The molecular basis of working mechanism of natural polyphenolic antioxidants

Antioxidant therapy: still in search of the 'magic bullet'

The therapeutic potential of natural phenolic antioxidants in human diseases associated with oxidative damage has received great attention to date. Appraisal of literature evidences that, in general, antioxidant therapy has enjoyed relative successes in preclinical studies but little benefits in human intervention studies or clinical trials. In fact, despite the huge, largely untapped potential therapeutic benefit of natural phenolic antioxidants, such as vitamins, non-flavonoid and flavonoid compounds, they appear not to be suitable drug candidates. The problem may be related, among others, to their non-drug-likeness properties. Though controversial the results obtained so far confirm the importance of exploring phenolic natural systems as safe templates for the design of new antioxidants. To support the assumption an outlook of the lead structural optimization process to improve ADME properties was given by means of natural hydroxycinnamic acids as a case study. The optimization of drug physicochemical properties and the development of appropriate delivery antioxidant systems can provide in the next future a way out to attain effective therapeutic antioxidant agents.

Theoretical and experimental exploration of the photochemistry of resveratrol: beyond the simple double bond isomerization

The photochemical isomerization of resveratrol has been the subject of recent studies in which contradictory results were reported. The photoproduct mixture of this reaction needs to be considered more complex than the coexistence of cis and trans isomers. An unidentified third product, at least, has been detected in various studies although its nature was unknown. In this work, we aim to provide a thorough description of the photochemical course of this reaction through experimental and computational approaches working in a synergetic association.

1,1,2,2-Tetra-kis[2,4-dichloro-6-(dieth-oxy-meth-yl)phen-oxy-meth-yl]ethene

In the title compound, C(50)H(60)Cl(8)O(12), the mol-ecules are disordered about an inversion center located at the mid-point of the central C=C bond. These atoms show disorder and were modelled with two different orientations with site occupancies of 0.828 (3) and 0.172 (3). The dihedral angle between the two benzene rings in the asymmetric unit is 52.80 (6)°. Intramolecular C-H⋯O and C-H⋯Cl interactions occur and the crystal packing features inversion dimers linked by pairs of C-H⋯O bonds, generating R(2) (2)(10) loops.

Anthraquinone profile, antioxidant and antimicrobial activity of bark extracts of Rhamnus alaternus, R. fallax, R. intermedia and R. pumila

The quantity of phenols, as well as antioxidant and antimicrobial activities, were investigated in bark of Rhamnus alaternus L., R. fallax Boiss., R. intermedia Steud. et Hochst., and R. pumila Turra from natural stands in Croatia. The most abundant anthraquinones in the investigated extracts were chrysophanol in R. alaternus (3.14 mg/g), emodin in R. pumila (0.339 mg/g), and physcion in R. fallax (2.70 mg/g) and R. intermedia (0.285 mg/g). The species exhibiting the highest antioxidant activity were R. fallax and R. pumila. A positive correlation was observed between total phenolic and flavonoid levels of the extracts and antioxidant activity in some of the assays. All species showed antimicrobial activity against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans, Aspergillus niger and Microsporum gypseum with minimal inhibitory concentrations equal to or below 2.500 mg/mL. The results indicate that the investigated Rhamnus species are a source of anthraquinones and other phenols, which act as multifunctional antioxidants with antimicrobial activity.

Tebrophen--an old polyphenol drug with anticancer potential

In vitro high-throughput screening was carried out in order to detect new activities for old drugs and to select compounds for the drug development process comprising new indications. Tebrophen, a known antiviral drug, was found to inhibit activities on inflammation and cancer related targets. In primary screening this semisynthetic halogenated polyphenol was identified to inhibit the activities of kinases ZAP-70 and Lck (IC₅₀ 0.34 µM and 16 µM, respectively), as well as hydrolase DPPIV (at 80 µM 41% inhibition). Next, it showed no cytotoxic effects on standard cell lines within 24 h. However, tebrophen slowed propagation of breast cancer (MDA-MB-231), osteosarcoma (U2OS) and cervical carcinoma (HeLa), through at least 35 population doublings in a dose-dependent manner. It completely stopped the division of the prostate cancer (PC3) cell line at 50 µM concentration and the cells entered massive cell death in less than 20 days. On the other hand, tebrophen did not influence the growth of normal fibroblasts. According to the measured oxidative burst and estimated in silico parameters its direct antioxidative ability is limited. The obtained results indicate that tebrophen can be considered a promising lead molecule for generating more soluble derivatives with specific anticancer efficacy.

