Chiral capillary electrophoresis as predictor for separation of drug enantiomers in continuous flow zone electrophoresis

From micro to macro: conversion of capillary electrophoretic separations of biomolecules and bioparticles to preparative free-flow electrophoresis scale

This invited contribution in the special issue of Electrophoresis published in celebration of the 30th Anniversary of this journal reflects the impact of our milestone paper [Prusík, Z., Kasicka, V., Mudra, P., Stepánek, J., Smékal, O., Hlavácek, J., Electrophoresis 1990, 11, 932-936] in the area of conversion of microscale analytical and micropreparative CE separations of biomolecules and bioparticles into (macro)preparative free-flow electrophoresis (FFE) scale on the basis of a correlation between CE and FFE methods. In addition to the survey of advances in the relatively narrow field of CE-FFE correlation and CE-FFE conversion, a comprehensive review of the recent developments of micropreparative CE and (macro)preparative FFE techniques is also presented and applications of these techniques to micro- and (macro)preparative separations and purifications of biomolecules and bioparticles are demonstrated. The review covers the period since the year of publication of the above paper, i.e. ca. the last 20 years.

True moving bed electrophoresis using stepped electric field gradients

True moving bed electrophoresis has been shown to be an effective technique for the bench-scale separation of enantiomers, and it is desired to increase the maximum possible throughput attainable with the process by using electric field gradients. Homatropine enantiomer separations were performed and results using a stepped electric field gradient were compared to those using a traditional non-gradient separation. In order to accomplish this, a newly designed stator was constructed for use with the Vortex-Stabilized Electrophoresis Apparatus that has three sets of electrode housings, one set at both ends and one in the middle of the chamber. There were several problems related to the membranes used at the middle electrode. The dialysis membranes were permeable to the homatropine enantiomers, and while a switch to anion exchange membranes prevented the permeation of the homatropine, this caused a pH shift that interrupted binding to the hydroxypropyl-ss-cyclodextrin chiral selector. These problems prevented any meaningful data from being collected using homatropine enantiomers, and due to this, a proof of concept study was conducted using two bovine proteins. The separations using fluorescein-labeled BSA and bovine hemoglobin showed that a 63% increase in the maximum processing rate was attainable. The maximum throughput using the non-gradient process was 30.6 mg/h and the maximum was 50.0 mg/h using an electric field gradient that was 10% lower than the non-gradient field in section II and 10% higher in section III.

Continuous Electrophoretic Purification of Individual Analytes from Multicomponent Mixtures

Individual analytes can be isolated from multicomponent mixtures and collected in the outlet vial by carrying out electrophoretic purification through a capillary column. Desired analytes are allowed to migrate continuously through the column under the electric field while undesired analytes are confined to the inlet vial by application of a hydrodynamic counter pressure. Using pressure ramping and buffer replenishment techniques, 18% of the total amount present in a bulk sample can be purified when the resolution to the adjacent peak is approximately 3. With a higher resolution, the yield could be further improved. Additionally, by periodically introducing fresh buffer into the sample, changes in pH and conductivity can be mediated, allowing higher purity (>or=99.5%) to be preserved in the collected fractions. With an additional reversed cycle of flow counterbalanced capillary electrophoresis, any individual component in a sample mixture can be purified providing it can be separated in an electrophoresis system.

Chiral separation of methadone, 2-ethylidene- 1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (EMDP) by capillary electrophoresis using cyclodextrin derivatives

A stereoselective method was developed for the simultaneous determination of methadone and its two major metabolites, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (EMDP) by capillary electrophoresis. Five beta-cyclodextrin (betaCD) background electrolyte (BGE) additives were evaluated for resolution efficiency. The conditions for baseline resolution of each of the three enantiomer pairs was determined to be 1 mM heptakis-(2,6-di-O-methyl)-beta-cyclodextrin (DMbetaCD) in 100 mM phosphate at pH 2.6. This method represents the first successful method for the resolution of the six enantiomers associated with the metabolism of methadone. The utilisation of doubly coated capillaries in conjunction with betaCD derivatives for a faster separation of the methadone-related enantiomers is also reported. The coated capillaries were prepared using a polycation of poly(diallyldimethylammonium chloride) (PDDAC) and a polyanion of dextran sulfate. Baseline resolution of the methadone enantiomers was achieved with a BGE of 8 mM (2-hydroxy)propyl-beta-cyclodextrin (HPbetaCD) in 100 mM phosphate at pH 2.6. The migration times for the stereoselective methadone separation were approximately 4 min, which represented a reduction by a factor of approximately three, compared to that attained using analogous conditions with the uncoated capillary.

Continuous fractionation of enantiomer pairs in free solution using an electrophoretic analog of simulated moving bed chromatography

Continuous fractionation of the left and right enantiomers of Piperoxan was performed in free solution in a vortex-stabilized electrophoresis apparatus. Sulfated beta-cyclodextrin was used as the chiral selector. A capillary electrophoresis (CE) study of the separation of Piperoxan enantiomers was carried out in order to find the buffer conditions that produce the maximum peak separation time between the two enantiomers and the optimal chiral selector concentration. These peak separation times were then used to calculate the electrophoretic mobilities of the enantiomer-ligand complexes. The difference in electrophoretic mobilities, when used in a preliminary model of the enantiomer separation, indicated that, by imposing a fluid flow opposite the direction of electromigration, it would be possible to force the fast and slow enantiomers to move in opposite directions within the vortex-stabilized apparatus. Using the predictions of the preliminary separation model, the vortex stabilized electrophoresis apparatus was configured with a feed port at the center of the chamber axis and offtake ports near the cathode and anode. This allowed for continuous operation of the apparatus. Continuous fractionations were completed at throughputs of 1.5 and 4.0 mg/h with both offtakes showing greater than 99% enantiomeric purity at 4.0 mg/h using CE. Fractionation was achieved at a throughput of 10 mg/h, but while the slow enantiomer was recovered with greater than 99% purity, only 96% enantiomeric purity of the fast stereoisomer was achieved. The loss of resolution at higher volumetric throughputs supports our hypothesis that a mobility-dependent "window" of operation exists in which two solutes can be completely separated.

