Variations in the Components and Antioxidant and Tyrosinase Inhibitory Activities of Styphnolobium japonicum (L.) Schott Extract during Flower Maturity Stages

Styphnolobium japonicum (L.) S chott is widely cultivated in China, and its flowers and flower buds (FFB‐SJ) are commonly used as traditional Chinese medicine. This work aimed to assess variations in the chemical components and antioxidant and tyrosinase inhibitory activities of S. japonicum extract during five flower maturity stages (ES1–ES5). The results showed that the contents of total flavonoids, rutin, and narcissin were highest at ES1, whereas the contents of quercetin and isorhamnetin were highest at ES3. ES1 presented considerable antioxidant activities in terms of reducing power (RP) and 1,1‐diphenyl‐2‐picrylhydrazyl radical (DPPH^.) and hydroxyl radical (^.OH) scavenging capacity, whereas ES3 showed excellent tyrosinase inhibitory activity and 2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid) radical (ABTS^.+)‐ and O₂^.?‐scavenging capacity. Rutin and quercetin are the main bioactive components of FFB‐SJ with antioxidant and tyrosinase inhibition, and the immature flower buds of S. japonicum (S2 and S3) with excellent biological activities and relatively high extract yields were the best for product development.     

Antioxidant constituents from rhubarb: structural requirements of stilbenes for the activity and structures of two new anthraquinone glucosides

The methanolic extracts from five kinds of rhubarb were found to show scavenging activity for DPPH radical and .O2-. Two new anthraquinone glucosides were isolated from the rhizome of Rheum undulatum L. together with two anthraquinone glucosides, a naphthalene glucoside, and 10 stilbenes. In the screening test for radical scavenging activity of rhubarb constituents, stilbenes and a naphthalene glucoside showed activity, but anthraquinones and sennosides did not. In addition, most stilbenes inhibited lipid peroxidation of erythrocyte membrane by tert-butyl hydroperoxide. Detailed examination of the scavenging effect on various related compounds suggested the following structural requirements; 1) phenolic hydroxyl groups are essential to show the activity; 2) galloyl moiety enhances the activity; 3) glucoside moiety reduces the activity; 4) dihydrostilbene derivatives maintain the scavenging activity for the DPPH radical, but they show weak activity for .O2-. In addition, several stilbenes with both the 3-hydroxyl and 4'-methoxyl groups inhibited xanthine oxidase.     

毛药忍冬花蕾抗氧化活性部位化学成分研究

毛药忍冬(Lonicera serreana)为忍冬属(Lonicera)植物,其花和果实入药,具有清热解毒、凉散风热之功效,但至今缺乏系统化学成分及药理活性研究。为了寻找毛药忍冬中天然抗氧化活性成分,进一步开发利用忍冬属药用植物资源,该研究以DPPH自由基清除法为活性指导,首次对毛药忍冬干燥花蕾75%乙醇提取物的不同极性萃取部位进行抗氧化活性测试,结果发现乙酸乙酯萃取物表现出最强的抗氧化活性(平均清除率为89.45%)。进一步应用现代色谱手段(硅胶柱色谱、Sephadex LH-20凝胶柱色谱等),从毛药忍冬花蕾的乙酸乙酯萃取物中分离单体化合物,运用现代光谱分析技术(MS、1 H-NMR、13 C-NMR、COSY、HSQC、HMBC、ROESY),并结合文献数据鉴定化合物的化学结构。结果表明:从毛药忍冬干燥花蕾75%乙醇提取物中共分离得到9个化合物,分别鉴定为4个酚酸类化合物:绿原酸(1)、绿原酸甲酯(2)、绿原酸乙酯(3)、咖啡酸(4);4个黄酮类化合物:木犀草素(5)、木犀草素-7-O-β-D-葡萄糖苷(6)、槲皮素(7)、槲皮素-3-O-β-D-葡萄糖苷(8);1个甾醇类化合物:β-谷甾醇(9)。所有化合物均为从毛药忍冬花蕾中首次分离得到。研究结果可为抗氧化类相关产品的开发提供科学依据。     

