Enantioseparation of chiral thiobarbiturates using cyclodextrin-modified capillary electrophoresis

The racemates of several chiral thiobarbiturates were separated by using different cyclodextrins in capillary electrophoresis (CE). Six neutral and negatively charged cyclodextrins 1 (CDs) were employed as chiral separators whereof five led to successful separation of enantiomeric thiobarbiturate pairs. The CDs used were the native alpha-CD, beta-CD, gamma-CD, and heptakis-(2,6-di-O-methyl)-beta-cyclodextrin (HDM) as well as heptakis-(2,3-di-O-methyl-6-sulfato)-beta-cyclodextrin (HDMS) and heptakis-(2,3-di-O-acetyl-6-sulfato)-beta-CD (HDAS). Five of the six chiral thiobarbiturates studied could be resolved at a basic pH value of 9.4 and a phosphate buffer concentration of 100 mM in a fused-silica capillary. Structurally related substances showed a similar behavior in separation: 1 and 2 bearing the center of chirality in the side chain at C5 can be best separated using gamma-CD, the N-alkyl-substituted compounds 3 and 4 as well as the N/S-dialkyl-substituted compound 5 could be resolved with HDM. Using the neutral CDs, the migration times were relatively small (< 11 min). 3 and 4 could be also resolved by means of the negatively charged HDMS. In the latter case, the migration time is twice as long as with HDM.     

Separation of multicomponent mixtures of 2,4-dinitrophenyl labelled amino acids and their enantiomers by capillary zone electrophoresis

The use of capillary zone electrophoresis (CZE) for the separation of a group of 33 2,4-dinitrophenyl labeled amino acids (DNP-AA), including DNP-AA racemates, DNP-L-AA enantiomers and achiral DNP-AAs, was investigated. Alpha-, beta- and gamma-cyclodextrins (CDs) and their derivatives (hydroxypropyl derivatives of alpha-, beta- and gamma-CDs, polymeric beta-CD and 6A-methylamino-beta-cyclodextrin (MA-beta-CD)) served as complexing agents and chiral selectors in this investigation. Although native alpha- and gamma-CDs and their derivatives influenced the effective mobilities of the studied DNP-AAs in different ways, they generally failed to resolve enantiomers of the individual DNP-AAs. On the other hand, beta-CD and all of its derivatives were found to be effective in this respect. Of these, the best results were achieved with a positively charged MA-beta-CD and this chiral selector resolved enantiomers of ten DNP-AA racemates available for this study. However, a complete resolution of these enantiomers in one CZE run required that the effect of the chiral selector be complemented by complexing effects of polyvinyl pyrrolidone (PVP) or gamma-CD. Complexing and chiral recognition capabilities of MA-beta-CD combined with complexing effects of gamma-CD and PVP provided separating conditions suitable for the CZE separations of multicomponent mixtures of DNP-AAs with preserved resolutions of the enantiomers. For example, a mixture consisting of 43 DNP-AA constituents was resolved using an MA-beta-CD/gamma-CD combination with three peak overlaps.     

Enantiomer separation of drugs by capillary electrophoresis using mixtures of beta-cyclodextrin sulfate and neutral cyclodextrins

Direct separation of enantiomers of drugs was investigated by capillary electrophoresis employing mixtures of charged cyclodextrin derivatives (CDs) and electrically neutral CDs (i.e., dual CD system). Among various charged CDs employed, it was found that beta-CD sulfate showed relatively wide enantioselectivity for a wide variety of drugs under acidic conditions. Then separation of enantiomers was performed by employing beta-CD sulfate and the effect of the addition of electrically neutral CDs to the buffers containing beta-CD sulfate was investigated. Through the addition of electrically neutral CDs to the buffers containing the charged CD, resolution of most of the enantiomers was improved, compared with those with the charged CD alone. It was also found that the ring size (alpha, beta, gamma,), the substitution groups and the concentration of the additional electrically neutral CDs affected the enantioselectivity. For example, alpha-CD addition was effective for the separation of enantiomers of chlorpheniramine and hydroxypropyl-beta-CD was effective for the enantiomer separation of trimetoquinol isomer. The application of the method in optical purity testing is also briefly mentioned.     

