Preparative separation of quinolines by centrifugal partition chromatography with gradient elution

Model-based design of gradient elution in liquid-liquid chromatography: Application to the separation of cannabinoids

Liquid-liquid chromatography (LLC) is a separation technique that utilizes a biphasic solvent system as the mobile and stationary phases. The components are separated solely due to their different distributions between the two liquid phases. Gradient change in the mobile phase composition during the chromatographic process is a powerful method for improving the resolution of separation or shortening the process time. Gradient elution readily applies to LLC with biphasic solvent systems in which the stationary phase composition remains nearly constant when the mobile phase composition changes. This work proposes a model-based approach to optimize gradients in LLC and circumvent tedious trial-and-error experiments. The solutes' distribution constant depends on the mobile phase composition. Thus, the distribution constants were described as a function of the content of one of the solvents (= modifier) in the mobile phase. The dispersive and mass-transfer effects in the tubing and the column are modeled with a stage model. Only a few experiments are required to determine the model parameters. After the validation of the model and its parameters, the model can be used for LLC gradient optimization. The proposed approach was demonstrated for a gradient LLC separation of a mixture of four cannabinoids. Two different gradient shapes, one-step and linear gradient, were considered. For a pre-selected minimal purity requirement, the gradient was optimized for maximum process efficiency, defined as the product of productivity and yield. An experiment conducted with the optimized gradient conditions was in good agreement with the simulation, showing the potential of the proposed method.

Lipophilicity assessement in drug discovery: Experimental and theoretical methods applied to xanthone derivatives

For the last several years, searching of new xanthone derivatives (XDs) with potential pharmacological activities has remained one of the main areas of interest of our group. The optimization of biological activity and drug-like properties of hits and leads is crucial at early stage of the drug discovery pipeline. Lipophilicity is one of the most important drug-like properties having a great impact in both pharmacokinetics and pharmacodynamics processes. In this work, we describe the lipophilicity of a small library of bioactive XDs, previously synthesized by our group, using different methods: computational, vortex-assisted liquid-liquid microextraction coupled with high-performance liquid chromatography (VALLME-HPLC), reversed-phase high-performance thin layer chromatography (RP-HPTLC), reversed-phase high-performance liquid chromatography (RP-HPLC), and biomembrane model by the partition between micelles and aqueous phase. The different results obtained by the used methods were compared and discussed. The methodologies and data gathered in this study will expand the investigation of lipophilicity of XDs, an important class of compounds in medicinal chemistry.

Preparative mass-spectrometry profiling of bioactive metabolites in Saudi-Arabian propolis fractionated by high-speed countercurrent chromatography and off-line atmospheric pressure chemical ionization mass-spectrometry injection

Propolis is a glue material collected by honeybees which is used to seal cracks in beehives and to protect the bee population from infections. Propolis resins have a long history in medicinal use as a natural remedy. The multiple biological properties are related to variations in their chemical compositions. Geographical settings and availability of plant sources are important factors for the occurrence of specific natural products in propolis. A propolis ethylacetate extract (800mg) from Saudi Arabia (Al-Baha region) was separated by preparative scale high-speed countercurrent chromatography (HSCCC) using a non-aqueous solvent system n-hexane-ACN (1:1, v/v). For multiple metabolite detection, the resulting HSCCC-fractions were sequentially injected off-line into an atmospheric pressure chemical ionization mass-spectrometry (APCI-MS/MS) device, and a reconstituted mass spectrometry profile of the preparative run was visualized by selected ion traces. Best ion-intensities for detected compounds were obtained in the negative APCI mode and monitored occurring co-elution effects. HSCCC and successive purification steps resulted in the isolation and characterization of various bioactive natural products such as (12E)- and (12Z)-communic acid, sandaracopimaric acid, (+)-ferruginol, (+)-totarol, and 3β-acetoxy-19(29)-taraxasten-20a-ol using EI-, APCI-MS and 1D/2D-NMR. Cycloartenol-derivatives and triterpene acetates were isolated in mixtures and elucidated by EI-MS and 1D-NMR. Free fatty acids, and two labdane fatty acid esters were identified by APCI-MS/MS. In total 19 metabolites have been identified. The novel combination of HSCCC fractionation, and APCI-MS-target-guided molecular mass profiling improve efficiency of lead-structure identification.

