Unusually large numbers of electrons for the oxidation of polyphenolic antioxidants

Unraveling Multiple Pathways of Electron Donation from Phenolic Moieties in Natural Organic Matter

Natural organic matter (NOM) exhibits a distinctive electron-donating capacity (EDC) that serves a pivotal role in the redox reactions of contaminants and minerals through the transformation of electron-donating phenolic moieties. However, the ambiguity of the molecular transformation pathways (MTPs) that engender the EDC during NOM oxidation remains a significant issue. Here, MTPs that contribute to EDC were investigated by identifying the oxidized products of phenolic model compounds and NOM samples in direct or mediated electrochemical oxidation (DEO or MEO, respectively) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). It was found that the oxidation of newly formed phenolic-OH (ArOH) and the oxidative coupling reaction of the phenoxy radical are the main MTPs that directly contribute to EDC, in addition to the transformation of hydroquinones to quinones. Notably, the oxidative coupling reaction of ArOH contributed at least 22-42% to the EDC. Ferulic acid-like structures can also directly contribute to EDC by incorporating H2O into their acrylic substituents. Furthermore, the opening of C rings can indirectly attenuate the EDC through structural alterations in the electron-donating process of NOM. Decarboxylation can either weaken or enhance the EDC depending on the structure of the phenolic moieties in NOM. These findings suggest that the EDC of NOM is a comprehensive result of multiple NOM MTPs, involving not only ArOH oxidation but also the addition of H2O to olefinic bonds and bond-breaking reactions. Our work provides molecular evidence that aids in the comprehension of the multiple EDC-associated transformation pathways of NOM.

Dissolved Organic Matter Composition Determines Its Susceptibility to Complete and Partial Photooxidation within Lakes

Dissolved organic matter (DOM) plays an important role in carbon cycling within inland surface waters. Under sunlight irradiation, DOM undergoes complete photooxidation to produce carbon dioxide (CO2) and partial photooxidation that alters the molecular composition of DOM. However, a mechanistic understanding of the relationship between DOM composition and its susceptibility to partial and complete photooxidation in surface waters is currently lacking. This work combines light exposure experiments with high-resolution mass spectrometry to investigate DOM photooxidation using two DOM isolates and DOM from 16 lakes that vary in trophic status and size. High ratios of oxygen consumption to dissolved inorganic carbon (DIC) production demonstrate that all samples undergo extensive partial photooxidation. At the molecular level, more oxidized, aromatic DOM formulas are associated with oxygen consumption and DIC production. Bulk level measurements indicate that DOM becomes less aromatic and lower in apparent molecular weight following partial photooxidation, and there is molecular level evidence of oxygen addition and loss of CO2 in all samples. However, formulas most susceptible to photooxidation vary depending on the initial DOM composition. Collectively, this work provides insights into the relationship between DOM composition and photooxidation, which has important implications for carbon cycling in diverse surface waters.

Unexpected Value of Honey Color for Prediction of a Non-Enzymatic H2O2 Production and Honey Antibacterial Activity: A Perspective

Hydrogen peroxide is the principal antibacterial compound of honey and its concentration determines honey bacteriostatic (MIC) and bactericidal (MBC) potencies. Levels of H₂O₂ produced are highly relevant to honey therapeutic potential, but they vary extensively among honey with reasons not immediately apparent. According to a traditional view, H₂O₂ is produced as a by-product of glucose oxidation by the honey bee enzyme, glucose oxidase; however, significant levels of H₂O₂ could be produced in a non-enzymatic way via polyphenol autooxidation. The aim of this study was to evaluate the potential for such an alternative pathway by re-examining evidence from many experimental and correlative studies in order to identify factors and compounds required for pro-oxidant activity. Unexpectedly, the color intensity was found to be the main indicator separating honey varieties based on the quantitative differences in the polyphenolic content, antioxidant activity and the content of transition metals, Fe, Cu and Mn, the main factors required for pro-oxidant effects. The color-impeding polyphenolics and their oxidation products (semiquinones and quinones) further contributed to color development through multiple chemical conjugations with proteins, phenolic oxidative polymerization, chelation or the reduction of metal ions. Moreover, quinones, as an intrinsic part of polyphenol redox activity, play an active role in the formation of higher-order structures, melanoidins and colloids in honey. The latter structures are also known to chelate metal ions, potentially contributing to H₂O₂ production. Thus, the color intensity appears as a major parameter that integrates polyphenol-dependent pro-oxidant reactions resulting in H₂O₂ generation.

Barium hexaferrite nanoplatelets with polyphenol coatings for versatile applications as a stable, magnetic, and antimicrobial colloid

Colloidal stabilization of magnetic nanoparticles is one of the most important steps in the preparation of magnetic nanoparticles for potential biomedical applications. A special kind of magnetic nanoparticle are barium hexaferrite nanoplatelets (BSHF NPLs) with a hexagonal shape and a permanent magnetic moment. One strategy for the stabilization of BHF in aqueous media is to use coatings. In our research, we used an eco-friendly tannic acid, as a coating on BSHF NPLs. As-prepared BSHF NPLs coated with tannic acid were examined with transmission electron microscopy, infrared and UV-Vis spectroscopy, electro-kinetic measurements, and their room-temperature magnetic properties were measured. Stable colloids were tested in two biological complex media and antimicrobial properties of the material were examined. To enhance the antimicrobial properties of our material, we used tannic acid as a platform for the in-situ production of silver on BSHF NPLs. New hybrid material with silver also possesses magnetic properties and excellent antimicrobial activity against Escherichia coli and Staphylococcus aureus.

