13C-CP-MAS-NMR studies of flavonoids. I. Solid-state conformation of quercetin, quercetin 5'-sulphonic acid and some simple polyphenols

Seleno-Functionalization of Quercetin Improves the Non-Covalent Inhibition of Mpro and Its Antiviral Activity in Cells against SARS-CoV-2

The development of new antiviral drugs against SARS-CoV-2 is a valuable long-term strategy to protect the global population from the COVID-19 pandemic complementary to the vaccination. Considering this, the viral main protease (M^pro) is among the most promising molecular targets in light of its importance during the viral replication cycle. The natural flavonoid quercetin 1 has been recently reported to be a potent M^pro inhibitor in vitro, and we explored the effect produced by the introduction of organoselenium functionalities in this scaffold. In particular, we report here a new synthetic method to prepare previously inaccessible C-8 seleno-quercetin derivatives. By screening a small library of flavonols and flavone derivatives, we observed that some compounds inhibit the protease activity in vitro. For the first time, we demonstrate that quercetin (1) and 8-(p-tolylselenyl)quercetin (2d) block SARS-CoV-2 replication in infected cells at non-toxic concentrations, with an IC₅₀ of 192 μM and 8 μM, respectively. Based on docking experiments driven by experimental evidence, we propose a non-covalent mechanism for M^pro inhibition in which a hydrogen bond between the selenium atom and Gln189 residue in the catalytic pocket could explain the higher M^pro activity of 2d and, as a result, its better antiviral profile.

Impact of intramolecular hydrogen bonding of gallic acid conformers on chemical shift through NMR spectroscopy

Intramolecular hydrogen bonding of gallic acid conformers was probed as a function of their dihedral angles using ¹³C nuclear magnetic resonance (NMR) chemical shift (δ_C). The quantum mechanically calculated ¹³C NMR chemical shift based on the most stable conformer (GA-I) in dimethylsulfoxide (DMSO) solution agrees to available measurement in the same solvent (RMSD = 0.95 ppm), better than to the measurement in solid phase (RMSD = 1.93 ppm). The accuracy of the calculated NMR chemical shift of the nominal but non-equivalent phenyl carbons C₍₃₎/C₍₇₎ and C₍₄₎/C₍₆₎ (ortho and meta to the acid -C₍₁₎OOH group) of GA may not be evaluated using the experimental measurements at room temperature. The splitting in chemical shift of the nominal phenyl carbons is able to be experimentally measured only in low temperature NMR and using quantum mechanical calculations. We further recognised that the C NMR chemical shifts of the nominal phenyl carbons (C₍₃₎/C₍₇₎ and C₍₄₎/C₍₆₎) encode information for intramolecular hydrogen bonding network formed by GA conformers. The ability to obtain accurate splitting of NMR chemical shifts for nominal carbons, therefore, determines the usefulness of the NMR technique as a probe for conformation of GA.

Anti-virulence activities of biflavonoids from Mesua ferrea L. flower

Based on the anti-virulence activity on Salmonella, the ethyl acetate extract (EAE) of Mesua ferrea flower was investigated for its chemical constituents. Ten purified compounds were identified and assayed for their inhibitory activity against Type III secretion system (T3SS) by polyacrylamide gel electrophoresis (SDS-PAGE) and Western blots experiments. We found the biflavonoids, rhusflavanone and mesuaferrone B, exhibited inhibitory effects on the secretion of Salmonella pathogenicity island 1 (SPI-1) effector proteins (SipA, B, C and D) without effecting the bacterial growth. In addition, 5, 6, 6'-trihydroxy-[1,1'-biphenyl]-3,3'-dicarboxylic acid (6) is a new natural product from M. ferrea flower.

