Hydrogen atom transfer versus proton coupled electron transfer mechanism of gallic acid with different peroxy radicals

Neutral Phenolic Contaminants Are Not Necessarily More Resistant to Permanganate Oxidation Than Their Dissociated Counterparts: Importance of Proton-Coupled Electron Transfer

Despite decades of research on phenols oxidation by permanganate, there are still considerable uncertainties regarding the mechanisms accounting for the unexpected parabolic pH-dependent oxidation rate. Herein, the pH effect on phenols oxidation was reinvestigated experimentally and theoretically by highlighting the previously unappreciated proton transfer. The results revealed that the oxidation of protonated phenols occurred via proton-coupled electron transfer (PCET) pathways, which can switch from ETPT (electron transfer followed by proton transfer) to CEPT (concerted electron-proton transfer) or PTET (proton transfer followed by electron transfer) with an increase in pH. A PCET-based model was thus established, and it could fit the kinetic data of phenols oxidation by permanganate well. In contrast with what was previously thought, both the simulating results and the density functional theory calculation indicated the rate of CEPT reaction of protonated phenols with OH- as the proton acceptor was much higher than that of deprotonated phenols, which could account for the pH-rate profiles for phenols oxidation. Analysis of the quantitative structure-activity relationships among the modeled rate constants, Hammett constants, and pKa values of phenols further supports the idea that the oxidation of protonated phenols is dominated by PCET. This study improves our understanding of permanganate oxidation and suggests a new pattern of reactivity that may be applicable to other systems.

Antioxidant properties of lipid concomitants in edible oils: A review

Edible oils are indispensable for human life, providing energy and necessary fatty acids. Nevertheless, they are vulnerable to oxidation via a number of different mechanisms. Essential nutrients deteriorate as well as toxic substances are produced when edible oils are oxidized; thus, they should be retarded wherever possible. Lipid concomitants have a strong antioxidant capacity and are a large class of biologically active chemical substances in edible oils. They have shown remarkable antioxidant properties and were documented to improve the quality of edible oils in varied ways. An overview of the antioxidant properties of the polar, non-polar, and amphiphilic lipid concomitants present in edible oils is provided in this review. Interactions among various lipid concomitants and the probable mechanisms are also elucidated. This review may provide a theoretical basis and practical reference for food industry practitioners and researchers to understand the underlying cause of variations in the quality of edible oils.

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.

Alizarin as a potential protector of proteins against damage caused by hydroperoxyl radical

Alizarin is a natural anthraquinone molecule with moderate antioxidative capacity. Some earlier investigations indicated that it can inhibit osteosarcoma and breast carcinoma cell proliferation by inhibiting of phosphorylation process of ERK protein (extracellular signal-regulated kinases). Several mechanisms of deactivation of one of the most reactive oxygen species, hydroperoxyl radical, by alizarin are estimated: hydrogen atom abstraction (HAA), radical adduct formation (RAF), and single electron transfer (SET). The plausibility of those mechanisms is estimated using density functional theory. The obtained results indicated HAA as the only thermodynamically plausible mechanism. For that purpose, two possible mechanistic pathways for hydrogen atom abstraction are studied in detail: hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET). Water and benzene are used as models of solvents with opposite polarity. To examine the difference between HAT and PCET is used kinetical approach based on the Transition state theory (TST) and determined rate constants (k). Important data used for a distinction between HAT and PCET mechanisms are obtained by applying the Quantum Theory of Atoms in Molecules (QTAIM), and by the analysis of single occupied molecular orbitals (SOMOs) in transition states for two examined mechanisms. The molecular docking analysis and molecular dynamic are used to predict the most probable positions of binding of alizarin to the sequence of ApoB-100 protein, a protein component of plasma low-density lipoproteins (LDL). It is found that alizarin links the nitrated polypeptide forming the π-π interactions with the amino acids Phenylalanine and Nitrotyrosine. The ability of alizarin to scavenge hydroperoxyl radical when it is in a sandwich structure between the polypeptide and radical species, as the operative reaction mechanism, is not significantly changed concerning its antioxidant capacity in the absence of polypeptide. Therefore, alizarin can protect the polypeptide from harmful hydroperoxyl radical attack, positioning itself between the polypeptide chain and the reactive oxygen species.

A DFT study on OH radical scavenging activities of eriodictyol, Isosakuranetin and pinocembrin

Flavonoids are natural compounds with antioxidant properties that have positive effects on human health, which reduce toxic effects of reactive oxygen species (ROS) and partially oxidative damage. In the work, the density functional theory (DFT/BMK) calculations were performed for antioxidant activity evaluation of pinocembrin (P), isosakuranetin (I) and eriodictyol (E). Four main mechanisms were examined: hydrogen atom transfer (HAT), radical adduct formation (RAF), single electron transfer-proton transfer (SET-PT) and Sequential proton loss electron transfer (SPLET). HAT and SPLET are thermodynamically the most probable process in gas phase and water. The three flavonoids examined + •OH HAT and RAF mechanisms for each possible location were investigated theoretically for the first time. The results were discussed by considering thermodynamic, kinetic and structural data of various reaction paths using IRC approach.Communicated by Ramaswamy H. Sarma.

