Enantioseparations of hydrobenzoin and structurally related compounds in capillary zone electrophoresis using heptakis(2,3-dihydroxy-6-O-sulfo)-beta-cyclodextrin as chiral selector and enantiomer migration reversal of hydrobenzoin with a dual cyclodextrin system in the presence of borate complexation

We investigated the enantioseparations of racemic hydrobenzoin, together with benzoin and benzoin methyl ether, in capillary electrophoresis (CE) using the single-isomer heptakis(2,3-dihydroxy-6-O-sulfo)-beta-cyclodextrin (SI-S-beta-CD) as a chiral selector in the presence and absence of borate complexation and enantiomer migration reversal of hydrobenzoin with a dual CD system consisting of SI-S-beta-CD and beta-CD in the presence of borate complexation at pH 9.0 in a borate buffer. The enantioselectivity of hydrobenzoin increased remarkably with increasing SI-S-beta-CD concentration and the enantioseparation depended on CD complexation between hydrobenzoin-borate and SI-S-beta-CD. The (S,S)-enantiomer of hydrobenzoin-borate complexes interacted more strongly than the (R,R)-enantiomer with SI-S-beta-CD. The enantiomers of hydrobenzoin could be baseline-resolved in the presence of SI-S-beta-CD at a concentration as low as 0.1% w/v, whereas the three test analytes were simultaneously enantioseparated with addition of 0.3% w/v SI-S-beta-CD or at concentrations >2.0% w/v in a borate buffer and 0.5% w/v in a phosphate background electrolyte at pH 9.0. Compared with the results obtained previously using randomly sulfated beta-CD (MI-S-beta-CD) in a borate buffer, enantioseparation of these three benzoin compounds is more advantageously aided by SI-S-beta-CD as the chiral selector. The enantioselectivity of hydrobenzoin depended greatly on the degree of substitution of sulfated beta-CD. Moreover, binding constants of the enantiomers of benzoin compounds to SI-S-beta-CD and those of hydrobenzoin-borate complexes to SI-S-beta-CD were evaluated for a better understanding of the role of CD complexation in the enantioseparation and chiral recognition. Enantiomer migration reversal of hydrobenzoin could be observed by varying the concentration of beta-CD, while keeping SI-S-beta-CD at a relatively low concentration. SI-S-beta-CD and beta-CD showed the same chiral recognition pattern but they exhibited opposite effects on the mobility of the enantiomers.     

Enantioseparation of phenothiazines in cyclodextrin-modified capillary zone electrophoresis using sulfated cyclodextrins as chiral selectors

In this study, enantioseparations of five phenothiazines, including promethazine, ethopropazine, trimeprazine, methotrimeprazine, and thioridazine, in CD-modified CZE using dual CD systems consisting of randomly sulfate-substituted CD (MI-S-beta-CD) and a neutral CD as chiral selectors in a citrate buffer (100 mM) at pH 3.0 were investigated. The results indicate that MI-S-beta-CD is an excellent chiral selector for enantioseparation of ethopropazine. The enantiomers of promethazine can also be baseline-resolved with MI-S-beta-CD at concentrations in the range of 0.5-1.0% w/v. On the other hand, thioridazine and trimeprazine interact strongly with neutral CDs. As a result, the enantioselectivity of these two phenothiazines is remarkably and synergistically enhanced with increasing the concentration of neutral CDs in the presence of MI-S-beta-CD and simultaneous enantioseparations of these phenothiazines, except for methotrimeprazine, could favorably be achieved with the use of dual CD systems. Moreover, by varying the concentration of beta-CD or gamma-CD at a fixed concentration of MI-S-beta-CD (0.75% w/v) reversal of the enantiomer migration order of promethazine occurred. This may be attributable to the opposite effects of charged and neutral CDs on the mobility of the enantiomers of promethazine.     