Intermolecular hydrogen bonds between catechin and theanine in tea: slow release of the antioxidant capacity by a synergetic effect

The health benefits of drinking tea stem from it being rich in polyphenols and other physiologically-active substances. Thus, exploring the synergistic effect between polyphenols and a variety of physiologically-active substances can contribute to our understanding of how tea benefits health. In this work, we have studied the interactions between catechin and theanine, exploring the synergetic antioxidant mechanism of the two molecules. Electrochemical characterization results showed that the oxidation peak current of catechin decreased gradually with the concentration of theanine, which is due to theanine spontaneously binding to catechin through intermolecular hydrogen bonds and forming molecular clusters via two hydrogen bonds. The binding constant is 4.75 at room temperature. The molecular clusters reduce the diffusion coefficient of catechin in solution, leading to the slow release of its antioxidant capacity (ability to effectively inhibit free radical oxidation reactions). Density functional theory calculations were also performed and verified the binding behavior. In identifying the synergistic effect between catechin and theanine on the antioxidant capacity of tea, this study adds to our understanding of the efficacy of tea polyphenols.

The health benefits of drinking tea stem from it being rich in polyphenols and other physiologically-active substances.

Availability and Metabolic Fate of Olive Phenolic Alcohols Hydroxytyrosol and Tyrosol in the Human GI Tract Simulated by the In Vitro GIDM–Colon Model

Hydroxytyrosol (HTyr) and tyrosol (Tyr) are the most well studied phenolic alcohols of olive oil and olive products demonstrating numerous and significant beneficial health effects. However, their activity in the human organism as food bioactives is strongly associated with their bioavailability and metabolism, while manifested through their metabolites. Nevertheless, there are limited studies investigating their biotransformation and mainly catabolism by gut microflora under a holistic interpretation close to the human organism. Thus, in the present study, the GastroIntestinal Dialysis (GIDM)-colon model, a continuous flow in vitro dialysis system mimicking physiological conditions during human gastrointestinal digestion, was used to explore the metabolism of HTyr and Tyr as pure compounds. The GIDM–colon model simulates absorption from the lumen to the mucosa, followed by the colon phase using pooled human fecal suspensions. Samples were collected at different time points and analyzed via LC–Orbitrap MS. An integrated approach combining Multivariate Data Analysis (MVA) and thorough dereplication procedures led to the identification of HTyr and Tyr metabolites in different phases (gastric, small intestine, and colon), yielding also valuable information about metabolites kinetics. To our knowledge, this is the first study reporting full spectrometric data of HTyr and Tyr metabolites along with possible transformation mechanisms in the GI tract.

Oxidant-reactive carbonous moieties in dissolved organic matter: Selective quantification by oxidative titration using chlorine dioxide and ozone

The application of oxidants for disinfection or micropollutant abatement during drinking water and wastewater treatment is accompanied by oxidation of matrix components such as dissolved organic matter (DOM). To improve predictions of the efficiency of oxidation processes and the formation of oxidation products, methods to determine concentrations of oxidant-reactive phenolic, olefinic or amine-type DOM moieties are critical. Here, a novel selective oxidative titration approach is presented, which is based on reaction kinetics of oxidation reactions towards certain DOM moieties. Phenolic moieties were determined by oxidative titration with ClO₂ and O₃ for five DOM isolates and two secondary wastewater effluent samples. The determined concentrations of phenolic moieties correlated with the electron-donating capacity (EDC) and the formation of inorganic ClO₂-byproducts (HOCl, ClO₂^-, ClO₃^-). ClO₂-byproduct yields from phenol and DOM isolates and changes due to the application of molecular tagging for phenols revealed a better understanding of oxidant-reactive structures within DOM. Overall, oxidative titrations with ClO₂ and O₃ provide a novel and promising tool to quantify oxidant-reactive moieties in complex mixtures such as DOM and can be expanded to other matrices or oxidants.

Gallic Acid Based Black Tea Extract as a Stabilizing Agent in ZnO Particles Green Synthesis

In this work, zinc oxide particles (ZnO NPs) green synthesis with the application of black tea extract (BT) is presented. A thorough investigation of the properties of the extract and the obtained materials was conducted by using Fourier transform infrared spectroscopy (FTIR), liquid chromatography-mass spectrometry (LC-MS), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and quadrupole mass spectroscopy (QMS). The obtained results indicated that the amount of used BT strongly influenced the morphology, chemical, and crystalline structure of the obtained particles. The investigation demonstrated that the substance present in black tea (BT) extract, which was adsorbed on the ZnO surface, was in fact gallic acid. It was found that gallic acid controls the crystallization process of ZnO by temporarily blocking the zinc cations. Additionally, these organic molecules interact with the hydroxide group of the precipitant. This blocks the dehydration process stabilizing the zinc hydroxide forms and hinders its transformation into zinc oxide. Performed measurements indicated that obtained ZnO particles have great antioxidant and antimicrobial properties, which are significantly correlated with ZnO-gallic acid interactions.