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.     

Separation of neutral and basic enantiomers by cyclodextrin electrokinetic chromatography using anionic cyclodextrin derivatives as chiral pseudo-stationary phases

Separations of neutral and basic racemates were performed using five different anionic cyclodextrin (CD) derivatives as chiral selectors, viz. carboxymethylated β-CD, β-CD phosphate sodium salt, sulfobutyl ether β-CD sodium salt, carboxymethylated γ-CD, and γ-CD phosphate sodium salt. For the separation of neutral racemates, an untreated fused silica capillary was employed and various neutral racemates were successfully separated. Since the pH of the buffer affected the electroosmotic flow (EOF), the resolution was improved by changing the buffer pH. A polyacrylamide coated capillary was employed for the separation of basic racemates to suppress EOF and to prevent adsorption of cationic analyte on the capillary surface. By choosing an appropriate type and concentration of anionic CD, about 40 basic racemates were successfully separated. Some rough binding constants of basic analytes with an anionic β-CD were measured to discuss the optimum concentration of the CD. The migration direction was dependent on the binding constants and the concentration of the CD. The analyte strongly bound to the anionic CD migrated towards the anode but the weakly bound one moved towards the cathode. Anionic γ-CDs were also very useful for the separation of basic enantiomers. Five neutral CDs were employed as chiral selectors to compare selectivity between charged and neutral CDs, and eleven racemates could only be resolved using anionic CDs. The separation of some basic racemates in human plasma was also described. The direct injection of plasma samples was possible for some enantiomers that did not interact strongly with plasma proteins.     

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.     

毛细管区带电泳法拆分手性药物萘普生和氟联苯丙酸

 70-72 -------------------------------------------------------------------------------- 以β-环糊精(CD)作为手性选择剂 ,用毛细管区带电泳法成功地拆分了两种弱酸性药物萘普生(naproxen)和氟联苯丙酸(flurb iprofen),并比较了4种环糊精［β-环糊精(β-CD)、二甲基-β-环糊精( DM-β-CD)、羟丙基-β-环糊精(HP-β-CD)和三甲基-β-环糊精( TM-β-CD)］对手性拆分的影响,同时测定了萘普生对映体在不同环糊精中的出峰次 序。通过实验,发现对于此类化合物拆分的最佳pH值为5左右,即接近于该类化合物的pK a值。该方法适用于酸性手性药物的拆分。     

Combination of cyclodextrins and polymeric surfactants for chiral separations

A cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) method was applied to the enantioseparation of three binaphthyl derivatives using neutral CDs (i.e., beta- and gamma-CD) in combination with various chiral amino acid-based polymeric surfactants (PSs). Both the D- and L-configurations of poly(sodium N-undecanoyl alaninate), poly(sodium N-undecanoyl leucinate), and poly(sodium N-undecanoyl valinate) (poly(L-SUV)) were synthesized. The retention behavior of the three binaphthyl derivatives under optimum electrophoretic conditions using a single chiral additive (PS or CD) is discussed. In addition, the effect of CD cavity size and stereochemical configuration of polymeric surfactants on selectivity (alpha) and resolution (Rs) was investigated. The enantioseparation of (+/-)1,1'-binaphthyl-2,2'-diamine gave a reversal of enantiomeric order when using beta-CD in combination with any of the three D-configuration PS. However, better enantioseparation is obtained when using the corresponding L-configuration PS with beta-CD. A reversal of migration order (RMO) for the enantiomers of (+/-)1,1'-bi-2-naphthol was observed upon the addition of 10 mM gamma-CD to poly(L-SUV). However, no RMO of (+/-)1,1'-bi-2-naphthol was seen when either beta-CD or gamma-CD was combined with D-PS. The enantiomers of (+/-)1,1'-binaphthyl-2,2'-diyl hydrogen phosphate showed little enantioselective behavior toward the PS alone. However, combined D- or L-PS and beta-CD or gamma-CD systems gave increased Rs and alpha values. The chiral recognition of binaphthyl derivatives observed resulting from the various combinations of two chiral selectors is discussed.     