Temperature effects on the enantioselectivity of basic analytes in capillary EKC using sulfated beta-CDs as chiral selectors

The thermodynamic processes were investigated to reveal the temperature effects during chiral separation by capillary EKC with reversed polarity mode using sulfated beta-CD (S-beta-CD) as chiral selectors. The temperature effects on enantioselectivities of basic analytes (ephedrine, norephedrine, synephrine, and epinephrine) were investigated in detail over a temperature range of 20-60 degrees C. An increase of the capillary temperature produced the decrease of enantioselectivities for ephedrine and norephedrine, but increase of enantioselectivities for synephrine and epinephrine. The thermodynamic variations showed that the interactions between the basic analytes and chiral selectors were always enthalpy- driven. However, the difference in enthalpy and entropy showed that the enantioseparation was an enthalpy-driven process for ephedrine and norephedrine, but an entropydriven process for synephrine and epinephrine. Just because of the different driving forces, there exist two kinds of temperature effects on enantioselectivities mentioned above.     

Strategies for enantioseparations of catecholamines and structurally related compounds by capillary zone electrophoresis using sulfated beta-cyclodextrins as chiral selectors

Strategies for simultaneous enantioseparations of three catecholamines (DL-norepinephrine, DL-epinephrine, and DL-isoproterenol) and three structurally related compounds (DL-octopamine, DL-synephrine, and DL-norephedrine) by CZE using sulfated beta-CDs as chiral selectors were investigated. Four different separation modes were attempted: (I) using randomly sulfate-substituted beta-CD (MI-S-beta-CD) at relatively low concentrations in a high-concentration phosphate buffer at low pH in the normal polarity mode, (II) using MI-S-beta-CD at high concentrations at low pH in the reversed polarity mode, (III) using MI-S-beta-CD at moderately high concentrations in a phosphate buffer at neutral pH in the normal polarity mode, and (IV) using the single isomer heptakis(2,3-dihydroxy-6-O-sulfo)-beta-CD (SI-S-beta-CD) at low to moderately high concentrations in a high-concentration BGE at low pH in the normal polarity mode. Among them, enantioseparation of these cationic solutes was best achieved under the conditions of mode (II). In mode (II) and mode (III), temperature is an important factor affecting the enantioresolution of norepinephrine. In mode (I) and mode (IV), the use of a high-concentration BGE (150-200 mM) is crucial for effective enantioseparation of these cationic solutes with sulfated beta-CDs. Comparative studies of enantioseparations of these cationic solutes with MI-S-beta-CD and SI-S-beta-CD reveal that the sulfate substituents of MI-S-beta-CD located at the C(2)- position interact strongly with the diol moiety of catecholamines.     

Chiral separation of hydroxyflavanones in cyclodextrin-modified capillary zone electrophoresis using sulfated cyclodextrins as chiral selectors

Chiral separations of three hydroxyflavanone aglycones, including 2'-, 3'-, and 4'-hydroxyflavanone, in capillary zone electrophoresis (CZE) using randomly sulfate-substituted beta-cyclodextrin (S-beta-CD) or dual cyclodextrin (CD) systems consisting of S-beta-CD and a neutral CD at low pH were investigated. The results indicate that S-beta-CD is an excellent chiral selector for enantioseparation of 2'-hydroxyflavanone and is a good chiral selector for 3'-hydroxyflavanone. Depending on the concentration of S-beta-CD ranging from 2.0 to 0.75% (w/v), the enantioresolution values were 10.5-19.5 and 1.8-3.4 for 2'- and 3'-hydroxyflavanone, respectively. The enantiomers of 4'-hydroxyflavanone could be effectively separated with S-beta-CD at a concentration of 2.0% (w/v) within 20 min. The enantioselectivity and enantioresolution follow the order 2'-hydroxyflavanone>3'-hydroxyflavanone>4'-hydroxyflavanone. Alternatively, with the addition of sodium dodecyl sulfate (SDS) monomers at low concentrations in the electrophoretic system, enantioselectivity of these hydroxyflavanone aglycones could be enhanced with dual CD systems. In this case, SDS monomer acted as a complexing agent probably first with S-beta-CD and then subsequently with the analytes for increasing the effective electrophoretic mobility of the analytes towards the anode and as a selectivity controller for affecting the selectivity of hydroxyflavanones. Better enantioseparation between 2'-hydroxyflavanone and 3'-hydroxyflavanone could be achieved with a dual CD system consisting of S-beta-CD and gamma-CD than that with S-beta-CD and beta-CD. In addition, possible chiral recognition mechanisms of hydroxyflavanones are discussed.     