Antimutagenic and antioxidant/prooxidant activity of quercetin

The present study has been performed to evaluate the antimutagenic activity of quercetin, ascorbic acid and their combination against an oxidative mutagen. An effort was also made to correlate this activity to the in vitro antioxidant activity of these agents. Antimutagenicity testing was done in Ames Salmonella Assay system using Salmonella typhimurium TA102 against t-butylhydroperoxide as an oxidative mutagen. In vitro antioxidant scavenging activity was tested for DPPH free radical, superoxide anion, hydrogen peroxide and hydroxyl radical in their specific test systems. Quercetin (0.5-8 nmole/plate) and ascorbic acid (0.1-100 micromole/plate) showed significant effect. Quercetin (4 and 8 nmole/plate) when combined with ascorbic acid (500 nmole/plate) showed an increase in the antimutagenic activity. In vitro antioxidant activity of quercetin was better than ascorbic acid in all the test systems used. The study indicated that the antimutagenic activity of quercetin was not solely accountable by its antioxidant nature. However, in vitro free radical scavenging activity of quercetin correlated well with the antimutagenic activity.     

Polyphenolic compounds in clove and pimento and their antioxidative activities

Two new polyphenolic glucosides, 6'-O-acetylisobiflorin (1) and (2S)-3-(4-hydroxy-3-methoxyphenyl)-propane-1,2-diol 1-O-(6'-O-galloyl)-β-D-glucoside (2), were respectively isolated from the flower buds of Syzygium aromaticum and berries of Pimenta dioica. Each structure was elucidated on the basis of spectral analyses (NMR, MS and [α](D)) and chemical conversion. A total of twenty-seven known compounds from the plants were also characterized. The antioxidative activity of their extracts and the twenty-nine isolates including gallo- and ellagitannins was estimated by oxygen radical absorbance capacity (ORAC) assay, and eugenol (3), which was the most abundant ingredient in each plant extract, showed the most potent antioxidative activity [ORAC value of 39,270 μmol TE (trolox equivalent)/g].     

Efficiency of absorption and metabolic conversion of quercetin and its glucosides in human intestinal cell line Caco-2

The efficiency of intestinal absorption and metabolic conversion of quercetin aglycone and its glucosides, quercetin-4'-O-beta-D-glucoside (Q4'G), quercetin-3-O-beta-D-glucoside (Q3G), and quercetin-3,4'-di-O-beta-D-glucoside (Q3,4'G), was estimated by using Caco-2 cell monolayers as an intestinal epithelial cell model. Aglycone was significantly lost from the apical side, resulting in the appearance of free and conjugated forms of quercetin and those of isorhamnetin in the cellular extracts. In the basolateral solution, the conjugated form of quercetin was predominant and increased with the elapse of incubation. As compared with quercetin aglycone, none of the quercetin glucosides were absorbed efficiently from apical side. However, Q4'G yielded conjugated quercetin and isorhamnetin in basolateral solution at higher amounts than Q3G or Q3,4'G. Lipophilicity of Q4'G was found to be higher than that of Q3G or Q3,4'G. This suggests that lipophilicity contributes to the relatively efficient absorption of Q4'G. It is likely that the occurrence of hydrolysis enhances the efficiency of intestinal absorption and metabolic conversion of dietary quercetin glucosides.     

DPPH radical scavengers from dried leaves of oregano (Origanum vulgare)

1,1-Dipehnyl-2-picrylhydrazyl (DPPH) radical scavenging activities were found in the extract of dried leaves of oregano (Origanum vulgare). The water-soluble active ingredients were isolated, and their structures were determined to be 4'-O-beta-D-glucopyranosyl-3',4'-dihydroxybenzyl protocatechuate and 4'-O-beta-D-glucopyranosyl-3',4'-dihydroxybenzyl 4-O-methylprotocatechuate by (1)H-, (13)C-NMR, DEPT, HMQC, and HMBC spectral analyses, and by NOE experiments. The DPPH radical scavenging activities of these compounds were compared with those of rutin, quercetin and rosmarinic acid at a concentration of 2 x 10(-5) M. The scavenging activity of 4'-O-beta-D-glucopyranosyl-3',4'-dihydroxybenzyl protocatechuate was almost the same as that of quercetin and rosmarinic acid, but that of 4'-O-beta-D-glucopyranosyl-3',4'-dihydroxybennzyl 4-O-methylprotocatechuate was less than that of quercetin, rosmarinic acid and 4'-O-beta-D-glucopyranosyl-3',4'-dihydroxybenzyl protocatechuate. The amount of 4'-O-beta-D-glucopyranosyl-3',4'-dihydroxybenzyl protocatechuate was estimated to be 3.8 mg/1 g of dried leaves by an HPLC analysis.     