Probing the influence of carboxyalkyl groups on the molecular flexibility and the charge density of apigenin derivatives

Apigenin is an important flavonoids due to its antidiabetic bioactivity. It was reported experimentally that the 7-substituent derivative of apigenin has higher biological activity than 4'- and 5-substituted derivatives while introducing sole carboxyalkyl group -(CH₂)₇COOH into the parent structure. Molecular docking studies indicated that the other two derivatives have lower binding affinities than the 7-substituent derivative (-7.52 kcal mol^-1), which is considered to be a better inhibitor than the parent molecule. Almost all of the carbon atoms and oxygen atoms are coplaner for all three molecules in solution phase, however, all carboxyalkyl groups bend inside into the parent molecules in the active site, and the jagged geometries of the carbon chains are destroyed correspondingly. In addition, most of the electron densities of the chemical bonds for all molecules are decreased, especially the 7-substituent derivative. In contrast, most of the Laplacian values for three molecules are increased in the active site, which suggests that the charge densities at the bond critical point (bcp) are much more depleted than the solution phase. Dipole moments of derivatives are all increased in the active site, suggesting strong intermolecular interactions. After interacting with the S. cerevisiae α-glucosidase, only the 7-substituent derivative has the lowest energy gap ΔE _HOMO-LUMO, which indicates the lowest stability and the highest inhibition activity. Graphical abstract Probing the influence of carboxyalkyl groups on the molecular flexibility and the charge density of apigenin derivatives.

Influence of heat curing on structure and physicochemical properties of phenolic acid loaded proteinaceous electrospun fibers

Effects of heat treatment on structure and physicochemical properties of zein (Ze) and gallic acid loaded zein (Ze-GA) electrospun fiber mats were investigated. The electrospun fiber mats displayed different surface and physicochemical properties after being heat-cured at 150 °C for 24 h, which were closely related to the initial amount of loaded gallic acid. The gallic acid was released from the Ze-GA fiber mats in a constant manner, but heat curing decreased the rate of release. Heat curing remarkably increased the molecular weight of the Ze and Ze-GA electrospun fiber mats. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis of the fiber mats indicated variations in zein protein secondary structure after heat curing. (13)C solid state NMR (SS-NMR) confirmed the presence of a different chemical environment among the fiber mats. The fabrication of heat-cured zein based electrospun fibers in this study may find applications in the food packaging industry.

Characterization of freeze-dried gallic acid/xyloglucan

BACKGROUND

Tamarind seed xyloglucan (TSX) is generally used for drug delivery systems. Gallic acid (GA) possesses various pharmacological activities. It has a good solubility and bioavailability but short half-life.

PURPOSE

To prepare a sustained-release of GA to overcome its relatively short half-life. GA was blended with TSX and freeze-dried. The physicochemical properties of freeze-dried GA and freeze-dried GA/TSX were characterized, and the release profiles of GA from these freeze-dried samples were investigated.

METHOD

All freeze-dried samples were characterized by PXRD, spectroscopic and thermal analyses. The dissolution studies were performed according to the United States Pharmacopeia (USP) XXX.

RESULTS

According to FTIR, FT-Raman and (13)C CP/MAS NMR, the spectra of freeze-dried GA were similar to that of the anhydrous form. Nevertheless, DRIFTS and DSC were able to differentiate these two forms. The crystallinity of GA in the freeze-dried GA/TSX was the same as that of the freeze-dried GA. DSC indicates that there were interactions between GA and TSX. It was of interest that a freeze-dried sample with low amount of GA, 0.2% GA/1% TSX was mostly in an amorphous form. Moreover, all freeze-dried GA/TSX preparations demonstrated a sustained-release of GA compared to GA alone. The freeze-dried 1% GA/1% TSX provided the best sustained-release of GA of up to 240 min.

CONCLUSIONS

TSX could change a crystal form of a small molecule to a mostly amorphous form. It was of importance that the freeze-dried GA/TSX could effectively retard the release of GA. These samples may be able to overcome the limitation for the therapeutic use of GA due to its short biological half-life.

