Preparation and chiral recognition of a novel chiral stationary phase for high-performance liquid chromatography, based on mono(6A-N-allylamino-6A-deoxy)-perfunctionalized β-cyclodextrin and covalently bonded silica gel

Light-assisted preparation of a cyclodextrin-based chiral stationary phase and its separation performance in liquid chromatography

A facile light-assisted preparation procedure of a cyclodextrin-based chiral stationary phase was developed for enantioseparations in HPLC.

Trimethyl-β-cyclodextrin-encapsulated monolithic capillary columns: Preparation, characterization and chiral nano-LC application

Trimethylated-β-cyclodextrin (TM-β-CD) was encapsulated within several polymer monolithic capillary columns for reversed-phase chiral nano-liquid chromatography (nano-LC). The monolithic phases were prepared using the one-pot in situ copolymerization of ethylene glycol dimethacrylate (EDMA), glycidyl methacrylate (GMA) monomers and 1-propanol, 1,4-butanediol as progenic solvents in presence of TM-β-CD solution within fused silica capillaries (150µm I.D.). The obtained chiral monolithic stationery phases were characterized by scanning electron microscopy (SEM), N₂ adsorption/desorption isotherms, wide angle x-ray diffraction (WAXRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The materials characterization demonstrated that monolithic phases with higher concentration of TM-β-CD have relatively larger surface area, smaller pore size and larger total pore volume compared to those with lower concentration TM-β-CD. The prepared columns were tested for their enantioseparation efficiency of a range of racemic pharmaceuticals. The screening results demonstrated the potential of functionalizing polymer monolithic stationary phases with TM-β-CD using the in situ encapsulation approach.

Enantioseparation of basic chiral drugs on a carbamoylated erythromycin-zirconia hybrid monolith using capillary electrochromatography

An organic-inorganic hybrid monolithic column was prepared within the confines of a capillary via a single-step in situ sol-gel approach using zirconium tetrabutoxide as a precursor to compose the inorganic backbone and 3-triethoxysilylpropyl carbamoylated derivative of erythromycin (TEOSPC-ERY) as a co-precursor to introduce the organic chiral selector moiety in the zirconia backbone. The resulting carbamoylated ERY-zirconia hybrid monolith (ERY-ZHM) showed homogeneous morphology with well-defined through pores and was tightly connected with the inner wall of the capillary. The column was employed for capillary electrochromatographic enantioseparation of six basic chiral drugs in mobile phases (MPs) consisting of acetonitrile (ACN) and triethylammonium acetate (TEAA) buffer. The effects of composition of MP and applied voltage on chiral separation were investigated by using propranolol as a representative analyte. The highest resolution (Rs=3.33) was obtained with a MP consisting of 10/90 (v/v) ACN/TEAA buffer (10mM, pH 7), 10 kV applied voltage and 25°C capillary temperature. The relative standard deviations for resolution values regarding run to run, day to day, column to column and batch to batch repeatability were 0.41%, 0.89%, 1.80% and 2.26% (for n=3), respectively, indicating satisfactory stability of columns and reproducibility of column preparation process.

Linkage of α-cyclodextrin-terminated poly(dimethylsiloxanes) by inclusion of quasi bifunctional ferrocene

We report the noncovalent linkage of terminally substituted oligo(dimethylsiloxanes) bearing α-cyclodextrins (α-CD) as host end groups for the cyclopentadienyl rings of ferrocene. This double complexation of unsubstituted ferrocene leads to a supramolecuar formation of the siloxane strands. Structural characterization was performed by the use of (1)H NMR and IR spectroscopy and by mass spectrometry. Electron microscopy studies and dynamic light scattering measurements show a significant decrease of the derivative size after the complexation with ferrocene. In addition, further evidence for the successful complexation of the end groups was verified by the shifts of the protons in the (1)H NMR spectra and in the correlation signals of the 2D ROESY NMR spectra.