Inhibitory effect of antioxidant-rich marinades on the formation of heterocyclic aromatic amines in pan-fried beef

The inhibitory effect of antioxidant-rich marinades containing beer and white wine (with/without alcohol) alone or mixed with herbs commonly used as meat flavoring (garlic, ginger, thyme, rosemary, and red chili pepper) on the formation of heterocyclic aromatic amines (HAs) in pan-fried beef was studied. Radical-scavenging activity was evaluated by DPPH assay, before the addition of meat to the marinade (T0) and after 4 h of meat marinating (T4). At T0, wine with herbs possessed the highest scavenging activity (73.5%), followed by wine (72.5%), dealcoholized wine with herbs (53.4%), beer and herbs (41.7%), dealcoholized wine (39.6%), and beer (25.9%). At T4, a decrease in the radical-scavenging activity of all marinades was observed, although with a similar radical-scavenging profile. All of the six marinades under the study reduced the total amount of HAs, keeping meat with good overall sensory quality. Beer marinades were more efficient than white wine marinades, and the addition of herbs provided a superior inhibitory effect, reducing around 90% of HAs. No correlation was observed between radical-scavenging activity of marinades and total or individual HAs formation. Herbs explained around 30% of inhibition of PhIP formation, whereas alcohol increased PhIP formation.

On the peroxyl scavenging activity of hydroxycinnamic acid derivatives: mechanisms, kinetics, and importance of the acid-base equilibrium

The peroxyl radical scavenging activity of four hydroxycinnamic acid derivatives (HCAD) has been studied in non-polar and aqueous solutions, using the density functional theory. The studied HCAD are: ferulic acid (4-hydroxy-3-methoxycinnamic acid), p-coumaric acid (trans-4-hydroxycinnamic acid), caffeic acid (3,4-dihydroxycinnamic acid), and dihydrocaffeic acid (3-(3,4-dihydroxyphenyl)-2-propionic acid). It was found that the polarity of the environment plays an important role in the relative efficiency of these compounds as peroxyl scavengers. It was also found that in aqueous solution the pH is a key factor for the overall reactivity of HCAD towards peroxyl radicals, for their relative antioxidant capacity, and for the relative importance of the different mechanisms of reaction. The H transfer from the phenolic OH has been identified as the main mechanism of reaction in non-polar media and in aqueous solution at acid pHs. On the other hand, the single electron transfer mechanism from the phenoxide anion is proposed to be the one contributing the most to the overall peroxyl scavenging activity of HCAD in aqueous solution at physiological pH (7.4). This process is also predicted to be a key factor in the reactivity of these compounds towards a large variety of free radicals.

A THEORETICAL STUDY ON THE ANTIOXIDANT PROPERTY OF GALLIC ACID AND ITS DERIVATIVES

We analyze the antioxidant property of four phenolic compounds i.e., gallic acid, methyl gallate, ethyl gallate, and 4, 5-dihydroxy 3-methoxy benzoic acid through the determination of bond dissociation enthalpy (BDE), vertical ionization potential ( IP v) in gas phase as well as in six different solvent medium. The theoretical trends of these phenolic compounds, based on BDE and IP, is compared with the scavenging activity towards HOCl . In addition, we compute and analyze the values of the density-based reactivity descriptors such as chemical potential, hardness, electrophilicity, and local softness.

Antioxidant activity of trans-resveratrol toward hydroxyl and hydroperoxyl radicals: a quantum chemical and computational kinetics study

In this work, we have carried out a systematic study of the antioxidant activity of trans-resveratrol toward hydroxyl ((•)OH) and hydroperoxyl ((•)OOH) radicals in aqueous simulated media using density functional quantum chemistry and computational kinetics methods. All possible mechanisms have been considered: hydrogen atom transfer (HAT), proton-coupled electron transfer (PCET), sequential electron proton transfer (SEPT), and radical adduct formation (RAF). Rate constants have been calculated using conventional transition state theory in conjunction with the Collins-Kimball theory. Branching ratios for the different paths contributing to the overall reaction, at 298 K, are reported. For the global reactivity of trans-resveratrol toward (•)OH radicals, in water at physiological pH, the main mechanism of reaction is proposed to be the sequential electron proton transfer (SEPT). However, we show that trans-resveratrol always reacts with (•)OH radicals at a rate that is diffusion-controlled, independent of the reaction pathway. This explains why trans-resveratrol is an excellent but very unselective (•)OH radical scavenger that provides antioxidant protection to the cell. Reaction between trans-resveratrol and the hydroperoxyl radical occurs only by phenolic hydrogen abstraction. The total rate coefficient is predicted to be 1.42 × 10(5) M(-1) s(-1), which is much smaller than the ones for reactions of trans-resveratrol with (•)OH radicals, but still important. Since the (•)OOH half-life time is several orders larger than the one of the (•)OH radical, it should contribute significantly to trans-resveratrol oxidation in aqueous biological media. Thus, trans-resveratrol may act as an efficient (•)OOH, and also presumably (•)OOR, radical scavenger.