Solid dispersion of quercetin in cellulose derivative matrices influences both solubility and stability

Amorphous solid dispersions (ASD) of quercetin (Que) in cellulose derivative matrices, carboxymethylcellulose acetate butyrate (CMCAB), hydroxypropylmethylcellulose acetate succinate (HPMCAS), and cellulose acetate adipate propionate (CAAdP) were prepared with the goal of identifying an ASD that effectively increased Que aqueous solution concentration. Crystalline quercetin and Que/poly(vinylpyrrolidinone) (PVP) ASD were evaluated for comparison. Powder X-ray diffraction (XRPD) and differential scanning calorimetry (DSC) were used to examine the crystallinity of ASDs, physical mixtures (PM) and quercetin. ASDs were amorphous up to 50 wt% Que. Que stability against crystallization and solution concentrations from these ASDs were significantly higher than those observed for physical mixtures and crystalline Que. PVP stabilizes against both Que degradation and recrystallization; in contrast, these carboxylated cellulose derivatives inhibit recrystallization but release Que slowly. PVP ASDs afforded fast and complete drug release, while ASDs using these three cellulose derivatives provide slow, incomplete, pH-triggered drug release.

Isolation and identification of compounds from Penthorum chinense Pursh with antioxidant and antihepatocarcinoma properties

Penthorum chinense Pursh, widely distributed in eastern Asia, has long been used in China for both food and medicine due to its various bioactivities. The aim of this study was to isolate its active compounds with antioxidant and antihepatocarcinoma properties. P. chinense was extracted with 95% ethanol, 70% ethanol, and water, respectively, and then the 70% ethanol extract was re-extracted, resulting in petroleum ether, ethyl acetate, n-butanol, and water fractions, subsequently. Results showed that the antioxidant and antihepatocarcinoma activities of ethanol extracts were stronger than those of aqueous extract, and the ethyl acetate fraction of 70% ethanol extract showed the highest activities. Four compounds, β-sitosterol, quercetin, pinocembrin-7-O-[3-O-galloyl-4″,6″-hexahydroxydiphenoyl]-β-glucose (PGHG), and thonningianins A (Th A), were isolated from the ethyl acetate fraction and identified by UV, MS, and NMR spectroscopic analysis. Th A was isolated from P. chinense for the first time. PGHG and Th A exhibited higher antioxidant and antihepatocarcinoma activities than did other isolated parts of P. chinense . The antihepatocarcinoma activity of Th A was much higher than that of positive control (5-fluorouracil). PGHG and Th A were suggested to be the active chemical compositions responsible for potent antioxidant and antihepatocarcinoma properties of P. chinense , which are worthy of further study.

Evaluation of the potential hypoglycemic and Beta-cell protective constituents isolated from Corni fructus to tackle insulin-dependent diabetes mellitus

Corni fructus is the fruit of Cornus officinalis Sieb. et Zucc. and has attracted much interest due to its traditional applications and active fraction that reportedly possesses antidiabetic effects. In this study, we isolated 12 compounds from Corni fructus including three flavonoids, two iridoid glycosides, three phenolic compounds, and two triterpenoids, together with cornuside (11) and 2-butoxybutanedioic acid (12). Chemical structures were identified by (1)H, (13)C NMR, DEPT, COSY, HSQC, and HMBC spectral analyses. Furthermore, the glucose uptake efficiency, messenger (m)RNA expression of phosphoenolpyruvate carboxykinase (PEPCK), and prevention of cytokine-mediated cytotoxicity in the presence of test agents were evaluated. While CH and CB significantly increased glucose uptake from muscle, compounds 3 and 8, each at 50 μM, significantly suppressed PEPCK mRNA expression. Finally, compound 5, at 50 and 100 μM, effectively attenuated β-cell death. In conclusion, those compounds could contribute to the antihyperglycemic and β-cell-protective actions of Corni fructus against diabetes mellitus.

