Thiophenols, Promising Scavengers of Peroxyl Radicals: Mechanisms and kinetics

Theoretical studies on the antioxidant activity of potential marine xanthones

In this study, a quantum chemical exploration was conducted to assess the antioxidant activity of xanthones isolated from marine sources, focusing on thermodynamics and kinetics within simulated physiological environments. DFT analysis revealed that xanthones such as 1,4,7-trihydroxy-6-methylxanthone (1), 1,4,5-trihydroxy-2-methylxanthone (2), arthone C (3), 2,3,4,6,8-pentahydroxy-1-methylxanthone (4), sterigmatocystin (5), oxisterigmatocystin C (6), and oxisterigmatocystin D (7) favor the SPLET pathway in water and the FHT pathway in lipid environments. The kinetic study of these xanthones reacting with the hydroperoxyl radical (HOO•) was conducted using the formal hydrogen atom transfer (FHT) mechanism and the single electron transfer (SET) mechanism. The results showed that compounds 1-4 exhibited antioxidant activities in aqueous environments surpassing that of the reference compound Trolox, with rate constants ranging from 2.02 x 105 to 9.44 x 107 M-1·s-1. In lipid environments, compounds 1 and 2 also demonstrated higher rate constants than Trolox. Additionally, molecular docking and molecular dynamics analysis suggested that xanthones 1-7 potentially inhibit the pro-oxidant effect of the Keap1 enzyme, highlighting their promise as both antiradicals and enzyme inhibitors.

The radical scavenging activity of 1-methyl-1,4-dihydronicotinamide: theoretical insights into the mechanism, kinetics and solvent effects

1,4-Dihydronicotinamide derivatives, including 1-methyl-1,4-dihydronicotinamide (MNAH), are derivatives of the active center of nicotinamide coenzyme (NADH) and are therefore potent radical scavengers. MNAH serves as a useful model of NADH that allows for modeling studies to address the activity of this important biomolecule. In this work, MNAH activity was evaluated against typical free radicals using quantum chemical calculations in physiological environments, with a secondary aim of comparing activity against two physiologically relevant radicals of markedly different stability, HO˙, and HOO˙, to establish which of these is a better model for assessing antioxidant capacity in physiological environments. The HO˙ + MNAH reaction exhibited diffusion-limited overall rate constants in all media, including the gas phase. The HOO˙ antiradical activity of MNAH was also good, with overall rate constants of 2.00 × 104 and 2.44 × 106 M-1 s-1, in lipid and aqueous media, respectively. The calculated rate constant in water (k overall(MNAH + HOO˙) = 3.84 × 105 M-1 s-1, pH = 5.6) is in good agreement with the experimental data (k exp(NADH + HOO˙) = (1.8 ± 0.2)×105 M-1 s-1). In terms of mechanism, the H-abstraction of the C4-H bond characterized the HOO˙ radical scavenging activity of MNAH, whereas HO˙ could react with MNAH at several sites and following either of SET (in polar media), RAF, and FHT reactions, which could be ascribed to the high reactivity of HO˙. For this reason the results suggest that activity against HOO˙ is a better basis for comparison of anti-radical potential. In the broader context, the HOO˙ scavanging activity of MNAH is better than that of reference antioxidants such as trans-resveratrol and ascorbic acid in the nonpolar environment, and Trolox in the aqueous physiological environment. Therefore, in the physiological environment, MNAH functions as a highly effective radical scavenger.

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.

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.

1,4-Diol Hq (TBHQ) vs 1,4-dithiol (TBDT); simulation of safe antioxidant with a lower carcinogenic activity

OBJECTIVES

tert-Butylhydroquinone (TBHQ) is an antioxidant and preservative used in unsaturated vegetable oils and processed foods. However, when consumed in higher doses daily, it may pose a threat to public health by potentially increasing the risk of cancer, as it has an affinity with both the aryl hydrocarbon receptor (AhR) and the estrogen receptor alpha (ERα).