pH-dependent cyclodextrin capillary electrophoresis resolution of atropisomers

Five noncommercial and four commercially available cyclodextrin (CD) derivatives were tested as chiral auxiliaries for the capillary electrophoretic (CE) resolution of racemic 1,1'-bi-(2-naphthol) (BN), 1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (BNHP), and 1,1'-binaphthyl-2,2'-diamine (BNA) at pH 4.0, 6.5, and 8.6. The noncommercial CDs were ethyloxycarbonyl-gamma-CD (ethylcarbonate-gamma-CD), dimethylamino ethyloxycarbonyl-beta-CD, a mercaptosuccinic acid derivative of beta-CD, a maleic acid derivative of beta-CD and heptakis(6-deoxy-6-amino)-beta-CD derivative with one amino group on the C-6 carbon of each glucose unit. Except for the latter, the remaining derivatives were synthesized for this work. Also commercially available methyl-beta-CD, hydroxypropyl-beta-CD and the native beta- and gamma-CDs were examined. Among the nine CDs tested, the maleic acid derivative of beta-CD gave the most interesting performances, since it resolved the atropisomers of BNA and BNHP in the same electrophoretic run at pH 4.0. It resolved the BNA racemate also at pH 6.5. Both the negatively charged CD tested were found to resolve anionic BNHP enantiomers, while positively charged CDs did not with cationic BNA. Several of the CDs investigated in this work were found to resolve the BNHP racemate, although at nonoptimal concentration. None of the experimented CDs was found to resolve the electrically neutral BN atropisomers pair at the three pHs considered, while some among these nine, experimented in a previous work, did so at higher pH.     

Separation of monomethyl-benz[a]anthracene isomers using cyclodextrin-modified electrokinetic chromatography

Cyclodextrin-modified electrokinetic chromatography (CD-EKC) was investigated for the separation of 12 monomethylbenz[a]anthracene (MBA) isomers. Combined use of a polymeric surfactant, poly(sodium 10-undecenyl sulfate) (poly-SUS), with various types of neutral cyclodextrins (CDs) [beta-CD, gamma-CD, dimethyl-beta-CD (DM-beta-CD), trimethyl-beta-CD (TM-beta-CD) and hydroxypropyl-beta-CD (HP-beta-CD)] were successful in CD-EKC separation of the MBA isomers. Baseline resolution of 10 of the 12 isomers, except for 9-MBA and 2-MBA, was achieved with gamma-CD at pH 9.75. The beta-CD, gamma-CD, and beta-CD derivatives (DM-beta-CD, TM-beta-CD, HP-beta-CD) were found to have different resolution and selectivity. Additionally, the tR/t0 values of isomers were found to be dependent on the type and concentration of the CD additives. In general, tR/t0 values of MBA isomers decrease with an increase in the concentration of beta-CD derivatives, whereas the reversed was true when the concentrations of native beta-CD and gamma-CD were varied. The combination of 5 mM gamma-CD, 0.5% (w/v) poly-SUS, 35% (v/v) acetonitrile at a pH of 9.75 provided the best selectivity and resolution of the MBA isomers with a separation time of 110 min. However, the use of 30 mM DM-beta-CD under similar EKC conditions resulted in much faster separation (ca. 16 min) of 10 MBA isomers.     

Resolution of ephedrine derivatives by means of neutral and sulfated heptakis(2,3-di-O-acetyl)beta-cyclodextrins using capillary electrophoresis and nuclear magnetic resonance spectroscopy.

Beta-cyclodextrin (beta-CD), heptakis(2,3-di-O-acetyl)beta-cyclodextrin (Diac-beta-CD) and heptakis (2,3-di-O-acetyl-6-sulfato)beta-cyclodextrin (HDAS-beta-CD) were tested for their ability to discriminate the enantiomers of ephedrine, pseudoephedrine, norephedrine and methylephedrine. Using capillary electrophoresis (CE) under optimized conditions, with the exception of norephedrine in presence of beta-CD, all racemates could be resolved. Utilizing Job's plot by means of UV spectroscopy revealed 1:1 complexes formed with beta-CD and Diac-beta-CD. HDAS-beta-CD gave curved plots indicating mixed stoichiometry. Inspection of the cyclodextrin-induced chemical shifts (CICS) of both the ligands and the CDs showed that the ephedrine sits deeply in the cavity of beta-CD and HDAS-beta-CD. In the case of Diac-beta-CD, the ephedrine is located closely to the wider rim of the CD cavity. In conclusion, comparing the pattern of CICS of the various CD derivatives clearly indicates the differences in the complex geometry.     

