Conformational analysis of flavonoids: crystal and molecular structures of morin hydrate and myricetin (1:2) triphenylphosphine oxide complex

Biodegradation of flavonoids - Influences of structural features

Flavonoids, a class of natural products with a variety of applications in nutrition, pharmacy and as biopesticides, could substitute more harmful synthetic chemicals that persist in the environment. To gain a better understanding of the biodegradability of flavonoids and the influence of structural features, firstly, the ultimate biodegradation of 19 flavonoids was investigated with the Closed Bottle Test according to the OECD guideline 301 D. Secondly, regarding the fast abiotic degradation reported for several flavonoids with severe concentration decrease within hours and its possible impacts on the processes behind the ultimate biodegradation, primary degradation of 4 selected flavonoids was compared at conditions representing biodegradation, abiotic degradation, and mixed substrates by monitoring the flavonoids' concentrations with HPLC-UV/vis. Our results showed that 17 out of the 19 tested flavonoids were readily biodegradable. Structural features like a hydroxy group at C3, the C2-C3 bond order, a methoxy group in the B ring, and the position of the B ring in regard to the chromene core did not affect biodegradation of the tested flavonoids. Only flavone without any hydroxy groups and morin with an uncommon 2',4' pattern of hydroxy groups were non-readily biodegradable. Monitoring the concentration of 4 selected flavonoids by HPLC-UV/vis revealed that biodegradation occurred faster than abiotic degradation at CBT conditions with no other carbon sources present. The presence of an alternative carbon source tends to increase lag phases and decrease biodegradation rates. At this condition, abiotic degradation contributed to the degradation of unstable flavonoids. Overall, as a first tier to assess the environmental fate, our results indicate low risks for persistence of most flavonoids. Thus, flavonoids could represent benign substitutes for persistent synthetic chemicals.

Molecular Structure, Antioxidant Potential, and Pharmacokinetic Properties of Plant Flavonoid Blumeatin and Investigating Its Inhibition Mechanism on Xanthine Oxidase for Hyperuricemia by Molecular Modeling

Hyperuricemia, which usually results in metabolic syndrome symptoms, is increasing rapidly all over the world and becoming a global public health issue. Xanthine oxidase (XO) is regarded as a key drug target for the treatment of this disease. Therefore, finding natural, nontoxic, and highly active XO inhibitors is quite important. To get insights into inhibitory potential toward XO and determine antioxidant action mechanism depending on the molecular structure, plant flavonoid blumeatin was investigated for the first time by Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT), ADME/Tox (absorption, distribution, metabolism, excretion, and toxicity) analysis, and molecular docking study. Theoretical findings indicated that blumeatin has high radical scavenging activity due to its noncoplanarity and over twisted torsion angle (-94.64°) with respect to its flavanone skeleton could explain that there might be a correlation between antioxidant activity and planarity of blumeatin. Based on the ADME/Tox analysis, it is determined that blumeatin has a high absorption profile in the human intestine (81.93%), and this plant flavonoid is not carcinogenic or mutagenic. A molecular docking study showed that Thr1010, Val1011, Phe914, and Ala1078 are the main amino acid residues participating in XO's interaction with blumeatin via hydrogen bonds.

Characterization and In Vivo Antiangiogenic Activity Evaluation of Morin-Based Cyclodextrin Inclusion Complexes

Morin (MRN) is a natural compound with antiangiogenic, antioxidant, anti-inflammatory, and anticancer activity. However, it shows a very low water solubility (28 μg/mL) that reduces its oral absorption, making bioavailability low and unpredictable. To improve MRN solubility and positively affect its biological activity, particularly its antiangiogenic activity, in this work, we prepared the inclusion complexes of MNR with sulfobutylether-β-cyclodextrin (SBE-β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD). The inclusion complexes obtained by the freeze-drying method were extensively characterized in solution (phase-solubility studies, UV-Vis titration, and NMR spectroscopy) and in the solid state (TGA, DSC, and WAXD analysis). The complexation significantly increased the water solubility by about 100 times for MRN/HP-β-CD and 115 times for MRN/SBE-β-CD. Furthermore, quantitative dissolution of the complexes was observed within 60 min, whilst 1% of the free drug dissolved in the same experimental time. 1H NMR and UV-Vis titration studies demonstrated both CDs well include the benzoyl moiety of the drug. Additionally, SBE-β-CD could interact with the cinnamoyl moiety of MRN too. The complexes are stable in solution, showing a high value of association constant, that is, 3380 M-1 for MRN/HP-β-CD and 2870 M-1 for MRN/SBE-β-CD. In vivo biological studies on chick embryo chorioallantoic membrane (CAM) and zebrafish embryo models demonstrated the high biocompatibility of the inclusion complexes and the effective increase in antiangiogenic activity of complexed MRN with respect to the free drug.

