Enantioselectivity of monolithic silica stationary phases immobilized with different concentrations cellulose tris (3,5-dimethylphenylcarbamate), analyzed with different mobile phases in capillary electrochromatography.

The 3,5-dimethylphenylcarbamate derivatives of cellulose bearing 3-(triethoxysilyl)propyl residues were immobilized in a capillary format onto a monolithic silica support by intermolecular polycondensation of the triethoxysilyl groups. The resulting columns were used for chiral separations using capillary electrochromatography. The effects of the synthesizing solvent, the selector coating procedure, the chiral selector concentration onto the silica monolith and the mobile phase pH value, on the separation of enantiomers were studied. The column-to-column reproducibility and stability also were evaluated. A test set of 14 chiral substances, including acidic, neutral, bifunctional and basic compounds, was used to investigate the effects of the factors mentioned above. Twelve pairs of enantiomers showed enantioselectivity at some of the different conditions tested. The column-to-column repeatability was satisfactory, and the prepared columns were stable under the adopted analysis conditions.

High-performance liquid chromatographic evaluation of a coated cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase having a small-pore silica support.

A two-step coating/precipitation synthetic procedure has been developed for the preparation of cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase (CSP) having a small-pore silica support. With this synthetic strategy, monodisperse, spherical CSP particles can be produced without the need for a wasteful and time-consuming sieving process. The performance of the synthesized CSP towards a variety of racemates was evaluated in the normal-phase HPLC mode. HPLC separation experiments revealed that the synthesized CSP exhibited a chiral recognition ability fully comparable to the corresponding commercial columns prepared using conventional large-pore silica as the support. Moreover, the synthesized CSP was successfully applied to semipreparative enantioseparation of a new triazole antifungal agent.

Synthesis of covalently bonded cellulose derivative chiral stationary phases with a bifunctional reagent of 3-(triethoxysilyl)propyl isocyanate.

A bifunctional reagent of 3-(triethoxysilyl)propyl isocyanate (TEPI) was initially adopted as a spacer reagent to prepare the bonded types of chiral stationary phases (CSPs) with cellulose derivatives. The silica-based CSPs were chemically prepared with non-regioselective and regioselective approaches and their chiral resolving capabilities were evaluated in terms of HPLC resolution of test enantiomers. It was observed that the chiral recognition capabilities of the non-regioselectively prepared CSPs were influenced by the amount of TEPI used. And also, the regioselectively prepared CSP generally showed a slightly higher resolution power than the non-regioselectively prepared CSP, while the non-regioselective procedures were highly advantageous to rapid preparation. In addition, chiral recognition of the prepared CSPs was affected by the properties of the used silica matrices.

Synthesis of chiral stationary phases with radical polymerization reaction of cellulose phenylcarbamate derivatives and vinylized silica gel.

Cellulose phenylcarbamate derivatives having methacrylate groups were synthesized with regioselective and non-regioselective procedures. These derivatives were chemically immobilized onto a vinylized silica gel, respectively, via a radical co-polymerization reaction. The immobilization was efficiently attained using a small amount of AIBN. The chiral recognition abilities of the prepared chiral stationary phases (CSPs) were evaluated by HPLC resolution of test enantiomers. It was observed that most of the enantiomers were completely resolved with markedly high column efficiency of 30,000-40,000 plates per metre for the eluted peaks. The effect of the amount of methacrylolyl chloride used for preparation on resolution was investigated. A direct comparison of the chiral recognition ability was made on the regioselectively and non-regioselectively prepared CSPs. In addition, the chemically bonded-type of CSPs were found to be relatively stable with addition of solvents such as tetrahydrofuran (THF) and chloroform into the mobile phase, which can lead to the dissolution of cellulose derivatives on the coated CSPs. Thus the choice of solvents used as the mobile phase is greatly extended and better resolution of several test enantiomers was observed on the prepared CSPs with THF and chloroform as a composition in the mobile phase. The batch-to-batch and run-to-run reproducibility was also discussed on the newly prepared CSPs.

