Do we know the mechanism of chiral recognition between cyclodextrins and analytes?

Characterization of binding properties of ephedrine derivatives to human alpha-1-acid glycoprotein

Most drugs, especially those with acidic or neutral moieties, are bound to the plasma protein albumin, whereas basic drugs are preferentially bound to human alpha-1-acid glycoprotein (AGP). The protein binding of the long-established drugs ephedrine and pseudoephedrine, which are used in the treatment of hypotension and colds, has so far only been studied with albumin. Since in a previous study a stereoselective binding of ephedrine and pseudoephedrine to serum but not to albumin was observed, the aim of this study was to check whether the enantioselective binding behavior of ephedrine and pseudoephedrine, in addition to the derivatives methylephedrine and norephedrine, is due to AGP and to investigate the influence of their different substituents and steric arrangement. Discontinuous ultrafiltration was used for the determination of protein binding. Characterization of ligand-protein interactions of the drugs was obtained by saturation transfer difference nuclear magnetic resonance spectroscopy. Docking experiments were performed to analyze possible ligand-protein interactions. The more basic the ephedrine derivative is, the higher is the affinity to AGP. There was no significant difference in the binding properties between the individual enantiomers and the diastereomers of ephedrine and pseudoephedrine.

Enantioseparation by Capillary Electrophoresis Using Cyclodextrins in an Amino Acid-Based Ionic Liquid Running Buffer

For enantioseparations of chiral drugs in capillary electrophoresis, chiral ionic liquids (CIL) can be employed instead of traditional running buffer containing a chiral selector. CILs can be applied solely or in addition to the often used cyclodextrin derivatives. Here the separation of phenethylamines, especially of ephedrine, is described using tetrabutylammonium L-argininate (125 mM) in phosphate buffer (75 mM, pH 1.5) in addition to β-cyclodextrin (30 mM). Using this dual-chiral running buffer system ephedrine, pseudoephedrine and methylephedrine, but not norephedrine, could be easily resolved.

Capillary electrophoresis separation of phenethylamine enantiomers using amino acid based ionic liquids

In recent years increasing interest was drawn towards ionic liquids in analytical separation science, such as capillary electrophoresis. Ionic liquids combining tetrabutylammonium cations with chiral amino acid based anions were prepared and investigated as capillary electrophoresis background electrolyte additives for the enantioseparation of ephedrine, pseudoephedrine, and methylephedrine isomers. For the optimization of buffer pH and ionic liquid concentration a design of experiments approach was performed. The best results for the separation of all enantiomers were achieved using 125mmol/L tetrabutylammonium l-argininate in a 75mmol/L phosphate buffer pH 1.5 containing 30mmol/L β-cyclodextrin.

Pre- and post-modification of mixed cyclodextrin-calixarene co-polymers: A route towards tunability

Various pre-modified and post-modified cyclodextrin-calixarene hyper-reticulated co-polymers were synthesized, fully characterized by different techniques (FT-IR, ¹³C{¹H} CP-MAS and LGFS solid-state NMR, thermogravimetry, porosimetry), and tested to assess their absorption abilities as nanosponges. The construction of the polymer network was accomplished exploiting the well-known CuAAC reaction between two different heptakis-6-azido-β-cyclodextrins and two different propargyloxy-calix[4]arenes. Post-modification was aimed to achieve the presence of ionizable (acidic or basic) groups on the polymer framework. Sequestration tests towards two model pollutant molecules surprisingly showed that both pristine and post-modified materials actually possess the abilities to act as pH-tunable nanosponges.

Cyclodextrin–calixarene co-polymers as a new class of nanosponges

Novel nanosponge materials, obtained by co-polymerization of cyclodextrin and calixarene derivatives, have been characterized by various techniques and tested for the removal of model pollutants from aqueous solution.

Chiral selectors in CE: recent developments and applications

This review article provides an overview of the recent advances in enantioanalysis by use of electrophoretic techniques. Due to the big number of publications in the subject mentioned above, this article is focused on chiral method developments and applications published from 2008 until 2011, and it demonstrates chiral selectors used in CE. Numerous chiral selectors have been used over the years, and these include the cyclic and the linear oligo- and polysaccharides, the branched polysaccharides, the polymeric and monomeric surfactants, the macrocyclic and other antibiotics, and the crown ethers. Different dual-selector systems are also presented in this article, and the results are compared with those obtained by use of a single chiral selector. Finally, several pharmaceutical and biomedical applications based on chiral recognition are summarized.