Preparation of perphenylcarbamoylated β-cyclodextrin-silica hybrid monolithic column with "one-pot" approach for enantioseparation by capillary liquid chromatography

Perphenylcarbamoylated β-cyclodextrin-silica (Ph-β-CD-silica) hybrid monolithic columns for enantioseparation in capillary liquid chromatography (cLC) have been prepared by a "one-pot" approach via the polycondensation of alkoxysilanes and in situ copolymerization of mono (6(A)-N-allylamino-6(A)-deoxy)-Ph-β-CD and vinyl group on the precondensed siloxanes. The morphologies of the Ph-β-CD-silica hybrid monolithic columns were characterized by optical microscopy and scanning electron microscopy (SEM), showing the uniform monolithic matrixes tightly bonded onto the capillary wall. The content of Ph-β-CD incorporated in monolithic matrix by the "one-pot" approach was ca. 2.9 times higher than that by postmodification method. The permeability of the Ph-β-CD-silica chiral hybrid monolithic column was 3.63 × 10(-14) m(2), and the minimum plate height was 12 μm corresponding to 83,300 theoretical plates/meter. Enantioseparations of 13 racemates were achieved by the Ph-β-CD-silica hybrid monolithic column. In this work, since the prepolymerization system mainly consisted of organic solvent (methanol (MeOH), N,N-dimethylformamide (DMF)), the limitation and difficulty of the use of water insoluble organic monomers in the previously reported "one-pot" method was circumvented. Therefore, various β-CD derivatives as well as other hydrophobic monomers could thus be used to prepare organic-silica hybrid monolithic columns with the "one-pot" process.

Application of Click-chemistry-based perphenylcarbamated beta-CD chiral stationary phase in CEC

A novel chiral stationary phase (CSP) was prepared by anchoring mono-6-azido-6-deoxyperphenylcarbamated beta-CD onto omega-alkynyl functionalized silica (5 microm) via organic soluble Cu(I)-catalyzed Click chemistry. The obtained CSP was thereafter packed into fused-silica capillary (100 microm id) with an effective length of 9 cm and tested in aqueous CEC to separate a series of racemic aryl alcohols. High separation factors with good resolution were achieved using the current novel CSP. Some pharmaceutical compounds could also be well resolved on this newly derived CSP. The analytical results demonstrate that Click-chemistry-based perphenylcarbamated CSP affords high stability in high electric field and depicts excellent enantioseparation in CEC. The effects of pH value, buffer concentration, applied voltage, concentration of organic modifier and analyte structure on the enantioseparation are also discussed.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Enantioseparation of a novel "click" chemistry derived native beta-cyclodextrin chiral stationary phase for high-performance liquid chromatography

A novel native beta-cyclodextrin chiral stationary phase was prepared via "click" chemistry with cuprous iodide-triphenylphosphine complex as the catalyst and applied for enantioseparation of Dns-amino acids, substituted phenyl and phenoxy group modified propionic acids, flavonoids, and some pharmaceutical compounds such as nimodipine, propranolol, brompheniramine and bendroflumethiazide in reversed-phase high-performance liquid chromatography. The studied analytes could be resolved under different separation conditions. The resolution of Dns-DL-Leu could reach 5.08 using a mobile phase consisting of 1% (w/w) triethylammonium acetate buffer (pH 4.11) and methanol (50:50 v/v). The effects of buffer pH and the content of organic modifier on enantioseparation of Dns-amino acids by this novel chiral phase were being investigated. The separation results demonstrate that click chemistry, a versatile reaction, affords a facile approach towards the preparation of stable chiral stationary phases.

Preparation of a new chiral stationary phase for HPLC based on the (R)- 1-phenyl-2-(4-methylphenyl)ethylamine amide derivative of (S)-valine and 2-chloro-3,5-dinitrobenzoic acid: Enantioseparation of amino acid derivatives and pyrethroid insecticides

A novel chiral stationary phase (CSP) for HPLC was prepared by bonding (R)-1-phenyl-2-(4-methylphenyl)ethylamine amide derivative of (S)-valine to aminopropyl silica gel through a 2-amino-3,5-dinitro-1-carboxamido-benzene unit. The CSP was used for the separation of some amino acid derivatives and pyrethroid insecticides by chiral HPLC. Satisfactory baseline separation required optimization of the variables of mobile phase composition. Use of dichloromethane as modifier in the mobile phase gave baseline separations of amino acid derivatives. The two enantiomers of fenpropathrin and four stereoisomers of fenvalerate were baseline separated using hexane-dichloromethane-ethanol as mobile phase. The results show that the enantioselectivity of the new CSP is better than Pirkle type 1-A column for these compounds. Only partial separations were observed for the stereoisomers of cypermethrin and cyfluthrin, which gave even and eight peaks, respectively.