Experimental and Theoretical Study of Fluorescent Properties of Morin

Morin (M) is one of the most widely distributed flavonoids with several beneficial effects on human health, and has the potential of being used as a possible treatment for COVID-19. To achieve a better understanding of the process of M dissolution, the fluorescent (FL) emission from M solutions prepared with different polar and nonpolar solvents (methanol, DMSO, and chloroform) was measured and compared with the FL emission from M powder and M crystals. In the FL spectra of the solutions with high M concentration, as well as in the spectra of M in solid state, two features, at 615 nm and 670 nm, were observed. As the solution concentration decreases, the maxima of FL spectra of the M solutions in all considered solvents shift to the blue side of the spectrum until reaching the value of 520 nm. To explain the experimental results, the TDDFT-M06-2X/6-31++G(d,p) method was used to determine the possible electronic transitions in the M molecule. The computations show that the FL emission in the spectral range of detection of our setup (405–800 nm) is related to the excited state intramolecular proton transfer (ESIPT). Comparison of the experimental data with the computations strongly suggests that in low-concentrated solutions, the FL emission is mostly due to electronic transitions in the keto OH3 form, whereas in aggregated states, the dominate contribution to the FL emission spectra is due to the transitions in keto OH5 form. Moreover, the time evolution of the M solutions FL spectra was observed, measured and explained for the first time.

Fluorescent Properties of Morin in Aqueous Solution: A Conversion from Aggregation Causing Quenching (ACQ) to Aggregation Induced Emission Enhancement (AIEE) by Polyethyleneimine Assembly

Unlike normal conversion from aggregation caused quenching (ACQ) to aggregation induced emission enhancement (AIEE) by introducing aromatic rotors tuning aggregation modes, in this study, it is achieved through a supramolecular assembly with polymer. Thus, it provides an easy approach for the inhibition of unwanted H-aggregation between luminogens. As a kind of flavonoid, morin has shown great potential in therapeutics. However, its poor solubility and weak emission in aqueous solution greatly limit its bioapplications. When morin is dissolved in aqueous solution, the presence of 30 × 10^-6 m polyethyleneimine (PEI) induces significant emission enhancement and bathochromic shift. Consequently, the quantum yield (QY) of 24.5% is either achieved by assembling with PEI, versus 0.76% of its ACQ state composed of H-aggregation in aqueous solution. Particularly, the in-depth mechanism studies reveal that it is the assembly with PEI that disassociates the H-aggregation in aqueous solution and further restricts the stretching and/or rotation of morin, which eventually reduce the nonradiative decays and enhance the emission. Therefore, the present study reports a unique phenomenon of AIEE effects on morin. Particularly the in-depth investigation on intrinsic mechanisms will highlight and greatly expand the development of more luminogens from traditional Chinese herbals.

Combined cereal and pulse flavonoids show enhanced bioavailability by downregulating phase II metabolism and ABC membrane transporter function in Caco-2 model

Predominant flavonoids in cereals and pulses are structurally different and may positively interact to enhance bioactivity in combined diet. This work investigated the effects of combined cereal 3-deoxyflavonoids (apigenin, naringenin) and pulse flavonols (quercetin), along with natural extracts, on their bioavailability and underlying mechanisms using Caco-2 monolayer model. Membrane permeability, phase II metabolism, and ATP binding cassette (ABC) membrane transporter expression and function were measured. Apparent absorption of quercetin and apigenin increased (p < 0.05) 3.3× and 1.5×, respectively, while both compounds were significantly less metabolized in combined treatments. Combinations with naringenin had insignificant effect, suggesting a role for flavonoid C2C3 conjugation. Both natural extracts and apigenin-quercetin combinations synergistically (3-40 fold) downregulated ABC transporter expression, and inhibited P-glycoprotein activity, suggesting direct binding and inhibition of ATPase. Combination of conjugated cereal and pulse flavonoids enhances their potential bioavailability through synergistic inhibition of membrane transporter and phase II enzyme function.

