[show abstract][hide abstract] ABSTRACT: The high-performance liquid chromatographic enantioresolution of β-substituted tryptophan analogues by direct and indirect
methods is reported. The direct separation was carried out on chiral crown ether-bonded, on the macrocyclic antibiotic teicoplanin-bonded
or on different derivatized β-cyclodextrin-bonded phases. The indirect resolution was achieved by applying pre-column derivatization
with the chiral reagents 1-fluoro-2,4-dinitrophenyl-5-L-alanine amide (FDAA) and 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl isothiocyanate (GITC). In the separation of all four isomers of the analytes, the indirect methods
seemed to be more efficient than the direct methods. In most cases, the FDAA derivatives of the stereoisomers could be separated
with better selectivity in methanol-containing mobile phases.
[show abstract][hide abstract] ABSTRACT: The enantiomers of 1-(α-aminobenzyl)-2-naphthol and 2-(α-aminobenzyl)-1-naphthol analogs were separated isothermally on a 3,5-dimethylphenylcarbamoylated β-cyclodextrin-based chiral stationary phase (Cyclobond DMP), with an n-hexane/alcohol modifier as mobile phase. Optimization of the separation was achieved by variation of combinations of the
polar mobile phase additives ethanol and methanol. The nature and position of the α-aminobenzyl substituent of the 1- and 2-naphthol analogs influenced the retention and the selectivity.
[show abstract][hide abstract] ABSTRACT: The comparisons of five different chiral stationary phases (CSPs) based on 2,6-dinitro-4-trifluoromethylphenyl (DNP-TFM) ether substituted β-cyclodextrin are presented. The five CSPs differ from each other in the linkage/spacer chemistry, or on the position of the substituents on β-cyclodextrin, or in the sequence of the synthetic procedure. The results show that there are two optimum combinations: (1) DNP-TFM randomly substituted on the β-cyclodextrin as the chiral selector along with a carbamate linkage chain bonding it to the silica support; and (2) β-cyclodextrin derivatized by DNP-TFM substituents only on the C-2 and C-3 positions of the cyclodextrin with an ether linkage chain anchoring it to the silica gel. These two combinations show complementary separations for some enantiomers. The spacer chain effect is much more pronounced for the CSP based on the β-cyclodextrin derivatives with DNP-TFM substituents only on C-2 and C-3 positions than its randomly substituted counterpart. The sequence of derivatizing the cyclodextrin and attaching it to silica gel also affects its selectivity and efficiency. The β-cyclodextrin should be derivatized before it is linked to the silica gel.
[show abstract][hide abstract] ABSTRACT: Direct reversed-phase high-performance liquid chromatographic methods were developed for the separation of enantiomers of
eighteen unnatural β-amino acids, including several β-3-homo-amino acids. The direct separations of the underivatized analytes were performed on chiral stationary phases containing
macrocyclic glycopeptide antibiotics such as teicoplanin (Chirobiotic T and T2), teicoplanin aglycone (Chirobiotic TAG), vancomycin
(Chirobiotic V and V2), and ristocetin A (Chirobiotic R) as chiral selectors. The effects of the organic modifier, mobile
phase composition and pH on the separations were investigated. A comparison of the separation performances of the macrocyclic
glycopeptide stationary phases revealed that the Chirobiotic T2 column exhibited better selectivity than the Chirobiotic T
column for the separation of β-3-homo-amino acid enantiomers; vancomycin or ristocetin A exhibited lower selectivity. The elution sequence was determined
in some cases: the S enantiomers eluted before the R enantiomers, with the exception of the Chirobiotic R column, where the elution sequence R < S was observed.
[show abstract][hide abstract] ABSTRACT: Direct reversed-phase high-performance liquid chromatographic methods were developed for the separation of enantiomers of
β-lactams. The enantiomers of 7 aryl-substituted β-lactams were separated on chiral stationary phases containing the macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic
T) and teicoplanin aglycone (Chirobiotic TAG) at 10-°C increments in the range 5–45 °C, using different compositions of 0.1%
aqueous triethylammonium acetate (pH 4.1)/methanol (v/v) as mobile phase. The mobile phase composition and temperature were varied to achieve baseline resolutions in a single chromatographic
run. The dependence of the natural logarithms of the selectivity factors ln α on the inverse of temperature, 1/T, was used to determine the thermodynamic data on the enantiomers. The thermodynamic data revealed that all the compounds
in this study undergo separation via the same enthalpy-driven chiral recognition mechanism. The different methods were compared
in systematic chromatographic examinations. The effects of the organic modifier, the mobile phase composition and the temperature
on the separation were investigated.
