Radical Adduct Formation Research Articles

Theoretical Insight into Antioxidant Mechanism of Caffeic Acid Against Hydroperoxyl Radicals in Aqueous Medium at Different pH-Thermodynamic and Kinetic Aspects

In this study, the DFT/M062X/PCM method was applied to investigate thermodynamic and kinetic aspects of reactions involved in possible mechanisms of antioxidant activity of caffeic acid against HOO● radicals in aqueous medium at different pH values. Kinetic parameters of the reactions involved in HAT (Hydrogen Atom Transfer), RAF (Radical Adduct Formation), and SET (Single Electron Transfer) mechanisms, including reaction energy barriers and bimolecular rate constants, were determined, and identification and characterization of stationary points along the reaction pathways within HAT and RAF mechanisms were performed. Inspection of geometrical parameters and spin densities of the radical products formed within HAT and RAF mechanisms revealed that they are stabilized by hydrogen bonding interactions and the odd electron originated through the reaction with the HOO● radical is spread over the entire molecule, resulting in significant radical stabilization. Thermodynamic and kinetic data collected in this study indicated that increasing pH of the medium boosts the antioxidant activity of caffeic acid by reducing the energy required to generate radicals within the RAF and/or HAT mechanism and, at extremely high pH, where the trianionic form of caffeic acid is a dominant species, by the occurrence of an additional fast, diffusion-limited electron-related channel.

Reactions of Diphenylamine with OH Radicals in the Environment: Theoretical Insights into the Mechanism, Kinetics, Temperature, and pH Effects.

Diphenylamine (DPL) has been widely utilized in industrial chemicals, but its degradation by HO• radicals in the environment has not been fully studied yet. The present study uses quantum chemical calculations to evaluate the reaction of DPL with HO• radicals in atmospheric and aqueous environments. The results showed that, in the atmosphere, the diphenylamine reacted with the HO• radical rapidly, with an overall rate constant of 9.24 × 1011 to 1.34 × 1011 M-1 s-1 and a lifetime of 0.17 to 1.55 h at 253-323 K. The calculated overall rate constant in water (koverall = 1.95 × 1010 M-1 s-1, pH = 3-14) is in excellent agreement with the experimental value (koverall = 1.00 × 1010-1.36 × 1010 M-1 s-1). The HO• + DPL reaction in water could occur following the hydrogen transfer (15.4%), single electron transfer (41.6%), and radical adduct formation (41.7%) mechanisms, clarifying that addition products were not exclusive products. Nevertheless, variations in temperature and pH within aqueous environments had an impact on the mechanisms, kinetics, and degradation products of the reaction of DPL with HO• radicals.

Theoretical insight into the removal process of isoquinoline by UV/Cl and UV/PDS: Oxidation mechanism and toxicity assessment

Isoquinoline (IQL), as a typical nitrogen-containing heterocyclic contaminant in coking wastewater, poses a serious threat to the aquatic environment and human health. Due to its chemical stability, traditional sewage treatment technology is not highly efficient in IQL removal. Advanced oxidation processes (AOPs) driven by ultraviolet radiation could be an effective treatment method, but it could generate toxic byproducts. In this work, the removal of IQL initiated by HO•, ClO•, Cl•, and SO4•- in UV/chlorine and UV/persulfate (PDS) process was comprehensively investigated, clarifying the degradation mechanism, reaction kinetics, and ecological toxicity. The findings indicate that the dominant oxidation mechanism of IQL by HO•, ClO•, and Cl• is radical adduct formation (RAF), while single electron transfer (SET) is the main reaction pathway of SO4•- with IQL. At 298 K and 1 atm, the order of rate constants for the reactions of IQL with active radicals is Cl• (6.23 × 1010 M−1 s−1) > SO4•- (8.81 × 109 M−1 s−1) > HO• (1.66 × 109 M−1 s−1) > ClO• (1.62 × 108 M−1 s−1). The acute and chronic toxicity of IQL and its degradation byproducts at three different trophic levels were evaluated using ECOSAR program. The byproducts produced by the oxidative degradation of IQL by HO• and SO4•- are mostly “not harmful”, and their toxicity shows a decreasing trend compared to that of IQL. The byproducts derived from the reaction of IQL with Cl• are all “toxic” or “harmful”, and the ranking of harm to three types of aquatic organisms is green algae > fish > daphnia. Hence, UV/PDS process could be more secure in pollutant disposal in wastewater. In actual water treatment process, merit attention should be paid to the potential hazards of the byproducts generated by various contaminants.

