Experimental verification of a predicted, previously unseen separation selectivity pattern in the capillary electrophoretic separation of noncharged enantiomers by octakis(2,3-diacetyl-6-sulfato)-gamma-cyclodextrin

Determination of enantiomeric impurity of linezolid by capillary electrophoresis using heptakis-(2,3-diacetyl-6-sulfato)-beta-cyclodextrin

A method for the enantioseparation of linezolid, the first compound of a truly new class of antibiotics-the oxazolidinones, was developed. The elaborated method of linezolid enantiomers separation was successfully performed using an anionic single-isomer cyclodextrin-heptakis-(2,3-diacetyl-6-sulfato)-beta-cyclodextrin (HDAS-beta-CD) as a resolving agent with the help of the charged resolving agent migration model (CHARM model). The best results were obtained with 27.5mM HDAS-beta-CD dissolved in 50mM borate buffer, pH 9.0, 15 degrees C, normal polarity. The facile strategies for the reversal of the enantiomers elution order are also described. Afterwards, the optimized method was validated in terms of sensitivity, linearity, accuracy and precision.

A Study on the Chiral Recognition Mechanism of Enantioseparation of Adrenaline and Its Analogues Using Capillary Electrophoresis

The purpose of this paper was to study the chiral recognition mechanism of enantioseparation of adrenaline and its analogues using capillary electrophoresis. The enantiomeric separation of adrenaline and its analogues has been developed using beta-cyclodextrins as the chiral selectors. All the tested compounds were separated under the same experimental conditions to study the chiral recognition mechanisms, using a low-pH buffer (50 mM Tris buffer at pH 2.5). By means of molecular docking the inclusion course between beta-cyclodextrins and enantiomers was investigated and thus the interaction energy was obtained by molecular mechanics calculations. The results suggest that the difference in interaction energy for the side chain part is most likely responsible for enantiomeric separation.

Operational control of stereoselectivity during the enzymatic hydrolysis of racemic organophosphorus compounds

The wild-type bacterial phosphotriesterase catalyzes the stereoselective hydrolysis of racemic pairs of organophosphorus compounds. The enzymatic stereoselectivity can be substantially enhanced via systematic alteration of the pKa for the leaving group phenol in the target substrates. These changes alter the rate-limiting step for substrate turnover from a diffusional event to phosphorus-oxygen bond cleavage. Turnover ratios in excess of 5000:1 were achieved using phenols with pKa values greater than 8.5. This method has enabled the isolation of the RP-enantiomer of 4-acetylphenyl methyl phenylphosphonate with an enantiomeric excess of >99% via a kinetic resolution of the racemate.

Capillary electrophoretic separation of enantiomers in a high-pH background electrolyte by means of the single-isomer chiral resolving agent octa(6-O-sulfo)-gamma-cyclodextrin

The new, alkali-stable, single-isomer, sulfated gamma-cyclodextrin, the sodium salt of octa(6-O-sulfo)-gamma-cyclodextrin (OS) was used for the first time to separate the enantiomers of non-ionic, acidic, basic and ampholytic analytes by capillary electrophoresis in high-pH aqueous background electrolytes. The effective mobilities and separation selectivities were found to follow trends similar to those observed earlier in acidic aqueous background electrolytes. OS proved to be a broadly applicable chiral resolving agent and afforded adequate peak resolution values with short separation times for a number of non-ionic, weak acid, weak base and ampholytic analytes.