METHODS

This study aimed to examine the impact of substituting the 1,4-diol of TBHQ with 1,4-dithiol, referred to as TBDT, on the carcinogenic and antioxidant systems using computational methods. The binding affinity of TBHQ and TBDT to the two carcinogenic receptors, AhR and ERα, as well as to the antioxidant receptor Keap1 alone and in connection with Nrf2 (Nrf2-Keap1) was investigated through docking analysis.

RESULTS

The results indicated a decrease in TBDT's binding strength to ERα and AhR when assessed using Molegro Virtual Docker (P-value: 0.0001 and 0.00001, respectively), AutoDock Vina (P-value: 0.0001 and 0.0001), and the online server Fast DRH (P-value: 0.0001 and 0.0001). However, TBDT's binding affinity to Keap1 was predicted to be significantly stronger than TBHQ's by both MVD and AutoDock Vina (P-value: 0.0001 and 0.04), while its binding to Nrf2-Keap1 assessed to be stronger only by MVD (P-value: 0.0001).

CONCLUSION

These findings suggest that TBDT not only exhibits higher antioxidant activity as a better ligand for the antioxidant system but also shows lower affinity with the AhR and ERα receptors. Therefore, TBDT can be considered a safer compound than TBHQ.

Bond analysis in meta- and para-substituted thiophenols: overlap descriptors, local mode analysis, and QTAIM

CONTEXT

This study delves into the chemical nuances of thiophenols and their derivatives through a comprehensive computational analysis, moving beyond traditional energetic perspectives such as bond dissociation enthalpy and S-H dissociation dynamics. By employing the overlap model along with its topological descriptors (OP/TOP), quantum theory of atoms in molecules (QTAIM), and local vibrational mode (LVM) theories, the research provides a deeper understanding of the S-H and C-S bonding scenarios in substituted thiophenols. The investigation follows the electron-donating capacity of S-H substituent variation with the nature and positioning of other ring substituents. Energy profile analyses indicate distinct stability differences in the cis and trans conformations of meta- and para-PhSH systems, influenced by the electron-donating strength of these substituents. The study also uncovers significant variations in S-H bond distances and descriptor values, particularly in para-substituted PhSH, reflecting the influence of electron-donating or withdrawing substituents. In contrast, alterations at the meta-position show minimal effects on C-S bond descriptors, while para-substitutions markedly influence C-S bond characteristics, demonstrating a clear correlation with the electron-donating or withdrawing capabilities of the substituents. This research sheds light on the intricate bond dynamics in aromatic systems with diverse substituents, highlighting the complex interaction between electronic effects and molecular conformation.

METHODS

The study employs the ω B97X-D/Def2TZVP level of theory for molecular geometries, ensuring accurate characterization of structures as true minima via analytical harmonic frequency determination. The electronic properties of S-H and C-S bonds in variously substituted thiophenols were analyzed using OP/TOP, QTAIM, and LVM methodologies. Computational processes, including conformational scans, geometry optimizations, and vibrational frequency calculations, were conducted using Gaussian 09, with ultra-fine integration grids and tight convergence criteria for the SCF procedure. Bond descriptors were computed utilizing ChemBOS, Multiwfn, and LModeA software, providing a robust and detailed examination of bond properties.

The radical scavenging activity of monocaffeoylquinic acids: the role of neighboring hydroxyl groups and pH levels