Differences in the Reaction Mechanisms of Chlorine Atom and Hydroxyl Radical with Organic Compounds: From Thermodynamics to Kinetics.

Hydroxyl radical (HO•) and chlorine atom (Cl•) are common reactive species in aqueous environments. However, the intrinsic difference in their reactions with organic compounds has not been revealed. This study compared the reaction mechanisms of HO• and Cl• with 13 aromatic and 11 aliphatic compounds by quantum chemical calculation and laser flash photolysis. Both HO• and Cl• can spontaneously react with aromatic compounds via radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) pathways. The SET reactions of Cl• were more thermodynamically favorable than HO•, but contrary results were obtained for HAT reactions. According to the free energy of activation (ΔGaq‡), the dominant oxidation mechanisms of aromatic compounds were RAF and SET by HO• and SET by Cl•. The important role of SET in the HO• reactions with aromatic compounds was further verified by accurately calculating the solvation free energy of HO•/HO- and experimentally tracking the radical cations, which were generally neglected in previous studies. Meanwhile, the ΔGaq‡ value of each reaction pathway of Cl• was lower than that of HO•, resulting in higher rate constants of Cl• with aromatic compounds than HO•. For saturated aliphatic compounds, HAT was found to be the only mechanism accounting for their transformation by HO• and Cl•. This study proposed general rules for the reaction mechanisms of HO• and Cl• and unraveled their differences in the aspects of thermodynamics and kinetics, providing fundamental information for understanding contaminant transformation in processes involving HO• and Cl•.

Exploring antioxidative properties of xanthohumol and isoxanthohumol: An integrated experimental and computational approach with isoxanthohumol pKa determination

This study explores the antioxidative activities of xanthohumol (XN) and isoxanthohumol (IXN), prenylated flavonoids from Humulus lupulus (family Cannabaceae), utilizing the oxygen radical absorption capacity (ORAC) and ferric reducing antioxidant power (FRAP) assays along with computational Density Functional Theory methods. Experimentally, XN demonstrated significantly higher antioxidative capacities than IXN. Moreover, we determined IXN pKa values using the UV/Vis spectrophotometric method for the first time, facilitating its accurate computational modeling under physiological conditions. Through a thermodynamic approach, XN was found to efficiently scavenge HOO• and CH3O• radicals via Hydrogen Atom Transfer (HAT) and Radical Adduct Formation (RAF) mechanisms, while CH3OO• scavenging was feasible only through the HAT pathway. IXN exhibited its best antioxidative activity against CH3O• via both HAT and RAF mechanisms and could also scavenge HOO• through RAF. Both Single Electron Transfer (SET) and Sequential Proton Loss-Electron Transfer (SPLET) mechanisms were thermodynamically unfavorable for all radicals and both compounds.

The inhibitory potential of 4,7-dihydroxycoumarin derivatives on ROS-producing enzymes and direct HOO•/o2 • – radical scavenging activity – a comprehensive kinetic DFT study