Phenolic contents, antioxidant potential and associated colour in sweet sorghum syrups compared to other commercial syrup sweeteners

BACKGROUND

Knowledge of the bioactive content of sweet sorghum syrups compared to other common food-grade syrups will expand their utilisation as a food source. Total phenolic content (TPC), phenolics evaluated by high-performance liquid chromatography, antioxidant 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activities and oxygen radical absorbance capacity (ORAC), as well as colour of high-fructose corn syrup (HFCS), corn, honey, maple, agave, rice and grain sorghum syrups, were compared to 10 commercial sweet sorghum syrups.

RESULTS

Sweet sorghum syrups contained markedly higher (P ≤ 0.05) TPC (6471 ± 1823 mg L^-1 ) compared to the other syrups (596 ± 497 mg L^-1 ). HFCS, corn, white grain sorghum and rice syrups had negligible and low DPPH radical scavenging activities and ORAC values, respectively. DPPH activities, ORAC and colour values of the sweet sorghum syrups were also markedly (P ≤ 0.05) higher than other syrups and highly related. The predominant phenolic components identified in sweet sorghum syrups were phenolic acids. Ellagic acid and protocatechuic acid were found in sorghum syrups ranging in concentration from 335-1177 and 53-485 μg g^-1 , respectively. Sinapic acid was detected in several sorghum syrups, ranging in concentrations between 21 and 3654 μg g^-1 .

CONCLUSION

HFCS, corn, white grain sorghum and rice syrups demonstrated low bioactivity with negligible and low DPPH activities and ORAC values, respectively. The TPC, DPPH, ORAC and colour values of the sweet sorghum syrups were related to each other and markedly (P ≤ 0.05) higher compared to the other syrups. Phenolic acids were the predominant phenolic compounds identified in sorghum syrups and represent potential for health benefits. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Towards using high-performance liquid chromatography at home

In order to make high-performance liquid chromatography (HPLC) more widely available at home and in small-scale settings, we have simplified two of its most costly modules, namely the pump and the detector. This should make the setup affordable for home or small laboratory use. A manual HPLC pump was constructed so as to fit into a caulk gun from a local hardware store enabling the generation of 100-150 bar of pressure. In order to limit the pressure drop during the running of a chromatogram, a pulse dampener was developed. We further modified the electrochemical detection (ECD) system so as to use a cheap boron-doped diamond electrode with an overlay of thin filter paper, causing an eluent flow over the electrode by wicking and gravity. Both the pump and the detector are at least ten times cheaper than conventional HPLC modules. Using a home-packed Jupiter^Ⓡ Proteo reversed phase capillary column we show how this low-cost HPLC system generates well resolving chromatograms after direct injection of fresh urine. The ECD did not lose its sensitivity during regular use over more than half a year. For homovanillic acid (HVA), which is of medical interest, we measured a linear dynamic range of two orders of magnitude, a detection limit of HVA in the injected sample of 3 μM and a coefficient of variation <10%. The contribution to peak broadening by the detector was much smaller than the contributions by the injector and by the column. After consumption of table olives containing hydroxytyrosol (HT), its metabolite HVA in the corresponding urine could be measured quantitatively. An approach to quantify HT in table olives is presented, as well. This method provides a new tool for investigating physiology of oneself or of dear ones at home.

Influence of Olive Pomace Blending on Antioxidant Activity: Additive, Synergistic, and Antagonistic Effects

Food innovation is moving rapidly and comprises new categories of food products and/or ingredients with a natural and ecological origin. Monocultivar olive pomaces, individually or combined, can be a source of natural bioactive compounds suitable for food or cosmetic applications. This work aimed to assess the phenolics content and antioxidant activity of four monocultivar olive pomaces (Arbosana, Koroneiki, Oliana, and Arbequina) and forty-nine blends prepared with different proportions of each. Additive, synergistic, and antagonistic effects were studied. Among the monocultivar pomaces, Koroneiki and Arbosana were the richest in total phenolics (~15 mg gallic acid eq./g). Most of the interactions found in the blends were additive or synergistic, while very few antagonistic effects were observed. The best results were obtained for those blends where the Koroneiki variety predominated: (i) 90% Koroneiki, 4.75% Oliana, 3.75% Arbequina, 1.5% Arbosana; (ii) 65% Koroneiki, 29% Oliana, 3.25% Arbequina, 2.75% Arbosana; and (iii) 85% Koroneiki, 8.75% Arbequina, 3.5% Arbosana, 2.75% Oliana. In sum, these combinations can be advantageous in comparison to the individual use of monocultivar pomaces, presenting a higher potential to be used as functional ingredients or for bioactive compounds extraction, having in view the obtention of natural preservatives or food/cosmetic formula enhancers.

Evaluation of Reducing Ability and Antioxidant Activity of Fruit Pomace Extracts by Spectrophotometric and Electrochemical Methods

The component profiles of apricot, grape, and black currant pomace extracts have been analyzed using HPLC coupled to diode-array detection and tandem mass spectrometry (HPLC-DAD-MS). The predominant components in grape, apricot, and black currant pomace extracts were phenolic acids and flavonols. The redox behavior of apricot, black currant, and grape pomace water extracts was evaluated by means of cyclic voltammetry. Also, individual substances mainly present in the extracts were analyzed. The results of electrochemical testing were compared to traditional chemical techniques of potassium ferricyanide reduction (FRAP) and phosphomolybdenum assay, and fair agreement was established. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging assays were applied in order to estimate antioxidant activity. The reducing power of the grape extract was found to be higher than that of the apricot and black currant extracts in both potassium ferricyanide reduction (FRAP) and phosphomolybdenum methods.

