Structure-antioxidant activity relationships of flavonoids: a re-examination

The antioxidant and prooxidant activities of flavonoids belonging to several classes were studied to establish their structure-activity relationships against different oxidants. Special attention was paid to the flavonoids quercetin (flavone), taxifolin (flavanone) and catechin (flavanol), which possess different basic structures but the same hydroxylation pattern (3,5,7,3',4'-OH). It was found that these three flavonoids exhibited comparable antioxidant activities against different oxidants leading to the conclusion that the presence of ortho -catechol group (3',4'-OH) in the B-ring is determinant for a high antioxidant capacity. The flavone kaempferol (3,5,7,4'-OH), however, in spite of bearing no catechol group, also presents a high antioxidant activity against some oxidants. This fact can be attributed to the presence of both 2,3-double bond and the 3-hydroxyl group, meaning that the basic structure of flavonoids becomes important when the antioxidant activity of B-ring is small.     

Structure-antioxidant Activity Relationships of Flavonoids: A Re-examination

The antioxidant and prooxidant activities of flavonoids belonging to several classes were studied to establish their structure-activity relationships against different oxidants. Special attention was paid to the flavonoids quercetin (flavone), taxifolin (flavanone) and catechin (flavanol), which possess different basic structures but the same hydroxylation pattern (3,5,7,3',4'-OH). It was found that these three flavonoids exhibited comparable antioxidant activities against different oxidants leading to the conclusion that the presence of ortho -catechol group (3',4'-OH) in the B-ring is determinant for a high antioxidant capacity. The flavone kaempferol (3,5,7,4'-OH), however, in spite of bearing no catechol group, also presents a high antioxidant activity against some oxidants. This fact can be attributed to the presence of both 2,3-double bond and the 3-hydroxyl group, meaning that the basic structure of flavonoids becomes important when the antioxidant activity of B-ring is small.     

Structure-radical scavenging activity relationships of flavonoids

The objective of this work is to establish the structural requirements of flavonoids for appreciable radical-scavenging activity (RSA) and elucidate a comprehensive mechanism that can explain their activity. To this end, the RSA of 52 flavonoids against 2,2-diphenyl-1-picrylhydrazyl was determined. The relative change in energy (DeltaH(f)) associated with the formation of various flavonoidal and other phenolic radicals and also the spin distribution in these radicals were determined using computational programmes. By correlating experimental data with DeltaH(f), structural features that affect activity have been identified and considered in perspective. It was shown with compelling evidences that the RSA of flavonoids could be mapped to one of their ring systems, making it possible to study their RSA by dissecting their structures and designing representative simpler models. Consequently, hydroxytoluene units were demonstrated to successfully account for the RSA of flavonoids due to ring B and also to satisfactorily do so for activities due to ring A. Further, a comprehensive model for the radical scavenging reactions of flavonoids (and in general, phenolic compounds), which could account for hydrogen atom donation and the termination of aroxyl radicals, was proposed. Finally, prediction of structural features that could endow flavonoids with appreciable radical scavenging capability was made by considering the stability data and the ease of termination. In conclusion, the underlying molecular phenomena of the RSA of flavonoids could be explained by the ease of hydrogen atom abstraction and the ease of the termination of the flavonoidal aroxyl radicals.     

Antioxidant properties of complexes of flavonoids with metal ions

The formation of complexes of metal ions with the flavonoids quercetin (L1), rutin (L2), galangin (L3) and catechin (L4) has been investigated by UV-visible spectroscopy. The antioxidant activities of the compounds were evaluated by using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicalscavenging method. In this work, we have shown that the complexed flavonoids are much more effective free radical scavengers than the free flavonoids. We suggest that the higher antioxidant activity of the complexes is due to the acquisition of additional superoxide dismutating centers. Radical scavenging activities of the compounds were also investigated from an electrochemical point of view. There is a relationship between the logarithm of the antioxidant activity (represented by EC50) and the oxidation potential. The synergic effect of the complexes and ascorbic acid were studied by [13C]-NMR analyses. The results show that ascorbic acid can protect flavonoids from oxidative degradation, and reveal antioxidant synergies between ascorbic acid and the compounds.     