Effect of cyclodextrins on the stability of adriamycin, adriamycinol, adriamycinone and daunomycin

The stability of adriamycin (ADR), adriamycinol, adriamycinone (ADR-ONE) and daunomycin in the presence of alpha-, beta- and gamma-cyclodextrins (CDs) was studied using high-performance liquid chromatography. It was found that alpha-CD did not affect the degradation of tested compounds, beta-CD caused a little effect and gamma-CD resulted in pronounced stabilizing effect. The formation of complexes between ADR and ADR-ONE with CDs was monitored by fluorescence spectroscopy. The fluorescence spectrum of ADR-gamma-CD complex had an activation maximum at 460 nm, emission maximum at 555 nm and a shoulder at 585 nm. A similar finding was observed in case of alpha-CD. In case of beta-CD, the fluorescence intensity at 580 nm peak enhanced less than in case of gamma-CD. With ADR-ONE, alpha-CD did not cause any significant change compared with the spectrum of free molecule. On the other hand, it was noticed that, the fluorescence spectra of ADR-ONE with both beta- and gamma-CD were the same but showed a significant difference to the spectrum of free molecule, especially the molar fluorescence of the 585 nm emission peak.     

Cyclodextrin-assisted capillary electrophoretic resolution of 1,1'-bi-2-naphthol atropisomers.

Native beta- and gamma-cyclodextrin (CD), neutral beta-CD derivatives and ethylcarbonate derivatives of beta- and gamma-CD were used as stereoselective additives for CD-capillary zone electrophoresis (CZE) resolution of atropisomers of 1,1'-bi-(2-naphthol) (BN). CZE experiments at variable CD concentration allowed calculating binding constants from electrophoretic mobility data, corrected for electroosmotic flow (EOF) and running buffer viscosity variations. The CDs were chosen on the basis of geometric examination of molecular models of BN and CDs that suggested the possibility of inclusion complexes formation. Optimum concentrations, with aqueous 25 mM phosphate running buffer at pH 10.5, 36 cm x 50 microm capillary and 10 kV applied potential, were 3.6, 3.9, 2.1, 2.2, 1.9 mM for beta-CD, gamma-CD, ethylcarbonate-beta-CD, methyl-beta-CD and hydroxypropyl-beta-CD, respectively.     

Migration order of dipeptide and tripeptide enantiomers in the presence of single isomer and randomly sulfated cyclodextrins as a function of pH

The present study was conducted in order to evaluate the cyclodextrin (CD)-mediated chiral separation of peptide enantiomers as uncharged analytes at pH 5.3 using randomly sulfated beta-cyclodextrin, heptakis-6-sulfato-beta-CD and heptakis-(2,3-diacetyl-6-sulfato)-beta-CD as chiral selectors. Although less effective compared to stronger acidic conditions, the CDs proved to be suitable chiral selectors for the present set of peptides at pH 5.3. The carrier ability of the negatively charged CDs upon reversal of the applied voltage may also be exploited leading to a reversal of the migration order. In addition, reversal of the enantiomer migration order upon increasing the buffer pH from 2.5 to 5.3 was also observed for Ala-Tyr in the presence of randomly sulfated beta-CD, for Ala-Phe, Ala-Tyr, Phe-Phe, Asp-PheNH(2) and Gly-Ala-Phe in the presence of heptakis-6-sulfato-beta-CD, and for Phe-Phe and Ala-Leu in the presence of heptakis-(2,3-diacetyl-6-sulfato)-beta-CD. The migration behavior could be explained on the basis of the complexation constants and the mobilities of the peptide-CD complexes. While a change in the affinity pattern of the CDs upon increasing the pH was observed for some peptides, complex mobility was the primary factor for other peptide-CD combinations affecting the enantiomer migration order at the two pH values studied.     