Cyclodextrin-mediated enantioseparation in microemulsion electrokinetic chromatography

Chiral separation by means of cyclodextrins has a long-standing tradition in capillary electrophoresis techniques. Here we present a chiral method utilizing the recently introduced microemulsion electrokinetic chromatography. The microemulsion consisting of 1.0% SDS, 4.0% 1-butanol, 3.0% 2-propanol, 0.5% ethylacetate, and 91.5% 20 mM phosphate buffer pH 2.5 serves as a pseudostationary phase which is complemented by sulfated cyclodextrin as a second phase. The analytes partition between the aqueous running buffer and both pseudostationary phases, the oil droplets and the cyclodextrins. Enantiomers are separated due to the formation of transient diastereomeric complexes with the cyclodextrins. For the racemates of ephedrine derivatives studied here sulfated β-cyclodextrin was successfully applied. The method is appropriate to resolve an entire series of chiral phenethylamines and can be used for separation of the racemates and impurity profiling, e.g., the determination of the enantiomeric excess.

Synthesis of mono-6-deoxy-6-N,N,N′,N′,N′-pentamethylethylenediammonio-cyclomaltoheptaose, a single-isomer, monosubstituted, permanently dicationicβ-CD and its use for enantiomer separations by CE

The dichloride salt of mono-6-deoxy-6-N,N,N',N',N'-pentamethylethylenediammonio-cyclomaltoheptaose (PEMEDA-BCD), the first single-isomer, monosubstituted, permanently dicationic beta-CD has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of analytes in acidic and basic BGEs. When the concentration of PEMEDA-BCD was changed in the BGEs, the resulting effective mobilities of the analytes and the respective separation selectivities followed the predictions of the ionic strength-corrected charged resolving agent migration model. Good separation selectivities and favorable normalized EOF mobilities allowed for the rapid, efficient separation of the enantiomers of anionic, weak acid and nonionic analytes in the low- and/or high-pH BGEs.

Synthesis of a single-isomer sulfated β-cyclodextrin carrying nonidentical substituents at all of the C2, C3, and C6 positions and its use for the electrophoretic separation of enantiomers in acidic aqueous and methanolic background electrolytes, Part 2: Heptakis(2-O-methyl-6-O-sulfo) cyclomaltoheptaose

The sodium salt of heptakis(2-O-methyl-6-O-sulfo)cyclomaltoheptaose (HMS), the second single-isomer, sulfated beta-CD carrying nonidentical substituents at all of the C2, C3, and C6 positions, has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of 23 weak base analytes in acidic aqueous and methanolic BGEs. HMS interacted strongly with only about half of the analytes studied. The good separation selectivities and favorable normalized EOF mobilities allowed for rapid, efficient separation of the enantiomers of 19 of the 23 weak base analytes in the aqueous BGEs, often with separation selectivity values complimentary to those obtained with other single-isomer sulfated CDs. HMS did not prove to be as good a resolving agent in acidic methanolic BGEs as its counterpart, heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)cyclomaltoheptaose.

Direct fluorimetric determination of ascorbic acid by the supramolecular system of AA with beta-cyclodextrin derivative

We established a simple and selective fluorimetric method for the determination of ascorbic acid (AA) with the novel mono-[6-N(4-carboxy-phenyl)]-beta-cyclodextrin (ACD). The method is based on the fluorescence intensity of ACD decreases as the ACD-AA supramolecular complexes form. The fluorescence intensity is measured at excitation and emission wavelengths of 270 and 352 nm, respectively. Under optimum condition, a linear relationship is obtained between the fluorescence intensity and the concentration of AA in the range of 0.05-8.0 microgml(-1). The detection limit is 0.012 microgml(-1). The method has been applied to the direct analysis of AA in real samples with satisfactory results.

