Enantioseparation of chiral aromatic acids by multiple dual mode counter-current chromatography using hydroxypropyl-β-cyclodextrin as chiral selector

Comparison of two different multiple dual-mode counter-current chromatograph for separation of ketoconazole enantiomers

In this work, two different multiple dual-mode (MDM) counter-current chromatography methods, conventional MDM and modified MDM elution modes, were compared for the chiral separation of the ketoconazole enantiomers. The biphasic solvent system which consisted of n-hexane: isobutyl acetate: 0.1 mol/L phosphate buffer (2:4:6, v/v) (pH = 8.5) was employed as stationary phase and mobile phase. And the hydroxypropyl-β-cyclodextrin (HP-β-CD) with a concentration of 100 mmol/L was dissolved in the phosphate buffer, as the chiral selector. Under two different methods, dual-mode (DM) elution was performed to determine the time of the transformed phase roles and multiple cycles were performed to isolate ketoconazole, respectively. The result indicated that the modified MDM elution had a significant improvement on the separation, increasing the resolution from 0.51 to 1.19, while the resolution was increased from 0.40 to 0.79 by the conventional MDM elution. Ultimately, baseline separation of ketoconazole enantiomers was essentially achieved by high-speed counter-current chromatography under optimized modified MDM separation conditions. The final recoveries of the two enantiomers, R-(K) and S-(K), were 92.5 % and 83.3 %, respectively, corresponding to enantiomeric excess values of 99.0 % and 97.0 %, as determined by HPLC.

An overview of recent progress in multiple dual-mode counter-current chromatography

Counter-current chromatography is a chromatographic separation and purification technique being developed. The development of different elution modes has significantly contributed to this field. Multiple dual-mode elution is a method developed based on dual-mode elution, which consists of a series of changing cycles of the phase role and the direction by switching between normal and reverse elution modes of counter-current chromatography. This dual-mode elution method takes full advantage of the liquid nature of stationary and mobile phases of counter-current chromatography and effectively improves the separation efficiency. So, this unique elution mode has gained extensive attention for separating complex samples. This review mainly describes and summarizes in detail its development, applications, and characteristics in recent years. Meanwhile, its advantages, limitations, and future outlook also have been discussed in this paper.

Enantioseparation of three constitutional isomeric 2-(methylphenyl)propanoic acids by countercurrent chromatography

2-(3-Methylphenyl)propanic acid and 2-(4-methylphenyl)propanoic acid were successfully enantioseparated by countercurrent chromatography using hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral selector. 2-(2-Methylphenyl)propanoic acid was also studied to compare the enantioseparation ability of three isomeric 2-(methylphenyl)propanoic acids. Totally 20 mg of 2-(3-methylphenyl)propanic acid and 20mg of 2-(4-methylphenyl)propanic acid were enantioseparated individually by countercurrent chromatography. Recovery for the (±)-2-(3-methylphenyl)propanic acid enantiomer was in the range of 85%-90% with 98.0%-98.8% purity and recovery for the (±)-2-(4-methylphenyl)propanic acid enantiomer was in the range of 80%-83% with 97.0%-98.0% purity. The enantioseparation factor in countercurrent chromatography for 2-(4-methylphenyl)propanic acid and 2-(3-methylphenyl)propanic acid were 1.31 and 1.26, and the peak resolution in HPLC reached 2.2 and 1.4. However, no enantioseparation could be found for 2-(2-methylphenyl)propanic acid. In addition, the inclusion complexes were investigated by UV spectrophotometer. The inclusion formation constant of inclusion complex between 2-(4-methylphenyl)propanic acid, 2-(3-methylphenyl)propanic acid, 2-(2-methylphenyl)propanic acid and HP-β-CD were determined as 121.73 mol/L, 78.12 mol/L and 53.18 mol/L, respectively. The present results showed that enantiorecognition was greatly affected by substituted positions of methyl group on the benzene ring. Combined with our previous results, the steric hindrance had a significant effect on inclusion interaction between HP-β-CD and racemic 2-(substitutedphenyl)propanoic acids. No enantiorecognition could be achieved for 2-(substitutedphenyl)propanoic acids with ortho-substituent group on benzene, while the influence of meta- and para- group on enantiorecognition varies with different substituent groups on benzene ring.