Complementary effects of cereal and pulse polyphenols and dietary fiber on chronic inflammation and gut health

Cereal grains and grain pulses are primary staples often consumed together, and contribute a major portion of daily human calorie and protein intake globally. Protective effects of consuming whole grain cereals and grain pulses against various inflammation-related chronic diseases are well documented. However, potential benefits of combined intake of whole cereals and pulses beyond their complementary amino acid nutrition is rarely considered in literature. There is ample evidence that key bioactive components of whole grain cereals and pulses are structurally different and thus may be optimized to provide synergistic/complementary health benefits. Among the most important whole grain bioactive components are polyphenols and dietary fiber, not only because of their demonstrated biological function, but also their major impact on consumer choice of whole grain/pulse products. This review highlights the distinct structural differences between key cereal grain and pulse polyphenols and non-starch polysaccharides (dietary fiber), and the evidence on specific synergistic/complementary benefits of combining the bioactive components from the two commodities. Interactive effects of the polyphenols and fiber on gut microbiota and associated benefits to colon health, and against systemic inflammation, are discussed. Processing technologies that can be used to further enhance the interactive benefits of combined cereal-pulse bioactive compounds are highlighted.

Properties and applications of flavonoid metal complexes

Flavonoid metal complexes have a wide spectrum of activities as well as potential and actual applications.

Related flavonoids cause cooperative inhibition of the sarcoplasmic reticulum Ca²⁺ ATPase by multimode mechanisms

Flavonoids are group of plant-derived hydroxylated polycyclic molecules found in fruit and vegetables. They are known to bio-accumulate within humans and are considered to have beneficial health effects, including cancer chemoprotection. One mechanism proposed to explain this is that they are able to induce apoptosis in cancer cells by inhibiting a variety of kinases and also the Ca²⁺ ATPase. An investigation was undertaken with respect to the mechanism of inhibition for three flavonoids: quercetin, galangin and 3,6 dihydroxyflavone (3,6-DHF). Each inhibited the Ca²⁺ ATPase with K(i) values of 8.7, 10.3 and 5.4 μM, respectively, showing cooperative inhibition with n ~ 2. Given their similar structures, the flavonoids showed several differences in their mechanisms of inhibition. All three flavonoids stabilized the ATPase in the E₁ conformation and reduced [³²P]-ATP binding. However, both galangin and 3,6-DHF increased the affinity of Ca²⁺ for the ATPase by decreasing the Ca²⁺-dissociation rate constant, whereas quercetin had little effect. Ca²⁺-induced changes in tryptophan fluorescence levels were reduced in the presence of 3,6-DHF and galangin (but not with quercetin), indicating that Ca²⁺-associated changes within the transmembrane helices are altered. Both galangin and quercetin reduced the rates of ATP-dependent phosphorylation and dephosphorylation, whereas 3,6-DHF did not. Modelling studies suggest that flavonoids could potentially bind to two sites: one directly where nucleotides bind within ATP binding site and the other at a site close by. We hypothesize that interactions of these two neighbouring sites may account for both the cooperative inhibition and the multimode mechanisms of action seen with related flavonoids.

PM6 and DFT study of free radical scavenging activity of morin

Flavonoids have long been recognised for their general health-promoting properties, of which their antioxidant activity may play an important role. In this work, we have studied the properties of flavonoid morin using semiempirical and density functional theory (DFT) methods in order to validate the application of the recently developed parametric method 6 (PM6). Reaction enthalpies related to mechanisms of free radical scavenging by flavonoid morin were calculated by DFT and PM6 methods in gas-phase, water, DMSO and benzene. It has been shown that fast semiempirical PM6 method can mimic results obtained by means of more accurate time consuming DFT calculations. Thermodynamically favoured mechanism depends on reaction medium: SPLET (sequential proton loss electron transfer) is preferred in water and DMSO, and HAT (hydrogen atom transfer) is predominant in gas-phase. In benzene these two mechanisms are competitive.

Mechanistic study of the structure-activity relationship for the free radical scavenging activity of baicalein

Density functional theory calculations were performed to evaluate the antioxidant activity of baicalein. The conformational behaviors of both the isolated and the aqueous-solvated species (simulated with the conductor-like polarizable continuum solvation model) were analyzed at the M052X/6-311 + G(d,p) level. The most stable tautomers of various forms of baicalein displayed three IHBs between O4 and OH5, O5 and OH6, and O6 and OH7. The most stable tautomer of the baicalein radical was obtained by dehydrogenating the hydroxyl at C6, while the most stable anion tautomer was obtained by deprotonating the C7 hydroxyl in gaseous and aqueous phases. The expected antioxidant activity of baicalein was explained by its ionization potentials (IPs) and homolytic O-H bond dissociation enthalpies (BDEs), which were obtained via the UM052X optimization level of the corresponding radical species. Heterolytic O-H bond cleavages (proton dissociation enthalpies, PDEs) were also computed. The calculated IP, BDE, and PDE values suggested that one-step H-atom transfer, rather than sequential proton loss-electron transfer or electron transfer-proton transfer, would be the most favorable mechanism for explaining the antioxidant activity of baicalein in the gas phase and in nonpolar solvents. In aqueous solution, the SPLET mechanism was more important.