Isolation of antioxidative phenolic glucosides from lemon juice and their suppressive effect on the expression of blood adhesion molecules

Phenolic glucosides having radical scavenging activity were examined from the fraction eluted with 20% methanol on Amberlite XAD-2 resin applied to lemon (Citrus limon) juice by using reversed phase chromatography. Four phenolic glucosides were identified as 1-feruloyl-beta-D-glucopyranoside, 1-sinapoyl-beta-D-glucopyranoside, 6,8-di-C-glucosylapigenin and 6,8-di-C-glucosyldiosmetin by (1)H-NMR, (13)C-NMR, and MS analyses. They exhibited radical scavenging activity for 1,1-diphenyl-2-picrylhydrazyl (DPPH) and superoxide, although the activity was low in comparison with eriocitrin, a potent antioxidant in lemon fruit, and the eriodictyol of its aglycone. The phenolic compounds in lemon juice were examined for their suppressive effect on the expression of blood adhesion molecules by measuring the expression of intercellular adhesion molecule-1 (ICAM-1) in human umbilical vein endothelial cells (HUVECs) induced by necrosis factor-alpha (TNF-alpha). 6,8-Di-C-glucosylapigenin, apigenin, and diosmentin of the flavones were found to significantly suppress the expression of ICAM-1 at 10 muM (P<0.05). The phenolic glucosides isolated in this study were contained in comparative abundance in daidai (Citrus aurantium) and niihime (Citrus unshiu x Citrus tachibana) among the sour citrus juices.     

Diversity of Essential Oil-Secretory Cells and Oil Composition in Flowers and Buds of Magnolia sirindhorniae and Its Biological Activities

Magnolia sirindhorniae Noot. & Chalermglin produces fragrant flowers. The volatile oil secretary cells, quantity and quality as well as antioxidant and antimicrobial activities of the oils extracted from buds and flowers, have been investigated. The distribution of essential oil secretory cell in bud and flower revealed that the density and size of the oil cells were significantly higher in flowers compared to buds. In different floral parts, carpel has a higher oil cell density followed by gynophore and tepal. The histochemical analysis revealed the essential oil is synthesized in oil secretory cells. The volatile oil yield was 0.25 % in the buds and 0.50 % in flowers. GC/FID and GC/MS analysis identified 33 compounds contributing 83.2–83.5 % of the total essential oil composition. Linalool is the main constituent contributing 58.9 % and 51.0 % in the buds and flowers oils, respectively. The essential oil extracted from the flowers showed higher antimicrobial efficacy against Klebsiella pneumoniae and Staphylococcus aureus. Similarly, the essential oil isolated from the flowers depicts higher free radical scavenging, and antioxidant activity compared to buds’ oil.     

Quantification of polyphenolic antioxidants and free radical scavengers in marine algae

While the health benefits of antioxidant compounds from terrestrial plants are widely accepted in Western counties, there is less recognition of the health benefits of marine algal antioxidant compounds. Oceans are an abundant source of biomaterials, with many natural antioxidants derived from marine algae being investigated as potential anti-aging, anti-inflammatory, anti-bacterial, anti-fungal, cytotoxic, anti-malarial, anti-proliferative, and anti-cancer agents. The aim of this work was to quantify and compare polyphenolic content and free radical scavenging activity of algal extracts using normal phase and reverse phase thin layer chromatography. Post-chromatographic derivatization with neutral ferric chloride (FeCl₃) solution and with 2,2-diphenyl-1-picrylhydrazyl (DPPH·) free radical were used to assess total polyphenolic content and free radical scavenging activities in algal samples. Total phenolic content quantified on normal phase plates was correlated to phenolic content established on reverse phase plates. Similarly, free radical scavenging activity established on normal phase and reverse phase plates were in good agreement. However, although free radical scavenging activities determined on normal phase plates were highly correlated with polyphenolic content, this correlation was low for reverse phase plates. Lipophilic reversed phase TLC plates do not effectively separate mixtures of highly polar compounds like flavonoids, phenolic compounds and their glucosides. Thus, although reversed phase plates are recommended for assessment of free radical scavengers, as they do not influence the free radical-antioxidant reaction, they may not provide the best separation of polar phenolic compounds, especially flavonoids, and therefore may not accurately quantify polyphenolic content and free radical scavenging potential.     