NMR of a series of novel hydroxyflavothiones

Alkylated hydroxyflavothiones, namely flavothione, 5-hydroxyflavothione, 5,7-dihydroxyflavothione (chrysinthione), 7-dodecyloxy-5-hydroxyflavothione, 7-butyloxy-5-hydroxyflavothione, 2',3,4',7-tetramethoxy-5-hydroxyflavothione, 3,3',4',7-tetramethoxy-5-hydroxyflavothione, 7-butyloxy-4',5-dihydroxyflavothione and 7-butyloxy-4',5-hydroxyflavanonethione have been synthesized from the corresponding hydroxyflavones in two steps, alkylation of the non-hydrogen-bonded hydroxyl groups by bromoalkanes or dimethyl sulfate followed by conversion of the carbonyl group to a thione using Lawesson's Reagent under microwave irradiation and solvent-free conditions. Part of the alkylated flavanone, 7-butyloxy-4',5-dihydroxyflavanone, was oxidized during the treatment with Lawesson's reagent to yield a second product 7-butyloxy-4',5-dihydroxyflavothione in addition to the target product butyloxy-4',5-hydroxyflavanonethione. Deuterium isotope effects on 13C chemical shifts have been measured in hydroxyflavones, isoflavones, flavanones and the thio analogs. Formal four-bond deuterium isotope effects on 13C chemical shifts, nDeltaC=S(OD) are very sensitive to variations in structures and substitution patterns. Density functional theory (DFT) calculations are carried out to obtain geometries. Correlations relating distances around the hydrogen bond system to the deuterium isotope effects on 13C chemical shifts are discussed. 13C chemical shifts are calculated by DFT methods. Effects of thiocarbonyl anisotropies are suggested.

Quantum mechanical calculation of aqueuous uranium complexes: carbonate, phosphate, organic and biomolecular species

BACKGROUND

Quantum mechanical calculations were performed on a variety of uranium species representing U(VI), U(V), U(IV), U-carbonates, U-phosphates, U-oxalates, U-catecholates, U-phosphodiesters, U-phosphorylated N-acetyl-glucosamine (NAG), and U-2-Keto-3-doxyoctanoate (KDO) with explicit solvation by H2O molecules. These models represent major U species in natural waters and complexes on bacterial surfaces. The model results are compared to observed EXAFS, IR, Raman and NMR spectra.

RESULTS

Agreement between experiment and theory is acceptable in most cases, and the reasons for discrepancies are discussed. Calculated Gibbs free energies are used to constrain which configurations are most likely to be stable under circumneutral pH conditions. Reduction of U(VI) to U(IV) is examined for the U-carbonate and U-catechol complexes.

CONCLUSION

Results on the potential energy differences between U(V)- and U(IV)-carbonate complexes suggest that the cause of slower disproportionation in this system is electrostatic repulsion between UO2 [CO3]3(5-) ions that must approach one another to form U(VI) and U(IV) rather than a change in thermodynamic stability. Calculations on U-catechol species are consistent with the observation that UO2(2+) can oxidize catechol and form quinone-like species. In addition, outer-sphere complexation is predicted to be the most stable for U-catechol interactions based on calculated energies and comparison to 13C NMR spectra. Outer-sphere complexes (i.e., ion pairs bridged by water molecules) are predicted to be comparable in Gibbs free energy to inner-sphere complexes for a model carboxylic acid. Complexation of uranyl to phosphorus-containing groups in extracellular polymeric substances is predicted to favor phosphonate groups, such as that found in phosphorylated NAG, rather than phosphodiesters, such as those in nucleic acids.

The biochemistry and medical significance of the flavonoids

Flavonoids are plant pigments that are synthesised from phenylalanine, generally display marvelous colors known from flower petals, mostly emit brilliant fluorescence when they are excited by UV light, and are ubiquitous to green plant cells. The flavonoids are used by botanists for taxonomical classification. They regulate plant growth by inhibition of the exocytosis of the auxin indolyl acetic acid, as well as by induction of gene expression, and they influence other biological cells in numerous ways. Flavonoids inhibit or kill many bacterial strains, inhibit important viral enzymes, such as reverse transcriptase and protease, and destroy some pathogenic protozoans. Yet, their toxicity to animal cells is low. Flavonoids are major functional components of many herbal and insect preparations for medical use, e.g., propolis (bee's glue) and honey, which have been used since ancient times. The daily intake of flavonoids with normal food, especially fruit and vegetables, is 1-2 g. Modern authorised physicians are increasing their use of pure flavonoids to treat many important common diseases, due to their proven ability to inhibit specific enzymes, to simulate some hormones and neurotransmitters, and to scavenge free radicals.