Caffeoylquinic acids (CQAs) are well-known antioxidants. However, a key aspect of their radical scavenging activity - the mechanism of action - has not been addressed in detail thus far. Here we report on a computational study of the mechanism of activity of CQAs in scavenging hydroperoxyl radicals. In water at physiological pH, the CQAs demonstrated ≈ 104 times higher HOO˙ antiradical activity than in lipid medium (k(lipid) ≈ 104 M-1 s-1). The activity in the aqueous solution was determined by the hydrogen transfer mechanism of the adjacent hydroxyl group (O6'-H) of the dianion states (Γ = 93.2-95.2%), while the single electron transfer reaction of these species contributed 4.8-6.8% to the total rate constants. The kinetics estimated by the calculations are consistent with experimental findings in water (pH = 7.5), yielding a kcalculated/kexperimental = 2.4, reinforcing the reliability and precision of the computational method and demonstrating its utility for evaluating radical reactions in silico. The results also revealed the pH dependence of the HOO˙ scavenging activity of the CQAs; activity was comparable for all compounds below pH 3, however at higher pH values 5CQA reacted with the HOO˙ with lower activity than 3CQA or 4CQA. It was also found that CQAs are less active than Trolox below pH 4.7, however over pH 5.0 they showed higher activity than the reference. The CQAs had the best HOO˙ antiradical activity at pH values between 5.0 and 8.6. Therefore, in the physiological environment, the hydroperoxyl antiradical capacity of CQAs exhibits similarity to renowned natural antioxidants including resveratrol, ascorbic acid, and Trolox.

Antiradical Activity of Lignans from Cleistanthus sumatranus: Theoretical Insights into the Mechanism, Kinetics, and Solvent Effects

Sumatranus lignans (SL) isolated from Cleistanthus sumatranus have demonstrated bioactivities, e.g., they were shown to exhibit immunosuppressive properties in previous research. Their structure suggests potential antioxidant activity that has not attracted any attention thus far. Consistently, a comprehensive analysis of the antioxidant activity of these compounds is highly desirable with the view of prospective medical applications. In this work, the mechanism and kinetics of the antiradical properties of SL against hydroperoxyl radicals were studied by using calculations based on density functional theory (DFT). In the lipid medium, it was discovered that SL reacted with HOO• through the formal hydrogen transfer mechanism with a rate constant of 101-105 M-1 s-1, whereas in aqueous media, the activity primarily occurred through the sequential proton loss electron transfer mechanism with rate constants of 102-108 M-1 s-1. In both lipidic and aqueous environments, the antiradical activity of compounds 6 and 7 exceeds that of resveratrol, ascorbic acid, and Trolox. These substances are therefore predicted to be good radical scavengers in physiological environments.

The alkoxy radical polymerization of N-vinylpyrrolidone in organic solvents: theoretical insight into the mechanism and kinetics

Poly(N-vinylpyrrolidone) (PVP) is a polymer with many applications in cosmetic, pharmaceutical, and biomedical formulations due to its minimal toxicity. PVP can be synthesized through radical polymerization in organic solvents; this well-known industrial process is thoroughly characterized experimentally, however, quantum chemical modeling of the process is scarce: the mechanism and kinetics have not been thoroughly investigated yet. In this work, the mechanism and kinetics of the alkoxy radical polymerization of N-vinylpyrrolidone in organic solvents, namely isopropanol (IP) and toluene (TL), were successfully modeled by computational chemistry. The initiator radicals di-tert-butyl peroxide (TBO˙) and dicumyl peroxide (CMO˙) as well as the solvents isopropanol and toluene, were shown to be capable of assisting in the initiation reactions. The rate constant was influenced by the combination of initiators and solvent and the values of the rate constant of propagation were approximately 101-103 M-1 s-1. The radical polymerization of NVP with dicumyl peroxide as an initiator was comparable to that of di-tert-butyl peroxide in all of the examined organic solvents, whereas the solvents had less of an effect.

Damage of amino acids by aliphatic peroxyl radicals: a kinetic and computational study

Absolute second-order rate coefficients for the reaction of the N- and C-protected amino acids tyrosine (Tyr), tryptophan (Trp), methionine (Met) and proline (Pro) with triethylamine-derived aliphatic peroxyl radical TEAOO˙, which was used as a model for lipid peroxyl radicals, were determined using laser flash photolysis. For Ac-Tyr-OMe a rate coefficient of 1.4 × 10⁴ M^-1 s^-1 was obtained, whereas the reactions with Ac-Trp-OMe and Ac-Met-OMe were slower by a factor of 4 and 6, respectively. For the reaction with Ac-Pro-OMe only an upper value of 10³ M^-1 s^-1 could be determined, suggesting that Pro residues are not effective traps for lipid peroxyl radicals. Density functional theory (DFT) calculations revealed that the reactions proceed via radical hydrogen atom transfer (HAT) from the Cα position, indicating that the rate is determined by the exothermicity of the reaction. In the case of Ac-Tyr-OMe, HAT from the phenolic OH group is the kinetically preferred pathway, which shuts down when hydrogen bonding with an amine occurs. In an alkaline environment, where the phenolic OH group is deprotonated, the reaction is predicted to occur preferably at Cβ, likely through a proton-coupled electron transfer (PCET) mechanism.