This study examined the antiradical activity of three synthesized coumarin derivatives: (E)-3-(1-((2-hydroxyphenyl)amino)ethylidene)-2,4-dioxochroman-7-yl acetate (A1-OH), (E)-3-(1-((3-hydroxyphenyl)amino)ethylidene)-2,4-dioxochroman-7-yl acetate (A2-OH), and (E)-3-(1-((4-hydroxyphenyl)amino)ethylidene)-2,4-dioxochroman-7-yl acetate (A3-OH) against HOO•/O2•− radical species. The investigation included electron spin resonance (ESR) measurements and a DFT kinetic study. Thermodynamic and kinetic parameters of antiradical mechanisms—Formal Hydrogen Atom Transfer (f-HAT), Radical Adduct Formation (RAF), Sequential Proton Loss followed by Electron Transfer (SPLET), and Single-Electron Transfer followed by Proton Transfer (SET-PT)—were evaluated using the Quantum Mechanics-based test for Overall Free Radical Scavenging Activity (QM–ORSA) under physiological conditions. ESR results indicated antiradical activity decreased in the sequence A1–OH (58.7%) > A2–OH (57.5%) > A3–OH (53.1%). Kinetic analysis revealed the f-HAT mechanism dominated HOO• inactivation. A newly formulated Sequential Proton Loss followed by Radical Adduct Formation (SPL-RAF) mechanism described interactions with O2 •−. The activity toward O2 •− was A2–OH (1.26 × 106 M−1s−1) > A3–OH (7.71 × 105 M−1s−1) > A1–OH (4.22 × 105 M−1s−1). Molecular docking and dynamics studies tested inhibitory capability against enzymes producing reactive species: Lipoxygenase (LOX), Myeloperoxidase (MPO), NAD(P)H oxidase (NOX), and Xanthine Oxidase (XOD). Affinity to enzymes decreased in the order: XOD > LOX > NOX > MPO.

On the dual role of (+)-catechin as primary antioxidant and inhibitor of viral proteases

Natural antioxidants have become the subject of many investigations due to the role that they play in the reduction of oxidative stress. Their main scavenging mechanisms concern the direct inactivation of free radicals and the coordination of metal ions involved in Fenton-like reactions. Recently, increasing attention has been paid to non-covalent inhibition of enzymes involved in different diseases by the antioxidants. Here, a computational investigation on the primary antioxidant power of (+)-catechin against the •OOH radical has been performed in both lipid-like and aqueous environments, taking into account the relevant species present in the simulated acid-base equilibria at the physiological pH. Hydrogen Atom Transfer (HAT), Single Electron Transfer (SET), and Radical Adduct Formation (RAF) mechanisms were studied, and relative rate constants were estimated. The potential inhibitory activity of the (+)-catechin towards the most important proteases from SARS-CoV-2, 3C-like (Mpro) and papain-like (PLpro) proteases was also investigated by MD simulations to provide deeper atomistic insights on the binding sites.Based on the antioxidant and antiviral properties also unravelled by comparison with other molecules having similar chemical scaffold, our results propose that (+)-CTc satisfies can explicate a dual action as antioxidant and antiviral in particular versus Mpro from SARS-CoV-2.

Trace Br- Inhibits Halogenated Byproduct Formation in Saline Wastewater Electrochemical Treatment.

The electrochemical technology provides a practical and viable solution to the global water scarcity issue, but it has an inherent challenge of generating toxic halogenated byproducts in treatment of saline wastewater. Our study reveals an unexpected discovery: the presence of a trace amount of Br- not only enhanced the electrochemical oxidation of organic compounds with electron-rich groups but also significantly reduced the formation of halogenated byproducts. For example, in the presence of 20 μM Br-, the oxidation rate of phenol increased from 0.156 to 0.563 min-1, and the concentration of total organic halogen decreased from 59.2 to 8.6 μM. Through probe experiments, direct electron transfer and HO• were ruled out as major contributors; transient absorption spectroscopy (TAS) and computational kinetic models revealed that trace Br- triggers a shift in the dominant reactive species from Cl2•- to Br2•-, which plays a key role in pollutant removal. Both TAS and electron paramagnetic resonance identified signals unique to the phenoxyl and carbon-centered radicals in the Br2•--dominated system, indicating distinct reaction mechanisms compared to those involving Cl2•-. Kinetic isotope experiments and density functional theory calculations confirmed that the interaction between Br2•- and phenolic pollutants follows a hydrogen atom abstraction pathway, whereas Cl2•- predominantly engages pollutants through radical adduct formation. These insights significantly enhance our understanding of bromine radical-involved oxidation processes and have crucial implications for optimizing electrochemical treatment systems for saline wastewater.