Changes in the mobile phase composition on a stepwise counter-current chromatography elution for the isolation of flavonoids from Siparuna glycycarpa

This paper describes the isolation of flavonoids and other aromatic compounds from an ethyl acetate extract of leaves of Siparuna glycycarpa using stepwise elution counter-current chromatography (CCC). The elution profile yielded the following compounds: diglycosylated flavonoids, quercetin 3-O-rutinoside and quercetin 7-O-rutinoside, followed by monoglycosylated flavonoids, kaempferol-3-O-β-glucopyranoside, kaempferol-3-O-β-rhamnopiranoside, kaempferol-3-O-β-6''(p-coumaroyl) glucopyranoside, and quercetin-3-O-β-glucopyranoside, and then free phenolics, protocatechuic acid, and 2',6'-dihydroxy-4, 4'-dimethoxydihydrochalcone, which shows that this type of elution covers a broader range of polarity than the traditional isocratic mode. This makes it more suitable to perform separations of mixtures containing large differences in hydrophobicity. A GC analysis of a blank CCC run was performed to determine if changes in the mobile phase composition affect the chromatographic process. Results showed a gradual variation of the composition of the mobile phase emerging after the step gradient, favoring the selectivity of the solvent system.

State-of-the-art separation of ginsenosides from Korean white and red ginseng by countercurrent chromatography

Ginseng (Panax ginseng C. A. Meyer) has been one of the most popular herbs used for nutritional and medicinal purposes by the people of eastern Asia for thousands of years. Ginsenosides, the mostly widely studied chemical components of ginseng, are quite different depending on the processing method used. A number of studies demonstrate the countercurrent chromatography (CCC) separation of ginsenosides from several sources; however, there is no single report demonstrating a one-step separation of all of these ginsenosides from different sources. In the present study, we have successfully developed an efficient CCC separation methodology in which the flow-rate gradient technique was coupled with a new solvent gradient dilution strategy for the isolation of ginsenosides from Korean white (peeled off dried P. ginseng) and red ginseng (steam-treated P. ginseng). The crude samples were initially prepared by extraction with butanol and were further purified with CCC using solvent gradients composed of methylene chloride-methanol-isopropanol-water (different ratios, v/v). Gas chromatography coupled with flame ionization detector was used to analyze the components of the two-phase solvent mixture. Each phase solvent mixture was prepared without presaturation, which saves time and reduces the solvent consumption. Finally, 13 ginsenosides have been purified from red ginseng with the new technique, including Rg1, Re, Rf, Rg2, Rb1, Rb2, Rc, Rd, Rg3, Rk1, Rg5, Rg6, and F4. Meanwhile, eight ginsenosides have been purified from white ginseng, including Rg1, Re, Rf, Rh1, Rb1, Rb2, Rc, and Rd by using a single-solvent system. Thus, the present technique could be used for the purification of ginsenosides from all types' ginseng sources. To our knowledge, this is the first report involving the separation of ginsenoside Rg2 and Rg6 and the one-step separation of thirteen ginsenosides from red ginseng by CCC.

Comparing two models of gradient elution in counter-current chromatography

Gradient change of mobile phase composition is commonly used in liquid chromatography to shorten analysis duration. Gradient elution is possible in counter-current chromatography if it is demonstrated that the mobile phase composition changes are not associated with liquid stationary phase composition changes. Also, the solute distribution ratios in initial (1) and final (2) eluting systems must be known. If the solute distribution ratios during the changing mobile phase composition are modeled, the full gradient separation can be modeled using classical equations of liquid chromatography. Another approach allowed modeling empirically the gradient step. It considered the decreasing volume of mobile phase 1 and the increasing volume of mobile phase 2 used at each gradient time to derive a very simple equation. The two equations were compared computing the retention volumes of five test solutes. A remarkable agreement was observed considering the very different mathematical expressions of the two models. The step-gradient method was also modeled and its results compared to those of the linear gradient method.

Modeling gradient elution in countercurrent chromatography: efficient separation of tanshinones from Salvia miltiorrhiza Bunge

Countercurrent chromatography (CCC) is a support-free liquid-liquid chromatography using centrifugal fields to hold the liquid stationary phase. CCC has been widely applied in the separation of various natural and synthetic components using a variety of biphasic liquid systems. The related hexane or heptane/ethyl acetate/methanol or ethanol/water biphasic liquid systems demonstrated their significance in CCC. Gradient is difficult in CCC since any composition change in one phase induces a composition change of the other phase to maintain phase equilibrium. This work provides a new insight into linear gradient elution in CCC that is feasible with some biphasic liquid systems such as selected compositions of the hexane/ethyl acetate/ethanol/water systems. The equations modeling solute motion inside the CCC column are proposed. Particular compositions of the liquid system, namely the hexane/ethyl acetate/ethanol/water 8:2:E:W compositions with E + W = 10, were studied from W = 1 to 9. They showed moderate changes in the upper organic phase compositions. The model is tested with the separation of tanshinones from the rhizome of Salvia miltiorrhiza Bunge. Different linear solvent gradient profiles were experimentally performed between 8:2:5:5 and 8:2:3:7 compositions and the results were evaluated using the proposed model. Five tanshinones including dihydrotanshinone I, cryptotanshinone, tanshinone I, 1,2-dihydrotanshinquinone, and tanshinone IIA have been successfully separated (>95% purities) using a gradient profile optimized by the developed model. The gradient model can be used only with biphasic liquid systems in which one phase shows minimum composition changes when the other phase composition changes notably. This case is not the general case for biphasic liquid systems but can be applied with specific compositions of the quaternary hexane or heptane/ethyl acetate/methanol or ethanol/water most useful CCC liquid systems.

