Interaction of quercetin with copper ions: complexation, oxidation and reactivity towards radicals

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.

Comparison of binding sites and affinity of flavonol-Cu(II) complexes with the same parent nucleus: Synthesis, DFT prediction, and coordination pattern

This study explores the coordination dynamics between dietary polyphenols, specifically kaempferol, quercetin, and myricetin, and Cu ions in aqueous environments. A novel synthesis method for flavonol-Cu(II) coordination compounds is introduced, effectively reducing interference from free metal ions. Our results reveal consistent binding patterns of Cu ions with flavonols (2:1 ratio of flavonol to Cu(II)), predominantly at the 4,5 sites. Various analytical techniques are used to validate these coordination ratios and sites. The binding affinity of the flavonols for Cu ions follows a descending sequence: myricetin > quercetin > kaempferol. Notably, coordination with Cu ions enhances the free-radical scavenging activities of these flavonols. These findings hold substantial importance for food chemistry, biology, and medicine, providing crucial insights into the way dietary flavonols form stable structures in environments similar to human body fluids and their interactions with metal ions, opening new possibilities for their application and understanding in diverse scientific domains.

Research progress of quercetin in cardiovascular disease

Quercetin is one of the most common flavonoids. More and more studies have found that quercetin has great potential utilization value in cardiovascular diseases (CVD), such as antioxidant, antiplatelet aggregation, antibacterial, cholesterol lowering, endothelial cell protection, etc. However, the medicinal value of quercetin is mostly limited to animal models and preclinical studies. Due to the complexity of the human body and functional structure compared to animals, more research is needed to explore whether quercetin has the same mechanism of action and pharmacological value as animal experiments. In order to systematically understand the clinical application value of quercetin, this article reviews the research progress of quercetin in CVD, including preclinical and clinical studies. We will focus on the relationship between quercetin and common CVD, such as atherosclerosis, myocardial infarction, ischemia reperfusion injury, heart failure, hypertension and arrhythmia, etc. By elaborating on the pathophysiological mechanism and clinical application research progress of quercetin's protective effect on CVD, data support is provided for the transformation of quercetin from laboratory to clinical application.

Hydroxytyrosol Counteracts Triple Negative Breast Cancer Cell Dissemination via Its Copper Complexing Properties

Polyphenols have gained increasing attention for their therapeutic potential, particularly in conditions like cancer, due to their established antioxidant and anti-inflammatory properties. Recent research highlights their ability to bind to transition metals, such as copper. This is particularly noteworthy given the key role of copper both in the initiation and progression of cancer. Copper can modulate the activity of kinases required for the epithelial-mesenchymal transition (EMT), a process fundamental to tumor cell dissemination. We have previously demonstrated the copper-binding capacity of oleuropein, a secoiridoid found in Olea europaea. In the present study, we investigated the effect of hydroxytyrosol, the primary oleuropein metabolite, on the metastatic potential of three triple-negative breast cancer cell lines (MDA-MB-231, MDA-MB-468, and SUM159). We found that hydroxytyrosol modulated the intracellular copper levels, influencing both the epithelial and mesenchymal markers, by downregulating copper-dependent AKT phosphorylation, a member of the EMT signaling cascade, through Western blot, RT-qPCR, and immunofluorescence. Indeed, by optical spectra, EPR, and in silico approaches, we found that hydroxytyrosol formed a complex with copper, acting as a chelating agent, thus regulating its homeostasis and affecting the copper-dependent signaling cascades. While our results bring to light the copper-chelating properties of hydroxytyrosol capable of countering tumor progression, they also provide further confirmation of the key role of copper in promoting the aggressiveness of triple-negative breast cancer cells.

Reaction kinetics of aspalathin degradation and flavanone isomer formation in aqueous model solutions: Effect of temperature, pH and metal chelators

The poor stability of aspalathin in aqueous solutions is a major challenge in delivering a shelf-stable ready-to-drink (RTD) green rooibos iced tea. The kinetics of aspalathin degradation and the formation of eriodictyol glucoside isomers [(S/R)-6-β-D-glucopyranosyleriodictyol and (S/R)-8-β-D-glucopyranosyleriodictyol] in aqueous buffers were modeled to understand and predict aspalathin losses during heat processing. The effects of temperature and pH on the rate constants of aspalathin degradation and eriodictyol glucoside isomer formation were determined in a 0.1 M phosphate buffer with 5.7 mM citric acid. The zero-order model best described the reaction kinetics of aspalathin degradation and eriodictyol glucoside isomer formation. Increasing the temperature and pH increased the reaction rate constants. The activation energies of the reactions were much lower at pH 6 than at pH 4, indicating that pH affected the temperature dependence of the reactions. The 8-C-glucosyl eriodictyol derivatives (RE8G and SE8G) formed at much lower rates than the 6-C-glucosyl eriodictyol derivatives (RE6G and SE6G). The metal chelators, citric acid, citrate and EDTA, drastically reduced the reaction rate constants, indicating the catalytic role of metal ions in aspalathin autoxidation. The results of the study could assist manufacturers to improve the shelf life of rooibos RTD beverages by changing the formulation and adjusting heat processing conditions.

The role of redox active copper(II) on antioxidant properties of the flavonoid baicalein: DNA protection under Cu(II)-Fenton reaction and Cu(II)-ascorbate system conditions

