Phenol Radical Research Articles

Oxidative Cross‐Coupling of Substituted Phenols with Unactivated Aromatics

A regioselective C–H arylation of substituted phenols or naphthols with substituted aromatics in the presence of K2S2O8 and Bu4N+·HSO4– in CF3COOH at ambient temperature providing unsymmetrical biaryls is described. The cross‐coupling reaction is compatible with various substituted phenols and aromatics. Based on oxidation potential studies, it was observed that phenols with oxidation potentials up to 0.68 V and naphthols up to 0.88 V were compatible with the reaction. The cross‐coupling reaction proceeds through the reaction of K2S2O8 with substituted phenol, providing a cationic phenol radical intermediate that was further converted into the phenoxy radical intermediate by the dissociation of H+. Subsequent radical‐anion coupling of the phenoxy radical intermediate with a substituted aromatic affords the cross‐coupling product. The formation of a cationic phenol radical intermediate and phenoxy radical intermediate were confirmed by UV/Vis and EPR studies.

Evaluation of antioxidant capacity and flavor profile change of pomegranate wine during fermentation and aging process

Antioxidant properties and flavor characteristic profile of pomegranate wine during winemaking were investigated. The total phenol content and radical scavenging activity exhibited a slightly decrease in the end edge. Punicalagins and gallic acid were revealed to be the most abundant phenolic compounds, followed by ellagic acid and vanillic acid. These constituents were mainly responsible for the effective antioxidant capacity of pomegranate wine. The major changes of flavor qualities occurred in the initial stage, particularly 0–4day of fermentation. Fermentation significantly reduced the relative content of aldehydes, ketones, heterocyclic and aromatic compounds, but promoted the generation of esters and alcohols. This is the first time of using E-nose and E-tongue to monitor odour and taste changes in the brewing process of pomegranate wine. The study may provide a promising instruction for improving functional features and quality control of the pomegranate wine.

Microsolvation of the 5-hydroxyindole cation (5HI+) with nonpolar and quadrupolar ligands: Infrared photodissociation spectra of 5HI+-Ln clusters with L = Ar and N2 (n ≤ 3)

Solvation of biomolecules and their building blocks has a strong influence on their structure and function. Herein we characterize the initial microsolvation of the 5-hydroxyindole cation (5HI+) in its 2A″ ground electronic state with nonpolar and quadrupolar ligands (L=Ar, N2) by infrared photodissociation (IRPD) spectroscopy of cold and mass-selected 5HI+-Ln (n≤3) clusters in a molecular beam and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). The isomer-selective OH and NH stretch frequency shifts (ΔνOH/NH) disentangle the competition between H-bonding to the acidic OH and NH groups and π-stacking to the conjugated bicyclic aromatic π-electron system, the intermolecular interaction strengths, and the cluster growth sequence. For 5HI+-Arn, H-bonding and π-stacking strongly compete, indicating that dispersion forces are important for the interaction of 5HI+ with nonpolar ligands. In contrast, for 5HI+-(N2)n clusters, the H-bonds are much stronger than the π-bonds and largely determine the initial solvation process. In all clusters, the OH…L bonds are stronger than the NH…L bonds followed by the π-bonds. The interaction of 5HI+ with N2 is roughly twice stronger than with Ar, mainly due to the additional quadrupole moment of N2. The nature and strength of the individual interactions are quantified by the noncovalent interaction approach. Comparison of 5HI+-L with the corresponding neutral clusters reveals the strong impact of ionization on the total and relative interaction strengths of the H-bonds and π-bonds. In addition, comparison of 5HI+-L with corresponding clusters of the phenol, indole, and pyrrole radical cations illustrates the effects of substitution of functional groups and the addition of aromatic rings to the various subunits of 5HI on the intermolecular potential.