Reactions of nicotine and the hydroxyl radical in the environment: Theoretical insights into the mechanism, kinetics and products

Nicotine (NCT) is a prevalent and highly poisonous tobacco alkaloid found in wastewater discharge. Advanced oxidative processes (AOP) are radical interactions between harmful pollutants and ambient free radicals that, theoretically, result in less toxic compounds. For a better understanding of the chemical transformations and long-term environmental effects of toxic discharges, the study of these processes is crucial. Here, quantum chemical calculations are used to investigate the AOP of the NCT in aqueous and lipidic environments. It was found that NCT interacted with HO^• in polar and nonpolar media, with an overall rate constant k_overall = 10⁶ - 10¹⁰ M^-1 s^-1. The computed kinetic data are reasonably accurate as seen by the comparison with the experimental rate constant in water (pH = 7.0), which results in a k_calculated/k_experimetal ratio of 1.4. The hydrogen transfer (C7, C9, C12)-single electron transfer pathways are the main mechanisms for the HO^• + NCT reaction in pentyl ethanoate solvent to form the cations as the primary products of the two-step reaction. However, in aqueous environments, the degradation of NCT by HO^• radicals increases with increasing pH levels. It is predicted that oxidation products are less toxic than nicotine itself, especially in an aqueous environment with a pH < 7.0.

Peroxyl radical scavenging activity measurement of antioxidants using histidine-stabilized and glutathione-capped fluorescent gold nanoclusters

A fluorescent gold nanocluster was used for determining peroxyl radical scavenging activity of antioxidants. Histidine was used as a green reducing and protective agent, and glutathione (GSH) enhanced the fluorescence intensity of histidine-stabilized gold nanoclusters (AuNCs) by ligand exchange process. When AAPH-induced oxidation of GSH occurred, the initial fluorescence intensity of GSH-capped AuNCs (λ_ex = 450 nm λ_em = 502 nm) was decreased with static quenching. The decline of fluorescence intensity of the GSH-capped AuNCs upon peroxyl radical attack is diminished with the addition of antioxidants to the reaction medium, the difference in fluorescence intensity being related to peroxyl radical scavenging activity of antioxidants. The 50 % inhibitive concentration of related antioxidant compounds were determined and compared to those of crocin bleaching assay. Inhibition % of sage (Salvia officinalis L.) and green tea (Camellia sinensis) infusions against peroxyl radicals were investigated. The proposed assay can be used for simple and selective estimation of the peroxyl radical scavenging activity in complex matrices, as histidine-stabilized GSH-capped AuNCs were selective toward peroxyl radicals, not affected by other ROS at the studied concentrations.

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.

7-O-Galloyltricetifavan: a promising natural radical scavenger

7-O-Galloyltricetifavan (7OGT), a natural flavonoid, is isolated from the leaves of Pithecellobium clypearia. The compound exhibits a variety of biological activities. This study details the evaluation of the HOO^• antiradical activity of 7OGT by quantum chemistry calculations. The HOO^• trapping activity of 7OGT in the gas phase (reference state) was discovered to follow the formal hydrogen transfer mechanism with a rate constant of k = 4.58 × 10⁸ M^-1 s^-1. In physiological environments, 7OGT is predicted to be an excellent HOO^• radical scavenger with k _overall = 2.65 × 10⁸ and 1.40 × 10⁴ M^-1 s^-1 in water and pentyl ethanoate solvents, respectively. The HOO^• antiradical activity of 7OGT in water at physiological pH is approximately 2000 times that of Trolox and substantially higher than that of other well-known natural antioxidants such as trans-resveratrol or ascorbic acid. Thus, 7OGT is an excellent natural antioxidant in polar environments.