Enantioseparation of 2-(4-chlorophenyl)succinic acid by countercurrent chromatography and investigation of injection volume on resolution

2-(4-Chlorophenyl)succinic acid was successfully enantioseparated by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as chiral selector. A two-phase solvent system composed of n-hexane-ethyl acetate-0.1 mol/L phosphate buffer with pH 2.65 (5:5:10, v/v) was selected. Enantioselective liquid-liquid extraction was used to optimize the enantioseparation conditions. Meanwhile, the influence of injection volume on resolution in countercurrent chromatography was investigated and a linear relationship between the inflection point of injection volume and sample loading was tentatively obtained. The peak resolution will decrease significantly when the injection volume over the inflection point was used. In addition, it could be found that the smaller amount of sample loading, the larger impact of injection volume on resolution could be observed, which might serve as a good reference for the selection of sample volume in enantioseparations by countercurrent chromatography. Under optimized conditions, 20 mg of 2-(4-chlorophenyl)succinic acid racemate dissolved in 10 mL of aqueous phase was successfully enantioseparated by countercurrent chromatography. The recovery for both of the enantiomer of (±)-2-(4-chlorophenyl)succinic acid reached within 70-75% with a purity of 99.0%.

Chiral counter-current chromatography: A survey of its instrument, mechanism, procedure, and applications

The chiral separation by counter-current chromatography has made great progress in the past three decades. It has become increasingly popular in the field of chiral separation, and many applications have been introduced during the last years. This review mainly focuses on the current topics, applications, and trends in chiral separation by counter-current chromatography. It contains the development of modern counter-current chromatography apparatus, theory of counter-current chromatography, overview of applications of chiral counter-current chromatography enantioseparation, its current situation, and challenges. At last, some conclusions and perspectives also have been discussed in this review.

Operating mode and parameter selection in liquid-liquid chromatography

The presence of a liquid stationary phase in liquid-liquid chromatography (LLC) allows for high versatility of operation as well as adaptability to different sample types and separation tasks. LLC, also known as countercurrent chromatography (CCC) or centrifugal partition chromatography (CPC), offers the user a variety of operating modes, many of which have no direct equivalent in conventional preparative liquid-solid chromatography. These operating modes have the potential to greatly improve LLC separation performance compared to the standard "classical" isocratic batch injection mode, and they often require minimal to no addition of equipment to the standard set-up. However, reports of the use of alternative LLC operating modes make up only a fraction of the literature. This is likely due, at least in part, to the lack of clear guidelines and methods for operating mode and parameter selection, leaving alternative process options to be avoided and underutilized. This review seeks to remedy this by providing a thorough overview of the available LLC operating modes, identifying the key characteristics, advantages and disadvantages, and areas of application of each. Additionally, the equations and short-cut models aiding in operating mode and parameter selection are presented and critiqued, and their notation is unified for clarity. By rendering LLC and its alternative operating modes more accessible to current and prospective users, it is hoped to help expand the application of this technology and support the achievement of its full potential.

Stereoselective separation of (1S, 4S)-sertraline from medicinal reaction mixtures by countercurrent chromatography with hydroxypropyl-β-cyclodextrin as stereoselective selector

Four stereoisomeric components were produced during the synthesis of the antidepressant drug (1S, 4S)-sertraline hydrochloride due to the two chiral carbon centers in its chemical structure, including (1S, 4S), (1R, 4R), (1S, 4R), and (1R, 4S)-isomer. Stereoselective separation of the target isomer (1S, 4S)-sertraline from the medicinal reaction mixtures by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as the stereoselective selector was investigated. A biphasic solvent system composed of n-hexane/0.20 mol/L phosphate buffer solution with pH 7.6 containing 0.10 mol/L of hydroxypropyl-β-cyclodextrin (1:1, v/v) was selected for separation of cis-sertraline and trans-sertraline using reverse phase elution mode and (1S, 4S)-sertraline was separated with (1R, 4R)-sertraline using recycling elution mode. A fabricated in-house analytical countercurrent chromatographic apparatus was used for optimization of the separation conditions. Stationary phase retention and peak resolution were investigated for separation of cis-sertraline and trans-sertraline by the analytical apparatus.