Liquid Chromatorgraphic Separation of the Enantiomers of Phenylthiohydantoin α-Amino Acids Derivatives on Polysaccharide-Based Chiral Stationary Phases

ABSTRACT

The liquid chromatographic enantioseparation of the phenylthiohydantoin (PTH) derivatives of various amino acids on four commercial polysaccharide-derived chiral stationary phases (CSPs) is described. Chiralcel OF and Chiralpak AS showed better performance than the other CSPs for resolution of the enantiomers of PTH amino acid derivatives. The enantiomers of all amino acids as their PTH derivatives were well separated on Chiralcel OF and/or Chiralpak AS. The (-)(L) or (-)-enantiomers of all analytes examined were preferentially retained on Chiralpak AS, whereas the (+)(D) or (+)-enantiomers of most of analytes were preferentially retained on Chiralcel OF.     

Enantioseparation of phenothiazines in CD-modified CZE using single isomer sulfated CD as a chiral selector

Enantioseparations of five chiral phenothiazines in CD-modified CZE using the single isomer sulfate-substituted beta-CD (heptakis(2,3-dihydroxy-6-O-sulfo)-beta-CD, SI-S-beta-CD) and dual CD systems consisting of SI-S-beta-CD and a neutral CD as chiral selectors in a citrate buffer at pH 3.0 were investigated. The results indicate that SI-S-beta-CD is an excellent chiral selector for enantioseparation of promethazine. The enantiomers of trimeprazine were well separated, while those of ethopropazine could also be baseline-resolved with SI-S-beta-CD. With dual CD systems, especially with hydroxypropyl-beta-CD (HP-beta-CD) as neutral CD, the enantioselectivity of thioridazine and ethopropazine was considerably enhanced. Effective enantioseparation of phenothiazines, except for methotrimeprazine, could thus be favorably and simultaneously achieved. Moreover, reversal of the enantiomer migration order of ethopropazine and thioridazine occurred by varying the concentration of gamma-CD in the presence of SI-S-beta-CD. These phenomena may be attributable to the opposite effects of sulfated beta-CD and gamma-CD on the mobility of the enantiomers of ethopropazine and of thioridazine. Comparative studies on the enantioseparations of phenothiazines with single CD and dual CD systems containing SI-S-beta-CD and randomly sulfate-substituted beta-CD (MI-S-beta-CD) were made.     

Semipreparative-scale gas-chromatographic separation of filbertone enantiomers

The enantioselectivity of heptakis(2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl-beta-CD) toward racemic filbertone (E-5-methyl-hep-2-en-4-one) was studied by performing the chiral separation on a capillary column, a thick-film wide-bore column and a semipreparative column. The semipreparative enantioseparation of filbertone was achieved at 80 degrees C by using a packed column providing (R)- and (S)-enantiomers of filbertone with ee 90 and 96%, respectively. The isolated enantiomers (approximately 250 microg each, ee = 90-96%) may be used for studies on the relationship of chirality and biological activity by olfactory screening and toxicological studies.     