Comprehensive kinetic investigation of antiradical activity of urolithins and their potential inhibitory effect on Xanthine Dehydrogenase

This work aimed to explore the antiradical activity of various ellagitannin metabolites, including Urolithin A (UroA), Urolithin B (UroB), Urolithin C (UroC), and Urolithin D (UroD), in a non-polar (benzene) and polar (water) medium. The antiradical effect of the investigated compounds against reactive oxygen species HO• was evaluated in water (physiological conditions) and benzene, while against CH3OO• and CCl3OO• it was assessed solely in benzene, using the Quantum Mechanics-based Test for Overall Free-radical Scavenging Activity (QM-ORSA) methodology. In non-polar media, compounds UroA, UroC, and UroD exhibit antiradical capabilities against the HO• through Proton-coupled electron transfer (PCET) and Radical Adduct Formation (RAF) processes. Conversely, UroB predominantly exhibits inhibitory activity on HO• through the RAF pathway. On the other hand, the reaction with CH3OO• and CCl3OO• only occurs through the Hydrogen Atom Transfer (HAT). Under physiological conditions, all investigated compounds show antiradical capacity against the HO•, primarily via the SPLET and RCF mechanisms. The reactivity of radical species in both solvents decreases in the following order: HO• > CCl3OO• > CH3OO•. The thermodynamic and kinetic parameters in both solvents indicate that the investigated compounds’ reactivity follows a decreasing order of UroD > UroA > UroC > UroB. Furthermore, newly formed radical species of urolithin exhibit lower reactivity towards the fatty acid model in comparison to investigated reactive radical species, potentially preventing lipid peroxidation initiation. The investigated compounds can regenerate glutathione (GSH) by providing a hydrogen atom to the glutathione radical (GS•), which forms during interaction with highly reactive radical species. In water, tested compounds also regenerate GSH by donating H atoms to GS•, as well as by transferring electrons from dominant acid-base species to GS•, followed by protonation of the resulting GS¯. Urolithins showed similar inhibitory effects as the conventional inhibitors Y-700, FYX-051, and Febuxostat on Xanthine Dehydrogenase (XDH). Reactivity towards XDH enzyme decreases in the following order UroC > UroB > UroD > UroA.

Chlorination mechanism of benzene series in aqueous system under the combined action of Cl- and multiple free radicals

In this study, non-targeted analysis revealed that the Fe2+/perodisulfate/Cl- system degraded aniline (AN) and nitrobenzene (NB), yielding 7 types and 4 types of aromatic chlorides, respectively. In order to explore their chlorination mechanism and the influence of different functional groups, various reactions (radical adduct formation (RAF), hydrogen atom abstraction (HAA) and single electron transfer (SET)) of benzene series with various functional group with sulfate radical (SO4·-), hydroxyl radical (·OH), chlorine radical (·Cl) and dichloride anion radicals (Cl2·-) were explored by Gibbs free energy (ΔG), and the subsequent reactions of SET and HAA products (organic free radical) were innovatively explored by ΔG. This study revealed that the reaction between benzene compounds containing electron-donating groups and ·Cl was dominated by HAA rather than RAF; the reaction between benzene compounds containing electron-withdrawing groups and ·Cl was dominated by RAF. Aromatic compounds containing electron-donating groups can undergo SET reaction and generate aromatic chlorides in the presence of O2 and Cl-. Combined with the product analysis, it was found that the HAA reaction of aromatic compounds will lead to the obvious formation of biphenyl compounds and aromatic chlorides in the presence of Cl-. Compared with the conditions of RAF of ·Cl and chlorination of SET products, the chlorination reaction between HAA products and Cl- is ubiquitous, which is more likely to be the key to the formation of aromatic chlorides in mixed systems. This study provides a theoretical basis for further study on the formation risk of chlorinated products in advanced oxidation process.

Primary Antioxidant Power and Mpro SARS-CoV-2 Non-Covalent Inhibition Capabilities of Miquelianin.

The antioxidant power of quercetin-3-O-glucuronide (miquelianin) has been studied, at the density functional level of theory, in both lipid-like and aqueous environments. In the aqueous phase, the computed pKa equilibria allowed the identification of the neutral and charged species present in solution that can react with the ⋅OOH radical. The Hydrogen Atom Transfer (HAT), Single Electron Transfer (SET) and Radical Adduct Formation (RAF) mechanisms were considered, and the individual, total and fraction corrected rate constants were obtained. Potential non-covalent inhibition of Mpro from SARS-CoV-2 by miquelianin has been also evaluated.