The hydroperoxyl radical scavenging activity of natural hydroxybenzoic acids in oil and aqueous environments: Insights into the mechanism and kinetics

Foods that contain hydroxybenzoic acid derivatives (HBA) include red fruits, black radish, onion, and potato peel. HBA are widely known for their anti-inflammatory, anti-cancer, and especially antioxidant capabilities; however, a comprehensive study of the mechanism and kinetics of the antiradical action of these compounds has not been performed. Here, we report a study on the mechanisms and kinetics of hydroperoxyl radical scavenging activity of HBA by density functional theory (DFT) calculations. According to the results, HBA exert low HOO^• antiradical activity in the nonpolar environment with overall rate constants in the range of k_overall = 5.90 × 10^-6 - 4.10 × 10³ M^-1 s^-1. However, most HBA exhibit significant HOO^• antiradical activity (k_overall = 10⁵ - 10⁸ M^-1 s^-1) by the single electron transfer (SET) reaction of the phenoxide anions in water at physiological pH. The overall rate constant increases with increasing pH values in the majority of the substances studied. At pH ≤ 4, gentisic acid had the best HOO^• antiradical activity (log(k_overall) = 3.7-4.8), however at pH > 4, the largest HOO^• radical scavenging activity (log(k_overall) = 4.8-9.8) was almost exclusively found for gallic and syringic acids. Salicylic and 5-sulphosalicylic acids have the lowest antiradical activity across most of the pH range. The activities of the majority of the acids in this study are faster than the reference compound Trolox. Thus, in the aqueous physiological environment, these HBA are good natural antioxidants.

Current Trends in Computational Quantum Chemistry Studies on Antioxidant Radical Scavenging Activity

The antioxidative nature of chemicals is now routinely studied using computational quantum chemistry. Scientists are constantly proposing new approaches to investigate those methods, and the subject is evolving at a rapid pace. The goal of this review is to collect, consolidate, and present current trends in a clear, methodical, and reference-rich manner. This paper is divided into several sections, each of which corresponds to a different stage of elaborations: preliminary concerns, electronic structure analysis, and general reactivity (thermochemistry and kinetics). The sections are further subdivided based on methodologies used. Concluding remarks and future perspectives are presented based on the remaining elements.

The implication of phenolic acid matrix effect on the volatility of ethyl acetate in alcohol-free wine model: Investigations with experimental and theoretical methods

Hydroxycinnamic acids and ethyl acetate were assessed in simulated alcohol-free wine solutions to explore the effect of phenolic acids on the aroma volatility of esters. The results showed that the phenolic acids could inhibit the volatilization of ethyl acetate, and the extent of inhibition was influenced by the concentration and structure of the phenolic compounds. The ultraviolet absorption spectra of the phenolic acids and ethyl acetate confirmed the interaction between the two compounds. The thermodynamic parameters of the interaction implied a spontaneous exothermic interaction, driven primarily by hydrophobic effects. Meanwhile, the results of the fluorescence-quenching analysis indicated electron transfer between the reactants. The quantum chemical investigations revealed negative and positive charge density distributions in the structures of ethyl acetate and the phenolic acids, respectively. These results will provide some data reference and theoretical support for further research on the effects of phenolic acid matrix on other structural esters.

A Statistically Supported Antioxidant Activity DFT Benchmark-The Effects of Hartree-Fock Exchange and Basis Set Selection on Accuracy and Resources Uptake

Polyphenolic compounds are now widely studied using computational chemistry approaches, the most popular of which is Density Functional Theory. To ease this process, it is critical to identify the optimal level of theory in terms of both accuracy and resource usage—a challenge we tackle in this study. Eleven DFT functionals with varied Hartree–Fock exchange values, both global and range-separated hybrids, were combined with 14 differently augmented basis sets to calculate the reactivity indices of caffeic acid, a phenolic acid representative, and compare them to experimental data or a high-level of theory outcome. Aside from the main course, a validation of the widely used Janak’s theorem in the establishment of vertical ionization potential and vertical electron affinity was evaluated. To investigate what influences the values of the properties under consideration, linear regression models were developed and thoroughly discussed. The results were utilized to compute the scores, which let us determine the best and worst combinations and make broad suggestions on the final option. The study demonstrates that M06–2X/6–311G(d,p) is the best fit for such research, and, curiously, it is not necessarily essential to include a diffuse function to produce satisfactory results.

A comprehensive review on polarity, partitioning, and interactions of phenolic antioxidants at oil-water interface of food emulsions

There has been a growing interest in developing effective strategies to inhibit lipid oxidation in emulsified food products by utilization of natural phenolic antioxidants owing to their growing popularity over the past decades. However, due to the complexity of emulsified systems, the inhibition mechanism of phenolic antioxidants against lipid oxidation is rather complicated and not yet fully understood. In order to highlight the importance of polarity of phenolic antioxidants in emulsified systems according to the polar paradox, this review covers the recent progress on chemical, enzymatic, and chemoenzymatic lipophilization techniques used to modify the polarity of antioxidants. The partitioning behavior of phenolic antioxidants at the oil-water interface, which can be influenced by the presence of synthetic surfactants and/or antioxidant emulsifiers (e.g., polysaccharides, proteins, and phospholipids), is discussed. In addition, the emerging phenolic antioxidants among phenolic acids, flavonoids, tocopherols, and stilbenes applied in food emulsions are elaborated. As well, the interactions of polar-nonpolar antioxidants are stressed as a promising strategy to induce synergistic interactions at oil-water interface for improved oxidative stability of emulsions.