Optimization of preparative electrophoretic chiral separation of ritalin enantiomers

Continuous free flow electrophoresis (CFFE) was applied to the preparative chiral separation of ritalin enantiomers. Sulfated beta-cyclodextrin (sbeta-CD) was used as the chiral additive. Liquid chromatography-mass spectrometry (LC-MS) experiments were applied to study the time averaged concentration of sbeta-CD in the separation chamber. The distribution of sbeta-CD in the separation chamber greatly influenced resolution and the angle of deflection. To optimize the separation, several parameters (methanol, concentration of sbeta-CD in the cathodic wash and in the separation buffer, and the introduction of a low conductivity zone) were investigated. The dependence of the resolution and deflection angles of ritalin enantiomers on the concentration of sbeta-CD in both the separation buffer and in the cathode wash solution appeared to be non-linear. Under close to optimal conditions, resolution of ritalin enantiomers was about 0.8 with an average processing rate of 0.5 mg/h. Overall, the enantiomeric purity of the individual isomers was approximately 83%; however, of the 20 vials containing ritalin, the presence of both enantiomers was only detected in three vials.

Chiral analysis of methylphenidate and dextromoramide by capillary electrophoresis

Capillary electrophoretic methods have been developed to separate the enantiomers of methylphenidate (MPH) and dextromoramide. For MPH separation was achieved with heptakis (2,6-di-O-methyl)-beta-cyclodextrin (DMCD) as chiral selector in a 100 mM phosphoric acid buffer adjusted to pH 3.0 with triethanolamine. Commercial samples of D,L-erytho-MPH HCl and D,L-threo-MPH HCl were analysed using the method. There was no evidence of the presence of D,L-threo-MPH HCl in D,L-erytho-MPH HCl and vice versa. The ratio of the enantiomers was determined for each diastereoisomer. Hydroxypropyl-beta-cyclodextrin was the chiral selector of choice for the chiral separation of the enantiomers of moramide. The separation which gave a resolution of about 3.5 was achieved in 4 min using only a 6 cm of length of capillary. In a sample of dextro-R-moramide tartrate only a small quantity (4.9% w/w) of levo-S-moramide was detected with this method.

Fundamental aspects of chiral separations by capillary electrophoresis

A review is presented that surveys the basic theory of direct separation of enantiomers by capillary electrophoretic (CE) techniques. These separations are based on the formation of diastereomeric complexes between the enantiomeric analytes and a chiral selector added to the electrolyte solution. The review covers a comprehensive treatment of the equations needed for optimization of selectivity coefficients, resolution and analysis time in the zone electrophoretic mode. In this context, it takes into account combined equilibria of complexation and protonation/deprotonation as well as complexation and paritition into micelles. On the basis of these equations, the benefits of charged selectors and the optimization potential inherent to pH tuning can be documented. In addition, the review deals with some basic aspects of chiral isoelectric focusing and briefly discusses indirect enantioseparation. In a subsequent section a survey is given on particularfeatures of the various types of chiral selectors. Finally, the recent developments in preparative enantioseparation in continuous free-flow system and by use of isoelectric membranes are discussed.

Use of dyes to investigate migration of the chiral selector in CFFE and the impact on the chiral separations

Continuous free flow electrophoresis was investigated as a tool for the preparative chiral separation of piperoxan enantiomers using sulfated beta-cyclodextrin (sbeta-CD) as the chiral additive. Bulk migration of sbeta-CD was confirmed using LC-MS analysis of the individual fractions collected and visualized with the addition of crystal violet to the separation buffer. In the absence of sbeta-CD, the crystal violet-containing buffer was reddish/purple and the crystal violet was deflected cathodically in the chamber. In the presence of sbeta-CD, the crystal violet-containing buffer was blue and was deflected anodically. However, formation of accumulation and depletion zones was apparent in both cases. The addition of sbeta-CD to the cathodic wash solution allowed for almost complete resolution of the piperoxan enantiomers with a processing rate of 0.45 mg/ h.

Improved preparative-scale. continuous, free-flow electrophoretic separation of the enantiomers of terbutaline utilizing equal-but-opposite enantiomer mobilities

The factors that influence yield and product purity in the continuous, preparative-scale electrophoretic separation of the enantiomers of terbutaline when using the principle of equal-but-opposite effective mobilities were studied. The sodium salt of heptakis-6-sulfato-beta-cyclodextrin was used as the resolving agent, in acidic, isopropanol-containing background electrolytes, in the continuous, free-flow, preparative electrophoretic instrument, the Octopus. By matching the linear velocity of the feed solution to that of the background electrolyte, lateral hydrodynamic dispersion was minimized resulting in a nonelectrophoresed feed band that was only three fractions (about 3 mm) wide as it exited the 0.5 m long separation channel. The multiple of residence time and applied potential was also optimized, constrained by migration of the front of heptakis-6-sulfato-beta-cyclodextrin out of the separation zone, leading to the recovery of 95% of both enantiomers in better than 99.99% purity, at a production rate of 0.1 mg/h.