"Sweet space" strategy for solvent system selection in countercurrent chromatography: A case study on separation of polyunsaturated fatty acids from borage oil

This study presents a comprehensive strategy for the selection and optimization of solvent systems in countercurrent chromatography (CCC) for the effective separation of compounds. With a focus on traditional organic solvent systems, the research introduces a "sweet space" strategy that merges intuitive understanding with mathematical accuracy, addressing the significant challenges in solvent system selection, a critical bottleneck in the widespread application of CCC. By employing a combination of volume ratios and graphical representations, including both regular and trirectangular tetrahedron models, the proposed approach facilitates a more inclusive and user-friendly strategy for solvent system selection. This study demonstrates the potential of the proposed strategy through the successful separation of gamma-linolenic acid, oleic acid, and linoleic acid from borage oil, highlighting the strategy's effectiveness and practical applicability in CCC separations.

Target-guided isolation and purification of cyclooxygenase-2 inhibitors from Meconopsis integrifolia (Maxim.) Franch. by high-speed counter-current chromatography combined with ultrafiltration liquid chromatography

Meconopsis integrifolia (Maxim.) Franch. is used extensively in traditional Tibetan medicine for its potent anti-inflammatory properties. In this study, six cyclooxygenase-2 (COX-2) inhibitors were purified from M. integrifolia using high-speed counter-current chromatography guided by ultrafiltration liquid chromatography (ultrafiltration-LC). First, ultrafiltration-LC was performed to profile the COX-2 inhibitors in M. integrifolia. The reflux extraction conditions were further optimized using response surface methodology, and the results showed that the targeted COX-2 inhibitors could be well enriched under the optimized extraction conditions. Then the six target COX-2 inhibitors were separated by high-speed countercurrent chromatography with a solvent system composed of ethyl acetate/n-butanol/water (4:1:4, v/v/v. Finally, the six COX-2 inhibitors, including 21.2 mg of 8-hydroxyluteolin 7-sophoroside, 29.6 mg of 8-hydroxyluteolin 7-[6'''-acetylallosyl-(1→2)-glucoside], 42.5 mg of Sinocrassoside D3, 54.1 mg of Hypolaetin 7-[6'''-acetylallosyll-(l→2)-3''-acetylglucoside, 30.6 mg of Hypolaetin 7-[6'''-acetylallosyll-(l→2)-6''-acetylglucoside and 17.8 mg of Hypolaetin were obtained from 500 mg of sample. Their structures were elucidated by 1 H-NMR spectroscopy. This study reveals that ultrafiltration-LC combined with high-speed counter-current chromatography is a robust and efficient strategy for target-guided isolation and purification of bioactive molecules. It also enhances the scientific understanding of the anti-inflammatory properties of M. integrifolia but also paves the way for its further medicinal applications.

Target-Guided Isolation and Purification of Antioxidants from Urtica laetevirens Maxim. by HSCCC Combined with Online DPPH-HPLC Analysis

Urtica laetevirens Maxim. is used extensively in traditional Chinese medicine (TCM) for its potent antioxidative properties. In this study, three antioxidants were purified from U. laetevirens. using HSCCC guided by online DPPH-HPLC analysis. Firstly, the online DPPH-HPLC analysis was performed to profile out the antioxidant active molecules in U. laetevirens. The ultrasonic-assisted extraction conditions were optimized by response surface methodology and the results showed the targeted antioxidant active molecules could be well enriched under the optimized extraction conditions. Then, the antioxidant active molecules were separated by high-speed countercurrent chromatography ethyl acetate/n-butanol/water (2:3:5, v/v/v) as the solvent system. Finally, the three targets including 16.8 mg of Isovitexin, 9.8 mg of Isoorientin, and 26.7 mg of Apigenin-6,8-di-C-β-d-glucopyranoside were obtained from 100 mg of sample. Their structures were identified by 1H NMR spectroscopy.

A Neglected Issue: Stationary Phase Retention Determination of Classic High-Speed Counter-Current Chromatography Solvent Systems

Obtaining an ideal solvent system for target compounds is still an obstacle to the wide application of high-speed counter-current chromatography (HSCCC). The partition coefficient and retention of the stationary phase are two key parameters for solvent system selection. The retention of the stationary phase of the solvent system is roughly judged by settling time using a test tube, which is subjective and inaccurate. In this study, we demonstrated that high-resolution separation of HSCCC is tightly connected with the retention of the stationary phase. Notably, unlike the in vitro test of settling time, we investigated the retention of the stationary phase of classical biphasic solvent systems by a TBE300C HSCCC apparatus. Our results revealed that settling time is not always inversely proportional to the retention of the stationary phase. The n-hexane–ethylacetate–methanol–water solvent systems showed the highest correlation coefficient of settling time and retention of the stationary phase (r = −0.91, n = 16). N-heptane–n-butanol–acetonitrile–water solvent system showed the lowest correlation coefficient (r = −0.26, n = 7). These results may be helpful for HSCCC solvent system selection and accelerate the application of this technique.