EVALUATION OF ANTIOXIDANT AND FREE RADICAL SCAVENGING ACTIVITIES OF DIFFERENT SOLVENT EXTRACTS OF LEAVES OF PIPER UMBELLATUM

Objective: This study was designed to determine the Total phenol, flavonoid content, antioxidant and free radical scavenging properties of different solvent extracts of Piper umbellatum. Methods: Different solvent extracts evaluated with DPPH radical scavenging activity and Reducing power activity.Results: The presence of phenol and flavonoid showed highly in the methanol extracts than ethyl acetate and acetone extracts. All the extracts have various level of antioxidant activity. Methanol solvent extract have good extraction and show significant antioxidant activity. The effect of reducing power of methanol extract revealed good antioxidant activity compare with other tested extracts.Conclusion: On the basis of the above results we concluded that methanol extract of Piper umbellatum whole plant extracts shows significant antioxidant activity than ethyl acetate and acetone extracts.Keywords: Piper umbellatum, Antioxidant, DPPH, FRAP assay, Polar, Non polar solvents

Non-covalent π-π stacking interactions turn off non-adiabatic effects in proton-coupled electron transfer reactions.

π-π stacking interactions do not necessarily change the mechanism involved in the H transfer reaction between phenol and phenoxyl radicals. We propose that, in such cases, the e- is transferred between the π delocalized moieties, while the H+ is transferred between the donor and acceptor atoms.

Phenol oxidation through its adduct formation with chromium complex of 1,4,8,11-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane: A theoretical study

Structural and electronic properties of [cis-[Cr(tmpcH)X2]n+ (n = 2 or 4; X = OH−, Cl−, Br− and H2O; tmpcH = 1,4,8,11-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane were analyzed by DFT and TD-DFT methods. The local reactivity active site of the ligand was determined by the condensed-to atom Fukui indexes (CAFI) f(r). In the study, the axial bond distance with metal ion undergoes a considerable change from shorter to longer as OH < Cl− < Br− < H2O, agreeing with the molecular orbital analysis where the dz2 energy is lowered for OH− compared to H2O at the axial position. After analyzing the geometrical data collected from literature for the complexes of Cr(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn (II) with tmpcH, it was found that the bond distance decreases with increasing number of d-electrons in the 3d orbital, suggesting that the over-lapping of oribital (π) from Npy with the metal d-orbital is more effective than those from Ncyclam with metal d-orbital. Therefore, the change of different oxidation states for [cis-[Cr(tmpcH)X2]n+ influences significantly the geometrical and electronic parameters. For cis-[Cr(tmpcH)Cl2]2+ the calculated bands are red shifted except for the lower energy band (595 nm) which agrees qualitatively with the experimental one; in addition, the effect of solvent on the electronic transition was analyzed. Furthermore, we collected the electronic data for several chromium complexes from the literature, and compared with our results by plotting the data against number of chromium compounds. Finally, the phenol oxidation properties of the chromium complexes were studied, and phenol forms an adduct with [Cr(tmpcH)Cl]3+ to yield [Cr(tmpcH)Cl-OPh]2+ which could produce the phenol radical, which is enhanced by the presence of -OCH3 group at para- position in the phenolic ring.

Biocatalytic trifluoromethylation of unprotected phenols.

Organofluorine compounds have become important building blocks for a broad range of advanced materials, polymers, agrochemicals, and increasingly for pharmaceuticals. Despite tremendous progress within the area of fluorination chemistry, methods for the direct introduction of fluoroalkyl-groups into organic molecules without prefunctionalization are still highly desired. Here we present a concept for the introduction of the trifluoromethyl group into unprotected phenols by employing a biocatalyst (laccase), tBuOOH, and either the Langlois' reagent or Baran's zinc sulfinate. The method relies on the recombination of two radical species, namely, the phenol radical cation generated directly by the laccase and the CF3-radical. Various functional groups such as ketone, ester, aldehyde, ether and nitrile are tolerated. This laccase-catalysed trifluoromethylation proceeds under mild conditions and allows accessing trifluoromethyl-substituted phenols that were not available by classical methods.

Proton-Coupled Electron Transfer in a Hydrogen-Bonded Charge-Transfer Complex.