Quantification of the electron donating capacity and UV absorbance of dissolved organic matter during ozonation of secondary wastewater effluent by an assay and an automated analyzer

Ozonation of secondary wastewater treatment plant effluent for the abatement of organic micropollutants requires an accurate process control, which can be based on monitoring ozone-induced changes in dissolved organic matter (DOM). This study presents a novel automated analytical system for monitoring changes in the electron donating capacity (EDC) and UV absorbance of DOM during ozonation. In a first step, a quantitative photometric EDC assay was developed based on electron-transfer reactions from phenolic moieties in DOM to an added chemical oxidant, the radical cation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS^·+). The assay is highly sensitive (limit of quantification ∼0.5 mg_DOC·L^-1) and EDC values of model DOM isolates determined by this assay were in good agreement with values determined previously by mediated electrochemical oxidation (slope = 1.01 ± 0.07, R² = 0.98). In a second step, the photometric EDC measurement method was transferred onto an automated fluidic system coupled to a photometer (EDC analyzer). The EDC analyzer was then used to monitor changes in EDC and UV absorbance of secondary wastewater effluent treated with ozone. While both parameters exhibited a dose-dependent decrease, a more pronounced decrease in EDC as compared to UV absorbance was observed at specific ozone doses up to 0.4 mg_O3·g_DOC^-1. The concentration of 17α-ethinylestradiol, a phenolic micropollutant with a high ozone reactivity, decreased proportionally to the EDC decrease. In contrast, abatement of less ozone-reactive micropollutants and bromate formation started only after a pronounced initial decrease in EDC. The on-line EDC analyzer presented herein will enable a comprehensive assessment of the combination of EDC and UV absorbance as control parameters for full-scale ozonation.

Reactivity, fate and functional roles of dissolved organic matter in anoxic inland waters

The transit of organic matter (OM) through the aquatic compartment of its global cycle has been intensively studied, traditionally with a focus on the processing and degradation of its dissolved fraction (dissolved organic matter, DOM). Because this is so intimately related to oxidation, the notion tenaciously persists that where oxygen is absent, DOM turnover is markedly slowed. In this Opinion Piece, we outline how diverse processes shape, transform and degrade DOM also in anoxic aquatic environments, and we focus here on inland waters as a particular case study. A suite of biogeochemical DOM functions that have received comparatively little attention may only be expressed in anoxic conditions and may result in enhanced biogeochemical roles of these deoxygenated habitats on a network scale.

ABTS/PP Decolorization Assay of Antioxidant Capacity Reaction Pathways

The 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•+) radical cation-based assays are among the most abundant antioxidant capacity assays, together with the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-based assays according to the Scopus citation rates. The main objective of this review was to elucidate the reaction pathways that underlie the ABTS/potassium persulfate decolorization assay of antioxidant capacity. Comparative analysis of the literature data showed that there are two principal reaction pathways. Some antioxidants, at least of phenolic nature, can form coupling adducts with ABTS^•+, whereas others can undergo oxidation without coupling, thus the coupling is a specific reaction for certain antioxidants. These coupling adducts can undergo further oxidative degradation, leading to hydrazindyilidene-like and/or imine-like adducts with 3-ethyl-2-oxo-1,3-benzothiazoline-6-sulfonate and 3-ethyl-2-imino-1,3-benzothiazoline-6-sulfonate as marker compounds, respectively. The extent to which the coupling reaction contributes to the total antioxidant capacity, as well as the specificity and relevance of oxidation products, requires further in-depth elucidation. Undoubtedly, there are questions as to the overall application of this assay and this review adds to them, as specific reactions such as coupling might bias a comparison between antioxidants. Nevertheless, ABTS-based assays can still be recommended with certain reservations, particularly for tracking changes in the same antioxidant system during storage and processing.

Potentiometric and UV-Vis spectrophotometric titrations for evaluation of the antioxidant capacity of chicoric acid

Potentiometric/spectrophotometric titrations have revealed the antioxidation behavior and mechanisms of chicoric acid (ChA, versus trolox) against ABTS˙⁺ radical cations.

Effects of Sorption on Redox Properties of Natural Organic Matter

Natural organic matter (NOM) is an important redox-active component of natural porous media and predominantly occurs in the sorbed state. Nevertheless, the effects of NOM sorption at minerals on its redox properties are unknown and thus are the major objective of this study. We report how adsorption of three different humic acids (HAs) to redox-inert sorbents (polar Al₂O₃ and nonpolar DAX-8 resin) affects their electron-exchange capacities (EEC) and redox states. The electron-donating capacity of HAs sorbed at Al₂O₃ increased by up to 200%, whereas the EEC of the remaining dissolved HA fractions decreased compared with their initial properties. Sorption at DAX-8, however, did not affect significantly the EEC of HAs. We rationalize these results by (i) preferential sorption of NOM components rich in redox-active groups (e.g., quinone, polyphenols) and (ii) surface-catalyzed polymerization of polyphenolic compounds. Our results demonstrate that even in the absence of electron exchange with the sorbent, adsorption to polar mineral surfaces considerably affects the redox properties of NOM. Quantification of the redox state and EEC of adsorbed NOM is thus crucial for assessing electron-transfer processes as well as organic carbon stabilization and sequestration in soils and sediments.