The antioxidant properties of flavonoids are mediated by their functional hydroxyl groups, which are capable of both chelating redox active metals such as iron, copper and scavenging free radicals. In this paper, the antioxidant vs. prooxidant and DNA protecting properties of baicalein and Cu(II)-baicalein complexes were studied under the conditions of the Copper-Fenton reaction and of the Copper-Ascorbate system. From the relevant EPR spectra, the interaction of baicalein with Cu(II) ions was confirmed, while UV-vis spectroscopy demonstrated a greater stability over time of Cu(II)-baicalein complexes in DMSO than in methanol and PBS and Phosphate buffers. An ABTS study confirmed a moderate ROS scavenging efficiency, at around 37%, for both free baicalein and Cu(II)-baicalein complexes (in the ratios 1:1 and 1:2). The results from absorption titrations are in agreement with those from viscometric studies and confirmed that the binding mode between DNA and both free baicalein and Cu-baicalein complexes, involves hydrogen bonds and van der Waals interactions. The DNA protective effect of baicalein has been investigated by means of gel electrophoresis under the conditions of the Cu-catalyzed Fenton reaction and of the Cu-Ascorbate system. In both cases, it was found that, at sufficiently high concentrations, baicalein offers some protection to cells from DNA damage caused by ROS (singlet oxygen, hydroxyl radicals and superoxide radical anions). Accordingly, baicalein may be useful as a therapeutic agent in diseases with a disturbed metabolism of redox metals such as copper, for example Alzheimer's disease, Wilson's disease and various cancers. While therapeutically sufficient concentrations of baicalein may protect neuronal cells from Cu-Fenton-induced DNA damage in regard to neurological conditions, conversely, in the case of cancers, low concentrations of baicalein do not inhibit the pro-oxidant effect of copper ions and ascorbate, which can, in turn, deliver an effective damage to DNA in tumour cells.

Exploring inhibitory effect and mechanism of hesperetin-Cu (II) complex against protein glycation

Inhibition of advanced glycation end products (AGEs) formed in protein glycosylation is crucial for minimizing diabetic complications. Herein, the anti-glycation potential of hesperetin-Cu (II) complex was investigated. Hesperetin-Cu (II) complex strongly inhibited three stages glycosylation products in bovine serum albumin (BSA)-fructose model, especially for the inhibition of AGEs (88.45%), which was stronger than hesperetin (51.76%) and aminoguanidine (22.89%). Meanwhile, hesperetin-Cu (II) complex decreased the levels of BSA carbonylation and oxidation products. 182.50 µg/mL of hesperetin-Cu (II) complex inhibited 66.71% β-crosslinking structures of BSA, and scavenged 59.80% superoxide anions and 79.76% hydroxyl radicals. Moreover, after incubating with methylglyoxal for 24 h, hesperetin-Cu (II) complex removed 85.70% methylglyoxal. The mechanisms of protein antiglycation by hesperetin-Cu (II) complex may be through protecting structure, trapping methylglyoxal, scavenging free radicals and interacting with BSA. This study may contribute to the development of hesperetin-Cu (II) complex as a functional food additive against protein glycation.

Preparation, antioxidant and antibacterial activities of cryptate copper(II)/sulfonate chitosan complexes

In this work, we synthesized cryptate copper(II) followed by complexed with sulfonate chitosan (SCS). After characterization, the evaluation of the antioxidant properties of resulting complexes were carried out by 1,1-Diphenyl-2-picrylhydrazyl radical (DPPH•), hydroxyl radical (•OH), and 2,2'-Azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS^•+), while the antibacterial and biofilm inhibitory activity against Pseudomonas aeruginosa PAO1 (P. aeruginosa PAO1) were also investigated. According to the results, cryptate copper(II) exhibited the best antioxidant activity followed by cryptate copper(II)/SCS complexes, and SCS. Significant antibacterial activity of cryptate copper(II) against P. aeruginosa PAO1 was observed with the minimum inhibitory concentration of MIC value 12.50 μg/mL and minimum bactericidal concentration of MBC value 100.00 μg/mL, followed by cryptate copper(II)/SCS complexes and SCS. Cryptate copper(II) and cryptate copper(II)/SCS exhibited antibacterial activity which copper ions might enter the interior of cells, and the intracellular ions made the killed bacteria serve as an antibacterial agent showing a zombie effect. The copper ions complexed with cryptate and SCS rendering potential unlimited biological activities, might become one of the most popular research areas because of their unique coordination chemistry and their long-term biological activities.

Opto Field-Effect Transistors for Detecting Quercetin-Cu2+ Complex

In this study, we explored the potential of applying biosensors based on silicon nanowire field-effect transistors (bio-NWFETs) as molecular absorption sensors. Using quercetin and Copper (Cu²⁺) ion as an example, we demonstrated the use of an opto-FET approach for the detection of molecular interactions. We found that photons with wavelengths of 450 nm were absorbed by the molecular complex, with the absorbance level depending on the Cu²⁺ concentration. Quantitative detection of the molecular absorption of metal complexes was performed for Cu²⁺ concentrations ranging between 0.1 μM and 100 μM, in which the photon response increased linearly with the copper concentration under optimized bias parameters. Our opto-FET approach showed an improved absorbance compared with that of a commercial ultraviolet-visible spectrophotometry.

Structure-activity assessment of flavonoids as modulators of copper transport

Flavonoids are polyphenolic small molecules that are abundant in plant products and are largely recognized for their beneficial health effects. Possessing both antioxidant and prooxidant properties, flavonoids have complex behavior in biological systems. The presented work investigates the intersection between the biological activity of flavonoids and their interactions with copper ions. Copper is required for the proper functioning of biological systems. As such, dysregulation of copper is associated with metabolic disease states such as diabetes and Wilson’s disease. There is evidence that flavonoids bind copper ions, but the biological implications of their interactions remain unclear. Better understanding these interactions will provide insight into the mechanisms of flavonoids’ biological behavior and can inform potential therapeutic targets. We employed a variety of spectroscopic techniques to study flavonoid-Cu(II) binding and radical scavenging activities. We identified structural moieties important in flavonoid-copper interactions which relate to ring substitution but not the traditional structural subclassifications. The biological effects of the investigated flavonoids specifically on copper trafficking were assessed in knockout yeast models as well as in human hepatocytes. The copper modulating abilities of strong copper-binding flavonoids were largely influenced by the relative hydrophobicities. Combined, these spectroscopic and biological data help elucidate the intricate nature of flavonoids in affecting copper transport and open avenues to inform dietary recommendations and therapeutic development.