Molecular transformation of petroleum compounds by hydroxyl radicals produced upon oxidation of reduced nontronite

At redox interfaces, the interaction between oxygen and structural Fe(II) in minerals produces hydroxyl radicals (●OH); however, the specific reactivity and mechanisms of oxidation of complex petroleum compounds by ●OH remain poorly understood. This study investigates the interaction mechanisms between ●OH generated from reduced nontronite, an Fe-rich clay mineral (NAu-2), and petroleum compounds by employing a suite of wet chemistry, gas chromatography mass spectrometry and Fourier transform-ion cyclotron resonance-mass spectrometry, as well as theoretical calculations. The results showed that the intrinsic structure, composition, solubility, and adsorption of petroleum molecules onto reduced NAu-2 significantly influenced their reactivity with ●OH. Specifically, the aromatic fraction of a crude oil sample exerted a minimal influence on the oxidation rate of structural Fe(II) in reduced NAu-2, but enhanced the production of ●OH by generating environmentally persistent free radicals. ●OH reacted more with water-soluble dissolved organic matter from oil compared to polar compounds. The newly generated molecules by ●OH exhibited a preference for the insoluble (polar) phase. The oxidation of petroleum hydrocarbons by ●OH was mediated through hydrogen atom abstraction and radical adduct formation, followed by dehydrogenation, hydroxylation, and carboxylation reactions. Consequently, the abundance of oxygen-containing compounds in polar compounds increased. However, specific oxidation pathways differed depending on the nature of petroleum compounds. Our results provide insights that oxidation of petroleum compounds by ●OH is vital for mitigating the negative impacts from hydrocarbon contamination and for evaluating the role of these processes in the carbon cycle in redox-active environments.

Transformative Removal of Aqueous Micropollutants into Polymeric Products by Advanced Oxidation Processes.

This perspective presents the latest advancements in selective polymerization pathways in advanced oxidation processes (AOPs) for removal of featured organic pollutants in wastewater. In radical-based homogeneous reactions, SO4• --based systems exhibit superior oxidative activity toward aromatics with electron-donating substituents via single electron transfer and radical adduct formation (RAF). The produced organic radical cations subsequently undergo coupling and polymerization reactions to produce polymers. For •OH-based oxidation, metal ions facilitate the production of monomer radicals via RAF. Additionally, heterogeneous catalysts can mediate both coupling and polymerization reactions via persulfate activation without generating inorganic radicals. Metal-based catalysts will mediate a direct oxidation pathway toward polymerization. In contrast, carbon-based catalysts will induce coupling reactions to produce low-molecular-weight oligomers (≤4 units) via an electron transfer process. In comparison to mineralization, polymerization pathways remarkably reduce peroxide usage, quickly separate pollutants from the aqueous phase, and generate polymeric byproducts. Thus, AOP-driven polymerization systems hold significant promise in reducing carbon emission and realizing carbon recycling in water treatment processes.

Identifying the Transients and Transformation Products in Hydroxyl Radical-Methimazole Reactions Using DFT and UPLC-Q-TOF MS/MS Approaches.

Oxidative reactions of the hydroxyl radical (·OH) with methimazole (MMI), an antithyroid drug, are crucial for understanding its fate in oxidizing environments. By synergistically integrating density functional theory and ultraperformance liquid chromatography-quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF MS/MS) techniques, we elucidated the transients and transformation products (TPs) arising from the ·OH-MMI reactions. We probed two hydrogen-atom abstraction (HA) reactions, three radical adduct formation reactions, and single electron transfer (SET) at the M06-2X/6-311++G(d,p)/SMD(water) level. All proposed reaction channels, except for HA from the methyl group and SET, were found to be barrier-free. SET is the dominant oxidation pathway, accounting for 44% of oxidations, as determined by branching ratio analysis. The selenium analogue, MSeI, exhibited minor reactivity differences compared to MMI, yet its overall patterns resembled those of ·OH-MMI reactions. TPs were generated experimentally by reacting MMI with ·OH produced by UV-photolysis of H2O2. Eight TPs were identified from an approximately 24% degradation of MMI using UPLC-Q-TOF MS/MS analysis, and an additional two TPs were identified from the approximately 52% degraded MMI sample. The exact identities of all of the TPs were established through their corresponding fragmentation patterns. This study elucidates the drug's susceptibility to free radical species under physiologically relevant conditions.