Monodispersed, molecularly imprinted polymers for cinchonidine by precipitation polymerization.

Three monodispersed, molecularly imprinted polymers (MIPs) for cinchonidine (CD) have been synthesized by precipitation polymerization. MIP1 was prepared using methacrylic acid (MAA) as a functional monomer and divinylbenzene (DVB) as a cross-linker and MIP2 was prepared with further addition of 2-hydroxyethyl methacrylate (HEMA) as a co-monomer. For the preparation of MIP3, core-shell type MIP, monodispersed DVB homopolymers, which are prepared by precipitation polymerization, were used as a core and CD-imprinted MAA-DVB copolymer phases were coated onto the core. Three MIPs synthesized gave monodispersed, spherical beads in micrometer sizes. The binding characteristics and molecular recognition properties of MIP1-3 were examined by Scatchard analysis and chromatographic studies. The association constant of CD with MIP1 was the highest among MIPs prepared, while that with MIP3 was the lowest. The template molecule, CD, was more retained than its stereoisomer, cinchonine, on the three MIPs, and the stereoseparation factor of 38 was obtained with MIP3.

Application of cross-linked beta-cyclodextrin polymer for adsorption of aromatic amino acids.

Beta-cyclodextrin (beta-CyD) was cross-linked by hexamethylene diisocyanate and the polymer was investigated for adsorption of aromatic amino acids (AAA) from phosphate buffer. High adsorption rates were observed at the beginning and the adsorption equilibrium was then gradually achieved in about 45 min. The adsorption of AAA decreased with the increase of initial concentration and also temperature. Under the same conditions, the adsorption efficiencies of AAA were in the order of L-tryptophan (L-Trp) > L-phenylalanine (L-Phe) > L-tyrosine (L-Tyr). Much higher adsorption values, up to 52.4 and 43.0 mg/g for L-Trp and L-Phe, respectively, at 50 mmol/L and 3.2 mg/g for L-Tyr at 2 mmol/L, were obtained with the beta-CyD polymer at 37 degrees C. It was shown that the adsorption of AAA on the beta-CyD polymer was consistent with the Freundlich isotherm equation. The adsorption of mixed aromatic amino acids and branched-chain amino acids (BCAA) showed that AAA were preferentially adsorbed with adsorption efficiencies 10-24%, while those of BCAA were lower than 2%. It seems that the structure and hydrophobicity of amino acid molecules are responsible for the difference in adsorption, by influencing the strength of interactions between amino acid molecule and the polymer.

Capillary electrochromatographic separation of enantiomers under aqueous mobile phases on a covalently bonded cellulose derivative chiral stationary phase.

A chemically bonded cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase (CSP) was prepared by a radical polymerization reaction. The prepared CSP was packed into fused-silica capillaries with inner diameter of 75 microm to perform enantiomer separations in CEC. The electrochromatographic behavior of the CSP was investigated. On the prepared CSP, high EOF could be generated under acidic mobile phases, which represented an advantage for the separation of acidic enantiomers. Several neutral, acidic, and basic enantiomers were resolved on the prepared CSP under aqueous mobile phases. The column efficiencies were between 20,000 and 100,000 plates/m, which were much higher than those of HPLC. In addition, it was observed that the separation of some enantiomers benefited from the adoption of THF as mobile phase modifier.

Enantiomer separation of dimethyl dicarboxy alpha-biphenyl (DDB) and its analogues on a covalently bonded cellulose tris-(3,5-dimethylphenylcarbamate) CSP.

Dimethyl dicarboxy alpha-biphenyl (DDB) and its analogues represent atropisomers which have been resolved on the covalently bonded cellulose tris-(3,5-dimethylphenylcarbamate) (CDMPC) CSP. Different kinds of alcohols, tetrahydrofuran (THF), and chloroform were employed as mobile phase modifiers (MPMs), and their influence on the retention and separation of the enantiomers was investigated. Ternary mobile phases (hexane/2-propanol/THF, hexane/2-propanol/chloroform) were employed to investigate the separation of the five enantiomers. The advantages of the broader choice of solvents offered by the covalently bonded CDMPC CSP were discussed. The effect of structural variation of the enantiomers on their retention and separation was investigated.