[show abstract][hide abstract] ABSTRACT: For the direct separation of enantiomers of tryptophan analogs, a native α-cyclodextrin-based Cyclobond III chiral stationary phase was examined. The separation conditions were optimized by variation
of the different chromatographic parameters, and data are reported as retention factors, separation factors and resolutions.
The nature and concentration of the buffers as mobile phase additivies exerted only slight effects on the retention and resolution.
In all cases, the erythro isomers could be separated significantly better than the threo ones. With increasing hydrophobicity of the analytes, the retention time increased, but no clear trend could be observed
between the resolution and the hydrophobicity. The sequence of elution of the enantiomers was in most cases determined by
spiking the racemic samples with enantiorich analytes.
[show abstract][hide abstract] ABSTRACT: Direct reversed-phase high-performance liquid chromatographic methods were developed for the separation of the enantiomers
of tricyclic β-lactams, cis-3,4-benzo-6-azabicyclo[3.2.0]heptan-7-one, cis-4,5-benzo-7-azabicyclo[4.2.0]-octan-8-one, cis-5,6-benzo-8-azabicyclo[5.2.0]nonan-9-one and new bicyclic β-amino acids, the six- and seven-membered homologues of cis-1-amino-4,5-benzocyclopentane-2-carboxylic acid (benzocispentacin), cis-1-amino-5,6-benzocyclohexane-2-carboxylic acid and cis-1-amino-6,7-benzocycloheptane-2-carboxylic acid. The direct separations of the analytes were performed on chiral stationary
phase (CSP) columns containing the macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T), teicoplanin aglycone (Chirobiotic
TAG), vancomycin (Chirobiotic V), vancomycin aglycone (Chirobiotic VAG), ristocetin A (Chirobiotic R) or a new dimethylphenyl
carbamate-derivatized β-cyclodextrin-based Cyclobond DMP. The results achieved with the different methods were compared in systematic chromatographic
examinations. The effects of an organic modifier and of the mobile phase composition on the separation and the separation
efficiency of different columns were investigated. The difference in enantioselective free energy between the aglycone CSP
and the teicoplanin CSP for these β-lactams and β-amino acids ranged between 0.3 and −1.1 kJmol−1. Better enantioseparations were attained in most cases on the aglycone CSP.
[show abstract][hide abstract] ABSTRACT: The enantiomeric separation of 12 β‐lactam compounds on 3 native cyclodextrin and 6 derivatized β‐cyclodextrin stationary phases was evaluated using high performance liquid chromatography (HPLC). The dimethylphenyl carbamate functionalized chiral stationary phase (CSP) (Cyclobond I 2000 DMP) separated 11 of the 12 β‐lactams in the reversed phase mode. The dimethylated β‐cyclodextrin column (Cyclobond I 2000 DM) was the second most effective CSP and it separated 8 of the 12 compounds. The reversed phase separation mode was the most effective approach. The effects of the composition and the flow rate on enantioseparations were studied. The effect of the structure of the substituents on the β‐lactams was examined.
Journal of Liquid Chromatography & Related Technologies - J LIQ CHROMATOGR RELAT TECHNO. 01/2006; 29(13):1847-1860.
[show abstract][hide abstract] ABSTRACT: A synthetic polymeric chiral stationary phase for liquid chromatography based on N, N′-[(1R,2R)-1,2-diphenyl-1,2-ethanediyl]bis-2-propenamide monomer was prepared via a simple solution initiated radical polymerization.
This stable chiral stationary phase showed enantioselectivities for a large number of racemates in polar organic and normal
phase modes and high sample loading ability. However, none of the generated data has been optimized in terms of column performance.
Different enantioselectivities were observed on this new chiral stationary phase compared with the commercial polymeric chiral
stationary phase based on N-(2-acryloylamino-(1R,2R)-cyclohexyl)-acrylamine monomer. Consequently, these two chiral stationary phases are considered
complementary to one another. Furthermore they utilize the same mobile phase and optimization procedures. This polymeric chiral
stationary phase appears to be useful for preparative separations since high amounts of analyte can be injected without loosing
[show abstract][hide abstract] ABSTRACT: The enantiomeric separation of 12 chiral dihydrobenzofurans is reported on derivatized -cyclodextrin stationary phases using high performance liquid chromatography. The hydroxypropyl -cyclodextrin chiral stationary phase (CSP) with acetonitrile/water mobile phases was the most effective combination as it baseline separated 9 of the 12 compounds. The acetyl -cyclodextrin and 2,3-dimethyl -cyclodextrin CSPs were also effective in the reverse phase mode. The native -cyclodextrin was far less effective than the non-aromatic derivatized CSPs. The aromatic functionalized CSPs showed no selectivity in the normal phase mode. Structural characteristics, such as substituent polarity and ring location, were important in the observed enantioselectivity.