Ice Chromatography. Characterization of Water−Ice as a Chromatographic Stationary Phase

Water-ice has been characterized as a stationary phase for liquid chromatography. Solutes having two or more polar groups are retained on this stationary phase with THF/hexane as the mobile phase, suggesting that multipoint interactions are required for measurable solute retention. Chromatographic separation of phenols or crown ethers on water-ice is possible. The ice surface is expected to provide two different adsorption sites coming from the OH and O dangling bonds. Although the solute partition into the quasiliquid layer is also considered, the dependence of the retention times on the THF concentration implies that the interaction of solutes with the water-ice surface rather than the partition into the quasiliquid layer is responsible for solute retention. A retention model suggests that the number of adsorption sites for a crown ether depends on its ring size, whereas two sites are involved for the retention of phenols having two hydroxyl groups. Although hydroxyl groups can act as both a hydrogen bond donor and an acceptor, the interaction with the ice OH sites, which are exposed to the surroundings in comparison with the ice O sites, is more important. However, when an acyclic polyether is added to the mobile phase, its adsorption onto the water ice surface allows the creation of the O sites that phenols can approach without steric hindrance. In the presence of the polyethers adsorbed on the ice surface, the retention of phenols is enhanced, whereas crown ethers become less retained due to the competitive adsorption of the polyethers.

Development of dinitrophenylated cyclodextrin derivatives for enhanced enantiomeric separations by high-performance liquid chromatography

The synthesis and evaluation of new dinitrophenyl (DNP) substituted beta-cyclodextrin (beta-CD) chiral stationary phases (CSPs) for the enantioseparation of various classes of chiral analytes by HPLC are presented. The dinitrophenyl substituted beta-CD derivatives are synthesized and covalently bonded to functionalized 5 microm spherical porous silica gel. These are the first reported derivatized cyclodextrin which contains pi-electron deficient substituents (i.e., pi-acidic moieties). The column performance in terms of their ability to separate enantiomers is evaluated. A variety of different dinitro-substituted aryl groups are investigated and compared. The pH of the mobile phase buffers, the buffer composition, the number and position of the dinitro groups on the phenyl ring substituent, the degree of substitution, and the bonding strategy all greatly affect the performance of the CSPs. A large variety of racemic compounds have been separated successfully on these CSPs. The bonded dinitrophenyl-derivatized cyclodextrins are stable in all three mobile phase modes, namely, the reversed-phase, polar organic, and normal phase modes. No degradation in column performance was observed in any mode of operation even after more than 1000 injections. The analytical applicability of these types of CSPs for enantiomeric separations is discussed in detail.

Click chemistry: a new facile and efficient strategy for preparation of functionalized HPLC packings

Click chemistry has been successfully extended into the field of preparation of functionalized HPLC packings, proving a novel facile and efficient strategy for covalently bonding stationary phases onto HPLC grade silica beads; the potential has been demonstrated by the preparation of "Click I-IV" columns and preliminary results in the separation of sugars.

Synthesis of polymer-type chiral stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two new chiral polymers of different molecular weights were synthesized by the copolymerization of (1R,2R)-(+)-1,2-diphenylethylenediamine, phenyl diisocyanate and terephthaloyl chloride. The polymers were immobilized on aminated silica gel to afford two chiral stationary phases. The polymers and the corresponding chiral stationary phases were characterized by Fourier transform-IR, elemental analysis, 1H and 13C NMR. The surface coverages of chiral structural units on the chiral stationary phases were estimated as 0.27 and 0.39 mmol/g, respectively. The enantioseparation ability of these chiral stationary phases was evaluated with a variety of chiral compounds by high-performance liquid chromatography. The effects of the organic additives, the composition of mobile phases, and the injection amount of sample on enantioseparation were investigated. A comparison of enantioseparation ability between these two chiral stationary phases was made. It was believed that the chain length of polymeric chiral selector significantly affected the enantioseparation ability of corresponding chiral stationary phase.