An overview of recent progress in solvent systems, additives and modifiers of counter current chromatography

Solvent systems are critical to counter current chromatography (CCC). Appropriate additives and modifiers can be used to improve the separation efficiency of CCC.

Separation of epimeric aromatic acid (-)-menthol esters by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as an additive

The separation of ten epimeric aromatic acid (-)-menthol esters by countercurrent chromatography with hydroxypropyl-β-cyclodextrin as the mobile phase additive was investigated, and methods for the analysis of all the epimeric esters by reversed-phase high-performance liquid chromatography were established. A biphasic solvent system composed of n-hexane/20-70% methanol containing 50 mmol/L of hydroxypropyl-β-cyclodextrin (1:1, v/v) was selected, which provided high separation factors for five of the epimeric esters, and successful separations by countercurrent chromatography were achieved. The complete separation of five pairs of epimeric ester was obtained with the purity being over 98% for each peak fractions, as determined by high-performance liquid chromatography. The recovery of each analyte from the eluted fractions reached around 80-88%.

Affinity purification of aprotinin from bovine lung

An affinity protocol for the purification of aprotinin from bovine lung was developed. To simulate the structure of sucrose octasulfate, a natural specific probe for aprotinin, the affinity ligand was composed of an acidic head and a hydrophobic stick, and was then linked with Sepharose. The sorbent was then subjected to adsorption analysis with pure aprotinin. The purification process consisted of one step of affinity chromatography and another step of ultrafiltration. Then purified aprotinin was subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis, trypsin inhibitor activity, gel-filtration, and thin-layer chromatography analysis. As calculated, the theoretical maximum adsorption (Qmax ) of the affinity sorbent was 25,476.0 ± 184.8 kallikrein inactivator unit/g wet gel; the dissociation constant of the complex "immobilized ligand-aprotinin" (Kd ) was 4.6 ± 0.1 kallikrein inactivator unit/mL. After the affinity separation of bovine lung aprotinin, reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis and gel-filtration chromatography revealed that the protein was a single polypeptide, and the purities were ∼ 97 and 100%, respectively; the purified peptide was also confirmed with aprotinin standard by gel-filtration chromatography and thin-layer chromatography. After the whole purification process, protein, and bioactivity recoveries were 2.2 and 92.6%, respectively; and the specific activity was up to 15,907.1 ± 10.2 kallikrein inactivator unit/mg.

Application and comparison of high-speed countercurrent chromatography and high performance liquid chromatography in preparative enantioseparation of α-substitution mandelic acids

Preparative enantioseparations of α-cyclopentylmandelic acid and α-methylmandelic acid by high-speed countercurrent chromatography (HSCCC) and high performance liquid chromatography (HPLC) were compared using hydroxypropy-β-cyclodextrin (HP-β-CD) and sulfobutyl ether-β-cyclodextrin (SBE-β-CD) as the chiral mobile phase additives. In preparative HPLC the enantioseparation was achieved on the ODS C₁₈ reverse phase column with the mobile phase composed of a mixture of acetonitrile and 0.10 mol L^-1 phosphate buffer at pH 2.68 containing 20 mmol L^-1 HP-β-CD for α-cyclopentylmandelic acid and 20 mmol L^-1 SBE-β-CD for α-methylmandelic acid. The maximum sample size for α-cyclopentylmandelic acid and α-methylmandelic acid was only about 10 mg and 5 mg, respectively. In preparative HSCCC the enantioseparations of these two racemates were performed with the two-phase solvent system composed of n-hexane-methyl tert.-butyl ether-0.1 molL^-1 phosphate buffer solution at pH 2.67 containing 0.1 mol L^-1 HP-β-CD for α-cyclopentylmandelic acid (8.5:1.5:10, v/v/v) and 0.1 mol L^-1 SBE-β-CD for α-methylmandelic acid (3:7:10, v/v/v). Under the optimum separation conditions, total 250 mg of racemic α-cyclopentylmandelic acid could be completely enantioseparated by HSCCC with HP-β-CD as a chiral mobile phase additive in a single run, yielding 105-110 mg of enantiomers with 95-98% purity and 85-90% recovery. But, no complete enantioseparation of α-methylmandelic acid was achieved by preparative HSCCC with either of the chiral selectors due to their limited enantioselectivity. In this paper preparative enantioseparation by HSCCC and HPLC was compared from various aspects.