Rapid and efficient purification of naphthodianthrones from St. John's wort extract by using liquid-liquid extraction and SEC

The isolation of water-soluble natural products - challenges, strategies and perspectives

Covering period: up to 2019Water-soluble natural products constitute a relevant group of secondary metabolites notably known for presenting potent biological activities. Examples are aminoglycosides, β-lactam antibiotics, saponins of both terrestrial and marine origin, and marine toxins. Although extensively investigated in the past, particularly during the golden age of antibiotics, hydrophilic fractions have been less scrutinized during the last few decades. This review addresses the possible reasons on why water-soluble metabolites are now under investigated and describes approaches and strategies for the isolation of these natural compounds. It presents examples of several classes of hydrosoluble natural products and how they have been isolated. Novel stationary phases and chromatography techniques are also reviewed, providing a perspective towards a renaissance in the investigation of water-soluble natural products.

Isothermal kinetics of ethanolic extraction of total hypericin from pre-extracted Hypericum perforatum flowers

INTRODUCTION

Hypericum perforatum L., Hypericaceae (St John's wort) is a highly familiar plant in the medicinal community which recently showed good antiviral activities including against some types of coronavirus.

OBJECTIVE

Establishing the kinetics of isothermal extraction of total hypericin (TH) from the pre-extracted flowers of H. perforatum flowers.

METHODS

The solvent extraction of TH from the remaining solid residue after the pre-extraction procedure was performed providing isothermal conditions at the temperature of 313 K, 323 K, and 328 K (±1 K) using ethanol as a solvent. The feasibility of mathematical modelling of the isothermal kinetics of TH extraction was explored applying some of the frequently used kinetic models of solvent extraction: first-order reaction model, film theory model, unsteady-state diffusion model, parabolic diffusion model, Elovich's equation. The kinetic complexity was examined using the differential isoconversional method.

RESULTS

The kinetics of isothermal solvent extraction of TH is a kinetic elementary process with the unique rate-determining step. It was found that the kinetics of isothermal extraction of TH can be best described employing the theoretical Jander three-dimensional (3D) diffusional model and its suitability for modelling the investigated extraction was confirmed with statistical parameters [adjusted linear correlation coefficient (R² _adj ) = 0.998-0.999 and the standard error (SE) = 0.005-0.006]. The values of the model kinetic parameters (rate constant (k_M /min^-1 ), activation energy (E_a  = 21.0 ± 4.9 kJ/mol) and pre-exponential factor (lnA = 3.1 ± 2.2 min^-1 ) were calculated.

CONCLUSIONS

Based on the model mechanism of the kinetics of the investigated extraction a new mathematical model is suggested and the controlling step of the overall process was found.

Interaction of small molecules with the SARS-CoV-2 papain-like protease: In silico studies and in vitro validation of protease activity inhibition using an enzymatic inhibition assay

The SARS-CoV-2 virus is causing COVID-19, an ongoing pandemic, with extraordinary global health, social, and political implications. Currently, extensive research and development efforts are aimed at producing a safe and effective vaccine. In the interim, small molecules are being widely investigated for antiviral effects. With respect to viral replication, the papain-like (PL^pro) and main proteases (M^pro), are critical for processing viral replicase polypeptides. Further, the PL^pro possesses deubiquitinating activity affecting key signalling pathways, including inhibition of interferon and innate immune antagonism. Therefore, inhibition of PL^pro activity with small molecules is an important research direction. Our aim was to focus on identification of potential inhibitors of the protease activity of SARS-CoV-2 PL^pro. We investigated 300 small compounds derived predominantly from our OliveNet™ library (222 phenolics) and supplemented with synthetic and dietary compounds with reported antiviral activities. An initial docking screen, using the potent and selective noncovalent PL^pro inhibitor, GRL-0617 as a control, enabled a selection of 30 compounds for further analyses. From further in silico analyses, including docking to scenes derived from a publicly available molecular dynamics simulation trajectory (100 μs PDB 6WX4; DESRES-ANTON-11441075), we identified lead compounds for further in vitro evaluation using an enzymatic inhibition assay measuring SARS-CoV-2 PL^pro protease activity. Our findings indicate that hypericin possessed inhibition activity, and both rutin and cyanidin-3-O-glucoside resulted in a concentration-dependent inhibition of the PL^pro, with activity in the micromolar range. Overall, hypericin, rutin, and cyanidin-3-O-glucoside can be considered lead compounds requiring further characterisation for potential antiviral effects in appropriate model systems.

Aromaticity and Electron Density of Hypericin

The influence of the substituents on the geometry of the central ring system of hypericin has been analyzed. Substitution that causes flattening of the hypericin central rings is connected with introducing the aromatic character of the empty rings. All the hypericin rings have an aromatic character illustrated by the Harmonic Oscillator Measure of Aromaticity (HOMA), Nucleus Independent Chemical Shift (NICS), Fluctuation Index (FLU), and Ellipticity Index (EL) indices. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analyses performed on 7,14-dihydrophenanthro[1,10,9,8-opqra]perylene, its substituted analogues, and hypericin show an influence of this substitution on electron density of the central rings.