Liquid-Liquid Chromatography: Current Design Approaches and Future Pathways

Since its first appearance in the 1960s, solid support-free liquid-liquid chromatography has played an ever-growing role in the field of natural products research. The use of the two phases of a liquid biphasic system, the mobile and stationary phases, renders the technique highly versatile and adaptable to a wide spectrum of target molecules, from hydrophobic to highly polar small molecules to proteins. Generally considered a niche technique used only for small-scale preparative separations, liquid-liquid chromatography currently lags far behind conventional liquid-solid chromatography and liquid-liquid extraction in process modeling and industrial acceptance. This review aims to expose a broader audience to this high-potential separation technique by presenting the wide variety of available operating modes and solvent systems as well as structured, model-based design approaches. Topics currently offering opportunities for further investigation are also addressed.

H2O-Induced Hydrophobic Interactions in MS-Guided Counter-Current Chromatography Separation of Anti-Cancer Mollugin from Rubia cordifolia

Counter-current chromatography (CCC) is a unique liquid–liquid partition chromatography and largely relies on the partition interactions of solutes and solvents in two-phase solvents. Usually, the two-phase solvents used in CCC include a lipophilic organic phase and a hydrophilic aqueous phase. Although a large number of partition interactions have been found and used in the CCC separations, there are few studies that address the role of water on solvents and solutes in the two-phase partition. In this study, we presented a new insight that H₂O (water) might be an efficient and sensible hydrophobic agent in the n-hexane-methanol-based two-phase partition and CCC separation of lipophilic compounds, i.e., anti-cancer component mollugin from Rubia cordifolia. Although the n-hexane-methanol-based four components solvent systems of n-hexane-ethyl acetate-methanol-water (HEMWat) is one of the most popular CCC solvent systems and widely used for natural products isolation, this is an interesting trial to investigate the water roles in the two-phase solutions. In addition, as an example, the bioactive component mollugin was targeted, separated, and purified by MS-guided CCC with hexane-methanol and minor water as a hydrophobic agent. It might be useful for isolation and purification of lipophilic mollugin and other bioactive compounds complex natural products and traditional Chinese medicines.

Target separation and antitumor metastasis activity of sesquiterpene-based lysine-specific demethylase 1 inhibitors from zedoary turmeric oil

Lysine-specific histone demethylase 1 (LSD1) was the first histone demethylase identified in epigenetics and has recently emerged as an attractive therapeutic target for treating tumors. To date, almost all reported LSD1 inhibitors have been chemosynthesized; however, natural products possess pharmacological and biological activity and can be sources for drug development. Here, we established a target separation countercurrent chromatography technique to isolate LSD1 inhibitors from zedoary turmeric oil. Four sesquiterpene-based LSD1 inhibitors were efficiently obtained with an inhibition ratio equal to or less than that of the positive control drug. Compound 2 showed the most potent inhibitory activity, with a half-maximal inhibitory concentration of 3.97 μM, and was further tested to determine its ability to inhibit LSD1 and its antitumor metastatic effects in MDA-MB-231 cells. These four compounds are the first sesquiterpene-based natural LSD1 inhibitors to be characterized. Our findings provide a new molecular framework for studying LSD1 inhibitors and offer a template for designing more sesquiterpene-based LSD1 inhibitors with potential antitumor activity.

Recent progress in separation prediction of counter-current chromatography

As a liquid-liquid partition chromatography, counter-current chromatography has advantages in large sample loading capacity without irreversible adsorption, which has been widely applied in separation and purification fields. The main factors, including partition coefficient, two-phase solvent systems, apparatus, and operating parameters greatly affect the separation process of counter-current chromatography. To promote the applications of counter-current chromatography, it is essential to develop theoretical research to master the principles of counter-current chromatographic separations so as to achieve predictions before laborious trials. In this article, recent progress about separation prediction methods are reviewed from a point of the steady and unsteady state of the mass transfer process of counter-current chromatography and its mass transfer characteristics, and then it is divided into three aspects: prediction of partition coefficient, modeling the thermodynamic process of counter-current chromatography, and modeling the dynamic process of counter-current chromatography.