Development of Poly(lipoic acid) Nanoparticles with Improved Oral Bioavailability and Hepatoprotective Effects of Quercetin

Quercetin (QUE)-loaded poly(lipoic acid) nanoparticles (QUE/pLA) were developed to improve chemical stability in the gastrointestinal (GI) tract, oral bioavailability (BA), and pharmacological properties of QUE. QUE/pLA was prepared by emulsion solvent evaporation with ultrasonication followed by freeze-drying. Its mean particle size was 185 nm, with a high encapsulation efficiency of QUE (84.8%). QUE/pLA exhibited sustained release of QUE with improved dissolution compared with crystalline QUE and significantly enhanced chemical stability under physiological pH in the GI tract. Orally dosed QUE/pLA (50 mg QUE/kg) in rats exhibited significantly prolonged systemic exposure, possibly due to the sustained release of QUE. The oral BAs of QUE in QUE/pLA and crystalline QUE groups were 29 and 0.19%, respectively, suggesting significant enhancement of oral absorbability, likely due to the improved stability and dissolution property of QUE in the GI tracts. In hepatic injury model rats, QUE/pLA (50 mg QUE/kg) led to marked reductions in the plasma biomarker levels of alanine aminotransferase and aspartate aminotransferase by 70 and 46%, respectively, compared with the vehicle group. QUE/pLA also showed improved antioxidant potential as evidenced by the enhanced activities of hepatic glutathione, superoxide dismutase, and a decrease in the level of malondialdehyde, a marker of lipid peroxidation. Based on these findings, QUE/pLA might be a promising option to improve both the nutraceutical and pharmaceutical properties of QUE.

Assessment of the Impact of the Application of a Quercetin-Copper Complex on the Course of Physiological and Biochemical Processes in Wheat Plants (Triticum aestivum L.) Growing under Saline Conditions

Salt stress is one of the main stressors limiting plant growth and yield. As a result of salt stress, unfavorable changes in the photosynthesis process take place, leading to a decrease in plant productivity. Therefore, it is necessary to use biologically active substances that reduce the effects of this stress. An example of such a substance is quercetin, classified as a flavonoid, which plays an important role in alleviating the effects of salt stress, mainly by the inactivation of reactive oxygen species (ROS) and by improvement of the photosynthesis process. A study was made of the effect of the quercetin–copper complex (Q-Cu (II)), which has a stronger antioxidant effect than pure quercetin. By means of a pot experiment, the influence of solutions of the Q-Cu (II) complex (100 mg∙L⁻¹ [Q1], 500 mg∙L⁻¹ [Q2] and 1000 mg∙L⁻¹ [Q3]) on the physiological and biochemical processes occurring in wheat plants subjected to salt stress was investigated. The plants were given two sprays of Q-Cu (II) solution, and their physiological parameters were examined both 1 and 7 days after each application of this solution. The level of ROS and the activity of antioxidant enzymes (catalase [CAT], superoxide dismutase [SOD] and guaiacol peroxidase [GPOX]) were also determined. It has been shown that spraying with Q2 and Q3 solutions improves the chlorophyll content, the values of chlorophyll fluorescence parameters (the photochemical efficiency of PS II [F_v/F_m], the maximum quantum yield of primary photochemistry [F_v/F₀], and the performance index of PS II [PI]), and gas exchange (net photosynthetic rate [P_n], stomatal conductance [g_s], transpiration rate [E] and intercellular CO₂ concentration [C_i]). As a result of the application of Q2 and Q3 solutions, the level of ROS and the activity of the antioxidant enzymes tested decreased, which means that these concentrations are most effective in counteracting the effects of salt stress.

Investigating Phragmites australis response to copper exposure using physiologic, Fourier Transform Infrared and metabolomic approaches

Phragmites australis (Cav.) Trin. ex Steud is a landscape plant with resistance to heavy metals that has significance in phytoremediation. However, little is known about the metabolomic background of the heavy metal resistance mechanisms of Phragmites . We studied copper stress on Phragmites and monitored physiological indicators such as malondialdehyde (MDA) and electrolyte leakage (EL). In addition, Fourier Transform Infrared (FTIR) was used to study the related chemical composition in the roots, stems, and leaves under copper stress. Furthermore, LC-MS technology was used to analyse the plants metabolic profile. Results showed that increased copper concentration in Phragmites led to the accumulation of MDA and EL. FTIR spectrum detected the presence of O-H and C=O stretching. O-H stretching was related to the presence of flavonoids, while C=O stretching reflected the presence of protein amide I. The latter was related to the change of amino acid composition. Both flavonoids and amino acids are regarded as contributors to the antioxidant of Phragmites under copper stress. Metabolomics analysis revealed that arginine and ayarin were accumulated and Phragmites leaves responded to copper stress with changes in the pool size of arginine and ayarin. It is speculated that they could improve resistance. Arginine is accumulated through two pathways: the citrulline decomposition and conversion pathway; and the circular pathway composed of ornithine, citrulline, l -argininosuccinate and arginine. Ayarin is synthesised through the quercetin methylation pathway. This study elucidates the antioxidant mechanisms for enhancing its resistance to heavy metal stress, thus improving of phytoremediation efficiency.

Sustainable Hues: Exploring the Molecular Palette of Biowaste Dyes through LC-MS Metabolomics

Underutilized biowaste materials are investigated for their potential as sustainable textile colorants through an approach based on mass spectrometry, bioinformatics, and chemometrics. In this study, colorful decoctions were prepared from the outer bark of Eucalyptus deglupta and fruit peels of Syzygium samarangense, Syzygium malaccense, Diospyros discolor, and Dillenia philippinensis. Textile dyeing was performed along with liquid chromatography-mass spectrometry (LC-MS)-based untargeted metabolomics to determine the small molecules responsible for the observed colors. Global Natural Products Social Molecular Networking (GNPS) guided the annotation of black-producing proanthocyanidins in D. philippinensis and E. deglupta through complexation with FeSO4 mordant. Flavonoids from the yellow-colored D. philippinensis extracts were found to be similar to those in Terminalia catappa, a known traditional dye source. A higher intensity of epicatechin in E. deglupta produced a red-brown color in the presence of Cu2+. Furthermore, Syzygium fruit peels have poor wash-fastness in cotton fibers, but bioactive chalcone unique to S. samarangense samples may be a potential nutritional food colorant. Unsupervised PCA and supervised OPLS-DA chemometrics distinguished chemical features that affect dyeing properties beyond the observed color. These findings, along with growing data on natural dyes, could guide future research on sustainable colorants.