Separation of lansoprazole enantiomers in human serum by HPLC

Summary A new and simple HPLC method is described for the separation and quantitative determination of the (+)-and (−)-enantiomers of lansoprazole. The analytes were extracted from serum as previously described for whole lansoprazole [K. Borner, Chromatographia 45, 450–452 (1997)]. The enantiomers were separated by chromatography on a CHIRAL-AGP^R column which contained covalently bound acid α₁-glycoprotein as chiral selector. In the pure drug the (−)/(+) ratio was 0.99:1.01. In serum of twelve human volunteers the concentration of the (−)-enantiomer was 3 to 5 times higher than that of the (+)-enantiomer. Both enantiomers differ remarkably in their pharmacokinetics.     

Fast enantioseparation of arylglycine amides by capillary electrophoresis with highly sulfated-beta-cyclodextrin as a chiral selector

Nine racemic arylglycine amides were synthesized and successfully enantioseparated by capillary electrophoresis (CE) using highly sulfated beta-cyclodextrin (HS-beta-CD) as a chiral selector. Baseline enantioseparation of the analytes was obtained around neutral pH but not in the acidic conditions that are commonly used. HS-beta-CD content, buffer pH, type and concentration, and organic modifier concentration were studied and optimized for fast and efficient separation. A chiral CE separation system composed of 1.5% (w/v) HS-beta-CD, 0 to 10% (v/v) methanol and 20 mM 3-(N-morpholino)propanesulfonic acid at pH 6.5 was shown suitable for baseline enantioseparation of the mentioned amides within 6 min, including simultaneous enantioseparation of three positional isomer series (methyl-, methoxyl or chloro-substituted). By using this system, D-enantiomers migrated ahead of the L-enantiomers and the enantiomeric resolution order of arylglycine amides was more or less parallel to the pK(a), order of the analytes.     

Enantioseparation of chiral sulfoxides and sulfinate esters by capillary electrophoresis

Forty-one chiral sulfoxides and sulfinate esters were separated using sulfated beta-cyclodextrin and carboxymethyl beta-cyclodextrin as chiral selectors. Binding constants of some analytes to both chiral selectors were measured in order to examine and help explain the observed migration behavior and enantioselectivity trends. Overall, sulfated beta-cyclodextrin separated a greater number of compounds, and had better separating capabilities than did carboxymethyl beta-cyclodextrin for these analytes. This was true even though all of the analytes showed much stronger binding to carboxymethyl beta-cyclodextrin than to sulfated beta-cyclodextrin. General procedures to optimize the separation, by varying pH, selector concentration, and organic modifier concentration were examined and discussed. Chiral selector concentration had the greatest effect on enantioseparation, with higher concentrations of selector giving better peak-to-peak separations. Organic modifier had an adverse affect on resolution, with increasing amounts giving lower mobility differences. Lastly, pH had only a minimal effect on separation.     

Structure of the 1 : 1 Complex of Hexakis(2,3,6-tri- O-methyl) α-Cyclodextrin with ( R)-(-)-1,7-Dioxaspiro[5.5]undecane

The crystal structure of the 1 : 1 inclusion complex of hexakis(2,3,6-tri-O-methyl)- -cyclodextrin (TM CD) with 1,7-dioxaspiro[5.5]undecane (spiroacetal) is orthorhombic, space group C2221, with a = 24.002(2), b = 14.812(1), c = 21.792(2) Å V = 7747.3(11) Å3 and Z = 8. The molecular six-fold axis of TM CD coincides with the a two-fold crystallographic axis and the guest is located at the secondary methoxy group side, disordered over two positions related by that axis. The guest model used during the refinement is that of the (R)-enantiomer alone because trials to either refine a 1 : 1 mixture of (R)- and (S)-enantiomers or the (S)-enantiomer alone failed. The crystallographic evidence of enantioselectivity towards the (R)-enantiomer of spiroacetal was confirmed by independent experiments and may be attributed to numerous non bonding interactions between host and guest involving non conventional H-bonds.     