A biphasic system based on guanidinium ionic liquid: Preparative separation of eicosapentaenoic acid ethyl ester and docosahexaenoic acid ethyl ester by countercurrent chromatography

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are high nutritional components. Evidence for unique effects of them is increasing. Further understanding of their independent biological functions urgently needs more efficient separation techniques. Nowadays, most of the commercially available fish oil products are the mixture of eicosapentaenoic acid ethyl ester (EPAEE) and docosahexaenoic acid ethyl ester (DHAEE). It will be convenient to directly separate esterified EPA and DHA without saponification pretreatment. However, it is of great challenge to separate EPAEE and DHAEE because of their extremely fat-soluble nature and the equivalent chain length rule. In this research, the suitability of green guanidinium ionic liquid (IL) in countercurrent chromatography (CCC) solvent system for the separation of them was evaluated for the first time. Compared with imidazolium IL and phosphonium IL, guanidinium IL based non-aqueous biphasic system showed more outstanding separation performance. The separation mechanism was elucidated in depth through quantum mechanical calculations. It was found that guanidinium IL acted a crucial role in the CCC separation, which resulted in difference of partition behavior of EPAEE and DHAEE via different hydrogen-bonding affinity. EPAEE and DHAEE were successfully separated by solvent system (n-heptane/methanol/propylguanidinium chloride ([C₃Gun]Cl, 1:1:5%, v/v/m)) with high purity (>95%) in one step, which was not achieved beforehand. Moreover, an easy recycling procedure of IL had also been devised, which significantly reduced waste generated. It opens up a new way for reasonable design water-free two-phase solvent system for efficient separation of very non-polar lipid compounds.

The applicability of high-speed counter current chromatography to the separation of natural antioxidants

Antioxidants play an essential role in human health, as they have been found to be capable of lowering the incidence of many diseases, such as cancer and angiocardiopathy. Currently, more attention is paid to natural antioxidants because of the possible insecurity of synthetic antioxidants. Thus, the development of efficient techniques or methods to separate antioxidants from natural sources is requested urgently. High-speed counter current chromatography (HSCCC) is a unique support-free liquid-liquid chromatographic technique and has been widely applied in the field of separation of natural products. In this review, we summarize and analyze the related researches on the application of HSCCC in the separation of various natural antioxidants so far. The purpose of the article is to provide a certain theoretical support for the separation of natural antioxidants by HSCCC, and to make full use of advantages of HSCCC in the separation of bioactive components. In particular, some key problems associated with the separation strategies, the structural categories of natural antioxidants, solvent system choices, and the application of different elution modes in HSCCC separation, are summarized and commented. We expect that the content reviewed can offer more evidence for the development of the field of natural antioxidants separation, so as to achieve large-scale preparation of natural antioxidants.

Ionic liquid-modified countercurrent chromatographic isolation of high-purity delphinidin-3-rutinoside from eggplant peel

Delphinidin-3-rutinoside, a high-value of anthocyanin, was isolated and purified by ionic liquid (IL)-modified countercurrent chromatography (CCC) from waste peel of eggplant (Solanum melongena), one of the most common vegetables consumed all around the world. Different conventional CCC and IL-CCC solvent systems were evaluated in respect of partition coefficient (K), separation factor (α), and stationary phase retention factor (S_f ) to separate polar target and other components. Basic solvent system, kind of ILs, and amount of ILs were systematically optimized by totally K-targeted strategy, which drastically reduced the experimental effort. Finally, a novel CCC two-phase solvent system (methyl tert-butyl ether-butanol-acetonitrile-1% trifluoroacetic acid water-1-butyl-3-methylimidazolium hexafluorophosphate ([C₄ MIM][PF₆ ]) [2:4:1:5:0.2; v/v/v/v/v]) was successfully established and applied. The baseline separation of target fraction was obtained in one cycle process. The purity of delphinidin-3-rutinoside was over 99%. Moreover, the distribution behavior of different kinds of ILs in biphasic solvent system and the removal method of ILs were explored. The results showed that hydrophobic IL significantly improved the partition of polar anthocyanin in organic solvent system, thereby the separation resolution and stationary phase retention through introducing intermolecular forces. This IL-modified CCC strategy may be applied for the separation of other anthocyanins from variety of natural food resources and waste.

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.

Development of online-storage inner-recycling counter-current chromatography for the preparative separation of complex components of alkylphenols from sarcotesta of Ginkgo biloba L

High-speed counter-current chromatography (HSCCC) is becoming an effective and non-absorptive separation method from natural products. Due to the insufficient separation efficiency, it is challenging to separate complex components, especially for compounds with similar K _D values. In this study, a novel and effective online-storage inner-recycling CCC method was used to separate alkylphenols from the sarcotesta of Ginkgo biloba L. A two-phase solvent system of n-heptane/ethyl acetate/methanol/acetic acid (5 : 4 : 1 : 1, v/v) was used for HSCCC separation of 500 mg crude extracts. After the inner-recycling of two fractions coupled with pre-HPLC, five main ginkgolic acids (C13:0, C15:1, C17:2, C15:1, C17:1) coupled with bilobol (C15:1) and a mixture were obtained from a non-stop separation using a storage loop and two six-way valves. This novel method was also evaluated and predicted by formula derivation. This method could be an effective, rapid, and simple approach to separate alkylphenols from the sarcotesta of G. biloba.