Unraveling Different Reaction Characteristics of Alkoxy Radicals in a Co(II)-Activated Peracetic Acid System Based on Dynamic Analysis of Electron Distribution.

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) have shown broad application prospects in organic wastewater treatment. Alkoxy radicals including CH3COO• and CH3COOO• are primary reactive species in PAA-AOP systems; however, their reaction mechanism on attacking organic pollutants still remains controversial. In this study, a Co(II)/PAA homogeneous AOP system at neutral pH was constructed to generate these two alkoxy radicals, and their different reaction mechanisms with a typical emerging contaminant (sulfacetamide) were explored. Dynamic electron distribution analysis was applied to deeply reveal the radical-meditated reaction mechanism based on molecular orbital analysis. Results indicate that hydrogen atom abstraction is the most favorable route for both CH3COO• and CH3COOO• attacking sulfacetamide. However, both radicals cannot react with sulfacetamide via the radical adduct formation route. Interestingly, the single-electron transfer reaction is only favorable for CH3COO• due to its lower ESUMO. In comparison, CH3COOO• can react with sulfacetamide via a similar radical self-sacrificing bimolecular nucleophilic substitution (SN2) route owing to its high ESOMO and easy escape of unpaired electrons from n orbitals of O atoms in the peroxy bond. These findings can significantly improve the knowledge of reactivity of CH3COO• and CH3COOO• on attacking organic pollutants at the molecular orbital level.

Reaction of methylene blue with OH radicals in the aqueous environment: mechanism, kinetics, products and risk assessment.

Methylene Blue (MB) is an industrial chemical used in a broad range of applications, and hence its discharge is a concern. Yet, the environmental effects of its degradation by HO˙ radicals have not been fully studied yet. This study employs quantum chemical calculations to investigate the two-step degradation of MB by HO˙ radicals in aqueous environments. It was found that MB undergoes a rapid reaction with the HO˙ radical, with an overall rate constant of 5.51 × 109 to 2.38 × 1010 M-1 s-1 and has a rather broad lifetime range of 11.66 hours to 5.76 years in water at 273-383 K. The calculated rate constants are in good agreement with the experimental values (k calculation/k experimental = 2.62, pH > 2, 298 K) attesting to the accuracy of the calculation method. The HO˙ + MB reaction in water followed the formal hydrogen transfer and radical adduct formation mechanisms, yielding various intermediates and products. Based on standard tests these intermediates and some of the products can pose a threat to aquatic organisms, including fish, daphnia, and green algae, they have poor biodegradability and have the potential to induce developmental toxicity. Hence MB in the environment is of moderate concern depending on the ratio of safe to harmful breakdown products.

Effect of pH on UV/H2O2-mediated removal of single, mixed and halogenated parabens from water

Adjusting pH values in aqueous environments can significantly improve the efficiency by which parabens and halo-parabens are removed. In this study, 20 neutral and deprotonated species were selected as models to investigate their pH-dependent removal mechanisms and kinetics by a UV/H2O2 process using density functional theory (DFT). Compared to neutral species, deprotonated species exhibit higher reactivity to HO• due to their high electron cloud density. H atom abstraction (HAA) reactions on the substitution groups are the most favorable pathways for neutral species, while radical adduct formation (RAF) reactions are the most favorable for deprotonated species. Single electron transfer (SET) reactions can be neglected for neutral species, while these reactions become a viable route for deprotonated molecules. The total reaction rate constants range from 1.63 × 109 to 3.74 × 1010 M− 1 s− 1 at pH 7.0, confirming the experimental results. Neutral and weakly alkaline conditions are favorable for the degradation of MeP and halo-parabens in the UV/H2O2 process. The order of removal efficiency at optimum pH is dihalo-parabens > mono-halo-parabens ≈ F, F-MeP > MeP. Furthermore, the transformation products must undergo oxidative degradation due to their high toxicity. Our findings provide new insights into the removal of parabens and their halogenated derivatives from wastewater.