The effect of Luteolin on DNA damage mediated by a copper catalyzed Fenton reaction

Luteolin has been reviewed as a flavonoid possessing potential cardioprotective, anti-inflammatory, anti-cancer activities. Having multiple biological effects, luteolin may act as either an antioxidant or a pro-oxidant. In this work, the protective role of copper(II)-chelation by luteolin on DNA damage via the Cu-Fenton reaction was studied. EPR and UV-vis spectroscopic data demonstrated that the luteolin, lacking 3-OH group, chelates to Cu(II) via the 5-OH and 4-CO groups, respectively. EPR spin trapping experiments using DMPO spin trap confirmed that the coordination of luteolin to Cu(II) significantly suppressed formation of hydroxyl and superoxide radicals (by 80%) in a Cu-Fenton system. Absorption titrations showed that the chelation of Cu(II) by luteolin slightly increased the mild intercalation strength of its interaction with DNA, as compared with free luteolin. Comparison with kaempferol and quercetin revealed, that the strength of the interaction between the free flavonoids/Cu-flavonoid complexes with DNA is only mildly affected by the presence/absence of 3-OH group. Due to the differences in the sensitivities of absorption titrations and viscometry, the latter confirmed weaker DNA intercalating efficiency of Cu-luteolin complex than does free luteolin. A dose dependent protective effect of luteolin against ROS-induced DNA damage was observed using gel electrophoresis. This effect was more pronounced compared to quercetin and kaempferol. In conclusion, the administration of luteolin to patients suffering from oxidative stress-related diseases with disturbed Cu-metabolism such as Alzheimer's diseases (antioxidant effect) and certain cancers (prooxidant effect) may have several health benefits.

Antioxidant properties of flavonoid metal complexes and their potential inclusion in the development of novel strategies for the treatment against neurodegenerative diseases

The increased oxidative stress in the acceleration of the aging process and development of the neuronal disorder are the common feature detected in neurodegenerative illness, such as Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. Searching for new treatment against these diseases, the inclusion of exogenous antioxidant agents has shown good results. Flavonoids are polyphenols compounds present in plants, fruits and vegetables that exhibit potent antioxidant and biological properties, which are related to their chemical structure that to confer an excellent radical scavenging ability. The design of metal-flavonoid complexes allows to obtain compounds with improved biological and physicochemical properties, generating important increase of the flavonoid antioxidant properties. This evidence we motive to propose that antioxidant properties of the metal flavonoids compounds can play an important role in the design of potential novel therapeutic strategies. This review presents the structure-activity relationship on the antioxidant properties of three series of metal-flavonoid complexes: M-(quercetin), M-(morin), and M-(rutin). In general, we observed that the coordination sites, the metal ion type used, and the molar ratio metal:flavonoid present in the complexes, are important factors for to increase the antioxidant activity. On these evidences we motive to propose that the development of metal-flavonoid compounds is a potentially viable approach for combating neurodegenerative diseases.

Vanadium-Flavonoid Complexes: A Promising Class of Molecules for Therapeutic Applications

Several reports have revealed the superior biological activity of metal ion-flavonoid complexes when compared with the parent flavonoid. Among the different metal ions explored, vanadium and its compounds are in the forefront because of their anticancer and antidiabetic properties. However, the toxicity of vanadium-based ions and their inorganic derivatives limits their therapeutic applications. Complexation of vanadium with flavonoids not only reduces its adverse effects but also augments its biological activity. This Review discusses the nature of coordination in vanadium-flavonoid complexes, their structure-activity correlations, with special emphasis on their therapeutic activities. Several investigations suggest that the superior biological activity of vanadium complexes arise because of their ability to regulate metabolic pathways distinct from those acted upon by vanadium alone. These studies serve to decipher the underlying molecular mechanism of vanadium-flavonoid complexes that can be explored further for generating a series of novel compounds with improved pharmacological and therapeutic performance.

Nutraceuticals as Potential Radionuclide Decorporation Agents

Exposure of individuals to radioactive material as a result of ingestion of contaminated food and water is an increasing public health concern. Unfortunately, there are limited treatment modalities for dealing with these types of potentially toxic exposures. Recent research suggests that many plant-based nutraceuticals may possess metal-binding properties. This preliminary study investigated the ability of genistein, curcumin, quercetin, and lentinan to bind metals considered internal contamination risks, namely cesium, uranium, cobalt, and strontium, in a variety of matrices. The efficacy of these nutraceuticals in protecting cultured cells from metal-induced toxicity was also explored. Results showed that none of the compounds bound cesium or strontium. However, genistein, curcumin, and quercetin could bind uranium. Curcumin and quercetin also bound cobalt and could also protect cultured cells from metal-induced cytotoxicity. Lentinan did not bind any of the metals tested. Metal binding was also pH dependent, with no binding observed at lower pH values. This project showed that nutraceuticals could function as chelators for metals considered internal radionuclide contamination hazards. Further investigations are required in order to determine whether these compounds will become a new nontoxic arsenal of pharmaceutical compounds with which to treat radionuclide contamination.

A Review on Coordination Properties of Al(III) and Fe(III) toward Natural Antioxidant Molecules: Experimental and Theoretical Insights

This review focuses on the ability of some natural antioxidant molecules (i.e., hydroxycinnamic acids, coumarin-3-carboxylic acid, quercetin, luteolin and curcumin) to form Al(III)- and Fe(III)-complexes with the aim of evaluating the coordination properties from a combined experimental and theoretical point of view. Despite the contributions of previous studies on the chemical properties and biological activity of these metal complexes involving such natural antioxidants, further detailed relationships between the structure and properties are still required. In this context, the investigation on the coordination properties of Al(III) and Fe(III) toward these natural antioxidant molecules might deserve high interest to design water soluble molecule-based metal carriers that can improve the metal’s intake and/or its removal in living organisms.

Design and optimization of quercetin-based functional foods

Quercetin is a flavonoid present in a wide variety of plant resources. Over the years, extensive efforts have been devoted to examining the potential biological effects of quercetin and to manipulating the chemical and physical properties of the flavonoid. However, limited studies have reviewed the opportunities and challenges of using quercetin in the development of functional foods. To address this necessity, in this review; we foremost present an overview of the chemical properties and stability of quercetin in food products followed by a detailed discussion of various strategies that enhance its oral bioavailability. We further highlight the areas to be practically considered during development of quercetin-based functional foods. By revisiting the current status of applied research on quercetin, it is anticipated that useful insights enabling research on quercetin can be potentially translated into practical applications in food product development.