The Effect of Deprotonation on the Reaction Kinetics of an Oxygen Scavenger Based on Gallic Acid

Oxygen scavengers based on gallic acid (GA) usually contain a base that establishes the alkaline conditions necessary for the humidity-induced scavenger reaction. Here we measured the effect of sodium carbonate (Na₂CO₃) and sodium hydroxide (NaOH) on the reaction kinetics of such scavengers. The time-dependent oxygen absorption of aqueous GA solutions (pH 2.9–13.8) was determined and the results were described using a second-order kinetic model. We calculated the degree of deprotonation (DoD) of GA in the solutions from the species distribution curves, and assessed the effect of the DoD on the reaction kinetics. This revealed that both the reaction rate coefficient k and the scavenger capacity n were significantly affected by the DoD. If the DoD fell below 0.25, there was no significant reaction. Although k increased with the DoD, n reached a maximum at DoD = 0.6–0.7. In principle, target DoD values can be achieved using any base, but a strong base is more efficient because lower quantities are required. In our experiments, the amount of Na₂CO₃ required to reach the maximum DoD was more than twice that of NaOH. Our results provide the basis for the functional design of active packaging systems incorporating optimized GA-based oxygen scavengers.

What is responsible for antioxidant properties of polyphenolic compounds from plants?

Due to the negative impact of reactive species (including free radicals) on humans and animals, the investigations to find effective substances (antioxidants), which protect living organisms against their damaging influence are carried out throughout the world. As most widespread synthetic antioxidants are suspected of having a noxious effect on the human body, more and more attention is paid to natural antioxidant compounds found in plants (especially phenolic compounds). The aim of this paper is to present the data about antioxidant activity of polyphenolic compounds with the emphasis on the main factors having influence on their antioxidant activity: chemical structure, ability to form hydrogen bonds, capability of metal ions chelation and reduction, adduct formation, kinetic solvents effect, mechanism of antioxidant reaction, capability of antioxidant enzyme activation and reduction potential.

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

The complex and indeterminant composition of NOM makes characterization of its redox properties challenging. Approaches that have been taken to address this challenge include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. In this study, we revisit the use of direct voltammetric methods in aprotic solvents by applying an expanded and refined suite of methods to a large set of NOM samples and model compounds (54 NOM samples from 10 different sources, 7 NOM model compounds, and 2 fresh extracts of plant materials that are high in redox-active quinonoid model compounds dissolved in DMSO). Refinements in the methods of fitting the data obtained by staircase cyclic voltammetry (SCV) provided improved definition of peaks, and square wave voltammetry (SWV), performed under the same conditions as SCV, provided even more reliable identification and quantitation of peaks. Further evidence is provided that DMSO improves the electrode response by unfolding some of the tertiary structure of NOM polymers, thereby allowing greater contact between redox active functional groups and the electrode surface. We averaged experimental peak potentials for all NOM compounds and calculated potentials in water. Average values for Epa1, Epc1, and Ep1 in DMSO were -0.866 ± 0.069, -1.35 ± 0.071, and -0.831 ± 0.051 V vs. Ag/Ag+, and -0.128, -0.613, and -0.0930 V vs. SHE in water. In addition to peak potentials, the breadth of SCV peaks was quantified as a way to characterize the degree to which the redox activity of NOM is due to a continuum of contributing functional groups. The average breadth values were 1.63 ± 0.24, 1.28 ± 0.34, and 0.648 ± 0.15 V for Epa1, Epc1, and Ep1 respectively. Comparative analysis of the overall dataset-from SCV and SWV on all NOMs and model compounds-revealed that NOM redox properties vary over a narrower range than expected based on model compound properties. This lack of diversity in redox properties of NOM is similar to conclusions from other recent work on the molecular structure of NOM, all of which could be the result of selectivity in the common extraction methods used to obtain the materials.

Voltammetric, spectroscopic, and cellular characterization of redox functionality of eckol and phlorofucofuroeckol-A: A comparative study

Interest in phlorotannins has increased in recent years largely due to antioxidant capacity, however, the redox mechanism of phlorotannins is still obscure. In the present study, the electrochemical oxidation mechanisms of eckol (EL) and phlorofucofuroeckol-A (PFF-A), two representative phlorotannin compounds, were comparatively analyzed in a wide pH range using cyclic and differential pulse voltammetry as well as spectroscopic assay. The voltammetric study revealed that EL and PFF-A were successively oxidized in three pH-dependent steps. Moreover, it was found that the PFF-A presented a stronger proton and electron transferring activity as compared to EL since PFF-A exhibited lower acid-base dissociation constant (pKa ) value and higher heterogeneous rate constant (kbh ) value in the first oxidation step. These property were further evidenced by comparison of direct antioxidant activity (i.e., superoxide anion and peroxide radicals) as well as indirect antioxidant activity (i.e., mRNA expression of two phase II enzymes) between EL and PFF-A. PRACTICAL APPLICATIONS: Phlorotannins from edible algae have been regarded as novel antioxidants those presented high application potential in food industry. Even though antioxidant activity of phlorotannin compounds have been widely investigated in both in vitro and in vivo studies, very few reports focused on electron transferring functionality which is chemical basis for antioxidant process. Herein, the oxidative mechanisms of two representative phlorotannins were comparatively analyzed using multiple electrochemical methods. This is hopefully to give information on the chemical meaning behind the antioxidant activity of dietary phlorotannins.