Migration behavior and enantioseparation of hydrobenzoin and structurally related compounds in capillary zone electrophoresis with a dual cyclodextrin system consisting of heptakis-(2,3-dihydroxy-6-O-sulfo)-beta-cyclodextrin and beta-cyclodextrin

Migration behavior and enantioseparation of racemic hydrobenzoin and structurally related compounds, including benzoin and benzoin methyl ether, in CZE with a dual CD system consisting of heptakis-(2,3-dihydroxy-6-O-sulfo)-beta-CD (SI-S-beta-CD) and beta-CD as chiral selectors in the presence and absence of borate complexation at pH 9.0 were investigated. The results indicate that enantioseparation of hydrobenzoin is mainly governed by CD complexation of hydrobenzoin-borate complexes with SI-S-beta-CD when SI-S-beta-CD concentration is relatively high. Whereas CD complexation of hydrobenzoin-borate complexes with beta-CD plays a significant role in enantioseparation when SI-S-beta-CD concentration is comparatively low. The (S,S)-enantiomer of the hydrobenzoin-borate complex was found to interact more strongly than the corresponding (R,R)-enantiomer with both SI-S-beta-CD and beta-CD. These two types of CD show the same chiral recognition pattern, but they exhibit opposite effects on the mobility of the enantiomers of hydrobenzoin-borate complexes. Enantiomer migration reversal of hydrobenzoin occurred in the presence of borate complexation when varying the concentration of beta-CD, while keeping SI-S-beta-CD at a relatively low concentration. Binding constants of the enantiomers of benzoin-related compounds to beta-CD and those of hydrobenzoin-borate complexes to SI-beta-CD were evaluated; the mobility contributions of all complex species to the effective mobility of the enantiomers of hydrobenzoin as a function of beta-CD concentration in a borate buffer were analyzed. In addition, comparative studies on the enantioseparation of benzoin-related compounds with SI-S-beta-CD and with randomly sulfate-substituted beta-CD were made.     

Enantioseparation of phenothiazines in cyclodextrin-modified capillary zone electrophoresis: reversal of migration order

Enantioseparations of phenothiazines with gamma-cyclodextrin (gamma-CD) as a chiral selector were investigated using citrate and phosphate buffer electrolytes at pH 3.0. Reversal of the enantiomer migration order of promethazine, ethopropazine, and trimeprazine was observed by varying gamma-CD concentration in the range of 5-9 mM, 2.5-4.5 mM and 1.5-2.8 mM, respectively, using 100 mM citrate buffer at pH 3.0. As in the case of beta-CD, the (+)-enantiomers of phenothiazines possess greater binding strength to gamma-CD than the (-)-enantiomers. The evaluation of the binding constants and limiting mobility of the complexes formed between the enantiomers of phenothiazines and gamma-CD reveals that the binding strength of phenothiazines to gamma-CD and the differences in the binding constants and limiting mobility of the complexes are responsible for the enantiomer migration reversal. Both the binding constants and limiting mobility of the complexes between the (+)-enantiomers of phenothiazine and gamma-CD are greater than those of the corresponding (-)-enantiomers in a citrate buffer, while the binding constants of the complexes primarily determined the migration order of the enantiomers in a phosphate buffer. Compared with the results obtained using a phosphate buffer, we may conclude that citrate buffer which involves competitive complexation with chiral selector plays a significant role in the enantiomer migration reversal.     

Enantioseparation of chiral N-imidazole derivatives by electrokinetic chromatography using highly sulfated cyclodextrins: mechanism of enantioselective recognition