Study on the interaction of methylene blue with cyclodextrin derivatives by absorption and fluorescence spectroscopy

The ability of beta-cyclodextrin (beta-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD), and carboxymethyl-beta-cyclodextrin (CM-beta-CD) to break the aggregate of the methylene blue (MB) and to form 1:1 inclusion complexes has been studied by absorption and fluorescence spectroscopy. Experimental conditions including concentrations of various cyclodextrins (beta-CD, HP-beta-CD and CM-beta-CD) and media acidity were investigated for the inclusion formation in detail. The formation constants are calculated by using steady-state fluorimetry, from which the inclusion capacity of different cyclodextrins (CDs) is compared. The results suggest that the charged beta-cyclodextrin (CM-beta-CD) is more suitable for inclusion of the cationic dye MB than the neutral beta-cyclodextrins (beta-CD, HP-beta-CD) at pH>5. A mechanism is proposed which is consistent with the stronger binding of MB with CM-beta-CD compared with the other CDs at pH>5.     

Chiral separations on multichannel microfluidic chips

Chiral separations of FITC-labeled basic drugs on multichannel microfluidic chips with LIF detector were investigated. A preliminary screening procedure for seven neutral CDs was performed under optimized conditions for chiral separations of three FITC-labeled drugs (baclofen, norfenefrine, and tocainide) on a mono-channel microfluidic chip. According to the results of screening, FITC-baclofen and FITC-norfenefrine as well as two chiral selectors including gamma-CD and dimethyl-beta-CD (DM-beta-CD) were selected as models to perform chiral separations on a two-channel chip. FITC-baclofen enantiomers were separated completely by gamma-CD in one channel, while resolution of FITC-norfenefrine enantiomers was achieved by DM-beta-CD in the other channel in the same run. Furthermore, the feasibility of using one chiral selector to separate multiple chiral samples was studied on a four-channel chip. These results show that multichannel chip has a potential for chiral high-throughput screening.     

Complexation study of brilliant cresyl blue with beta-cyclodextrin and its derivatives by UV-vis and fluorospectrometry

The complexation reactions of brilliant cresyl blue (BCB) with beta-cyclodextrin (beta-CD), mono[2-O-(2-hydroxypropyl)]-beta-CD (2-HP-beta-CD), mono[2-O-(2-hydroxyethyl)]-beta-CD (2-HE-beta-CD), and heptakis(2,6-di-methyl) -beta-CD (DM-beta-CD) were investigated using UV-vis and fluorospectrometry. The complexation between BCB and CDs could inhibit the aggregation of BCB molecules and could cause its absorbance at 634nm gradually increasing. The fluorescence of BCB was also enhanced with the addition of CDs. The fluorescence enhancement was more notable in neutral and acidic media than in basic media. Hildebrand-Benesi equation was used to calculate the formation constants of beta-CDs with BCB based on the fluorescence differences in the CDs solution. The stoichiometry ratio was found to be 1:1. The complexing capacities of beta-CD and its three derivatives were compared and the results followed the order: 2-HP-beta-CD>2-HE-beta-CD>DM-beta-CD>beta-CD. The effect of temperature on the formation of BCB-beta-CD inclusion complexes has also been examined. The results revealed that the formation constants decreased with the increase of temperature from 1038.9 to 491.6l/mol. Enthalpy and entropy values were calculated and the values were -25.77kJ/mol and 35.04J/kmol, respectively. The thermodynamic measurements suggest that the inclusive process was enthalpic favor. The release of high-energy water molecules and Van der Waals force played an important role in the inclusive process.     