Phenolic metabolites from Hypericum kelleri Bald., an endemic species of Crete (Greece)

Thirteen compounds were isolated from the aerial parts of Hypericum kelleri Bald., growing as an endemic on the island of Crete (Greece). These compounds comprise four previously unknown prenylated xanthones 1,2-dihydro-3,8-dihydroxy-6-methoxy-1,1,5-tri(3-methylbut-2-enyl)xanthen-2,9-dione (kellerine A), 1,2-dihydro-3,6,8-trihydroxy-1,1,5-tri(3-methylbut-2-enyl)xanthen-2,9-dione (kellerine B), 1,2-dihydro-3,8-dihydroxy-6-methoxy-1,1-bi(3-methylbut-2-enyl)xanthen-2,9-dione (6-methylpatulone), (R/S)-1,3,5-trihydroxy-2-(3-methyl-2-buten-1-yl)-4-[2-(3-methylbut-2-enyl)-3-methylbut-3-enyl]-6-methoxy-9H-xanthen-9-one ((2″R/S)-kellerine C) and the hitherto undescribed depsidone (R/S)-1,3,6-trihydroxy-5-methoxy-2-(3-methyl-2-buten-1-yl)-4-[2-(3-methylbut-2-enyl)-3-methylbut-3-enyl]-11Η-dibenzo[b,e] [1,4]dioxepin-9-one ((2″R/S)-creticine). As known compounds, brevipsidone D, 4-geranyl-2-(2'-isobutyryl)-phloroglucinol, 4-geranyl-2-(2'-methylbutyryl)-phloroglucinol, I3, II8-biapigenin, quercetin, avicularin, pseudohypericin and neochlorogenic acid have been isolated. The structures were elucidated on the basis of their 1D, 2D NMR, CD and MS data. The study confirms the typical occurrence of xanthones in Hypericum section Oligostema (Boiss.) Stef., and is also the first report on the simultaneous isolation of acylphloroglucinols in this section. Furthermore the first evidence of depsidones in the genus Hypericum L. is reported. Cytotoxicity was investigated in HeLa cells for prenylated xanthones and the depsidones. Both triprenylated 1,2-dihydroxanthones (kellerine A and B) showed significant in vitro cytotoxicity with IC₅₀ values of 2.5 ± 0.1 (kellerine A) and 5.9 ± 0.9 (kellerine B) μM, whereas other compounds were less cytotoxic (IC₅₀ > 20 μM).

Isolation and purification of series bioactive components from Hypericum perforatum L. by counter-current chromatography

Counter-current chromatography (CCC) combined with pre-separation by ultrasonic solvent extraction was successively used for the separation of series bioactive compounds from the crude extract of Hypericum perforatum L. The petroleum ether extract was separated by the solvent system of n-heptane-methanol-acetonitrile (1.5:0.5:0.5, v/v) and n-heptane-methanol (1.5:1, v/v) in gradient elution, yielding a phloroglucinol compound, hyperforin with HPLC purity over 98%. The ethyl acetate extract was separated by using the solvent system composed of hexane-ethyl acetate-methanol-water (1:1:1:1 and 1:3:1:3, v/v) in gradient through both reverse phase and normal phase elution mode, yielding a naphthodianthrone compound, hypericin with HPLC purity about 95%. The n-butanol extract was separated with the solvent system composed of n-butanol-ethyl acetate-water (1:4:5 and 1.5:3.5:5, v/v) in elution and back-extrusion mode, yielding two of flavones, rutin and hyperoside, with HPLC purity over 95%. HPLC-MS, reference sample and UV spectrum were selectively used in separation to search for target compounds from HPLC-DAD profiles of different sub-extracts. The structures of isolated compounds were further identified by ESI-MS, ¹HNMR and ¹³CNMR.

Hypericins as potential leads for new therapeutics

70 years have passed since the first isolation of the naphthodianthrones hypericin and pseudohypericin from Hypericum perforatum L. Today, they continue to be one of the most promising group of polyphenols, as they fascinate with their physical, chemical and important biological properties which derive from their unique chemical structure. Hypericins and their derivatives have been extensively studied mainly for their antitumor, antiviral and antidepressant properties. Notably, hypericin is one of the most potent naturally occurring photodynamic agents. It is able to generate the superoxide anion and a high quantum yield of singlet oxygen that are considered to be primarily responsible for its biological effects. The prooxidant photodynamic properties of hypericin have been exploited for the photodynamic therapy of cancer (PDT), as hypericin, in combination with light, very effectively induces apoptosis and/or necrosis of cancer cells. The mechanism by which these activities are expressed continues to be a main topic of discussion, but according to scientific data, different modes of action (generation of ROS & singlet oxygen species, antiangiogenesis, immune responces) and multiple molecular pathways (intrinsic/extrinsic apoptotic pathway, ERK inhibition) possibly interrelating are implicated. The aim of this review is to analyse the most recent advances (from 2005 and thereof) in the chemistry and biological activities (in vitro and in vivo) of the pure naphthodianthrones, hypericin and pseudohypericin from H. perforatum. Extracts from H. perforatum were not considered, nor pharmakokinetic or clinical data. Computerised literature searches were performed using the Medline (PubMed), ChemSciFinder and Scirus Library databases. No language restrictions were imposed.