A proton-coupled electron transfer (PCET) reaction in a hydrogen-bonded charge-transfer (CT) complex of 4-([2,2'-bipyridin]-4-yl)phenol (bpy-phenol) with a F- ion has been investigated by ultrafast time-resolved transient absorption spectroscopy. The phenolic receptor molecule, bpy-phenol, binds to the F- ion through a hydrogen bond and senses the F- ion via the Stokes-shifted CT band. Upon photoexcitation, CT from the phenol residue to the bpy residue promotes proton transfer from the phenol radical cation (ArOH•+) to the fluoride ion at ultrafast time scales of <150 fs (instrument response function limited) and 3 ps, separately. The fast and slow proton-transfer times are linked to two different types of hydrogen-bonding networks between the phenol residue and fluoride ion. Crystalline water in the fluoride salt hydrates mediates the proton-transfer reaction. This work demonstrates the participation of a hydrogen-bonded water bridge within a PCET reaction in a water-restricted environment.

Effect of phenolic radicals on the geometry and electronic structure of DNA base pairs: computational study

In this paper, we show the reaction of a hydroxyl, phenyl and phenoxy radicals with DNA base pairs by the density functional theory (DFT) calculations. The influence of solvation on the mechanism is also presented by the same DFT calculations under the continuum solvation model. The results showed that hydroxyl, phenyl and phenoxy radicals increase the length of the nearest hydrogen bond of adjacent DNA base pair which is accompanied by decrease in the length of furthest hydrogen bond of DNA base pair. Also, hydroxyl, phenyl and phenoxy radicals influenced the dihedral angle between DNA base pairs. According to the results, hydrogen bond lengths between AT and GC base pairs in water solvent are longer than vacuum. All of presented radicals influenced the structure and geometry of AT and GC base pairs, but phenoxy radical showed more influence on geometry and electronic properties of DNA base pairs compared with the phenyl and hydroxyl radicals.

Characterization and antioxidant capacity of sweet chestnut honey produced in North-West Spain

Abstract In recent years, authentic foodstuffs have became a major requirement for consumers and producers worldwide. Honey has increased in popularity since it is associated with a natural diet, and because of honey’s authentic origin. The present study investigated the palynological characteristics, physicochemical parameters, total phenol content, flavonoid content, and radical scavenging activity of 41 sweet chestnut (Castanea sativa) honeys from the northwestern part of Spain. These honeys were characterised by high values of electrical conductivity, pH, diastase content, and colour. All the samples showed a pollen combination of Castanea sativa-Rubus-Cytisus type-Erica. Fructose and glucose were 37.2% and 25.9%, while other sugars were less than 5%. Regarding the mineral content K, was the main with a mean value of 260.2 mg/100g. Other elements as Mg with a mean value of 17.1 mg/100g, Ca (mean value of 15.8 mg/100g) and P (mean value of 12.8 mg/100g) were well represented in this honey type. The phenol and flavonoid content were high (mean values of 129.8 mg/100g and 9.0 mg/100g, respectively). Multivariate statistical techniques showed the close relationship of colour, Mg, P, phenols, melezitose, and flavonoids, and the radical scavenging activity.

Browning susceptibility of commercial monovarietal white wines under accelerated oxidation conditions: correlation with compositional data and effect on total phenol content and radical scavenging activity

A set of seven commercial white wine samples (W1–7) produced from the cultivar Malagousia and obtained directly from the cellar 4 months after winemaking were subjected to accelerated oxidation (55 °C, 11 days). The correlation of browning susceptibility with compositional data, as well as the effect on phenolic content using simple assays, and the radical scavenging activity (DPPH·, ABTS·+) were examined. Calculated browning rate constants (k) varied by 1.03–2.66-fold among pairs of samples. Thus, despite the fact that samples originated from the same cultivar, differences were significant in some cases. No correlation was observed between browning and total or free sulfur dioxide, total iron, copper or other compositional parameters (total phenol, total hydroxycinnamates, total flavanol, catechin, epicatechin content, etc.), except for the grape reaction product/caftaric acid ratio (0.802, p = 0.017). The later suggested that wine must elaboration might be important for white wine oxidative stability, and its contact with oxygen should be avoided or minimized. Addition of ascorbic acid at the maximum level detected in the tested wines (70 mg/L), to one of the samples free of ascorbic acid but containing the highest level of total (100 mg/L) and free SO2 (30 mg/L) among these, had a negative effect. However, one of the samples containing ascorbic acid (33 mg/L) was resistant to oxidation. The molar ratio of free SO2 to that of ascorbic may affect browning susceptibility; therefore, the use of ascorbic acid should be made with caution. Chemometrics could be useful to provide prediction of browning susceptibility in relation to compositional data. The radical scavenging activity decreased with a significant correlation between the loss in DPPH· scavenging and ascorbic acid content (r = 0.845, p = 0.008). The changes in total flavanols did not follow a clear trend, whereas the spectrophotometric monitoring of total hydroxycinnamate was not informative due to interferences with browning products. Present findings are expected to be useful for those involved in white wine production.