Antioxidant vs. Prooxidant Properties of the Flavonoid, Kaempferol, in the Presence of Cu(II) Ions: A ROS-Scavenging Activity, Fenton Reaction and DNA Damage Study

Kaempferol is a flavonoid that occurs in tea and in many vegetables and fruits, including broccoli, cabbage, beans, grapes, apples, and strawberries. The efficacy of Kaempferol has been demonstrated in the treatment of breast, esophageal, cervical, ovarian, and liver cancers and leukemia, which very likely arises from its prooxidant properties and the activation of pro-apoptotic pathways. Indeed, this matter has already been the focus of a number of published studies and reviews. The aim of the present study was to elucidate the antioxidant vs. prooxidant properties of flavonoids in the presence of the redox-active metal, copper (II) ion, by means of the Fenton reaction. The specific motivation of this work is that, since an increased level of Cu(II) ions is known to be associated with many disease states such as neurological conditions (Alzheimer’s disease) and cancer, any interaction between these ions and flavonoids might affect the outcome of therapeutic uses of the latter. The structure of the Cu-kaempferol complex in DMSO was investigated by means of low temperature EPR spectroscopy, which confirmed the existence of at least two distinct coordination environments around the copper (II) ion. UV vis-spectra of kaempferol and its Cu(II) complex in DMSO revealed an interaction between the 5-OH (A ring) group and the 4-CO (C ring) group of kaempferol with Cu(II) ions. An ABTS assay confirmed that kaempferol acted as an effective radical scavenger, and that this effect was further enhanced in the form of the Cu(II)-kaempferol complex. Quantitative EPR spin trapping experiments, using DMPO as the spin trap, confirmed suppression of the formation of a mixture of hydroxyl, superoxide, and methyl radicals, in a Fenton reaction system, upon coordination of kaempferol to the redox-active Cu(II) ions, by 80% with respect to the free Cu(II) ions. A viscometric study revealed a better DNA-intercalating ability of the Cu-kaempferol complex than for free kaempferol, essential for conferring anticancer activity of these substances. The results of the viscometric measurements were compared with those from a DNA damage study of Cu-kaempferol complexes in a Fenton reaction system, using gel electrophoresis. At low concentrations of kaempferol (Cu–kaempferol ratios of 1:1 and 1:2), a very weak protective effect on DNA was noted, whereas when kaempferol was present in excess, a significant DNA-protective effect was found. This can be explained if the weakly intercalated kaempferol molecules present at the surface of DNA provide protection against attack by ROS that originate from the Fenton reaction involving intercalated Cu(II)-kaempferol complexes. Following the application of ROS scavengers, L-histidine, DMSO, and SOD, gel electrophoresis confirmed the formation of singlet oxygen, hydroxyl radicals, and superoxide radical anions, respectively. We propose that the prooxidant properties of Cu-kaempferol complexes may provide anticancer activity of these substances. When present in excess, kaempferol displays antioxidant properties under Cu-Fenton conditions. This suggests that kaempferol might prove a suitable candidate for the prevention or treatment of oxidative stress related medical conditions that involve a disturbed metabolism of redox metals such as copper, for example, Menkes disease, and neurological disorders, including Alzheimer’s disease. For the potential use of kaempferol in clinical practice, it will be necessary to optimize the dose size and critical age of the patient so that this flavonoid may be beneficial as a preventive drug against cancer and neurological disorders.

Dextran-modified Quercetin-Cu(II)/hyaluronic acid nanomedicine with natural poly(ADP-ribose) polymerase inhibitor and dual targeting for programmed synthetic lethal therapy in triple-negative breast cancer

Serious side effects from chemotherapies are the main problem with cancer treatments. To solve these issues, precision cancer nanomedicine based on natural therapeutic materials is developed, which enables specifically apoptosis by interacting with genetic mutation in cancer cells, while leaving normal cells unaffected. Here, we report a novel nanomedicine (CuQDA/IO@HA) composed of hyaluronic acid (HA) / copper ion (Cu(II))-chelated dextran-aldehyde (DA)-quercetin (Q) with dual targeting for synthetic lethal therapy. The CuQDA/IO@HA prepared using a ratio of metal/Q at 0.5:1 resulted in a stable particle structure with uniform particle distribution. The CuQDA/IO@HA can specifically target and induce specific cytotoxicity in BRCA-mutant cancer cells in vitro. Combination treatment with CuQDA/IO@HA and magnetic navigation can induce poly (ADP-ribose) polymerase (PARP) inhibition and DNA damage in BRCA-mutant triple-negative breast cancer (TNBC) via CD44 targeting. The dual-targeting CuQDA/IO@HA can extend the median survival of the BRCA-mutant xenograft mice from 34 to 61 days in comparison to Q treatment alone in vivo, which is attributed to the significant increase in γH2AX, leading to significant apoptosis. More importantly, the CuQDA/IO@HA displayed biocompatibility and no obvious side-effect in normal organs. These results demonstrate the promising potential of integrating natural and metal ions into a nanomedicine that can provide precision medicine through synthetic lethality.

Spectroscopic, computational and molecular docking study of Cu(ii) complexes with flavonoids: from cupric ion binding to DNA intercalation

Copper(ii) complexes with flavonoids as perspective therapeutic agents with DNA as a target molecule.

Targeting Microglia for Therapy of Parkinson's Disease by Using Biomimetic Ultrasmall Nanoparticles

Microglia as an important type of innate immune cell in the brain have been considered as an effective therapeutic target for the treatment of central nervous degenerative diseases. Herein, we report cell membrane coated novel biomimetic Cu_2-xSe-PVP-Qe nanoparticles (denoted as CSPQ@CM nanoparticles, where PVP is poly(vinylpyrrolidone), Qe is quercetin, and CM is the cell membrane of neuron cells) for effectively targeting and modulating microglia to treat Parkinson's disease (PD). The CSPQ nanoparticles exhibit multienzyme activities and could effectively scavenge the reactive oxygen species and promote the polarization of microglia into the anti-inflammatory M2-like phenotype to relieve neuroinflammation. We reveal that biomimetic CSPQ@CM nanoparticles targeted microglia through the specific interactions between the membrane surface vascular cells adhering to molecule-1 and α4β1 integrin expressed by microglia. They could significantly improve the symptoms of PD mice to result in an excellent therapeutic efficacy, as evidenced by the recovery of their dopamine level in cerebrospinal fluid, tyrosine hydroxylase, and ionized calcium binding adapter protein 1 to normal levels. Our work demonstrates the great potential of these robust biomimetic nanoparticles in the targeted treatment of PD and other central nervous degenerative diseases.