Effect of hydroxyl position on antioxidant ability of hydroxycoumarin derivatives: A theoretical investigation

Effect of the OH group position on the antioxidant ability of hydroxycoumarin derivatives (HOC) was investigated by quantum chemical computation. Frontier molecular orbitals, molecular electrostatic potential, and thermodynamic parameters were analyzed in detail. The results indicate that 3-HOC, 6-HOC, and 7-HOC exhibit as the best reactants with free radicals through analysis of global reactivity descriptor values, meanwhile H (in OH groups) of 4-HOC, 5-HOC, 6-HOC, and 7-HOC molecules are the most preferable sites for nucleophilic attacks. Besides, thermodynamic and kinetic studies reveal hydrogen atomic transfer is the primary mechanism when hydroxycoumarin derivatives react with HOO• free radical in the gas phase and pentyl ethanoate. Hydrogen atomic transfer products account for over 97 % of the total products of hydrogen atomic transfer and radical adduct formation mechanisms. According to the study, 6-HOC is the most effective scavenger of free radicals when compared to other derivatives, with a rate constant of 1.38 × 104 M−1s−1 in the gas phase and 6.5 × 103 M−1s−1 in pentyl ethanoate. The interaction of O20∙∙∙H21 in the transition state of 6-HOC is crucial in stabilizing its structure. This finding suggests that 6-HOC can be a potential candidate for developing antioxidants for various applications in the medical and industrial fields. The results of this study provide valuable insights into the mechanisms by which antioxidants can scavenge free radicals and protect against oxidative stress.

Revealing the crucial role of Carbon-Centered radicals in UV/PAA process for trace amounts of organic contaminants removal

Peracetic acid (PAA) is a promising peroxyacid to combine with ultraviolet (UV) to eliminate trace amounts of organic contaminants (TrOCs), with growing importance in water treatment. In spite of a large number of research on UV/PAA process, the underlying mechanism still require further investigation, due to the difficulties in analyzing the carbon-centered radicals. Here we demonstrated the crucial roles of the carbon-centered radicals (namely CH3C(O)OO), via electron paramagnetic resonance (EPR) and kinetic models, with concentration of 4 orders magnitude higher than that of OH. Then, the theoretical calculation using density functional theory (DFT) showed that CH3C(O)OO and OH tended to attack phenol mostly through the route of radical adduct formation. The generated CH3C(O)OO and OH adducts may undergo further reactions to produce catechol and hydroquinone. Further, the environmental applicability of UV/PAA process for TrOC abatement was evaluated, including the effects of pH, PAA dosage, water constituents, as well as the variation of acute toxicity. Different from the OH-dominated process, kinetic analysis revealed that CH3C(O)OO was almost immune to the presence of water constituents, leading to the outperformance of UV/PAA process in actual water samples. Our present study could inform the application of UV/PAA process in water and wastewater treatment.

Radical scavenging activity of three Scirpusins: A kinetic and mechanistic study

The strong antioxidant activity of Scirpusin, a polyphenolic compound also known as resveratrol or piceatannol dimer, toward •OH/•OOH in gas phase and solvents was investigated through systematic thermodynamics and kinetics analyses using the density functional theory method. All mechanisms, including hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), sequential proton loss electron transfer (SPLET), and radical adduct formation (RAF), were investigated. The transition state theory was used to calculate potential energy surfaces and rate constants for HAT and RAF. The results revealed that Scirpusins exhibited considerable antioxidant activity, which followed the sequence: Scirpusin-B > Scirpusin-C ≈ Scirpusin-A, the amount of o-bisphenol is essential to enhance their antioxidant activity. Furthermore, Scirpusin-A, Scirpusin-B, and Scirpusin-C with structures of 13′-OH, 12′-OH, and 2-OH, respectively, shared the same parent nucleus but exhibited distinct strongest reactive sites. This result can provide a guidance for Scirpusin application in the future.