Calculating bond dissociation energies of X-H (X=C, N, O, S) bonds of aromatic systems via density functional theory: a detailed comparison of methods

In this study, the performance of 17 different density functional theory functionals was compared for the calculation of the bond dissociation energy (BDE) values of X-H (X=C, N, O, S) bonds of aromatic compounds. The effect of the size of the basis set (expansions of 6-31(G)) was also assessed for the initial geometry and zero-point energy calculations, followed by the single-point BDE calculations with different model chemistries with the 6-311 + (3df,2p) basis set. It was found that the size of the basis set for geometry optimization has a much smaller effect on the accuracy of BDE than the choice of functional for the following single-point calculations. The M06-2X, M05-2X and M08-HX functionals yielded highly accurate BDE values compared to experimental data (with the average mean unsigned error MUE = 1.2-1.5 kcal mol^-1), performing better than any of the other functionals. The results suggest that geometry optimization may be performed with B3LYP functional and a small basis set, whereas the M06-2X, M05-2X and M08-HX density functionals with a suitably large basis set offer the best method for calculating BDEs of ArX-H (X=C, N, O, S) bonds.

Analytical and Theoretical Studies of Antioxidant Properties of Chosen Anthocyanins; A Structure-Dependent Relationships

The relationship between the structure and the antiradical and antioxidant activities of three anthocyanidins, namely peonidin, petunidin, and delphinidin, and their glucosides was investigated in this study. The ability of anthocyanins to scavenge free radicals was determined using DPPH● assay, whereas the inhibition of peroxidation in liposomes in relation to a model membrane that imitated the composition of a lipid membrane in tumor cells was specified using the fluorimetric method. To explore this issue at the atomistic level, density functional theory studies were applied. It was shown that glycosides performed better than anthocyanidins in protecting membranes against oxidation. The highest redox potential was demonstrated by anthocyanidins with the highest number of hydroxyl groups in the B ring in the order as follows: (Dp > Pt > Pn), and the same relationship was proven for their glucosides. The majority of the compounds studied here proved to be better antioxidants than ascorbic acid. They showed consistent electrodonating properties and though the f-HAT mechanism became more feasible with each consecutive deprotonation. Glycosylation did not have a direct impact on reactivity, apart from peonidin and petunidin in the study of which it was found that this process was responsible for lifting off steric hindrance between B and C rings and rendering certain pathways more feasible. Kinetic and molecular dynamics are essential to properly describe the membrane’s lipid oxidation.

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.

Chemical repair of damaged leucine and tryptophane by thiophenols at close to diffusion-controlled rates: Mechanisms and kinetics

Thiophenols are chemical species with multiple desirable biological properties, including their primary and secondary antioxidant capacity. In this work, the repairing antioxidant activity of eight different thiophenols has been investigated for damaged leucine and tryptophane. The investigation was carried out employing quantum mechanical and transition state methods to calculate the thermodynamic and kinetic data of the reactions involved, while simulating the biological conditions at physiological pH and aqueous and lipidic medium. The analysis of the atomic charges and the spin densities at each of the points on the potential energy surface was the tool that allowed the elucidation of the reaction mechanisms through which thiophenols repair the oxidative damage caused to the amino acids leucine and tryptophan. It was found that thiophenols can repair leucine via a hydrogen atom transfer mechanism in a manner which is similar to the one used by glutathione to repair the carbon-centered radicals of guanosine. In addition, thiophenols can also restore tryptophane, a nitrogen-centered radical, via proton-coupled electron transfer and single electron transfer mechanisms. Moreover, both processes occur at close to diffusion-controlled rates.