Synthesis and application of mono-2A-azido-2A-deoxyperphenylcarbamoylated beta-cyclodextrin and mono-2A-azido-2A-deoxyperacetylated beta-cyclodextrin as chiral stationary phases for high-performance liquid chromatography

Two novel chiral stationary phases (CSPs) were prepared based upon the regioselective immobilizations of beta-cyclodextrin (beta-CD) at its C2 position to the silica support. The mono-2A-azido-2A-deoxyperphenylcarbamoylated beta-cyclodextrin and mono-2A-azido-2A-deoxyperacetylated beta-cyclodextrin were synthesized by selective tosylation and azidolysis followed by perfunctionalisation. The derivatised cyclodextrins were then immobilized onto the aminised silica gel via the Staudinger reaction to provide new chiral stationary phases. Their application to high-performance liquid chromatography (HPLC) enantioseparation of racemic compounds was demonstrated using beta-adrenergic blockers, flavonone compounds, benzodiazepinones, antihistamines and weakly protolytic compounds, of which good separations were achieved for some racemic compounds, for instance, bendroflumethiazide (Rs 6.26), oxazepam (Rs 5.99), temazepam (Rs 2.85) and althiazide (Rs 1.13) when compared with the corresponding CSPs where the beta-CD molecule was regioselectively immobilized at the C6 position. The enantiodiscriminatory properties of these CSPs were found to be affected by the orientation of the CD cavity under reversed-phase conditions, and also by the derivitising groups of the CD. The HPLC results inferred that the mono-6A-azido-6A-deoxyperphenylcarbamoylated CD CSP (CD bonded at C6 position to silica) exhibited slightly better chiral recognition ability than mono-2A-azido-2A-deoxyperphenylcarbamoylated CD CSP under the normal-phase and reversed-phase modes on the separation of 31 different racemic compounds and drugs. On the contrary, higher chiral recognition abilities were observed on the mono-2(A)-azido-2A-deoxyperacetylated CD CSP compared to mono-6A-azido-6A-deoxyperacetylated CD CSP.

Chiral recognition of aromatic compounds by β-cyclodextrin based on bimodal complexation

The chiral recognition of the selected aromatic chiral compounds by native beta-cyclodextrin (beta-CD) based on bimodal complexation was studied using a flexible docking algorithm FDOCK. A quantitative empirical free energy relationship model was employed to predict the complex stability constants and the preferred binding modes. The results showed that the calculated complex stability constants are in good agreement with the experimental data. Furthermore, the main force responsible for host-guest complexation is the van der Waals force and the chiral molecules are completely included into the beta-CD cavity. The chiral recognition for the selected aromatic chiral compounds is the result of the van der Waals force counterbalancing with the other effects, such as the electrostatic interaction and the hydrophobic effect.

Enantioseparation on mono(6A-N-allylamino-6A-deoxy)permethylated β-cyclodextrin covalently bonded silica gel

A chiral selector, mono(6A-N-allylamino-6A-deoxy)permethylated beta-cyclodextrin, was synthesized through a facile synthetic route and chemically immobilized onto porous silica gel via hydrosilylation to afford a cyclodextrin based chiral stationary phase MeCD-CSP. This chiral stationary phase exhibited good enantioselectivity under standard HPLC conditions. The optimal resolution of 1-(p-bromophenyl)ethanol and bromopheniramine was achieved under normal-phase conditions using a mobile phase comprising n-hexane (hexane) and 2-propanol (IPA). The enantioseparation of warfarin, suprofen and a series of flavanones under reversed-phase conditions were optimized and efficient enantioseparations for these analytes were obtained.