Toward the prediction of the activity of antioxidants: experimental and theoretical study of the gas-phase acidities of flavonoids

The relative gas-phase acidities were determined for eight flavonoids, applying the kinetic method, by means of electrospray-ion trap mass spectrometry. The experimental acidity order, myricetin > luteolin > quercetin > (+/-)-taxifolin > kaempferol > apigenin > (+)-catechin > (+/-)-naringenin shows good agreement with the order obtained by theoretical calculations at the B3LYP/6-311 + G(2d,2p)//HF/6-31G(d) level. Moreover, these calculations provide the gas-phase acidities of the different OH groups for each flavonoid. The calculated acidity values (Delta(ac)H), corresponding to the most favorable deprotonation, cover a narrow range, 314.8-330.1 kcal/mol, but the experimental method is sensitive enough to differentiate the acidity of the various flavonoids. For all the flavones and the flavanol, catechin, the 4'-hydroxyl group is the most favored deprotonation site whereas for the flavanones studied, taxifolin and naringenin, the most acidic site is the 7-hydroxyl group. On the other hand, the 5-hydroxyl, in flavones and naringenin, and the 3-hydroxyl, in taxifolin and catechin, are always the less acidic positions. The acidity pattern observed for this family of compounds mainly depends on the following structural features: The ortho-catechol group, the 2,3 double bond and the 4-keto group.

Kinetic and stoichiometric assessment of the antioxidant activity of flavonoids by electron spin resonance spectroscopy

There is current interest in the use of naturally occurring flavonoids as antioxidants for the preservation of foods and the prevention of diseases such as atherosclerosis and cancers. To establish the molecular characteristics required for maximum antioxidant activity, electron spin resonance (ESR) spectroscopy has been used to determine the stoichiometry and kinetics of the hydrogen-donating ability of 15 flavonoids and d-alpha-tocopherol to galvinoxyl, a resonance-stabilized, sterically protected aryloxyl radical. The second-order reaction rates, which will be governed by O-H bond dissociation energies, were myricetin > morin > quercetin > fisetin approximately catechin > kaempferol approximately luteolin > rutin > d-alpha-tocopherol > taxifolin > tamarixetin > myricetin 3',4',5'-trimethyl ether > datiscetin > galangin > hesperitin approximately apigenin. Reactivity is highly dependant on the configuration of OH groups on the flavonoid B and C rings, there being little contribution from the A ring to antioxidant effectiveness. Highest reaction rates and stoichiometries were observed with flavonols capable of being oxidized to orthoquinones or extended paraquinones. However, rates and stoichiometries did not always correlate and the data suggest that kinetic factors may be of greater importance within a biological context.

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

13C-CP-MAS-NMR spectra were measured in order to characterise the orientation of hydroxyl groups of quercetin, quercetin-5'-sulphonic acid and polyphenol-type compounds such as catechol, pyrogallol and gallic acid. The locked conformation of the OH group in the solid results in an increased shielding of carbon proxime to C-OH hydrogen. Carbon shieldings suggest that there is orientational disorder of three OH groups of pyrogallol and gallic acid and that two OH groups of catechol are not equivalent. In solid quercetin and quercetin-5'-sulphonic acid the C7-OH points towards C6-H and the C3'-OH hydrogen is near C2'-H.

A quantum chemical explanation of the antioxidant activity of flavonoids

Flavonoids are a group of naturally occurring antioxidants, which over the past years have gained tremendous interest because of their possible therapeutic applicability. The mechanism of their antioxidant activity has been extensively studied over several decades. However, there is still much confusion about the molecular mechanism of radical scavenging and the relationship between structure and activity. Therefore, we have calculated the heat of formation and the geometry of both the parent compound and the corresponding radical using the ab initio program GAMESS. We have compared their differences in energy in order to gain insight into the stability of the radical and the ease with which it is formed. We have also investigated the spin density of the radical, to determine the delocalization possibilities. These calculated data were compared with experimental data from ESR (hyperfine coupling constants) and electrochemical oxidation (Ep/2) and were found to be in good agreement. By comparing the geometries of several flavonoids, we were able to explain the structural dependency of the antioxidant action of these compounds. The extremely good antioxidant activity of the flavonols could be explained by the formation of an intramolecular hydrogen bond.