Octakis-6-sulfato-gamma-cyclodextrin as additive for capillary electrokinetic chromatography of dibenzoazepines: carbamazepine, oxcarbamazepine and their metabolites

Single isomer octakis-(2,3-dihydroxy-)6-sulfato-gamma-cyclodextrin used as pseudostationary phase of the background electrolyte interacts with dibenzo[b,f]azepines (consisting of a condensed 3-ring system) and forms negatively charged complexes. Hydroxygroups in position 2 and 3 at carbamazepine increase the extent of interaction, whereas substitution by oxygen at position 10 and/or 11 reduces it. The complex constants for the analytes are ranging from few tens L/mol (10-hydroxycarbamazepine, 10,11-dihydroxycarbamazepine, 10,11-epoxycarbamazepine, oxcarbazepine) to several hundreds L/mol (carbamazepine, 2-hydroxycarbamazepine, 3-hydroxycarbamazepine), and are much larger than those of the analytes with octakis-(2,3-dimethyl-)-6-sulfato-gamma-cyclodextrin. Full enantiomeric separation of the chiral metabolites of carbamazepine and oxcarbazepine is obtained at octakis-(2,3-dihydroxy-)-6-sulfato-gamma-cyclodextrin concentrations of about 10 mM (3 mM borate buffer, pH 8.5). Compared to heptakis-6-sulfato-beta-cyclodextrin, selectivity differs and stereoselectivity is more pronounced.

Recent developments in chiral capillary electrophoresis and applications of this technique to pharmaceutical and biomedical analysis

This paper provides an overview of the current status of chiral capillary electrophoresis (CE). The emphasis is placed on the application of CE in chiral separation of various racemic compounds. During the last two years about 280 papers, several review articles, and two entire issues, edited by S. Fanali (Electrophoresis 1999, 20, 2577-2798, and H. Nishi and S. Terabe (J. Chromatogr. A 2000, 879, 1-471.) have been devoted to chiral CE. Enantiomeric separations of various compounds, e.g., pharmaceuticals, drug candidates, drugs and related metabolites in biological fluids, amino acids, di- and tri peptides, pesticides and fungicides, have been performed using different chiral selectors. Native and derivatized cyclodextrins continue to be the most widely used chiral selectors. Other chiral selectors such as natural and synthetic chiral micelles, crown ethers, chiral ligands, proteins, oligo- and polysaccharides, and macrocyclic antibiotics have also been applied to chiral CE separations.

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.

pH-dependence of complexion constants and complex mobility in capillary electrophoresis separations of dipeptide enantiomers

The chiral separation of the LL- and DD-enantiomers of the dipeptides Ala-Tyr, Phe-Phe, and Asp-PheOMe has been investigated at pH 2.5 and pH 3.5 using beta-cyclodextrin (beta-CD), heptakis-(2,6-di-O-methyl)-beta-cyclodextrin, and heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin as chiral selectors. According to electrospray mass spectrometry, heptakis-(2,6-di-O-methyl)-beta-cyclodextrin was a mixture of six isomers. Reversal of the enantiomer migration order upon increasing the buffer pH from 2.5 to 3.5 was observed for all peptides with beta-cyclodextrin, for Ala-Tyr and Phe-Phe in the presence of heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin, and for Ala-Tyr using heptakis-(2,6-di-O-methyl)-beta-cyclodextrin. The migration behavior could be explained on the basis of the complexation constants and the mobilities of the peptide-cyclodextrin complexes. Both, the binding constants and complex mobilities decreased with increasing pH as the overall-charge of the peptides decreased. While the complexation constants primarily determined the migration order at pH 2.5, complex mobility dominated in most cases at pH 3.5.

Stereoselective detoxification of chiral sarin and soman analogues by phosphotriesterase