Structure–Activity Relationships for Antioxidant Activities of a Series of Flavonoids in a Liposomal System

Structurally diverse plant phenolics were examined for their abilities to inhibit lipid peroxidation induced either by Fe(II) and Fe(III) metal ions or by azo-derived peroxyl radicals in a liposomal membrane system. The antioxidant abilities of flavonoids were compared with those of coumarin and tert-butylhydroquinone (TBHQ). The antioxidant efficacies of these compounds were evaluated on the basis of their abilities to inhibit the fluorescence intensity decay of an extrinsic probe, 3-(p-(6-phenyl)-1,3,5-hexatrienyl)phenylpropionic acid (DPH-PA), caused by the free radicals generated during lipid peroxidation. All the flavonoids tested exhibited higher antioxidant efficacies against metal-ion-induced peroxidations than peroxyl-radical-induced peroxidation, suggesting that metal chelation may play a larger role in determining the antioxidant activities of these compounds than has previously been believed. Distinct structure–activity relationships were also revealed for the antioxidant abilities of the flavonoids. Presence of hydroxyl substituents on the flavonoid nucleus enhanced activity, whereas substitution by methoxy groups diminished antioxidant activity. Substitution patterns on the B-ring especially affected antioxidant potencies of the flavonoids. In cases where the B-ring could not contribute to the antioxidant activities of flavonoids, hydroxyl substituents in an catechol structure on the A-ring were able to compensate and become a larger determinant of flavonoid antioxidant activity.     

Hepatoprotection of quercetin against oxidative stress by induction of metallothionein expression through activating MAPK and PI3K pathways and enhancing Nrf2 DNA-binding activity

Flavonoids are natural phenolic substances widely found in fruit, vegetables, grains, and wine. Most of these compounds exert health-promoting effects seem to attribute to their antioxidant activity. Metallothioneins (MT) has been suggested to protect against acute heavy metal toxicity in the liver, and the proteins of MT can be induced by various stimuli including antioxidant. Measuring the induction of MT genes may provide an efficient approach to understand the chemopreventive mechanisms of flavonoids. The antioxidant activity of eight flavonoids was determined by TEAC and ORAC assays and their effects on MT protein were also measured. HepG2 cells were employed to explore the mechanisms underlying flavonoid-induced MT induction. Statistical analysis revealed a positive correlation between the antioxidant activity of flavonoids and MT expression. Quercetin-induced MT expression may function by activating the phosphorylation of JNK, p38 and PI3K/Akt as well as by enhancing Nrf2 DNA-binding activity. Moreover, quercetin exhibited a potential protective effect on t-BHP-caused injury in hepatocytes through the induction of MT. These results suggest that quercetin is a natural antioxidant in the diet and the consumption of foods that are rich in quercetin could be beneficial for the prevention of environmental oxidant-induced liver damage.     

Induction of PC12 cell differentiation by flavonoids is dependent upon extracellular signal-regulated kinase activation

Many of the physiological benefits attributed to flavonoids are thought to stem from their potent antioxidant and free radical scavenging properties. Recently, it was shown that flavonoids protect nerve cells from oxidative stress by multiple mechanisms, only one of which is directly related to their antioxidant activity, suggesting that specific flavonoids may have other properties that could make them useful in the treatment of conditions that lead to nerve cell death. In particular, it was asked if any flavonoid could mimic neurotrophic proteins. To examine this possibility, we looked at the ability of flavonoids to induce nerve cell differentiation using PC12 cells. PC12 cells were treated with a variety of flavonoids to determine if there was a correlation between their neuroprotective activity and their neurite outgrowth-promoting activity. In addition, the signaling pathways required for flavonoid-induced differentiation were examined. We found that only a small subset of the flavonoids that were neuroprotective could induce neurite outgrowth by an extracellular signal-regulated kinase-dependent process. There was a strong correlation between the concentrations of the flavonoids that were neuroprotective and the concentrations that induced differentiation. These results suggest that the consumption of specific flavonoids could have further beneficial effects on nerve cells following injury, in pathological conditions or in normal aging.     