[show abstract][hide abstract] ABSTRACT: Twenty chiral isochromene derivatives have been chromatographed on native and derivatized cyclodextrin stationary phases using HPLC. The most effective CSPs for the enantioresolution of these analytes in the reverse phase mode are the hydroxypropyl--cyclodextrin (Cyclobond RSP), the 2,3-dimethyl--cyclodextrin (Cyclobond DM), and the -cyclodextrin (Cyclobond II) stationary phases. The -cyclodextrin (Cyclobond III), -cyclodextrin (Cyclobond I), acetyl--cyclodextrin (Cyclobond AC), S-1-naphthylethyl carbamate--cyclodextrin (Cyclobond SN), and 3,5-dimethylphenyl carbamate--cyclodextrin (Cyclobond DMP) stationary phases also show enantioselectivities for some analytes. No enantioseparations were observed in the polar organic mode and only a few separations were found in the normal phase mode. The Cyclobond RSP CSP provided the best overall separations of these analytes in the reverse phase mode. The pH of the mobile phase and the nature of organic modifiers have little effect on the enantioresolution. The substituents on the isochromene ring greatly affect the chiral recognition.
[show abstract][hide abstract] ABSTRACT: The enantioselectivity of native and derivatized cyclodextrin stationary phases and macrocyclic glycopeptides for chiral pterocarpans was evaluated using high performance liquid chromatography (HPLC). All enantiomers could be baseline resolved in the reverse phase mode on cyclodextrin‐based Cyclobond chiral stationary phases (CSPs). The hydroxypropyl‐β‐cyclodextrin, acetyl‐β‐cyclodextrin, and gamma‐cyclodextrin CSPs show the broadest enantioselectivity in the reverse phase mode. Some compounds were baseline separated on the ristocetin A and vancomycin macrocyclic glycopeptide chiral stationary phases in the reverse phase mode. Separations on the ristocetin A columns produced the highest resolutions (up to ∼7.1) in this study. The 3,5‐dimethylphenyl carbamate derivatized cyclodextrin column showed the broadest enantioselectivity in normal phase LC. Of the macrocyclic glycopeptide CSPs, ristocetin A and teicoplanin aglycone (Chirobiotic R and TAG, respectively) showed enantioselectivity for the most compounds in the normal phase mode. However, baseline separations were only achieved with the teicoplanin and teicoplanin aglycone.
Journal of Liquid Chromatography & Related Technologies - J LIQ CHROMATOGR RELAT TECHNO. 01/2005; 28(6):823-834.
[show abstract][hide abstract] ABSTRACT: Direct and indirect reversed-phase high-performance liquid chromatographic methods were developed for the separation of enantiomers of seventeen unnatural -amino acids, including several -3-homo amino acids. The direct separations of the underivatized analytes were performed on chiral stationary phases containing macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T column) and teicoplanin aglycone (Chirobiotic TAG column). The indirect method involved pre-column derivatization with two new chiral derivatizing agents, (1S,2S)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propylisothiocyanate, (S,S)-DANI and (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester, (S)-NIFE. The different methods were compared in systematic chromatographic examinations. The effects of organic modifier, mobile phase composition, pH and flow rate on the separation were investigated.
[show abstract][hide abstract] ABSTRACT: 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF(6)) was synthesized and purified to be used as a ionic liquid solvent. Its physicochemical properties were studied. The ionic liquid/water (P(il/water)) and ionic liquid/heptane (P(il/heptane)) distribution coefficients of a set of 40 compounds with various functionalities, including organic acids, organic bases, amino acids, antioxidants, and neutral compounds, were measured using liquid chromatography. For ionizable compounds, the P(il/water) values measured at pH 2, 5.1, and 10 were very different. These allowed the determination of both the molecular P(o)il(/water) values and the ion P(-)il(/water) value for each compound. These coefficients were compared to the corresponding P(oct/water) coefficients. Marked differences in the partitioning behavior of basic, acidic, and neutral compounds were observed. The relationship between P(il/water) and P(oct/water) is different from that reported previously. By using the linear free energy solvation approach and the descriptors found for 12 solutes, the BMIM-PF(6) solvent parameters were calculated for the ionic liquid/water and ionic liquid/heptane biphasic systems. The regression parameters show a low basicity of the BMIM-PF(6) solvent compared to octanol. The high cohesion of the ions in the ionic liquid phase is also indicated by the regression equations obtained. Ionized phenols (phenoxide ions) associate more strongly with BMIM-PF(6) than most other ionized molecules. Amino acids were not soluble in ionic liquid; however, it is possible to extract them partially by adding a crown ether to the ionic liquid phase and working at pH 1. The positive form of amino acids is complexed by the crown ether and the complex is extracted in the ionic liquid phase.