Synthesis of heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)-cyclomaltoheptaose, a single-isomer, sulfated β-cyclodextrin carrying nonidentical substitutents at all the C2, C3, and C6 positions and its use for the capillary electrophoretic separation of enantiomers in acidic aqueous and methanolic background electrolytes

The sodium salt of heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)cyclomaltoheptaose (HMAS), the first single-isomer, sulfated beta-cyclodextrin carrying nonidentical substituents at all of the C2, C3, and C6 positions, has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of 24 weak base pharmaceuticals in acidic aqueous and acidic methanolic background electrolytes. HMAS interacted more strongly with most of the analytes studied than heptakis(2,3-di-O-methyl-6-O-sulfo)cyclomaltoheptaose, but less strongly than heptakis(2,3-di-O-acetyl-6-O-sulfo)cyclomaltoheptaose, the respective analogs with identical substituents at the C2 and C3 positions. The good separation selectivities and favorable normalized electroosmotic flow mobilities allowed for rapid, efficient separation of the enantiomers of 19 of the 24 weak base analytes in the aqueous and methanolic background electrolytes. The trends in the effective mobilities and separation selectivities as a function of the HMAS concentration closely followed the predictions of the ionic strength-corrected charged resolving agent migration model.

Non-aqueous capillary electrophoretic enantiomer separations using the tetrabutylammonium salt of heptakis(2,3-O-diacetyl-6-O-sulfo)-cyclomaltoheptaose, a single-isomer sulfated β-cyclodextrin highly-soluble in organic solvents

The tetrabutylammonium salt of heptakis(2,3-di-O-acetyl-6-O-sulfo)-cyclomaltoheptaose, a single-isomer sulfated beta-cyclodextrin that is adequately soluble in a number of protic and aprotic polar solvents was synthesized on the large scale and used for the capillary electrophoretic separation of the enantiomers of weak bases in acidic acetonitrile background electrolytes. The effective mobilities and separation selectivities observed for these analytes followed trends similar to those found with other single-isomer sulfated cyclodextrins in acidic methanol background electrolytes. Enantiomer separations obtained with the tetrabutylammonium and sodium salts of heptakis(2,3-di-O-acetyl-6-O-sulfo)-cyclomaltoheptaose were different indicating, for the first time, that selection of the counter ion of the single-isomer sulfated cyclodextrin is also of importance for the separation of enantiomers.

Characterization of sulfatedβ-cyclodextrins and determination of enantiomeric purity of (1R,2S)-ephedrine by capillary zone electrophoresis

Matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy were employed to characterize two different sulfated beta-cyclodextrins (HS-beta-CD). It was found that the HS-beta-CDs have broad heterogeneity in terms of sulfation degree. The average sulfate contents were in the range of 6 to 8 per CD molecule. Furthermore the sugar moieties of both HS-beta-CDs were sulfated either at position 6 or at positions 2 and 6, but not at position 3. Enantiomeric separation by capillary zone electrophoresis (CZE) using the HS-beta-CDs as chiral selectors showed that these CDs exhibited similar chiral selectivity and resolution of the ephedrine enantiomers. One of the CDs was employed for the enantiomeric purity evaluation of (1R,2S)-ephedrine (or (-)-ephedrine) by capillary zone electrophoresis. Quantification was done by comparison between the corrected peak areas of the minor enantiomer and (-)-ephedrine.

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.

Quantitative approach for the screening of cyclodextrins by nuclear magnetic resonance spectroscopy in support of chiral separations in liquid chromatography and capillary electrophoresis enantioseparation of norgestrel with alpha-, beta- and gamma-cyclodextrins

A quantitative NMR approach is proposed for the screening of cyclodextrins with regard to their enantioselectivity as chiral mobile phase additives in column reversed-phase chromatography and capillary electrophoresis. Similarities and differences between the mechanism of enantiomeric peak-separation in NMR and HPLC and CE are interpreted. The affinity of d-norgestrel to bind to (alpha-, beta-, gammay-) cyclodextrins in aqueous solution was quantified and compared by determining the association constants from chemical shift data. The association constant of l-norgestrel was estimated from titration of the racemate. Differences between the apparent association constants of the enantiomerically pure drug and the racemate are discussed from the point of view of enantiomeric competition for the cyclodextrin. The apparent association constants and chiral selectivities determined by 'H NMR for dl-norgestrell/gamma-CD system at various water-methanol ratios are correlated with the corresponding chromatographic results found in the literature. The pitfalls of previously proposed screening methods based on comparison of chemical shift differences with separation parameters are discussed.