Electron-Donating Phenolic and Electron-Accepting Quinone Moieties in Peat Dissolved Organic Matter: Quantities and Redox Transformations in the Context of Peat Biogeochemistry

Electron-donating phenolic and electron-accepting quinone moieties in peat dissolved organic matter (DOM) are considered to play key roles in processes defining carbon cycling in northern peatlands. This work advances a flow-injection analysis system coupled to chronoamperometric detection to allow for the simultaneous and highly sensitive determination of these moieties in dilute DOM samples. Analysis of anoxic pore water and oxic pool water samples collected across an ombrotrophic bog in Sweden demonstrated the presence of both phenolic and quinone moieties in peat DOM. The pore water DOM had higher quantities of phenolic but not quinone moieties compared with commonly used model aquatic and terrestrial DOM isolates. Significantly lower phenol content in DOM from oxic pools than DOM from anoxic pore waters indicated oxidative DOM processing in the pools. Consistently, treatment of peat DOM with laccase, a phenol-oxidase, under oxic conditions resulted in an irreversible removal of phenols and reversible oxidation of hydroquinones to quinones. Electron transfer to peat DOM was fully reversible over an electrochemical reduction and subsequent O₂-reoxidation cycle, supporting that quinones in peat DOM serve as regenerable microbial electron acceptors in peatlands. The results advance our understanding of redox processes involving phenolic and quinone DOM moieties and their roles in northern peatland carbon cycling.

Comparative analysis of oxidative mechanisms of phloroglucinol and dieckol by electrochemical, spectroscopic, cellular and computational methods

Numerous studies have been carried out on the redox activities of phenolic compounds from terrestrial plants, however, the redox pathway of phlorotannins, a type of marine algae-derived polyphenol, is far from clear. In the present study, the redox mechanisms of two phlorotannins, phloroglucinol (PL) and dieckol (DL), were comparatively scrutinized. Differential pulse voltammetry was conducted in the pH range 2.0-12.0 to determine the acid-base dissociation constant (pK a) and the number of electrons and protons involved in the redox reactions of two phlorotannins. Cyclic voltammetry was applied to obtain the heterogeneous electron transfer rate constant (k 0). By means of computational calculation, UV-vis spectroscopy, and electrochemical analysis, it is proposed that PL oxidation in the whole pH range undergoes two steps which are dominated by proton-coupled electron transfer (PCET) (pH ≤ 9) and sequential proton-loss electron transfer (SPLET) mechanisms (pH > 9), respectively. In contrast, the multiple steps taking place in the DL oxidation process rely on PCET (pH ≤ 5), mixed SPLET/PCET (5 < pH ≤ 10), and electron transfer (pH > 10) mechanisms, respectively. Also, the lower proton affinity and ionization potential values of DL, which are attributed to its conjugated Cπ-O-Cπ moieties, lead to relatively higher redox activity as compared to PL in various chemical and cellular models. These findings may provide useful insights into the oxidative conversion of phlorotannins in their biological and chemical processes.

A mystery of a cup of coffee; an insight look by chemist

Fruits, vegetables as well as processed food products of plant origin are a rich source of beneficial for human health constituents. Among them the polyphenols constitute a large group of compounds. The presented literature survey is devoted to chlorogenic acid the most abundant representative of cinnamate acids esters. Its chemical as well as biological properties are described. © 2017 BioFactors, 43(5):621-632, 2017.

Chemical composition and antioxidant-prooxidant potential of a polyphenolic extract and a proanthocyanidin-rich fraction of apple skin

The apple is a food rich in diverse classes of polyphenols (PP), among which the proanthocyanidins (PCs), which are primarily concentrated in the skin, are one of the most abundant. These compounds are of considerable interest for their possible positive health effects because of their antioxidant properties. However, depending on the classes of PP present (chemical composition) and their relative concentrations in the apple skin, their antioxidant effects vary and some of their components can even generate prooxidant effects. This work determined the chemical composition and antioxidant-prooxidant potential of a polyphenolic extract (PPE) and a proanthocyanidin-rich fraction (PRF) of apple skin, along with the contribution of their most abundant individual compounds, based on their copper chelating ability, ease in reducing peroxidase-generated free radicals and TEAC (Trolox-Equivalent Antioxidant Capacity) assay. For this purpose, chromatographic and colorimetric methods were used. The majority compounds identified in PPE were flavan-3-ols (44.58%), flavonols (42.89%) and dihydrochalcones (11.60%). In PRF, we detected monomers and oligomers from dimers to heptamers, which were composed of 97% (−)-epicatechin and 3% (+)-catechin. The antioxidant potential was notably higher in PRF than in PPE. The (−)-epicatechin monomer and the procyanidin B2 dimer showed more ease in reducing peroxidase-generated free radicals compared to other compounds of the apple skin, whereas phloridzin dihydrochalcone produced prooxidant effects.