THE DIFFERENTIATION OF PIGMENTATION IN FLOWER PARTS. I. THE FLAVONOID PIGMENTS OF IMPATIENS BALSAMINA,GENOTYPE IIHHPrPr,AND THEIR DISTRIBUTION WITHIN THE PLANT

Chromatographic analysis of stems, sepals and petals of inbred Impatiens balsamina of the red-flowered genotype llHHP^rP^r has revealed a characteristic assemblage of flavonoid pigments in each organ. The more conspicuous compounds have been identified or partially characterized. The stems possess 3-monoglucosides of kaempferol, quercetin, pelargonidin, cyanidin and, presumably, delphinidin. The variety of pigments is less in flower parts than in stems, and less in petals than in sepals, but the flower parts exhibit a greater elaboration of substituents on the aromatic nuclei. The paired petals of mature flowers are pigmented by p-coumaroyl and feruloyl esters of pelargonidin-3, 5-diglucoside supplemented by more highly substituted derivatives of pelargonidin and by large amounts of kaempferol as the aglycone and two glucosides. The distribution of pigments has significance in the biology of the plant as well as providing an approach to studies of factors which control flower differentiation.     

Radical scavenging activities of Heracleum aquilegifolium Wight (Apiaceae) fruit oils in vitro

The fruits of Heracleum aquilegifolium Wight (Apiaceae) were collected from Western Ghats of the Indian Peninsula. The essential oils were extracted by hydrodistillation. The chemical composition of the essential oils was analysed by gas chromatography and gas chromatography-mass spectrometry (GC-MS). Beta-Pinene (22.3%), 1,8-cineole (20.3%), and beta-phellandrene (12.4%) were the main components of H. aquilegifolium fruit oils. The antioxidant properties of essential oils of H. aquilegifolium were examined by different procedures namely reducing power ability, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, nitric oxide radical scavenging activity, hydrogen peroxide scavenging activity, hydroxyl radical scavenging activity, superoxide anion scavenging activity, and metal chelating activity. The antioxidant activities were compared with those of synthetic antioxidants and standard drugs such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), ascorbic acid, alpha-tocopherol, curcumin, and quercetin. The study confirmed the possible antioxidant potential of essential oils tested with various in vitro antioxidant methods. The presence of monoterpenes in combination with other components in the oils could be responsible for the activity.     

Purification and identification of flavonoids from the yellow green cocoon shell (Sasamayu) of the silkworm,Bombyx mori

Three quercetin glycosides, quercetin 5-O-beta-D-glucoside, quercetin 7-O-beta-D-glucoside, and quercetin 4'-O-beta-D-glucoside, and two kaempferol glycosides, kaempferol 5-O-beta-D-glucoside and kaempferol 7-O-beta-D-glucoside, along with their aglycones, quercetin and kaempferol, were isolated from an ethanolic extract of Sasamayu cocoon shells. The chemical structures were characterized by chemical and spectroscopic methods including UV spectrometry and HPLC-ESI-MS. The five flavonol glycosides of the shell are different structurally from those of the leaves of mulberry (Morus alba). It was suggested that potent antioxidative activity in the cocoon is mainly due to flavonoid compounds since free radical scavenging activity was found in the cocoon flavonoids identified here.     

Inhibitory effect of quercetin metabolites and their related derivatives on copper ion-induced lipid peroxidation in human low-density lipoprotein