Decomposition of Flavonols in the Presence of Saliva

In this study, the LC-MS/MS was applied to explore the stability of four common dietary flavonols, kaempferol, quercetin, isorhamnetin, and myricetin, in the presence of hydrogen peroxide and saliva. In addition, the influence of saliva on the representative quercetin glycosides, rutin, quercitrin, hyperoside, and spiraeoside was examined. Our study showed that, regardless of the oxidative agent used, flavonols stability decreases with increasing B-ring substitution. The decomposition of analyzed compounds was based on their splitting by the opening the heterocyclic C-ring and realizing more simple aromatic compounds. The dead-end products corresponded to different benzoic acid derivatives derived from B-ring. Kaempferol, quercetin, isorhamnetin, and myricetin were transformed into 4-hydroxybeznoic acid, protocatechuic acid, vanillic acid, and gallic acid, respectively. Additionally, for quercetin and myricetin, two intermediate depsides and 2,4,6-trihydroxybenzoic acid derived from A-ring were detected. All analyzed glycosides were resistant to hydrolysis in the presence of saliva. Based on our data, saliva was proven to be a next oxidative agent which leads to the formation of corresponding phenolic acids. Hence, studies on flavonols’ metabolism should take into consideration that the flavonols decomposition starts in the oral cavity; hence, in subsequent parts of the human digestive tract, they could be present not in their parent form but as phenolic acids. Further analyses of the influence of saliva on flavonols glycosides need to be performed due to the possible interindividual fluctuations.

Production of Antioxidant Molecules in Polygonum aviculare (L.) and Senecio vulgaris (L.) under Metal Stress: A Possible Tool in the Evaluation of Plant Metal Tolerance

Plants growing on heavy metal (HM)-polluted soils show toxicity symptoms, such as chlorosis and growth reduction, and undergo oxidative stress due to the formation of reactive oxygen species (ROS). Plants overcome oxidative stress by producing a wide range of antioxidant molecules, such as polyphenols and flavonoids. The aim of the present work was to study the accumulation of these molecules in response to increasing concentrations of Cd, Cr, Cu, Ni, Pb and Zn and to assess whether they can be used as a tool in assessing metal-related stress in Polygonum aviculare and Senecio vulgaris. On average, P. aviculare shoots accumulated lower amounts of metals than S. vulgaris shoots. The uptake of all six elements was correlated and proportional to their concentration in the nutrient solution (ρ > 0.9), with the bioaccumulation factor (BAF) being >1 for most of them. The present research demonstrated that 82% of the samples showed a good correlation (|ρ| > 0.5) between the level of polyphenols, flavonoids and antioxidant activity and the metal concentration in plant shoots, confirming that the metal stress level and production of phenolic compounds having antioxidant activity were strictly connected. Nonetheless, the mere quantification of these molecules cannot identify the type of metal that caused the oxidative stress, neither determine the concentration of the stressors. The five tested populations of each species did not show any specific adaptation to the environment of origin.

PI3K/Akt and ERK1/2 Signalling Are Involved in Quercetin-Mediated Neuroprotection against Copper-Induced Injury

Copper, a transition metal with essential cellular functions, exerts neurotoxic effects when present in excess by promoting production of reactive oxygen species (ROS). The aim of the present study was to investigate potential benefits of flavonoid quercetin against copper-induced toxicity. Results obtained with MTT assay indicate that the effects of quercetin are determined by the severity of the toxic insult. In moderately injured P19 neuronal cells, concomitant treatment with 150 μM quercetin improved viability by preventing ROS formation, caspase-3 activation, and chromatin condensation. Western blot analysis revealed that quercetin reduced copper-induced increase in p53 upregulated modulator of apoptosis (PUMA) expression and promoted upregulation of nucleoside diphosphate kinase NME1. Levels of p53 and Bax proteins were not affected by both copper and quercetin. UO126 and wortmannin, inhibitors of ERK1/2 and PI3K/Akt signalling pathways, respectively, prevented neuroprotective effects of quercetin. In severely injured neurons, 30 μM quercetin exerted strong prooxidative action and exacerbated cytotoxic effects of copper, whereas 150 μM quercetin failed to affect neuronal survival. These results demonstrate the dual nature of quercetin action in copper-related neurodegeneration. Hence, they are relevant in the context of considering quercetin as a possible therapeutic for neuroprotection and imply that detailed pharmacological and toxicological studies must be carried out for natural compounds capable of acting both as antioxidants and prooxidants.

Synthesis, physicochemical characterization and biological properties of two novel Cu(II) complexes based on natural products curcumin and quercetin

Curcumin and quercetin are two of the most prominent natural polyphenols with a diverse spectrum of beneficial properties, including antioxidant, anti-inflammatory, chemopreventive and chemotherapeutic activity. The complexation of these natural products with bioactive transition metal ions can lead to the generation of novel metallodrugs with enhanced biochemical and pharmacological activities. Within this framework, the synthesis and detailed structural and physicochemical characterization of two novel complex assemblies of Cu(II) with curcumin and quercetin and the ancillary aromatic chelator 2,2'-bipyridine is presented. The two complexes represent the only crystallographically characterized structures with Cu(II) as the central metal ion and curcumin or quercetin as the ligands. The new complexes were biologically evaluated in vitro for their antioxidant potential, both exhibiting strong scavenging activity in the 2,2-diphenyl-1-picrylhydrazyl assay, and their plasmid DNA binding/cleavage properties. Both complexes appear to be non-toxic in the eukaryotic experimental model Saccharomyces cerevisiae and merit further investigation of their pharmacological profile.