Comparison of cyclodextrin-barbiturate noncovalent complexes using electrospray ionization mass spectrometry and capillary electrophoresis

Various noncovalent complexes between native and derivatized cyclodextrins (CDs) and barbiturates were studied using capillary electrophoresis (CE) and electrospray ionization mass spectrometry (ESI-MS). This paper involves the study of four aspects of CD-barbiturate noncovalent inclusion complexes. The first study focused on determining the formation of CD-barbiturate inclusion complexes in ESI-MS. This determination was accomplished by the comparison of migration data from CE with ESI-MS inclusion complex peak abundances, which were found to be complementary. The second study found the possibility of predicting native beta-CD mediated CE elution orders for barbiturates using data from ESI-MS. A third study focused on the formation of barbiturate inclusion complexes with derivatized beta-CD and gamma-CD. As part of this study, the effect of the extent of side chain substitution on native CD complexation behavior was investigated. The results indicated that the number of side chains on the CD does not affect the formation of barbiturate complexes with the hydrophobic CD cavity. Finally, a comparison of the hydroxypropyl-beta-CD-barbiturate and hydroxypropyl-gamma-CD-barbiturate complexes in CE and ESI-MS was made to study the relationship between strength of drug-CD binding and enantioresolution. The results from the above studies indicated that the gas phase and the solution state complexes showed comparable behavior indicating that similar interactions played a role in stabilizing these complexes. While it was possible to use the ESI-MS data to determine drug binding to the CDs, it was not possible to predict whether a separation of the enantiomers of a chiral barbiturate would occur. However, the ESI-MS data could be used to eliminate certain CDs from consideration as chiral selectors.     

Influence of the amino acid sequence and nature of the cyclodextrin on the separation of small peptide enantiomers by capillary electrophoresis using randomly substituted and single isomer sulfated and sulfonated cyclodextrins

The separation of dipeptide and tripeptide enantiomers using negatively charged single isomers as well as randomly sulfated and sulfonated cyclodextrins (CDs) was investigated with respect to the amino acid sequence of the peptides and the nature of the CDs. Standardized conditions concerning buffer pH and molarity, CD concentration, and separation voltage were applied. Compared to suffobutylether-beta-CD and heptakis-(2,3-dimethyl-6-sulfato)-beta-CD, randomly sulfated beta-CD as well as the single isomer derivatives heptakis-6-sulfato-beta-CD and heptakis-(2,3-diacetyl-6-sulfato)-beta-CD were the more universal CDs for enantioseparations. The enantiomer migration order depended to a greater extent on the CD than on the amino acid sequence of the peptide although small structural differences such as formation of a peptide amide or ester affected the chiral recognition by the randomly substituted CD derivatives. Using sulfobutylether-beta-CD or heptakis-(2,3-diacetyl-6-sulfato)-beta-CD the DD enantiomers migrated before the LL enantiomers for most peptides while the opposite migration order, i.e. LL before DD, was observed when heptakis-6-sulfato-beta-CD was applied as chiral selector.     

Cyclodextrin-modified monolithic columns for resolving dansyl amino acid enantiomers and positional isomers by capillary electrochromatography

We describe beta- and gamma-cyclodextrins (beta- and gamma-CD)-modified monolithic columns prepared by sol-gel process and chemical modifications. The monolithic silica column was fabricated inside a fused-silica capillary with 100 microm inner diameter by sol-gel process. The monolithic silica matrix was chemically modified chiral selectors of beta- or gamma-CDs with a spacer of 3-glycidoxypropyltrimethoxysilane by on-column reactions. Gamma-CD-modified monolithic column has successfully been applied for the separation of dansyl amino acid enantiomers. Beta-CD-modified monolithic column has been used for the separation of the positional isomers of o-, m-, and p-cresols and the enantioseparation of racemates of benzoin and several dansyl amino acids by capillary electrochromatography, respectively. For the separation of neutral positional isomers, a positive electric field was applied. However, for the separation of negatively charged analytes, a negative electric field was applied at the inlet of column. The separation efficiency of 5.0 x 10(4) theoretical plates/m for dansyl-L-threonine was obtained at electric field strength of -300 V/cm in the mobile phase of 50 mM 2-(N-morpholino)ethanesulfonic acid (MES)-Tris/methanol (70/30) buffer at pH 7.0. L-enantiomers were eluted as the first peak. Scanning electron micrograph showed that monolithic columns have the morphology of continuous skeleton and large through-pores.