Antioxidant Activity/Capacity Measurement. 1. Classification, Physicochemical Principles, Mechanisms, and Electron Transfer (ET)-Based Assays

Because there is no widely adopted "total antioxidant parameter" as a nutritional index for labeling food and biological fluids, it is desirable to establish and standardize methods that can measure the total antioxidant capacity (TAC) level directly from plant-based food extracts and biological fluids. In this review, we (i) present and classify the widely used analytical approaches (e.g., in vitro and in vivo, enzymatic and nonenzymatic, electron transfer (ET)- and hydrogen atom transfer (HAT)-based, direct and indirect assays) for evaluating antioxidant capacity/activity; (ii) discuss total antioxidant capacity/activity assays in terms of chemical kinetics and thermodynamics, reaction mechanisms, and analytical performance characteristics, together with advantages and drawbacks; and (iii) critically evaluate ET-based methods for analytical, food chemical, biomedical/clinical, and environmental scientific communities so that they can effectively use these assays in the correct places to meet their needs.

pH dependent antioxidant activity of lettuce (L. sativa) and synergism with added phenolic antioxidants

Influence of pH on the antioxidant activities of combinations of lettuce extract (LE) with quercetin (QC), green tea extract (GTE) or grape seed extract (GSE) was investigated for both reduction of Fremy's salt in aqueous solution using direct electron spin resonance (ESR) spectroscopy and in L-α-phosphatidylcholine liposome peroxidation assay measured following formation of conjugated dienes. All examined phenolic antioxidants showed increasing radical scavenging effect with increasing pH values by using both methods. QC, GTE and GSE acted synergistically in combination with LE against oxidation of peroxidating liposomes and with QC showing the largest effect. The pH dependent increase of the antioxidant activity of the phenols is due to an increase of their electron-donating ability upon deprotonation and to their stabilization in alkaline solutions leading to polymerization reaction. Such polymerization reactions of polyphenolic antioxidants can form new oxidizable -OH moieties in their polymeric products resulting in a higher radical scavenging activity.

Quantitative comparison of caffeoylquinic acids and flavonoids in Chrysanthemum morifolium flowers and their sulfur-fumigated products by three-channel liquid chromatography with electrochemical detection

For the determination of seven caffeoylquinic acids [neochlorogenic acid (NcA), cryptochlorogenic acid (CcA), chlorogenic acid (CA), caffeic acid (CfA), isochlorogenic acid A (Ic A), isochlorogenic acid B (Ic B), isochlorogenic acid C (Ic C)] and two flavonoids [luteolin 7-O-glucoside (LtG) and luteolin (Lt)], a three-channel liquid chromatography with electrochemical detection (LC-3ECD) method was established. Chromatographic peak heights were proportional to each concentration, ranging from 2.5 to 100 ng/mL for NcA, CA, CcA, and CfA, and ranging from 2.5 to 250 ng/mL for LtG, Ic B, Ic A, Ic C, and Lt, respectively. The present LC-3ECD method was applied to the quantitative analysis of caffeoylquinic acids and flavonoids in four cultivars of Chrysanthemum morifolium flowers and their sulfur-fumigated products. It was found that 60% of LtG and more than 47% of caffeoylquinic acids were lost during the sulfur fumigation processing. Sulfur fumigation showed a destructive effect on the C. morifolium flowers. In addition, principle component analyses (PCA) were performed using the results of the quantitative analysis of caffeoylquinic acids and flavonoids to compare the "sameness" and "differences" of these analytes in C. morifolium flowers and the sulfur-fumigated products. PCA score plots showed that the four cultivars of C. morifolium flowers were clearly classified into four groups, and that significant differences were also found between the non-fumigated C. morifolium flowers and the sulfur-fumigated products. Therefore, it was demonstrated that the present LC-3ECD method coupled with PCA is applicable to the variation analysis of different C. morifolium flower samples.

Detoxification Processes from Vanadate at the Root Apoplasm Activated by Caffeic and Polygalacturonic Acids

In the root apoplasm, V(V) and V(IV) toxicity can be alleviated through redox and complexation reactions involving phenolic substances and the polyuronic components. In such context we report the role of polygalacturonic acid (PGA) on the reducing activity of caffeic acid (CAF) towards V(V). The redox reaction was particularly effective at pH 2.8 leading to the formation of oxidation products with redox activity towards V(V). An o-quinone was identified as the first product of the reaction which is further involved in the formation of CAF dimers. At pH ≥ 3.6 the redox activity decreased and a yield in V(IV) equal to 38, 31, 21 and 14% was found at pH 3.6, 4.0. 5.0 and 6.0 respectively compared with that obtained at pH 2.8. The redox reaction was faster in the presence of PGA and a higher yield of V(IV) was found in the 4.0-6.0 pH range with respect to the CAF-V(V) binary system. The higher efficiency of the redox reaction in the presence of PGA was related with the ability of PGA to bind V(IV). The biological significance of the redox reaction between CAF and V(V), as well as the role of PGA in such reaction, was established "in vivo" using triticale plants. Results showed that PGA reduced significantly the phytotoxic effects of the V(V)-CAF system.

Study of coumarin-resveratrol hybrids as potent antioxidant compounds

In the present work we synthesized a selected series of hydroxylated 3-phenylcoumarins 5–8, with the aim of evaluating in detail their antioxidant properties. From an in depth study of the antioxidant capacity data (ORAC-FL, ESR, CV and ROS inhibition) it was concluded that these derivatives are very good antioxidants, with very interesting profiles in all the performed assays. The study of the effect of the number and position of the hydroxyl groups on the antioxidant activity was the principal aim of this study. In particular, 7-hydroxy-3-(3'-hydroxy)phenylcoumarin (8) proved to be the most active and effective antioxidant of the selected series in four of the performed assays (ORAC-FL = 11.8, capacity of scavenging hydroxyl radicals = 54%, Trolox index = 2.33 and AI₃₀ index = 0.18). However, the presence of two hydroxyl groups on this molecule did not increase greatly the activity profile. Theoretical evaluation of ADME properties of all the derivatives was also carried out. All the compounds can act as potential candidates for preventing or minimizing the free radical overproduction in oxidative-stress related diseases. These preliminary findings encourage us to perform a future structural optimization of this family of compounds.