Experimental and Theoretical Study of the Stability of the Complex Fisetin-Cu(II) and A Comparative Study of Free Ligand and Complex Interaction with Molecular Singlet Oxygen

In this work, the flavonol fisetin was selected in order to study its reactivity against Cu(II), a metal ion of interest in biological media and industry. The stoichiometry and apparent formation constant of the complex in ethanolic medium at 25°C were evaluated using spectrophotometric techniques. The resulting stoichiometry was a 1:1 ligand:metal complex, and a log K = 5.17 ± 0.12 was determined. Since two possible chelation sites can be proposed for the complex formation, quantum chemistry calculations were performed on these structures. Calculations suggest that the hydroxyl-keto site is more stable for the complex formation than the catechol site. Flavonoids could exert protection against oxidative damage caused by reactive oxygen species, and this biological activity could be affected by chelation with metal ions. This led us to perform a study on the interaction of both, free flavonoid and complex, with reactive oxygen species. Our results showed both compounds quench molecular singlet oxygen photogenerated with visible light, mainly in a physical fashion. In order to analyze a possible protective effect of flavonoid and its complex against oxidative damage in biological environments, the amino acid tryptophan was selected as a model oxidation system. Free flavonoid does not have a marked protective effect, whereas its complex showed a relevant protective effect.

The Significance of Utilizing In Vitro Transfer Model and Media Selection to Study the Dissolution Performance of Weak Ionizable Bases: Investigation Using Saquinavir as a Model Drug

This study investigated the dissolution behavior of BCS class II ionizable weak base Saquinavir and its mesylate salt in the multi-compartment transfer setup employing different composition of dissolution media. The dissolution behavior of Saquinavir was studied by using a two-compartment transfer model representing the transfer of drug from the stomach (donor compartment) to the upper intestine (acceptor compartment). Various buffers like phosphate, bicarbonate, FaSSIF, and FeSSIF were employed. The dissolution was also studied in the concomitant presence of the additional solute, i.e., Quercetin. Further, the dissolution profiles of Saquinavir and its mesylate salt were simulated by GastroPlus^TM, and the simulated dissolution profiles were compared against the experimental ones. The formation of in situ HCl salt and water-soluble amorphous phosphate aggregates was confirmed in the donor and acceptor compartments of the transfer setup, respectively. As the consequence of the lower solubility product of HCl salt of Saquinavir, the solubility advantage of mesylate salt was vanished leading to the lower than the predicted dissolution in the acceptor compartment. However, the formation of water-soluble aggregates in the presence of the phosphate salts was observed leading to the higher than the predicted dissolution of the free base in the transfer setup. Interestingly, the formation of such water-soluble aggregates was found to be hindered in the concomitant presence of an ionic solute resulting in the lower dissolution rates. The in situ generation of salts and aggregates in the transfer model lead to the inconsistent prediction of dissolution profiles by GastroPlus^TM.

The Stability and Activity Changes of Apigenin and Luteolin in Human Cervical Cancer Hela Cells in Response to Heat Treatment and Fe2+/Cu2+ Addition

Flavonoids are natural polyphenolic compounds with desired bio-functions but with chemical instability and sensitivity to temperature, oxygen, and other factors. Apigenin and luteolin, two flavones of the flavonoid family in plant foods, were; thus, assessed and compared for their stability, especially the changes in anti-cancer activity in response to the conducted heat treatments and the addition of ferrous or cupric ions. The two flavones in aqueous solutions showed first-order degradation at 20 and 37 °C. The addition of ferrous or cupric ions (except for Cu2+ at 37 °C) enhanced luteolin stability via forming the luteolin-metal complexes; however, Fe/Cu addition (especially at 37 °C) consistently impaired apigenin stability. Using the human cervical cancer Hela cells and two cell treatment times (24 and 48 h), it was evident that heat treatments (37 and 100 °C) or Fe/Cu addition could endow apigenin and luteolin with decreased activities in growth inhibition, DNA damage, intracellular reactive oxygen species (ROS) generation, and apoptosis induction. In general, higher temperature led to greater decrease in these activities, while Fe2+ was more effective than Cu2+ to decrease these activities. The correlation analysis also suggested that the decreased ROS generation of the two flavones in the Hela cells was positively correlated with their decreased apoptosis induction. It is; thus, concluded that the two treatments can influence the two flavones' stability and especially exert an adverse impact on their anti-cancer activities.

Influence of Mineral Supplementation on the Results from Analysis of Flavonol Glycoside Content in Ginkgo biloba Dietary Supplements

Flavonoids are a major component of Ginkgo biloba extract (GBE). Several studies have investigated chelate formation and the redox reaction between flavonoids and metal ions; however, the effect of mineral supplements on the results from the analysis of the flavonol glycoside content in products containing GBE dietary supplement remains unknown. In this study, the effects of commonly used mineral supplements on the recovery of quercetin from GBE-containing dietary supplements were investigated using conventional methods of flavonol glycoside determination. Mineral supplements containing Zn (II), Mn (II), and Fe (II) did not affect quercetin recovery, whereas Cu (II) and Fe (III) significantly reduced recovery (P<0.05). Quercetin oxidation was prevented by adding an antioxidant to the diluent (extraction solvent). Among the tested synthetic antioxidants, tert-butyl hydroquinone (TBHQ) promoted the greatest increase in quercetin recovery. The flavonol glycoside content of commercially available GBE-containing dietary supplements was analyzed using a conventional diluent or a diluent containing 20 mg/mL TBHQ. The amount of quercetin recovered from products containing Cu (II) was found to decrease with increasing hydrolysis duration and the duration in the final test solution state using the conventional diluent, while the TBHQ-containing diluent yielded consistent quercetin contents (P<0.05). These findings suggest that quercetin, a major aglycone of GBE flavonol glycosides, can be oxidized by Cu (II) and Fe (III) during the analytical process and, therefore, the total flavonol glycoside content may be underestimated. The addition of TBHQ to the diluent can improve the accuracy and reproducibility of flavonol glycoside content analysis in GBE-containing dietary products supplemented with minerals.