Baseline separation of ten new substituted [1-(imidazo-1-yl)-1-phenylmethyl)] benzothiazolinone and benzoxazolinone derivatives, with one chiral center, was achieved by CD-EKC using highly sulfated CDs (alpha, beta, gamma highly S-CDs) as chiral selectors. The influence of the type and concentration of the chiral selectors on the enantioseparations was investigated. The highly S-CDs exhibit a very high enantioselectivity power since they allow excellent enantiomeric resolutions compared to those obtained with the neutral CDs. The enantiomers were resolved with analysis times inferior to 2.5 min and resolution factors R(s) of 3.73, 3.90, 1.40, and 4.35 for compounds 1, 2, 3, and 5, respectively, using 25 mM phosphate buffer at pH 2.5 containing either highly S-alpha-CD, highly S-beta-CD, and highly S-gamma-CD (3 or 4% w/v) at 298 K, with an applied field of 0.30 kV/cm. The determination of the enantiomer migration order for the various analytes and the study of the analyte structure-enantioseparation relationships display the high contribution of the interactions between the analytes phenyl ring and the CDs to the enantiorecognition process. The thermodynamic study of the analyte-CD affinities permits us to improve our knowledge about the enantioseparation mechanism.     

Effect of temperature on the enantioselectivity of basic analytes using CE with sulfated β‐cyclodextrins

Twelve basic analytes, including ephedrine and its structurally related compounds, were used to study the influence of capillary temperature on enantioselectivity in CE enantioseparations under reversed polarity mode using sulfated β‐CD (S‐β‐CD) as chiral selectors. All of the effective mobility changes of (+)‐enantiomers between 35 and 20°C were higher than those of (–)‐enantiomers whosoever enantioselectivity increased or decreased with an increase in temperature. However, the unusual temperature effect that enantioselectivity was increased with an increase of temperature was observed for the compounds with hydroxyl substitution on phenyl ring and had relationship with the molecular structures. With performing NMR studies on the selected selector‐analyte complexes, the results led to a deeper understanding of the chiral discrimination process. Inspection of the complexation‐induced chemical shifts (CICS) of the enantiomers showed that the phenyl ring sits deeply and slantways in the cavity of S‐β‐CD and alkyl chain pointed out of the wider rim of S‐β‐CD with ion–ion and hydrogen bond interactions between analytes and S‐β‐CD.     

Separation of α-cyclohexylmandelic acid enantiomers using biphasic chiral recognition high-speed counter-current chromatography

This work concentrates on a chiral separation technology named biphasic recognition applied to resolution of α-cyclohexylmandelic acid enantiomers by high-speed counter-current chromatography (HSCCC). The biphasic chiral recognition HSCCC was performed by adding lipophilic (−)-2-ethylhexyl tartrate in the organic stationary phase and hydrophilic hydroxypropyl-β-cyclodextrin in the aqueous mobile phase, which preferentially recognized the (−)-enantiomer and (+)-enantiomer, respectively. The two-phase solvent system composed of n-hexane-methyl tert-butyl ether–water (9:1:10, v/v/v) with the above chiral selectors was selected according to the partition coefficient and separation factor of the target enantiomers. Important parameters involved in the chiral separation were investigated, namely the types of the chiral selectors (CS); the concentration of each chiral selector; pH of the mobile phase and the separation temperature. The mechanism involved in this biphasic recognition chiral separation by HSCCC was discussed. Langmuirian isotherm was employed to estimate the loading limits for a given value of chiral selectors. Under optimum separation conditions, 3.5–22.0 mg of α-cyclohexylmandelic acid racemate were separated using the analytical apparatus and 440 mg of racemate was separated using the preparative one. The purities of both of the fractions including (+)-enantiomer and (−)-enantiomer from the preparative CCC separation were over 99.5% determined by HPLC and enantiomeric excess reached 100% for the (±)-enantiomers. Recovery for the target compounds from the CCC fractions reached 85–88% yielding 186 mg of (+)-enantiomer and 190 mg of (−)-enantiomer. The overall experimental results show that the HSCCC separation of enantiomer based on biphasic recognition, in which only if the CSs involved will show affinity for opposite enantiomers of the analyte, is much more efficient than the traditional monophasic recognition chiral separation, since it utilizes the cooperation of both of lipophilic and hydrophilic chiral selectors.     