Theoretical study on isomerization of α-acids: A DFT calculation

The α-acids contained in hops are one of the ingredients of beer. The isomerization of α-acids produces iso-α-acids, the main source of bitterness in beer. In this study, the isomerization mechanism of the α-acid, cohumulone, was elucidated by using density functional theory in conjunction with the polarizable continuum model or 3D-RISM integral equation theory of liquids. The calculated reaction diagram is consistent with experimental results; the activation free energy difference between the cis and trans isomers is in good agreement with the experimental estimate. The activation energy difference results from solvation energy. Additionally, a calculation of NMR chemical shifts showed that the proton position of isocohumulone is different from that proposed previously. The effect of Mg²⁺ cation on the isomerization was also investigated. Both the activation and reaction free energy are stabilized by the presence of Mg²⁺, which is consistent with experimental results. Water solvation reduces the activation free energy.

The radical scavenging activity of abietane diterpenoids: Theoretical insights

Salvia species are frequently used in traditional medicine and are a source of diterpenoid antioxidants. In this study, the hydroperoxide radical scavenging activity of seven known abietane diterpenoids (ADs), isolated from Salvia barrelieri, are investigated using a quantum chemical approach. The ADs are 7-oxoroyleanone-12-methyl ether (1), 7a-acetoxyroyleanone-12-methyl ether (2), royleanone (3), horminone (4), 7-acetylhorminone (5), cryptojaponol (6), and inuroyleanol (7). It was found that formal hydrogen transfer is the main mechanism of the antiradical activity of these ADs in nonpolar environments, whereas the single electron transfer mechanism of anion states is favored in aqueous environment. The antioxidant activity of compounds 1-5 involves H-abstraction at the C7(15)-H bonds whereas for the compounds 6 and 7 the H abstraction takes place at the O12-H bond. The HOO^• scavenging activity of compounds 1-5 is minor in all of the studied media, however 6 and 7 exhibit excellent antiradical activity in aqueous solution. Remarkably, the HOO^• scavenging activity of compound 7 is substantially higher than that of Trolox, the reference antioxidant: the calculated rate constant was 122.3 times higher in polar and 6.1 times higher in nonpolar environments, respectively. Consistently 7 is a promising radical scavenger in physiological environments.

Density Functional Theory Studies on the Antioxidant Mechanism and Electronic Properties of Some Bioactive Marine Meroterpenoids: Sargahydroquionic Acid and Sargachromanol

Certain meroterpenoids isolated from brown alga of the genus Sargassum are known to be antioxidant agents. Herein, density functional theory has been performed to analyze the preferred antioxidant mechanism of the two reactive antioxidant compounds derived from the Sargassum genus, that is, Sargahydroquinoic acid and Sargachromanol and some of their derivatives. Their global reactivity descriptors have been calculated to reveal their reactivity as an antioxidant. Molecule 1 is the most reactive antioxidant according to calculated descriptors. The results of molecule 1 are comparable to that of Trolox, suggesting their similar activity. The calculated descriptors are closely matched with experimental pieces of evidence. It has been found that hydrogen atom transfer (HAT) is more favored in gas media. Also, the effect of solvent polarity on the antioxidant activity has been explored for molecule 1. The results disclose that the polarity of the solvent increases the contribution of two other mechanisms, that is, single-electron transfer, followed by proton transfer and sequential proton loss electron transfer.

Flavones' and Flavonols' Antiradical Structure-Activity Relationship-A Quantum Chemical Study

Flavonoids are known for their antiradical capacity, and this ability is strongly structure-dependent. In this research, the activity of flavones and flavonols in a water solvent was studied with the density functional theory methods. These included examination of flavonoids’ molecular and radical structures with natural bonding orbitals analysis, spin density analysis and frontier molecular orbitals theory. Calculations of determinants were performed: specific, for the three possible mechanisms of action—hydrogen atom transfer (HAT), electron transfer–proton transfer (ETPT) and sequential proton loss electron transfer (SPLET); and the unspecific—reorganization enthalpy (RE) and hydrogen abstraction enthalpy (HAE). Intramolecular hydrogen bonding, catechol moiety activity and the probability of electron density swap between rings were all established. Hydrogen bonding seems to be much more important than the conjugation effect, because some structures tends to form more intramolecular hydrogen bonds instead of being completely planar. The very first hydrogen abstraction mechanism in a water solvent is SPLET, and the most privileged abstraction site, indicated by HAE, can be associated with the C3 hydroxyl group of flavonols and C4’ hydroxyl group of flavones. For the catechol moiety, an intramolecular reorganization to an o-benzoquinone-like structure occurs, and the ETPT is favored as the second abstraction mechanism.

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.