A Comparative Study Between Antioxidant Potential of Ripe and Pre-ripe Mango Using Conventional and Physiological Extraction

ABSTRACTMango is distinctive among all fruits considering its exceptional flavors. All varieties of mango have their unique essence. It is one of the largest consumed fruits worldwide. Its antioxidant properties are also well known due to the presence of carotenoids and phenolic compounds. Unlike other studies, this study considers that the methanolic extraction may not well reflect the antioxidant activity of mango after consumption, i.e., in vivo. Thus, an in vitro gastrointestinal model has been used for the simulation of digestion of pre-ripe and ripe mango samples. Both samples thus extracted are compared with their conventional methanolic extracts for their antioxidant compounds and antioxidant activity. Interesting differences in the two types of extracts can be observed. The simulated gastrointestinal extracts show significantly higher antioxidant properties in parameters, such as total phenol content, flavonoid content, flavonol content, carotenoid content, reducing power, and radical scavenging activity. From the results, it can be concluded that the conventional extracts under rate the antioxidant value of mangoes and they may have much higher health significance than they are believed to have.

Stabilization of Phenolic Radicals on Graphene Oxide: An XPS and EPR Study.

A graphene oxide-gallic acid hybrid material was synthesized by the immobilization of gallic acid (3,4,5-trihydroxobenzoic acid) on graphene oxide. The grafting was achieved via the formation of amide bonds between the amine groups on the organofunctionalized graphite oxide surface and the carboxyl groups of the gallic acid molecules. The EPR signal of the gallic acid radicals in this hybrid material remained almost unaltered over at least 500 days, with less than 3% signal decay over that period, pointing to the truly remarkable stability of these radicals. The produced material was characterized by Fourier transform infrared, X-ray photoelectron, and electron paramagnetic resonance spectroscopies as well as by thermogravimetric analysis and the Kaiser test. The stability of the radicals in the material was studied in powder form and in aqueous solution vs pH. We demonstrate that in the graphene oxide-gallic acid hybrid material a radical is favorably stabilized on the ring-O while the oxidation of the second OH is precluded, and this results in long-term stabilization of the gallic acid radicals in solid hybrid material. Thus, in applications where it will be used under O2-free and humidity-free conditions, the graphene oxide-gallic acid hybrid material is a reliable spintronics scaffold.

Theoretical and experimental studies of phenol oxidation by ruthenium complex with N,N,N-tris(benzimidazol-2yl-methyl)amine.

The ruthenium complex with (N,N,N-tris(benzimidazol-2yl-methyl)amine, L(1)) was prepared, and characterized. Fukui data were used to localize the reactive sites on the ligand. The structural and electronic properties of the complex were analyzed by DFT in different oxidation states in order to evaluate its oxidant properties for phenol oxidation. The results show that the hard Ru(IV) cation bonds preferentially with a hard base (Namine = amine nitrogen, or axial chloride ion), and soft Ru(II) with a soft base (Nbzim = benzimidazole nitrogen or axial triphenyl phosphine). Furthermore, the Jahn-Teller effect causes an elongation of the axial bond in the octahedral structure. The bonding nature and the orbital contribution to the electronic transitions of the complex were studied. The experimental UV-visible bands were interpreted by using TD-DFT studies. The complex oxidizes phenol to benzoquinone in the presence of H2O2 and the intermediate was detected by HPLC and (13)C NMR. A possible mechanism and rate law are proposed for the oxidation. The adduct formation of phenol with [Ru(O)L(1)](2+) or [Ru(OH)L(1)](+) is theoretically analyzed to show that [Ru(OH)L(1)-OPh](+) could produce the phenol radical.