Experimental and theoretical evaluation on the antioxidant activity of a copper(ii) complex based on lidocaine and ibuprofen amide-phenanthroline agents

A new copper(ii) complex, [Cu(LC)(Ibu-phen)(H2O)2](ClO4)2 (LC: lidocaine, Ibu-phen: ibuprofen amide-phenanthroline), was synthesized and characterized. The antioxidant activities of the free ligands and the copper(ii) complex were evaluated by in vitro experiments and theoretical calculations using density functional theory (DFT). Structures of the ligand Ibu-phen and the complex were identified by 1H and 13C NMR, FT-IR spectroscopies, mass spectrometry, thermogravimetric analysis and elemental analysis. The antioxidant potentials of LC and Ibu-phen ligands as well as copper(ii) complex were also evaluated by DPPH˙, ABTS˙+, HO˙ essays and EPR spectroscopy. The experimental results show that the radical scavenging activity (RSA) at various concentrations is decreased in the following order: copper(ii) complex > ascorbic acid > LC > Ibu-phen. Structural and electronic properties of the studied compounds were also analyzed by DFT approach at the M05-2X/6-311++g(2df,2p)//M05-2X/LanL2DZ level of theory. ESP maps and NPA charge distributions show that the highly negative charge regions found on the N and O heteroatoms make these sites more favorable to bind with the central copper ion. Frontier orbital distributions of copper(ii) complex indicate that HOMOs are mainly localized at Ibu-phen, while its LUMOs are distributed at LC. Based on natural bond orbitals (NBO) analyses, Cu(ii) ion plays as electron acceptor in binding with the two ligands and two water molecules. Thermochemical properties including bond dissociation enthalpy (BDE), ionization energy (IE), electron affinity (EA), proton affinity (PA) characterizing three common antioxidant mechanisms i.e. hydrogen transfer (HT), single electron transfer (SET) and proton loss (PL) were finally calculated in the gas phase and water solvent for two ligands and the copper(ii) complex at the same level of theory. As a result, the higher EA and lower BDE and PA values obtained for copper(ii) complex show that the complex shows higher antioxidant potential than the free ligands.

A neutral Cu-based MOF for effective quercetin extraction and conversion from natural onion juice

A new neutral metal–organic framework can efficiently extract natural product quercetin (QT) from fresh QT-rich onion juice and rapidly convert it into Cu–QT with a relatively high conversion rate.

Empirical Kinetic Modelling of the Effect of l-Ascorbic Acid on the Cu(II)-Induced Oxidation of Quercetin

This study aimed at investigating the effect of l-ascorbic acid on the Cu2+-induced oxidation of quercetin, within a pH range from 6.7 to 8.3 and temperatures varying from 53 to 87 °C. Initial examinations showed that quercetin degradation obeyed apparent first-order kinetics and it was significantly affected by temperature. Modelling of the effect of l-ascorbic acid by implementing response surface methodology suggested that l-ascorbic acid did not impact quercetin oxidation significantly (p < 0.05) and led to an empirical kinetic model based on temperature (T) and pH. Liquid chromatography–diode array–mass spectrometry analyses revealed the presence of typical quercetin degradation and oxidation products, including protocatechuic acid and 2-(hydroxybenzoyl)-2-hydroxybenzofuran-3(2H)-one. It was concluded that the formation of l-ascorbyl or other radicals (superoxide anion) may be involved in quercetin oxidation and this fact merits further attention to illuminate the possible beneficial or adverse nutritional consequences of such reactions in foods.

Protective role of quercetin against copper(II)-induced oxidative stress: A spectroscopic, theoretical and DNA damage study

The radical scavenging and metal chelating properties of flavonoids indicate that they may play a protective role in diseases with perturbed metal homeostasis such as Alzheimer's disease. In this work we investigated the effect of the coordination of quercetin to copper(II) in view of the formation of ROS in Cu-catalyzed Fenton reaction. ABTS and DPPH assays confirmed that the copper(II)-quercetin complex exhibits a stronger radical scavenging activity than does quercetin alone. EPR spin trapping experiments have shown that chelation of quercetin to copper significantly suppressed the formation of hydroxyl radicals in the Cu(II)-Fenton reaction. DNA damage experiments revealed a protective effect for quercetin, but only at higher stoichiometric ratios of quercetin relative to copper. DNA protective effect of quercetin against ROS attack was described by two mechanisms. The first mechanism lies in suppressed formation of ROS due to the decreased catalytic action of copper in the Fenton reaction, as a consequence of its chelation and direct scavenging of ROS by free quercetin. Since the Cu-quercetin complex intercalates into DNA, the second mechanism was attributed to a suppressed intercalating ability of the Cu-quercetin complex due to the mildly intercalating free quercetin into DNA, thus creating a protective wall against stronger intercalators.

Genistein Binding to Copper(II)-Solvent Dependence and Effects on Radical Scavenging

Genistein, but not daidzein, binds to copper(II) with a 1:2 stoichiometry in ethanol and with a 1:1 stoichiometry in methanol, indicating chelation by the 5-phenol and the 4-keto group of the isoflavonoid as demonstrated by the Jobs method and UV-visible absorption spectroscopy. In ethanol, the stability constants had the value 1.12 × 10¹¹ L²∙mol^-2 for the 1:2 complex and in methanol 6.0 × 10⁵ L∙mol^-1 for the 1:1 complex at 25 °C. Binding was not detected in water, as confirmed by an upper limit for the 1:1 stability constant of K = 5 mol^-1 L as calculated from the difference in solvation free energy of copper(II) between methanol and the more polar water. Solvent molecules compete with genistein as demonstrated in methanol where binding stoichiometry changes from 1:2 to 1:1 compared to ethanol and methanol/chloroform (7/3, v/v). Genistein binding to copper(II) increases the scavenging rate of the stable, neutral 2,2-diphenyl-1-picrylhydrazyl radical by more than a factor of four, while only small effects were seen for the short-lived but more oxidizing β-carotene radical cation using laser flash photolysis. The increased efficiency of coordinated genistein is concluded to depend on kinetic rather than on thermodynamic factors, as confirmed by the small change in reduction potential of -0.016 V detected by cyclic voltammetry upon binding of genistein to copper(II) in methanol/chloroform solutions.