Enantiomer-specific high-performance liquid chromatography with fluorescence detection of methamphetamines in abusers' hair and urine

Enantiomer-specific high-performance liquid chromatography with fluorescence detection using 4-(4,5-diphenyl-1H-imidazol-2-yl)-benzoyl chloride as a fluorescence labeling reagent was applied to determine methamphetamine and its metabolites in abusers' hair and urine. Hair samples were segmentally analyzed based on 1 cm long segments. In four hair samples, only the S(+)-enantiomers of methamphetamine and its N-demethylated metabolite, S(+)-amphetamine were detected. Satisfactory correlation (r = 0.901) between the results of high-performance liquid chromatography-fluorescence and those of gas chromatography-nitrogen phosphorous detection was obtained (n = 19). In an abuser's urine sample, the S(+)- and R(-)-enantiomers of methamphetamine, amphetamine and para-hydroxymethamphetamine were detected. The degree of N-demethylation of S(+)-methamphetamine into the corresponding metabolite of amphetamine was significantly higher than that of the R(-)-enantiomer.     

Characterization of complexes between phenethylamine enantiomers and β‐cyclodextrin derivatives by capillary electrophoresis—Determination of binding constants and complex mobilities

To optimize chiral separation conditions and to improve the knowledge of enantioseparation, it is important to know the binding constants K between analytes and cyclodextrins and the electrophoretic mobilities of the temporarily formed analyte‐cyclodextrin‐complexes. K values for complexes between eight phenethylamine enantiomers, namely ephedrine, pseudoephedrine, methylephedrine and norephedrine, and four different β‐cyclodextrin derivatives were determined by affinity capillary electrophoresis. The binding constants were calculated from the electrophoretic mobility values of the phenethylamine enantiomers at increasing concentrations of cyclodextrins in running buffer. Three different linear plotting methods (x ‐reciprocal, y ‐reciprocal, double reciprocal) and nonlinear regression were used for the determination of binding constants with β‐cyclodextrin, (2‐hydroxypropyl)‐β‐cyclodextrin, methyl‐β‐cyclodextrin and 6‐O‐α‐maltosyl‐β‐cyclodextrin. The cyclodextrin concentration in a 50 mM phosphate buffer pH 3.0 was varied from 0 to 12 mM. To investigate the influence of the binding constant values on the enantioseparation the observed electrophoretic selectivities were compared with the obtained K values and the calculated enantiomer‐cyclodextrin‐complex mobilities. The different electrophoretic mobilities of the temporarily formed complexes were crucial factors for the migration order and enantioseparation of ephedrine derivatives. To verify the apparent binding constants determined by capillary electrophoresis, a titration process using ephedrine enantiomers and β‐cyclodextrin was carried out. Furthermore, the isothermal titration calorimetry measurements gave information about the thermal properties of the complexes.     

The degree of substitution affects the enantioselectivity of sulfobutylether‐β‐cyclodextrin chiral stationary phases

Three chiral stationary phases were prepared by dynamic coating of sulfobutylether‐β‐cyclodextrin (SBE‐β‐CD) with different degrees of substitution, onto strong anion‐exchange stationary phases. The enantioselective potential and stability of newly prepared chiral stationary phases were examined using a set of structurally different chiral analytes. Measurements were performed in RP‐HPLC. Mobile phases consisted of methanol/formic acid, pH 2.10, and methanol/10 mM ammonium acetate buffer, pH 4.00, in various volume ratios. SBE‐β‐CDs with degrees of substitution (DS) 4, 6.3, and 10 proved suitable for the enantioseparation of 14, 11, and 8 analytes, respectively. The SBE‐β‐CD DS 4 based chiral stationary phase enabled the enantioseparation of the nearly all basic and neutral compounds. Chiral stationary phases with higher sulfobutylether‐β‐cyclodextrin substitution (especially DS 10) yielded higher enantioresolution values for acidic compounds.