One‐Step Selective Hydroxylation of Benzene to Phenol

AbstractRecent developments in the one‐step thermal and photochemical catalytic hydroxylation of benzene to phenol using various oxidants are described, focusing on the catalytic mechanisms. Hydroxylation of benzene with hydrogen peroxide is catalyzed by homogeneous metal complexes and heterogeneous metal oxides to produce phenol when high‐valent metal‐oxo complexes are reactive species The selective hydroxylation of benzene to phenol is achieved by using mesoporous silica‐alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. Photocatalytic oxidation of benzene with dioxygen and water is made possible by using organic photocatalysts via photoinduced electron transfer from benzene to the excited states of the photocatalysts, followed by the hydroxylation of the benzene radical cation with water to yield phenol selectively. The further oxidation of phenol is prohibited because of the fast back electron transfer from the one‐electron reduced species of the photocatalyst to the phenol radical cation, whereas back electron transfer from the one‐electron reduced species to the benzene radical cation is slowed down because of the large driving force of the back electron transfer, which is in the Marcus inverted region.

Hawthorn ethanolic extracts with triterpenoids and flavonoids exert hepatoprotective effects and suppress the hypercholesterolemia-induced oxidative stress in rats.

The current study was aimed to determine the bioactive constituents and biological effects of the Crataegus monogyna ethanolic extracts from bark, leaves and berries on hypercholesterolemia. Oleanolic acid, ursolic acid, quercetin and lupeol concentrations were quantified by HPLC. Total phenol content and radical scavenging activity of extracts were also measured. The hypocholesterolemic, antioxidant, and hepatoprotective effects of the extracts were examined in hypercholesterolemic rats and compared with orlistat. The highest phenol content, oleanolic acid, quercetin and lupeol levels and free radical scavenging potency were found in the bark extract, and the highest ursolic acid level was found in the berries extract. Orlistat and extracts significantly (P<0.05) lowered the hypercholesterolemia-increased serum level of hepatic enzymes and lipid peroxidation level. Hawthorn's extracts protected from hepatic thiol depletion and improved the lipid profile and hepatic damages. Data suggested that hawthorn's extracts are able to protect from hypercholesterolemia-induced oxidative stress and hepatic injuries. Moreover, the hypocholesterolemic effect of extracts was found comparable to orlistat.

A three-enzyme-system to degrade curcumin to natural vanillin.

The symmetrical structure of curcumin includes two 4-hydroxy-3-methoxyphenyl substructures. Laccase catalyzed formation of a phenol radical, radical migration and oxygen insertion at the benzylic positions can result in the formation of vanillin. As vanillin itself is a preferred phenolic substrate of laccases, the formation of vanillin oligomers and polymers is inevitable, once vanillin becomes liberated. To decelerate the oligomerization, one of the phenolic hydroxyl groups was protected via acetylation. Monoacetyl curcumin with an approximate molar yield of 49% was the major acetylation product, when a lipase from Candida antarctica (CAL) was used. In the second step, monoacetyl curcumin was incubated with purified laccases of various basidiomycete fungi in a biphasic system (diethyl ether/aqueous buffer). A laccase from Funalia trogii (LccFtr) resulted in a high conversion (46% molar yield of curcumin monoacetate) to vanillin acetate. The non-protected vanillin moiety reacted to a mixture of higher molecular products. In the third step, the protecting group was removed from vanillin acetate using a feruloyl esterase from Pleurotus eryngii (PeFaeA) (68% molar yield). Alignment of the amino acid sequences indicated that high potential laccases performed better in this mediator and cofactor-free reaction.

Physicochemical Characterization and Enhancement of the Antioxidant Potential of &lt;em&gt;Ocimum gratissimum&lt;/em&gt; Enriched Pepper Soup Mix