To determine the antioxidant activity of dietary quercetin (3,3',4', 5,7-pentahydroxyflavone) in the blood circulation, we measured the inhibitory effect of quercetin metabolites and their related derivatives on copper ion-induced lipid peroxidation of human low-density lipoprotein (LDL). Conjugated quercetin metabolites were prepared from the plasma of rat 1 h after oral administration of quercetin aglycone (40 micromol/rat). The rate of cholesteryl ester hydroperoxide (CE-OOH) accumulation and the rate of alpha-tocopherol consumption in mixtures of LDL solution (0.4 mg/ml) with equal volumes of this preparation were slower than the rates in mixtures of LDL with preparations from control rats. The concentrations of CE-OOH after 2 h oxidation in the mixtures of LDL with preparations of conjugated quercetin metabolites were significantly lower than those in the control preparation. It is therefore confirmed that conjugated quercetin metabolites have an inhibitory effect on copper ion-induced lipid peroxidation in human LDL. Quercetin 7-O-beta-glucopyranoside (Q7G) and rhamnetin (3,3',4', 5-tetrahydroxy-7-methoxyflavone) exerted strong inhibition and their effect continued even after complete consumption, similarly to quercetin aglycone. The effect of quercetin 3-O-beta-glucopyranoside (Q3G) did not continue after its complete consumption, indicating that the antioxidant mechanism of quercetin conjugates lacking a free hydroxyl group at the 3-position is different from that of the other quercetin conjugates. The result that 4'-O-beta-glucopyranoside (Q4'G) and isorhamnetin (3,4',5, 7-tetrahydroxy-3'-methoxyflavone) showed little inhibition implies that introduction of a conjugate group to the position of the dihydroxyl group in the B ring markedly decreases the inhibitory effect. The results of azo radical-induced lipid peroxidation of LDL and the measurement of free radical scavenging capacity using stable free radical, 1,1,-diphenyl-2-picrylhydrazyl, demonstrated that the o-dihydroxyl structure in the B ring is required to exert maximum free radical scavenging activity. It is therefore likely that conjugation occurs at least partly in positions other than the B ring during the process of metabolic conversion so that the inhibitory effect of dietary quercetin is retained in blood plasma after absorption.     

In vitro antioxidant activity of banana (Musa spp. ABB cv. Pisang Awak)

The methanolic extract of Musa ABB cv Pisang Awak was investigated for the polyphenolic contents and antioxidant activity. The total phenol and flavonoid contents of the fruit extract were found to be 120 mg gallic acid equivalents (GAE) and 440 mg quercetin equivalents (QE)/100 g of sample dry weight, respectively. The antioxidant activity of the Pisang Awak methanol extract (PAME) (20-500 microg/ml) was determined using 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical scavenging activity, reducing capacity, 2-2'-azinobis-3-ethylbenzothiozoline-6-sulfonic acid (ABTS) radical cation decolourization and hydroxyl radical scavenging capacity (OH*). The EC50 values of DPPH, ABTS and OH* activities of the PAME and butylated hydroxy toluene (BHT) were found to be 65 and 9 microg/ml, 29 and 6 microg/ml, 36 and 42 microg/ml respectively. The reducing capacity increased with increasing concentration (31.5-1000 mg/ml) of the fruit extract and the activity was comparable with the standard BHT. The high performance thin layer chromatography (HPTLC) analysis of the extract revealed the presence of polyphenols. The strong and positive correlations were obtained between total phenol/flavonoid contents (R2 = 0.693-1.0) and free radical scavenging ability was attributed to the polyphenols as the major antioxidants.     

New antioxidant flavonoid isolated from Leuzea carthamoides

A new natural flavonoid patuletin 3′-β-xylofuranoside was isolated from Leuzea carthamoides leaves. The antioxidant activity of this compound was evaluated by the DPPH radical assay and ferric reducing antioxidant power (FRAP) assay, and the results were compared with those for trolox and quercetin. DPPH radical scavenging activity of the tested compounds was expressed by the parameter EC₅₀: patuletin 3′-β-xylofuranoside (56.0 μM), trolox (27.8 μM), and quercetin (25.3 μM). The ferric reducing activity of the compounds was demonstrated as FRAP values at 4 and 60?min: patuletin 3′-β-xylofuranoside (28.4 μM, 35.8 μM), trolox (19.3 μM, 20.2 μM), and quercetin (54.3 μM, 79.9 μM). The structure/activity relationship of the flavonoid is also discussed. The results indicate significant antioxidant potency of patuletin 3′-β-xylofuranoside.     

Flavonoids in flowers of 16 Kalanchoë blossfeldiana varieties

Kalancho? blossfeldiana varieties with orange, pink, red and magenta flowers were found to contain 3,5-O-beta-D-diglucosides of pelargonidin, cyanidin, peonidin, delphinidin, petunidin and malvidin. Pink, red and magenta varieties contained relatively high amounts of quercetin based flavonols. Four distinct quercetin flavonols were identified, namely quercetin 3-O-beta-D-glucoside and three that were quercetin 3-O-alpha-L-rhamnoside based, with either glucose, xylose or arabinose attached to position 2 of the rhamnose. In addition, the presence of at least three kaempferol based diglycosides was suggested from LC-MS analyses. Orange varieties contained very low amounts of flavonol co-pigments and of delphinidin derivatives. The flower extracts of the varieties 'Diva' (magenta) and 'Molly' (red) had identical anthocyanin ratios but differed significantly in flavonol content. The magenta variety contained four times as much quercetin relative to anthocyanidin as the red variety. This difference was mainly due to a larger content of quercetin 3-O-(2'-O-beta-D-glucopyranosyl-alpha-L-rhamnopyranoside). Based on pigment and co-pigment analyses, approaches for molecular breeding towards blue flower colour are discussed.     