The catalytic activity of the bacterial phosphotriesterase (PTE) toward a series of chiral analogues of the chemical warfare agents sarin and soman was measured. Chemical procedures were developed for the chiral syntheses of the S(P)- and R(P)-enantiomers of O-isopropyl p-nitrophenyl methylphosphonate (sarin analogue) in high enantiomeric excess. The R(P)-enantiomer of the sarin analogue (k(cat)=2600 s(-1)) was the preferred substrate for the wild-type PTE relative to the corresponding S(P)-enantiomer (k(cat)=290 s(-1)). The observed stereoselectivity was reversed using the PTE mutant, I106A/F132A/H254Y where the k(cat) values for the R(P)- and S(P)-enantiomers were 410 and 4200 s(-1), respectively. A chemo-enzymatic procedure was developed for the chiral synthesis of the four stereoisomers of O-pinacolyl p-nitrophenyl methylphosphonate (soman analogue) with high diastereomeric excess. The R(P)R(C)-stereoisomer of the soman analogue was the preferred substrate for PTE. The k(cat) values for the soman analogues were measured as follows: R(P)R(C,) 48 s(-1); R(P)S(C), 4.8 s(-1); S(P)R(C), 0.3 s(-1), and S(P)S(C), 0.04 s(-1). With the I106A/F132A/H254Y mutant of PTE the stereoselectivity toward the chiral phosphorus center was reversed. With the triple mutant the k(cat) values for the soman analogues were found to be as follows: R(P)R(C,) 0.3 s(-1); R(P)S(C), 0.3 s(-1); S(P)R(C), 11s(-1), and S(P)S(C), 2.1 s(-1). Prior investigations have demonstrated that the S(P)-enantiomers of sarin and soman are significantly more toxic than the R(P)-enantiomers. This investigation has demonstrated that mutants of the wild-type PTE can be readily constructed with enhanced catalytic activities toward the most toxic stereoisomers of sarin and soman.

Separation of dipeptide and tripeptide enantiomers in capillary electrophoresis using carboxymethyl-beta-cyclodextrin and succinyl-beta-cyclodextrin: influence of the amino acid sequence, nature of the cyclodextrin and pH

The separation of the LL and DD enantiomers of dipeptides and tripeptides using cyclodextrins (CDs) containing carboxyl groups was investigated with respect to the amino acid sequence of the peptides, the nature of the cyclodextrin and the buffer pH. Compared to succinyl-beta-cyclodextrin, carboxymethyl-beta-cyclodextrin was the more universal CD for enantioseparations. Reversal of the enantiomer migration order upon increasing the buffer pH from 2.5 to 3.5 was observed in some cases. As shown for Phe-Phe reversal of the migration order also occurred between pH 3.5 and 5.3. Complexation constants and complex mobilities change with pH as both, the charge of the peptide and the charge of the CD vary depending on the pH. The complexation constants and complex mobilities of the dipeptides Ala-Phe and Phe-Phe were determined in order to explain the enantiomer migration behavior in the pH range 2.5-5.3. While the complexation constants determined the migration order at pH 2.5 and 5.3, complex mobility had a strong influence around pH 3.5-3.8.

Separation selectivity patterns in the capillary electrophoretic separation of anionic enantiomers by octakis-6-sulfato-gamma-cyclodextrin

The capillary electrophoretic separation of anionic enantiomers with multiply-charged, single-isomer, anionic resolving agents was reexamined with the help of the charged resolving agent migration model. Three general model parameters were identified that influence the shape of the separation selectivity and enantiomer mobility difference curves: parameter b, the binding selectivity (K(RCD)/K(SCD)), parameter s, the size selectivity (micro0RCD/micro0SCD), and parameter a, the complexation-induced alteration of the analyte's mobility (micro0SCD/micro0). Function analysis of the model indicates that there are six unique separation selectivity vs. resolving agent concentration patterns: in two of the patterns, separation selectivity asymptotically increases to the limiting value set by parameter b; in two other patterns, separation selectivity passes a local maximum and asymptotically decreases to the limiting value set by parameter b; and in the last two patterns, separation selectivity passes a local maximum, decreases to unity, then, after reversal of the intrinsic migration order, asymptotically increases to the limiting value set by parameter b. Though the patterns with asymptotically increasing selectivities were observed in earlier work, this paper reports the first experimental verification of the existence of the local selectivity maximum during the capillary electrophoretic separation of the enantiomers of several weak acids in high pH background electrolytes with octakis-6-sulfato-gamma-cyclodextrin as the resolving agent.