Scent leaf (Ocimum gratissimum) enriched pepper-soup mix continues to remain an indigenous soup delicacy consumed for its nutritional and health benefits. There is however paucity of scientific data underlying its significance which could be further explored to advantage. In this study, dry soup mix was prepared from scent leaf (15%), ginger (30%), black pepper (25%), African nutmeg (17.5%) and scent pepper (12.5%) and evaluated for proximate, anti-oxidative and sensory properties using standard methods. The mix was reformulated to include 10% modified cocoyam (Colocasia esculenta) flour (thickener), 5% fresh and dried tea (Camellia sinensis) leaf following which it was re-characterized Concentration-dependent scavenging activity was observed for the entire samples with nitric oxide radical scavenging activity of scent leaf enriched soup (750��������g/mL) exhibiting the highest value 77% compared to 57.7% for soup containing cocoyam. Total phenol, flavonoid contents and hydrogen peroxide radical scavenging activity differed in all the reformulated samples (p＜0.05). A significant increase (＞50%) in DPPH radical scavenging activity was observed as a result of tea leaf addition. The moisture, fat, crude fiber, ash, protein of the enhanced scent leaf enriched pepper-soup mix was 7.55, 28.4, 8.0, 6.2% and 11.2%, respectively. There was a significant difference (p＜0.05) in the sensory qualities (color, aroma, taste, consistency and overall acceptability) between scent leaf and enhanced tea leaf pepper soup mix. The antioxidant potential, nutrient content and sensory attributes of this enhanced pepper soup mix holds great promise in further combating oxidative stress as in some disease conditions and rapid ageing.

Enzymatic Hydrolysis of Oleuropein from Olea europea (Olive) Leaf Extract and Antioxidant Activities

Oleuropein (OE), the main polyphenol in olive leaf extract, is likely to decompose into hydroxytyrosol (HT) and elenolic acid under the action of light, acid, base, high temperature. In the enzymatic process, the content of OE in olive leaf extract and enzyme are key factors that affect the yield of HT. A selective enzyme was screened from among 10 enzymes with a high OE degradation rate. A single factor (pH, temperature, time, enzyme quantity) optimization process and a Box-Behnken design were studied for the enzymatic hydrolysis of 81.04% OE olive leaf extract. Additionally, enzymatic hydrolysis results with different substrates (38.6% and 81.04% OE) were compared and the DPPH antioxidant properties were also evaluated. The result showed that the performance of hydrolysis treatments was best using hemicellulase as a bio-catalyst, and the high purity of OE in olive extract was beneficial to biotransform OE into HT. The optimal enzymatic conditions for achieving a maximal yield of HT content obtained by the regression were as follows: pH 5, temperature 55 °C and enzyme quantity 55 mg. The experimental result was 11.31% ± 0.15%, and the degradation rate of OE was 98.54%. From the present investigation of the antioxidant activity determined by the DPPH method, the phenol content and radical scavenging effect were both decreased after enzymatic hydrolysis by hemicellulase. However, a high antioxidant activity of the ethyl acetate extract enzymatic hydrolysate (IC50 = 41.82 μg/mL) was demonstated. The results presented in this work suggested that hemicellulase has promising and attractive properties for industrial production of HT, and indicated that HT might be a valuable biological component for use in pharmaceutical products and functional foods.

Nutrient profile of porridge made from Eleusine coracana (L.) grains: effect of germination and fermentation.

Porridge (koozh) is one of the traditional foods made from Eleusine coracana L. grains (Finger millet). It is a soft food prepared from processed (germinated & fermented) finger millet flour (FMF). However, in the modern world of fast food, koozh is usually prepared from non-processed (non-germinated & non-fermented) FMF. Hence, present study was undertaken to evaluate the macro and micro nutrient contents in koozh prepared from germinated (fermented & non-fermented) and non-germinated (fermented & non-fermented) FMF. Highest protein, carbohydrate and glycoprotein contents were found in koozh prepared from germinated & non-fermented FMF. The free amino acid contents are higher in germinated & fermented condition when compare to other preparations. No significant change was observed in the calorific value of all preparations. There is no statistical difference in macro-nutrients & micro-nutrients minerals such as calcium, iron, magnesium, manganese, phosphorous and zinc among all the preparations. However, copper content is higher in non-germinated condition, whereas selenium, silicon and sulphur are higher in germinated FMF when compared to others. Significant level of total phenol, total flavonoid and free radical scavenging activity was observed in all preparations, which increased further during fermentation. The present observations, lead us to conclude that koozh prepared from germinated & non-fermented FMF contains higher level of carbohydrate, protein and glycoprotein, however germinated & fermented koozh has increased aminoacids, phytochemicals and free radical scavenging activity. Hence it is suggested that the consumption of koozh made from germinated & fermented FMF may provide easily digestible and energetic nutrients for healthier life.