Antioxidant Generation during Coffee Roasting: A Comparison and Interpretation from Three Complementary Assays

Coffee is a major source of dietary antioxidants; some are present in the green bean, whereas others are generated during roasting. However, there is no single accepted analytical method for their routine determination. This paper describes the adaption of three complementary assays (Folin-Ciocalteu (FC), ABTS and ORAC) for the routine assessment of antioxidant capacity of beverages, their validation, and use for determining the antioxidant capacities of extracts from coffee beans at different stages in the roasting process. All assays showed a progressive increase in antioxidant capacity during roasting to a light roast state, consistent with the production of melanoidins having a higher antioxidant effect than the degradation of CGAs. However, the three assays gave different numbers for the total antioxidant capacity of green beans relative to gallic acid (GA), although the range of values was much smaller when chlorogenic acid (CGA) was used as reference. Therefore, although all three assays indicated that there was an increase in antioxidant activity during coffee roasting, and the large differences in responses to GA and CGA illustrate their different sensitivities to different types of antioxidant molecule.

Urolithins display both antioxidant and pro-oxidant activities depending on assay system and conditions

The biological effects of polyphenolic ellagitannins are mediated by their intestinal metabolites, urolithins. This study investigated redox properties of urolithins A and B using ORAC assay, three cell-based assays, copper-initiated pro-oxidant activity (CIPA) assay, and cyclic voltammetry. Urolithins were strong antioxidants in the ORAC assay, but mostly pro-oxidants in cell-based assays, although urolithin A was an antioxidant in cell culture medium. Parent compound ellagic acid was a strong extracellular antioxidant, but showed no response in the intracellular assay. The CIPA assay confirmed the pro-oxidant activity of ellagitannin metabolites. In the cell proliferation assay, urolithins but not ellagic acid decreased growth and metabolism of HepG2 liver cells. In cyclic voltammetry, the oxidation of urolithin A was partly reversible, but that of urolithin B was irreversible. These results illustrate how strongly measured redox properties depend on the employed assay system and conditions and emphasize the importance of studying pro-oxidant and antioxidant activities in parallel.

Defensive strategies in Geranium sylvaticum, Part 2: Roles of water-soluble tannins, flavonoids and phenolic acids against natural enemies

Geranium sylvaticum is a common herbaceous plant in Fennoscandia, which has a unique phenolic composition. Ellagitannins, proanthocyanidins, galloylglucoses, gallotannins, galloyl quinic acids and flavonoids possess variable distribution in its different organs. These phenolic compounds are thought to have an important role in plant-herbivore interactions. The aim of this study was to quantify these different water-soluble phenolic compounds and measure the biological activity of the eight organs of G. sylvaticum. Compounds were characterized and quantified using HPLC-DAD/MS, in addition, total proanthocyanidins were determined by BuOH-HCl assay and total phenolics by the Folin-Ciocalteau method. Two in vitro biological activity measurements were used: the prooxidant activity was measured by the browning assay and antioxidant activity by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. Organ extracts were fractionated using column chromatography on Sephadex LH-20 and the activities of fractions was similarly measured to evaluate which polyphenol groups contributed the most to the biological activity of each organ. The data on the activity of fractions were examined by multivariate data analysis. The water-soluble extracts of leaves and pistils, which contained over 30% of the dry weight as ellagitannins, showed the highest pro-oxidant activity among the organ extracts. Fraction analysis revealed that flavonoids and galloyl quinic acids also exhibited high pro-oxidant activity. In contrast, the most antioxidant active organ extracts were those of the main roots and hairy roots that contained high amounts of proanthocyanidins in addition to ellagitannins. Analysis of the fractions showed that especially ellagitannins and galloyl quinic acids have high antioxidant activity. We conclude that G. sylvaticum allocates a significant amount of tannins in those plant parts that are important to the fitness of the plant and susceptible to natural enemies, i.e. pistil and leaf tannins protect against insect herbivores and root tannins against soil pathogens.

Reducing power: the measure of antioxidant activities of reductant compounds?

Electrochemical assay has been employed recently to study the activity of antioxidants; however, there is controversy as to whether reducing power fully characterizes the antioxidant activity. This study provides some essential further evidence on this point based on the reported data and mechanisms underlying the antioxidant functions as well as the anodic oxidation of phenolic antioxidants, indicating that further consideration and investigation should be made before reducing power is used as the absolute measure of antioxidant activity.

Effects of molecular structure on kinetics and dynamics of the trolox equivalent antioxidant capacity assay with ABTS(+•)

Reaction kinetics in the Trolox equivalent antioxidant capacity (TEAC) assay between ABTS(+•) [2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) cation radical] and compounds with different structure, molecular weight, number of OH groups, and redox potential were investigated by recording loss of ABTS(+•) absorbance (734 nm) continuously over time. Curves showed six distinguishable kinetic patterns, including both immediate and extended reaction components. Radical quenching rates in the immediate component most relevant to reactions in foods and tissues depended on phenol structure and steric accessibility to the hindered radical, while reaction stoichiometry correlated with the number of phenol groups (>0.81) but not redox potential. Current assay procedures measure antioxidant capacity under conditions not relevant to actual applications and do not determine radical quenching rates. Results raise serious questions regarding the ability of reactions with the hindered ABTS(+•) to rank actual radical quenching by compounds with different structures and invalidate reporting antioxidant activity as Trolox equivalents.