DPPH radical scavenging activity of tricin and its conjugates isolated from "Njavara" rice bran: a density functional theory study

Structural, electronic, and energetic characteristics of tricin, tricin-4'-O-(erythro-β-guaiacylglyceryl)ether (TEGE), and tricin-4'-O-(threo-β-guaiacylglyceryl)ether (TTGE), isolated from "Njavara" rice bran have been studied using DFT to explain their experimentally determined radical scavenging activity (EC(50) values) in comparison with known standards such as quercetin, myricetin, and catechin. Among the three mechanisms proposed for explaining the antioxidant activity, proton coupled-electron transfer (PC-ET), sequential proton loss electron transfer (SPLET), and electron transfer-proton transfer (ET-PT), our results support the second one. The O-H bond dissociation enthalpy (BDE) and the spin density on the oxygen with the radical character are excellent descriptors of radical scavenging activity. BDE (in kcal/mol) increased in the order myricetin (74.6) < quercetin (78.1) < catechin (78.3) < tricin (81.5) < TTGE (90.6) < TEGE (91.1), while the EC(50) increased exponentially with increase in BDE, 20.51, 42.98, 45.07, 90.39, 208.01, and 352.04 μg/mL for myricetin, quercetin, catechin, tricin, TTGE, and TEGE, respectively.

PM6 and DFT study of free radical scavenging activity of morin

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

Development of a phenol-enriched olive oil with both its own phenolic compounds and complementary phenols from thyme

Besides affecting the oil's sensorial characteristics, the presence of herbs and spices has an impact on the nutritional value of the flavored oils. The aim of the study was to develop a new product based on the phenol-enrichment of a virgin olive oil with both its own phenolic compounds (secoiridoid derivatives) plus additional complementary phenols from thyme (flavonoids). We studied the effect of the addition of phenolic extracts (olive cake and thyme) on phenolic composition, oxidative stability, antioxidant activity, and bitter sensory attribute of olive oils. Results showed that flavonoids from thyme appeared to have higher transference ratios (average 89.7%) from the phenolic extract to oil, whereas secoiridoids from olive presented lower transference ratios (average 35.3%). The bitter sensory attribute of the phenol-enriched oils diminished with an increase of the concentration of phenols from thyme, which might denote an improvement in the consumer acceptance.

Improving grape phenolic content and wine chromatic characteristics through the use of two different elicitors: methyl jasmonate versus benzothiadiazole

Benzothiadiazole (BTH) and methyl jasmonate (MeJ) have been described as exogenous elicitors of some plant defense compounds, polyphenols among them. The objective of this study was to determine whether the application of BTH or MeJ to grape clusters at the beginning of the ripening process had any effect on the accumulation of the main flavonoid compounds in grapes (anthocyanins, flavonols, and flavanols) and the technological significance of these treatments in the resulting wines. The results obtained after a 2 year experiment indicated that both treatments increased the anthocyanin, flavonol, and proanthocyanidin content of grapes. The wines obtained from the treated grapes showed higher color intensity and total phenolic content than the wines made from control grapes. The exogenous application of these elicitors, as a complement to fungicide treatments, could be an interesting strategy for vine protection, increasing, at the same time, the phenolic content of the grapes and the resulting wines.

Significance of dietary antioxidants for health

Since evidence became available that free radicals were involved in mechanisms for the development of major diseases, including cardiovascular disease and cancer, there has been considerable research into the properties of natural dietary antioxidants. However, it has become clear that dietary antioxidants can only have beneficial effects in vivo by radical scavenging or effects on redox potential if they are present in tissues or bodily fluids at sufficient concentrations. For many dietary components, absorption is limited or metabolism into derivatives reduces the antioxidant capacity. For many dietary phytochemicals, direct antioxidant effects may be less important for health than other effects including effects on cell signalling or gene expression in vivo.

Quantum-chemical investigation of the structure and the antioxidant properties of α-lipoic acid and its metabolites

Quantum-chemical computations were used to investigate the structure-antioxidant parameter relationships of α-lipoic acid and its natural metabolites bisnorlipoic acid and tetranorlipoic acid in their oxidized and reduced forms. The enantiomers of lipoic and dihydrolipoic acid were optimized using the B3LYP/6-311+G(3df,2p), B3LYP/aug-cc-pVDZ and MP2(full)/6-31+G(d,p) levels of theory as isolated molecules and in the presence of water. The geometries of the metabolites and the values of their antioxidant parameters (proton affinity, bond dissociation enthalpy, adiabatic ionization potential, spin density, and the highest occupied molecular orbital energy) were calculated at the B3LYP/6-311+G(3df,2p) level of theory. The results obtained reveal similarities between these structures: a pentatomic, nonaromatic ring is present in the oxidized forms, while an unbranched aliphatic chain (as found in saturated fatty acids) is present in both the oxidized and the reduced forms. Analysis of the spin density and the highest occupied molecular orbital energy revealed that the SH groups exhibited the greatest electron-donating activities. The values obtained for the proton affinity, bond dissociation enthalpy and adiabatic ionization potential indicate that the preferred antioxidant mechanisms for α-lipoic acid and its metabolites are sequential proton loss electron transfer in polar media and hydrogen atom transfer in vacuum.