Evaluation of the antioxidant activity of flavonoids by "ferric reducing antioxidant power" assay and cyclic voltammetry

Flavonoids, naturally occurring phenolic compounds, have recently been studied extensively for their antioxidant properties. The structure-antioxidant activity relationships (SAR) of flavonoids have been evaluated against different free radicals, but "ferric reducing antioxidant power" (FRAP) assay, which determines directly the reducing capacity of a compound, has not been used for this purpose. In this study, the antioxidant activities of 18 structurally different flavonoids were evaluated by FRAP assay modified to be used in 96-well microplates. Furthermore, their oxidation potentials were also measured, which were in the range of +0.3 V (myricetin) to +1.2 V (5-hydroxy flavone) and were in good agreement with FRAP assay results. Quercetin, fisetin and myricetin had the lowest oxidation potentials and appeared the most active compounds in FRAP assay and were 3.02, 2.52 and 2.28 times more active than Trolox, respectively. Indications were found that the o-dihydroxy structure in the B ring and the 3-hydroxy group and 2,3-double bond in the C ring give the highest contribution to the antioxidant activity.     

Characterization of plant phenolic compounds by cyclic voltammetry

Phenolic acids and flavonoids were characterized by cyclic voltammetry and total antioxidant activity in the reaction with the ABTS cation radical. Anode peak voltages (Eap) and their pH dependences were determined for the studied phenolic acids and flavonoids. The Eap and Trolox equivalent antioxidant capacity (TEAC) values were found to correlate for polyphenols, which react with the ABTS cation radical in two steps. Correlation between the half-wave potential (Ep/2) and TEAC was determined for electrochemically irreversible compounds. Mechanisms of the reaction of phenolics on the electrode involving one- and two-electron oxidation are proposed.     

Evaluation of the antioxidant activity of flavonoids by “ferric reducing antioxidant power” assay and cyclic voltammetry

Flavonoids, naturally occurring phenolic compounds, have recently been studied extensively for their antioxidant properties. The structure–antioxidant activity relationships (SAR) of flavonoids have been evaluated against different free radicals, but “ferric reducing antioxidant power” (FRAP) assay, which determines directly the reducing capacity of a compound, has not been used for this purpose. In this study, the antioxidant activities of 18 structurally different flavonoids were evaluated by FRAP assay modified to be used in 96-well microplates. Furthermore, their oxidation potentials were also measured, which were in the range of +0.3 V (myricetin) to +1.2 V (5-hydroxy flavone) and were in good agreement with FRAP assay results. Quercetin, fisetin and myricetin had the lowest oxidation potentials and appeared the most active compounds in FRAP assay and were 3.02, 2.52 and 2.28 times more active than Trolox, respectively. Indications were found that the o-dihydroxy structure in the B ring and the 3-hydroxy group and 2,3-double bond in the C ring give the highest contribution to the antioxidant activity.     

Radiation protection by the ocimum flavonoids orientin and vicenin: mechanisms of action