Preparation of a positively charged cellulose derivative chiral stationary phase with copolymerization reaction for capillary electrochromatographic separation of enantiomers.

A positively charged chiral stationary phase (CSP) was prepared by chemically immobilizing cellulose 3,5-dimethylphenylcarbamate onto methacryloyldiethylenetriaminopropylated silica (MCDEAPS) via a radical copolymerization reaction. The prepared CSP was evaluated for enantiomer separation in nonaqueous capillary electrochromatography (CEC). Electroosmotic flow (EOF) generated on the prepared CSP could be significantly improved with introduction of positive charges into the CSP, and separation of enantiomers in CEC has been achieved with mobile phases of ethanol and hexane-ethanol, respectively. In addition, we investigated the solvent versatility of the immobilized CSP on enantioseparations in CEC and capillary liquid chromatography (CLC) due to the elimination of dissolution of chiral selector in a number of solvents. Chiral resolution of some enantiomers was improved by adopting tetrahydrofuran (THF) and chloroform as mobile-phase modifiers, respectively.

Capillary electrochromatographic separation of enantiomers on chemically bonded type of cellulose derivative chiral stationary phases with a positively charged spacer.

Positively charged chiral stationary phases (CSPs) were prepared for capillary electrochromatography (CEC) separation of enantiomers by chemically immobilizing cellulose derivatives onto diethylenetriaminopropylated silica (DEAPS) with tolylene-2,4-diisocyanate (TDI) as a spacer reagent. Anodic electroosmotic mobility was observed in both nonaqueous and aqueous mobile phases due to the positively charged amines on the surface of the prepared CSPs. For comparison, the traditionally used 3-aminopropyl silica (APS) was also adopted as the base material instead of DEAPS to prepare CSP. It was observed that the EOF on the DEAPS-based CSP was 18%-60% higher than that on the APS-based CSP under nonaqueous mobile phase conditions. Separation of enantiomers in CEC was performed on the positively charged CSPs with the nonaqueous mobile phases of pure ethanol or mixture of hexane-alcohol and the aqueous phases of acetonitrile-water or 95% ethanol. Fast separation of enantiomers was achieved on the newly prepared CSPs.

[Preparation of covalently bonded cellulose tris (4-methylbenzoate) derivative chiral stationary phases through a polymerization reaction].

Cellulose tris (4-methylbenzoate) derivatives (CTMB) having methacryloyl groups were synthesized with regio-selective or nonselective procedures, and were immobilized on gamma-methacrylatepropylated silica (gamma-MAPS) through a polymerization reaction. The obtained chiral stationary phases ( CSPs) were evaluated by high performance liquid chromatographic (HPLC) resolution of 9 racemates (3-butyl-phthalide, ketoprofen, trans-stilbene oxide, benzoin, benzoin analogue, Tröger' s base, warfarin, 4,4'-dimethoxy-5,6,5',6'-dimethylenedioxy-biphenyl-2,2'-dicarboxylate and 4,4'-dimethoxy-5,6,5',6'-dimethylenedioxy-biphenyl-2-methylcarboxylate-2'-ethylcarboxylate). For regio-nonselective procedure, increasing the content of vinyl group on CTMB would lead to higher immobilization efficiency and enantio-selectivity. CSPs prepared by regio-selective procedure possessed slightly higher enantioselectivities than those prepared by regio-nonselective procedure, while the latter showed some advantages for rapid and facile preparation. CSPs prepared with silica gel II (Fuji, 5 microm, 30 nm, ca. 150 m2/g) showed higher enantioselectivities than those prepared with silica gel I (Kromasil, 5 microm, 20 nm, ca. 240 m2/g). The prepared CSPs could keep stable under different eluents, even with 40% of tetrahydrofuran (THF) in mobile phases.