Analysis of alkaloids from different chemical groups by different liquid chromatography methods

Alkaloids are biologically active compounds widely used as pharmaceuticals and synthesised as secondary methabolites in plants. Many of these compounds are strongly toxic. Therefore, they are often subject of scientific interests and analysis. Since alkaloids — basic compounds appear in aqueous solutions as ionized and unionized forms, they are difficult for chromatographic separation for peak tailing, poor systems efficiency, poor separation and poor column-to-column reproducibility. For this reason it is necessity searching of more suitable chromatographic systems for analysis of the compounds. In this article we present an overview on the separation of selected alkaloids from different chemical groups by liquid chromatography thus indicating the range of useful methods now available for alkaloid analysis. Different selectivity, system efficiency and peaks shape may be achieved in different LC methods separations by use of alternative stationary phases: silica, alumina, chemically bonded stationary phases, cation exchange phases, or by varying nonaqueous or aqueous mobile phase (containing different modifier, different buffers at different pH, ion-pairing or silanol blocker reagents). Developments in TLC (NP and RP systems), HPLC (NP, RP, HILIC, ion-exchange) are presented and the advantages of each method for alkaloids analysis are discussed.

Gradient elution in counter-current chromatography: a new layout for an old path

Gradient elution in CCC is a powerful tool, which needs further systematic development to become robust and easy to use. The first attempt to build a correlation between gradient elution profile and distribution ratio (K(D)) values for model mixtures containing typical representatives of pharmaceutical compounds is presented in this paper. The three step estimation of the solvent system composition of a heptane-ethyl acetate-methanol-water (HEMWat) series is described. The estimation is based on simple measurements of initial and final stationary phase retention for gradient elution run, calculating gradient distribution ratio and correlating it with static K(D) against HEMWat number.

Applications of counter-current chromatography in organic synthesis purification of heterocyclic derivatives of lapachol

This work describes the application of counter-current chromatography (CCC) as a useful, fast and economic alternative for the isolation and purification of heterocyclic derivatives from lapachol and beta-lapachone, two naturally occurring compounds from Tabebuia species, and nor-beta-lapachone, a synthetic congener of lapachol. The discussed data comprise four examples of purification of synthetic reactions with different solvent systems - the mixture of the oxazole and the imidazole from beta-lapachone; the quinoxaline from nor-beta-lapachone; and the purification of the N-oxides from the quinoxaline and the phenazine from nor-beta-lapachone from their respective not fully reacted substrates by means of aqueous reversed- and normal-phase elution modes and non-aqueous solvent systems. Traditional purification of these reaction products by silica gel column chromatography demanded a large amount of solvent and time and, in some cases, serious degradation of the products occurred, leading to low yield of the reaction. High-speed counter-current chromatography (HSCCC) was used as an alternative to optimize the process and raise the yield of the reactions.

Determination of liquid–liquid partition coefficients by separation methods

By essence, all kinds of chromatographic methods use the partitioning of solutes between a stationary and a mobile phase to separate them. Not surprisingly, separation methods are useful to determine accurately the liquid-liquid distribution constants, commonly called partition coefficient. After briefly recalling the thermodynamics of the partitioning of solutes between two liquid phases, the review lists the different methods of measurement in which chromatography is involved. The shake-flask method is described. The ease of the HPLC method is pointed out with its drawback: the correlation is very sensitive to congeneric effect. Microemulsion electrokinetic capillary electrophoresis has become a fast and reliable method commonly used in industry. Counter-current chromatography (CCC) is a liquid chromatography method that uses a liquid stationary phase. Since the CCC solute retention volumes are only depending on their partition coefficients, it is the method of choice for partition coefficient determination with any liquid system. It is shown that Ko/w, the octanol-water partition coefficients, are obtained by CCC within the -1 < log Ko/w < 4 range, without any correlation or standardization using octanol as the stationary phase. Examples of applications of the knowledge of liquid-liquid partition coefficient in the vast world of solvent extraction and hydrophobicity estimation are presented.