In previous studies, flavonoids, orientin and vicenin, that were isolated from the leaf extract of Ocimum sanctum, were found to protect mice against radiation injury. Several flavonoids are known to be good antioxidants. Therefore, the effect of orientin and vicenin on radiation-induced lipid peroxidation in vivo and their antioxidant activity in vitro were studied. Adult mice were injected intraperitoneally with 50 microgram/kg of orientin or vicenin and exposed whole-body to 3 Gy of gamma radiation. Lipid peroxidation was measured in the liver 15 min to 8 h postirradiation. The antioxidant activity of orientin/vicenin (10-500 microM) was studied by measuring inhibition of hydroxyl radicals generated by the Fenton reaction (Fe(3+)-EDTA-ascorbic acid-H(2)O(2)) in vitro. The compounds were also tested for possible pro-oxidant and iron chelation activities at the above concentrations in the in vitro system. Orientin and vicenin provided almost equal protection against radiation-induced lipid peroxidation in mouse liver. Both compounds showed a significantly greater free radical-inhibiting activity in vitro than DMSO. Neither orientin nor vicenin showed any pro-oxidant activity at the concentrations tested. Both compounds inhibited free radical formation in the absence of EDTA. Free radical scavenging appears to be a likely mechanism of radiation protection by these flavonoids.     

In vitro scavenger activity of some flavonoids and melanins against O2-(.).

The scavenger activity against O2-. of some flavonoids and melanins (synthetic melanins and melanins isolated from animal tissues, vegetable seeds, and mushroom spores) has been studied by ESR spectrometry. All these substances, except flavon and flavanone, diminish the signal of O2-. generated in vitro by a system containing H2O2 and acetone in an alkaline medium. It is shown that the presence of hydroxyl groups in the B ring of flavonoids is essential for their scavenger activity. Moreover, the presence of a hydroxyl at C-3 enhances the scavenger ability of flavonoids. Generally, aglycons are more active than their glycosides. It seems plausible that the antioxidant property of these substances comes from their scavenger activity against O2-(.). It is also pointed out that the scavenger activity shown by melanins, is strictly correlated with their nature of stable free radical.     

Characterization of plant phenolic compounds by cyclic voltammetry

Phenolic acids and flavonoids were characterized by cyclic voltammetry and total antioxidant activity in the reaction with the ABTS cation radical. Anode peak voltages (E_ap) and their pH dependences were determined for the studied phenolic acids and flavonoids. The E_ap and Trolox equivalent antioxidant capacity (TEAC) values were found to correlate for polyphenols, which react with the ABTS cation radical in two steps. Correlation between the half-wave potential (E_1/2) and TEAC was determined for electrochemically irreversible compounds. Mechanisms of the reaction of phenolics on the electrode involving one-and two-electron oxidation are proposed.     

Structure-antioxidant activity relationships of flavonoids and phenolic acids

The recent explosion of interest in the bioactivity of the the flavonoids of higher plants is due, at least in part, to the potential health benefits of these polyphenolic components of major dietary constituents. This review article discusses the biological properties of the flavonoids and focuses on the relationship between their antioxidant activity, as hydrogen donating free radical scavengers, and their chemical structures. This culminates in a proposed hierarchy of antioxidant activity in the aqueous phase. The cumulative findings concerning structure-antioxidant activity relationships in the lipophilic phase derive from studies on fatty acids, liposomes, and low-density lipoproteins; the factors underlying the influence of the different classes of polyphenols in enhancing their resistance to oxidation are discussed and support the contention that the partition coefficients of the flavonoids as well as their rates of reaction with the relevant radicals define the antioxidant activities in the lipophilic phase.     

Cyclic voltammetric analysis of 2-styrylchromones: relationship with the antioxidant activity