Analytical and Bioanalytical Chemistry 02/2003; 375(2):191-9. · 3.66 Impact Factor
[show abstract][hide abstract] ABSTRACT: Twenty-four chiral dihydrofurocoumarin derivatives and structurally related compounds were separated using super/subcritical fluid chromatography (SFC) on three macrocyclic glycopeptide chiral stationary phases (CSPs). All the dihydrofurocoumarin derivatives could be resolved on at least one of the macrocyclic glycopeptide CSPs, and the Chirobiotic T column showed the best enantioselectivity for 21 of the dihydrofurocoumarins studied. The Chirobiotic TAG and Chirobiotic R CSPs separated similar numbers of chiral dihydrofurocoumarin derivatives (16 on Chirobiotic TAG and 15 on Chirobiotic R), but the Chirobiotic TAG column baseline resolved more samples than the Chirobiotic R column under similar experimental conditions. All separations were done isocratically with an outlet pressure regulated at 100 bar, a column temperature of 31 C, and a flow rate of 3.0 mL min-1. Various amounts of methanol ranging from 2 to 25% (v/v) were added to the supercritical carbon dioxide in order to optimize the enantiomeric separations. All of the separations were completed in less than 15 min, and 81% of the separations were done in less than 10 min. The separation speed is one of the main advantages of SFC.
[show abstract][hide abstract] ABSTRACT: Reversed-phase high-performance liquid chromatographic methods were developed for the separation of the enantiomers of five
glycine and twelve alanine analogues. The enantioselective separation involved two methods. The direct separations were performed
on chiral stationary phases containing a macrocyclic glycopeptide antibiotic: teicoplanin (Chirobiotic T column), ristocetin
A (Chirobiotic R column) or chiral crown ether (Crownpak CR(+) column). The indirect methods involved pre-column derivatization
with the chiral derivatizing agents 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl isothiocyanate andN-α-(2,4-dinitro-5-fluorophenyl)-L-alaninamide (Marfey's reagent). The different methods were compared in systematic chromatographic
examinations. The effects of organic modifier content, mobile phase composition, pH and flow rate on the separation were investigated.
[show abstract][hide abstract] ABSTRACT: The difect and indirect separation of enantiomers of secondary amino acids was studied by high-performance liquid chromatography.
Direct separation was by using a macrocyclic glycopeptide, teicoplanin or ristocetin A, as chiral stationary phase. Indirect
separation was via pre-column derivatization with (S)-N-(4-nitrophenoxycarbonyl)phenylalanine methoxyethyl ester [(S)-NIFE] as a new chiral derivatizing agent. Both direct and indirect separations were performed by means of reversed-phase
HPLC. Conditions for separations were optimized. The methods described offer good enantioselectivity for synthetic chiral
[show abstract][hide abstract] ABSTRACT: Teicoplanin is a macrocyclic glycopeptide that is highly effective as a chiral selector for LC enantiomeric separations. Two possible interaction paths were investigated and related to solute retention and selectivity: (1) interactions with the only teicoplanin amine group and (2) role of hydrogen bonding interactions. Mobile phases containing 0.5 and 5 mM copper ions were used to try to block the amine group. In the presence of copper ions, it was found that the teicoplanin stationary phase has a decreased ability to separate most underivatized racemic amino acids. However, it maintained its ability to separate enantiomers that were not alpha-amino acids. It is established that there is little copper-teicoplanin complex formation. The effect of Cu2+ on the enantioseparation of some alpha-amino acids appears to be due to the fact that these solutes are good bidentate ligands and form complexes with copper ions in the mobile phase. Isotopic exchange with deuterium oxide was performed using acetonitrile-heavy water mobile phases. It was found that the retention times of all amino acids were lower with deuterated mobile phases. The retention times of polar or apolar molecules without amine groups were higher with deuterated mobiles phases. In all cases, the enantioselectivity factors were unaffected by the deuterium exchange. It is proposed that the electrostatic interactions are decreased in the deuterated mobile phases and the solute-accessible stationary-phase volume is somewhat swollen by deuterium oxide. The balance of these effects is a decrease in the amino acid retention times and an increase in the apolar solute retention time. The enantioselectivity factors of all of the molecules remain unchanged because all of the interactions are changed equally. We propose a new global quality criterion (the E factor) for comparing and evaluating enantiomeric separations.