consisting of 50 mM phosphate in the pH range of 4-6 by adding a relatively low concentration of chiral selector (less than 10 mM). The effects of the concentration of CD and the pH of the electrolyte on the resolution of these compounds were studied.

Sensitive chiral analysis by capillary electrophoresis

In this review, an updated view of the different strategies used up to now to enhance the sensitivity of detection in chiral analysis by CE will be provided to the readers. With this aim, it will include a brief description of the fundamentals and most of the recent applications performed in sensitive chiral analysis by CE using offline and online sample treatment techniques (SPE, liquid-liquid extraction, microdialysis, etc.), on-column preconcentration techniques based on electrophoretic principles (ITP, stacking, and sweeping), and alternative detection systems (spectroscopic, spectrometric, and electrochemical) to the widely used UV-Vis absorption detection.

Multivariate optimization approach for chiral resolution of drugs using human serum albumin in affinity electrokinetic chromatography-partial filling technique

The enantiomeric resolution of chiral compounds using HSA by means of affinity EKC (AEKC)-partial filling technique is the result of a delicate balance between different experimental variables such as protein concentration, running pH (background electrophoretic buffer, protein and compound solutions) and protein solution plug length. In this paper multivariate optimization approaches for chiral separation of four basic drugs (alprenolol, oxprenolol, promethazine and propranolol) using HSA as chiral selector in AEKC-partial filling technique are studied. The experimental conditions to achieve maximum resolution are optimized using the Box-Behnken experimental design. Partial least squares and pareto charts are used to analyse the main effects on the resolution. The experimental resolutions observed for all compounds studied in optimum conditions agree with the estimated values based on response surface models. The results obtained show that the range of experimental conditions that provided enantioresolution narrows as hydrophobicity of analytes decreases. This fact can be explained by assuming that hydrophobicity controls the interaction of basic compounds with HSA.

Chiral separation of oxprenolol by affinity electrokinetic chromatography-partial filling technique using human serum albumin as chiral selector

The intrinsic characteristics of capillary electrophoresis have made this technique a powerful tool in the chiral separation field. The present paper deals with the enantiomeric separation of oxprenolol enantiomers by affinity electrokinetic chromatography-partial filling technique using human serum albumin (HSA) as chiral selector. Several experimental conditions and variables affecting the separation such as pH, HSA concentration and plug length, background electrolyte concentration, temperature and voltage were studied. Baseline separation of oxprenolol enantiomers was obtained in less than 8 min under the following selected conditions: electrophoretic buffer composed of 50 mM Tris-(hydroximethyl)-aminomethane (Tris) at pH 8.5; 190 microM HSA solution applied at 50 mbar for 225 s as chiral selector; oxprenolol samples contained 190 microM HSA solution injected hydrodynamically at 30 mbar for 2s and the electrophoretic runs performed at 30 degrees C applying 15 kV voltage. The proposed methodology was applied for the analysis of two pharmaceutical preparations. Resolution, accuracy, reproducibility, speed and cost of the proposed method make it suitable for quality control of the enantiomeric composition of oxprenolol in drugs. The results show that a different affinity between oxprenolol enantiomers and HSA exists and can contribute to the pharmacokinetic differentiation of these enantiomers.

Enantiomeric purity determination of tamsulosin by capillary electrophoresis using cyclodextrins and a polyacrylamide-coated capillary

The chiral separation of racemic tamsulosin hydrochloride (TH) was carried out using cyclodextrin (CD)-mediated capillary electrophoresis (CE) with DAD at 200 nm. The best separation of enantiomers of the studied compound was achieved at 20 kV with 30 cm x 50 microm I.D. polyacrylamide (PAA)-coated fused-silica capillary (effective length 20 cm) and running buffer with sulfated-beta-CD (S-beta-CD) as chiral selector. Other selected native or derivatized CDs were also tested: beta-CD (5, 15 mmol l(-1)), carboxymethyl-beta-CD (5, 30 mmol l(-1)), dimethyl-beta-CD (15 mmol l(-1)) and hydroxypropyl-beta-CD (5, 30 mmol l(-1)). Several parameters such as capillary pretreatment, buffer type and concentration, pH of background electrolyte, methanol content, separation temperature and voltage, were optimized. The excellent baseline separation of chiral TH was successfully achieved within 12 min using 100 mmol l(-1) phosphate buffer with pH 2.5 containing 1.7 mmol l(-1) S-beta-CD. Rectilinear calibration range was 50.0-500.0 mumol l(-1) of each enantiomer (r = 0.9993-0.9996). The method was applied to the assay of R-TH in Omnic, capsules (nominal content 0.4 mg per capsule) with R.S.D. 2.75% (n = 6), recovery 99.3-101.7% and it was suitable for the chiral purity control of the active enantiomer in the pharmaceutical.