2-Styrylchromones (2-SC) are a chemical family of oxygen heterocyclic compounds, vinylogues of flavones (2-phenylchromones), whose occurrence in nature has been reported. Recently, several 2-SC derivatives were demonstrated to have antioxidant properties, namely, xanthine oxidase inhibition, hepatoprotection against pro-oxidant agents in cellular and non-cellular systems and scavenging activity against reactive oxygen and reactive nitrogen species (ROS and RNS). Considering these antioxidant properties, it may be hypothesised that the electrochemical redox behaviour of 2-SC contributes significantly to their activity. To test this hypothesis, the electrochemical behaviour of different 2-SC was studied, together with a number of flavonoids with well-known antioxidant activities, by cyclic voltammetry, and the results correlated to their ability to scavenge ROS and RNS. The results obtained showed that 2-SC with a catecholic B-ring have a low oxidation peak potential corresponding to the oxidation of the 3',4'-OH (catechol) moiety. The compounds with a phenolic B-ring have a common peak, with oxidation potential values of about +0.4/+0.5 V versus Ag/AgCl, corresponding to the oxidation of the 4'-OH. The oxidation of the hydroxyl substituents in the A-ring generated peaks of higher potentials (+0.7/+0.8 V vs Ag/AgCl). The results from the scavenging assays were in agreement with those obtained from the cyclic voltammetry, that is, higher scavenging effects corresponded to lower values of oxidation potentials, with significant correlation coefficients. The values obtained for the studied flavonoids are in accordance with the literature, and reflect their relative antioxidant activity, when compared to the studied 2-SC. Thus, in this family of compounds, oxidation potentials obtained by cyclic voltammetry seem to be applicable as a general indicator of radical scavenging activity.     

刺梨茶总黄酮的提取工艺的建立及其抗氧化活性

为进一步开发并利用贵州独特野生资源刺梨茶,研究其总黄酮的提取工艺及体外抗氧化活性,本文采用单因素实验和响应曲面分析法优化刺梨茶中总黄酮的提取工艺,并经1,1-二苯基-2-三硝基苯肼(DPPH)法初步评价其抗氧化活性。结果表明,刺梨茶总黄酮最佳提取工艺为料液比(g∶mL)1∶26,乙醇浓度74%,提取温度75℃,提取时间120min。此条件下,刺梨茶总黄酮得率为2.190%,与预测值2.216%接近。刺梨茶总黄酮对DPPH具有较好的清除能力,其EC50值为0.012 27mg/mL,其清除DPPH的能力微弱于Vc,但强于2,6-二叔丁基对甲酚(BHT)。刺梨茶总黄酮表现出良好的抗氧化活性,可作为天然抗氧化剂开发利用。     

滁菊总黄酮抗氧化作用研究

通过乙醇浸提,AB-8大孔树脂纯化制备滁菊总黄酮,紫外分光光度法测定总黄酮含量.取一定量的滁菊总黄酮配制成溶液,采用烘箱储藏法测定滁菊总黄酮的体外抗油脂氧化作用,采用Decker法等测定滁菊总黄酮对Fe2+的螯合能力,对亚硝酸盐、DPPH自由基的清除能力.结果表明:经醇提、AB-8大孔树脂纯化,紫外分光光度计检测滁菊中总黄酮含量为2.62%,滁菊总黄酮对油脂过氧化具有明显的抑制作用,对Fe2+具有较强强的螯合能力,对亚硝酸盐、DPPH自由基具有较好的清除能力,即滁菊总黄酮具有良好的抗氧化作用.     

Oxidation of flavonoids by hypochlorous acid: reaction kinetics and antioxidant activity studies

Flavonoids, plant polyphenols, ubiquitous components of human diet, are excellent antioxidants. Hypochlorous acid (HOCl), produced by activated neutrophils, is highly reactive chlorinating and oxidizing species. It has been reported earlier that flavonoids are chlorinated by HOCl. Here we show that flavonoids from flavonol subclass are also oxidized by HOCl, but only if the latter is in a large molar excess (≥?10). The kinetics of this reaction was studied by stopped-flow spectrophotometry, at different pH. We found that flavonols were oxidized by HOCl with the rate constants of the order of 10⁴–10⁵ M^?1 s^?1 at pH 7.5. Antioxidant activity of HOCl-modified flavonoids was measured by 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) method. Slightly higher antioxidant activity, compared to parent compounds, was observed for flavonols after their reaction with equimolar or moderate excess of HOCl whereas flavonols treated with high molar excess of HOCl exhibited decrease in antioxidant activity. The mechanism of flavonoid reaction with HOCl at physiological pH is proposed, and biological consequences of this reaction are discussed.