Detailed Investigation of the OH Radical Quenching by Natural Antioxidant Caffeic Acid Studied by Quantum Mechanical Models

The effectiveness of naturally occurring antioxidant caffeic acid in the inactivation of the very damaging hydroxyl radical has been theoretically investigated by means of hybrid density functional theory. Three possible pathways by which caffeic acid may inactivate free radicals were analyzed: hydrogen abstraction from all available hydrogen atoms, hydroxyl radical addition to all carbon atoms in the molecule, and single electron transfer. The reaction paths were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory including the contribution of tunneling. The more reactive sites in caffeic acid are the C4OH phenolic group and the C4 carbon atom, for the hydrogen abstraction and radical addition, respectively. The single electron transfer process seems to be thermodynamically unfavored, in both polar and nonpolar media. Both hydrogen abstraction and radical addition are very feasible, with a slight preference for the latter, with a rate constant of 7.29 × 10(10) M(-1) s(-1) at 300 K. Tunnel effects are found to be quite unimportant in both cases. Results indicate caffeic acid as a potent natural antioxidant in trapping and scavenging hydroxyl radicals.

Phenolic profile and antioxidant activities of olive mill wastewater

Olive trees play an important role in the Moroccan agro-economy, providing both employment and export revenue. However, the olive oil industry generates large amounts of wastes and wastewaters. The disposal of these polluting by-products is a significant environmental problem that needs an adequate solution. On one hand, the phytotoxic and antimicrobial effects of olive mill wastewaters are mainly due to their phenolic content. The hydrophilic character of the polyphenols results in the major proportion of natural phenols being separated into the water phase during the olive processing. On other hand, the health benefits arising from a diet containing olive oil have been attributed to its richness in phenolic compounds that act as natural antioxidants and are thought to contribute to the prevention of heart diseases and cancers. Olive mill wastewater (OMW) samples have been analysed in terms of their phenolic constituents and antioxidant activities. The total phenolic content, flavonoids, flavanols, and proanthocyanidins were determined. The antioxidant and radical scavenging activity of phenolic extracts and microfiltred samples was evaluated using different tests (iron(II) chelating activity, total antioxidant capacity, DPPH assays and lipid peroxidation test). The obtained results reveal the considerable antioxidant capacity of the OMW, that can be considered as an inexpensive potential source of high added value powerful natural antioxidants comparable to some synthetic antioxidants commonly used in the food industry.

Antiradical properties of red wine portisins

The free radical chemistry of two polyphenolic pigments from red wine, belonging to the family of portisins, has been investigated after reaction with O(2)(•-) radicals using electron paramagnetic resonance (EPR) spectroscopy. Two portisins derived from malvidin-3-glucoside and cyanidin-3-glucoside were used for this study. Stable free radicals were detected with both portisins and correspond to unpaired electrons localized on the B as well as F rings of the portisins. Interpretations were confirmed by comparison with the spectra of free radicals formed by oxidation of the model compounds cyanidin-3-glucoside, malvidin-3-glucoside, and catechin. These results concur with previous work reporting the higher antiradical properties of these pigments compared to their anthocyanin precursors.

Encapsulation of natural polyphenolic compounds; a review

Natural polyphenols are valuable compounds possessing scavenging properties towards radical oxygen species, and complexing properties towards proteins. These abilities make polyphenols interesting for the treatment of various diseases like inflammation or cancer, but also for anti-ageing purposes in cosmetic formulations, or for nutraceutical applications. Unfortunately, these properties are also responsible for a lack in long-term stability, making these natural compounds very sensitive to light and heat. Moreover, polyphenols often present a poor biodisponibility mainly due to low water solubility. Lastly, many of these molecules possess a very astringent and bitter taste, which limits their use in food or in oral medications. To circumvent these drawbacks, delivery systems have been developed, and among them, encapsulation would appear to be a promising approach. Many encapsulation methods are described in the literature, among which some have been successfully applied to plant polyphenols. In this review, after a general presentation of the large chemical family of plant polyphenols and of their main chemical and biological properties, encapsulation processes applied to polyphenols are classified into physical, physico-chemical, chemical methods, and other connected stabilization methods. After a brief description of each encapsulation process, their applications to polyphenol encapsulation for pharmaceutical, food or cosmetological purposes are presented.