Affinity capillary electrophoresis on microchips

The present study shows that the application of the method of affinity capillary electrophoresis (ACE) to investigate interactions between ligands and their substrates can be realized on microchips. With ACE it is possible to characterize non-covalent molecular interactions (complexation and partition equilibria). Binding constants (K(B)) provide a measured value of the affinity of a ligand molecule to a substrate, which is basic information for the understanding of hormones, drugs and their targets, e.g. receptors in the human body. A microchip electrophoresis instrument equipped with a UV-detector and a home-built chip-station with electrochemical detection were used. ACE could be achieved with model solutions of neurotransmitters using sulfated beta-cyclodextrin (sCD) as substrate in a background buffer. This paper describes the advantages of microchip-ACE (MC-ACE) to traditional affinity capillary electrophoresis on a capillary. The results show that MC-ACE has great potential as a tool for fast scanning of interactions and to calculate binding constants of ligands with their substrates.

Chiral separation of highly negatively charged enantiomers by capillary electrophoresis

The separation of two highly negatively charged enantiomeric organic disulfates containing two chiral centers was investigated by capillary electrophoresis using cyclodextrin based chiral selectors added to the run buffer. The optimum separation for the enantiomers was achieved in less than 3 min at 25 degrees C with a run buffer of 10 mM glycine pH 2.4 and 5 mM QA-beta-CD, which is a positively charged quaternary ammonium beta-cyclodextrin derivative. The method resulted in baseline resolution, excellent linearity, and highly reproducible migration times allowing facile evaluation of the enantiomeric purity of the individual isomers. Detection limits for the enantiomeric pair were determined to be 0.3 ng/microl (S/N = 3). The nature of the selector-enantiomer interaction and a quantitative measurement of the apparent stability constants that governed chiral discrimination of the enantiomers with QA-beta-CD were also investigated by UV-Vis spectroscopy and electrospray ionization mass spectrometry.

Polymerised bicontinuous microemulsions as stationary phases for capillary electrochromatography

Monolithic columns for capillary electrochromatography (CEC) were prepared by in situ polymerisation of bicontinuous microemulsions containing butyl methacrylate. The resulting monoliths were found to be permeable to mobile phase flow and their behaviour as CEC stationary phases was investigated. It was found that the monoliths were able to separate a simple test mixture of phthalates, but that efficiencies were low. However, several advantages of the monoliths compared to conventional ODS packed columns were found: preparation time is short, many columns can be prepared from the same batch of microemulsion and column conditioning is much faster. The columns show promise as stationary phases for CEC, but more development is required to improve efficiencies.

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.

Drug affinity to immobilized target bio-polymers by high-performance liquid chromatography and capillary electrophoresis

This review addresses the use of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) as affinity separation methods to characterise drugs or potential drugs-bio-polymer interactions. Targets for the development of new drugs such as enzymes (IMERs), receptors, and membrane proteins were immobilized on solid supports. After the insertion in the HPLC system, these immobilized bio-polymers were used for the determination of binding constants of specific ligands, substrates and inhibitors of pharmaceutical interest, by frontal analyses and zonal elution methods. The most used bio-polymer immobilization techniques and methods for assessing the amount of active immobilized protein are reported. Examples of increased stability of immobilized enzymes with reduced amount of used protein were shown and the advantages in terms of recovery for reuse, reproducibility and on-line high-throughput screening for potential ligands are evidenced. Dealing with the acquisition of relevant pharmacokinetic data, examples concerning human serum albumin binding studies are reviewed. In particular, papers are reported in which the serum carrier has been studied to monitor the enantioselective binding of chiral drugs and the mutual interaction between co-administered drugs by CE and HPLC. Finally CE, as merging techniques with very promising and interesting application of microscale analysis of drugs' binding parameters to immobilized bio-polymers is examined.