Stable free radical scavenging and antiperoxidative properties of resveratrol compared in vitro with some other bioflavonoids

Stable free radical scavenging and antiperoxidative activities of resveratrol, a component of grapes and red wine, were evaluated and compared with some other known bioflavonoids (quercetin, catechin, kaempferol, myricetin, fisetin, ellagic acid and naringenin) widely present in the plant kingdom. Free radical scavenging activity was measured in an in vitro chemical system (DPPH assay), while for antiperoxidative activity, biological system comprising of hepatic and pulmonary homogenates was employed. Antiradical activity assay showed quercetin and myricetin to be stronger antiradical agents than resveratrol. Structure-activity study revealed that O-dihydroxy group on ring B of flavonoid plays a crucial role. A double bond at 2-3 position conjugated with a 4-oxo function and hydroxy groups at positions 3 and 5 also contribute towards antiradical activity of flavonoids. Resveratrol exhibited stronger antiradical activity than kaempferol and naringenin and was also more efficient than alpha-tocopherol, a known strong endogenous non-flavonoid antioxidant, used for comparison. In vitro antiperoxidative assay showed fisetin as the strongest and kaempferol as the weakest antioxidant. Resveratrol was found to be stronger antioxidant than catechin, myricetin, kaempferol and naringenin, but was weaker than quercetin, fisetin and alpha-tocopherol. Antiradical and antiperoxidative activities of resveratrol may explain its beneficial effects in disease states. Assays exhibited no direct correlation between antiradical and antiperoxidative activities of the phenolics.     

Study on Degradation of Flavonols in Mutants of PoppyPapaver somniferumL.

We studied flavonol-degrading activity of cell-free extracts from petals of the flower color and structure mutants. The relationship between degradation of flavonols (kaempferol, quercetin, and myricetin) and biosynthesis of anthocyanins has been revealed. The white-flower mutant proved to have the highest flavonol-degrading activity toward all substrates, particularly quercetin. The mutations inhibiting synthesis of pelargonidin, an anthocyanin, provide for synthesis of various amounts of cyanidin in the petals. The flavonol-degrading activity considerably increases proportionally to the content of cyanidin. A similar relationship has been revealed in the mutants synthesizing both cyanidin and pelargonidin. The plants accumulating considerable amounts of pelargonidin in their petals have accordingly higher flavonol-degrading activity and predominantly hydrolyze kaempferol. The plants forming additional pods in their flower (pistillody) have higher flavonol-degrading activity as compared to the anther-in-petal and doubleness mutants     

The effect of catechol O-methylation on radical scavenging characteristics of quercetin and luteolin--a mechanistic insight

The biological effect of flavonoids can be modulated in  vivo due to metabolism. The O-methylation of the catechol group in the molecule by catechol O-methyl transferase is one of the important metabolic pathways of flavonoids. In the present study, the consequences of catechol O-methylation for the pH-dependent radical scavenging properties of quercetin and luteolin were characterized both experimentally and theoretically. Comparison of the pKa values to the pH-dependent TEAC profiles reveals that O-methylation not only affects the TEAC as such but also modulates the effect of changing pH on this radical scavenging activity due to an effect on the pKa for deprotonation. The pH-dependent TEAC curves and computer calculated electronic parameters: bond dissociation energy (BDE) and ionisation potential (IP) even indicate that O-methylation of the luteolin catechol group affects the radical scavenging potential only because it shifts the pKa for deprotonation. O-Methylation of the quercetin catechol moiety affects radical scavenging capacity by both an effect on the pKa, and also by an effect on the electron and hydrogen atom donating properties of the neutral (N) and the anionic (A) form of the molecule. Moreover, O-methylation of a catechol OH-group in quercetin and luteolin has a similar effect on their TEAC profiles and on calculated parameters as replacement of the OH-group by a hydrogen atom. Altogether, the results presented provide new mechanistic insight in the effect of catechol O-methylation on the radical scavenging characteristics of quercetin and luteolin.