Role of Sulphur and Heavier Chalcogens on the Antioxidant Power and Bioactivity of Natural Phenolic Compounds

The activity of natural phenols is primarily associated to their antioxidant potential, but is ultimately expressed in a variety of biological effects. Molecular scaffold manipulation of this large variety of compounds is a currently pursued approach to boost or modulate their properties. Insertion of S/Se/Te containing substituents on phenols may increase/decrease their H-donor/acceptor ability by electronic and stereo-electronic effects related to the site of substitution and geometrical constrains. Oxygen to sulphur/selenium isosteric replacement in resveratrol or ferulic acid leads to an increase in the radical scavenging activity with respect to the parent phenol. Several chalcogen-substituted phenols inspired by Vitamin E and flavonoids have been prepared, which in some cases prove to be chain-breaking antioxidants, far better than the natural counterparts. Conjugation of catechols with biological thiols (cysteine, glutathione, dihydrolipoic acid) is easily achieved by addition to the corresponding ortho-quinones. Noticeable examples of compounds with potentiated antioxidant activities are the human metabolite 5-S-cysteinyldopa, with high iron-induced lipid peroxidation inhibitory activity, due to strong iron (III) binding, 5-S-glutathionylpiceatannol a most effective inhibitor of nitrosation processes, and 5-S-lipoylhydroxytyrosol, and its polysulfides that proved valuable oxidative-stress protective agents in various cellular models. Different methodologies have been used for evaluation of the antioxidant power of these compounds against the parent compounds. These include kinetics of inhibition of lipid peroxidation alkylperoxyl radicals, common chemical assays of radical scavenging, inhibition of the OH• mediated hydroxylation/oxidation of model systems, ferric- or copper-reducing power, scavenging of nitrosating species. In addition, computational methods allowed researchers to determine the Bond Dissociation Enthalpy values of the OH groups of chalcogen modified phenolics and predict the best performing derivative. Finally, the activity of Se and Te containing compounds as mimic of glutathione peroxidase has been evaluated, together with other biological activities including anticancer action and (neuro)protective effects in various cellular models. These and other achievements are discussed and rationalized to guide future development in the field.

The hydroperoxyl antiradical activity of natural hydroxycinnamic acid derivatives in physiological environments: the effects of pH values on rate constants

Quantum chemistry calculations suggest that hydroxycinnamic acid derivatives are good natural antioxidants in aqueous solutions.

The radical scavenging activity of glycozolidol in physiological environments: a quantum chemical study

Glycozolidol (GLD), derived from Glycosmis pentaphylla, is predicted to be a good radical scavenger in physiological environments.

Radical Scavenging Potential of the Phenothiazine Scaffold: A Computational Analysis

The reactivity of phenothiazine (PS), phenoselenazine (PSE), and phenotellurazine (PTE) with different reactive oxygen species (ROS) has been studied using density functional theory (DFT) in combination with the QM-ORSA (Quantum Mechanics-based Test for Overall Free Radical Scavenging Activity) protocol for an accurate kinetic rate calculation. Four radical scavenging mechanisms have been screened, namely hydrogen atom transfer (HAT), radical adduct formation (RAF), single electron transfer (SET), and the direct oxidation of the chalcogen atom. The chosen ROS are HO. , HOO. , and CH3 OO. . PS, PSE, and PTE exhibit an excellent antioxidant activity in water regardless of the ROS due to their characteristic diffusion-controlled regime processes. For the HO. radical, the primary active reaction mechanism is, for all antioxidants, RAF. But, for HOO. and CH3 OO. , the dominant mechanism strongly depends on the antioxidant: HAT for PS and PSE, and SET for PTE. The scavenging efficiency decreases dramatically in lipid environment and remains only significant (via RAF) for the most reactive radical (HO. ). Therefore, PS, PSE, and PTE are excellent antioxidant molecules, especially in aqueous, physiological environments where they are active against a broad spectrum of harmful radicals. There is no advantage or significant difference in the scavenging efficiency when changing the chalcogen since the reactivity mainly derives from the amino hydrogen and the aromatic sites.

The hydroperoxyl and superoxide anion radical scavenging activity of anthocyanidins in physiological environments: Theoretical insights into mechanisms and kinetics

The HOO^• and O₂^•- scavenging activities of 12 natural anthocyanidins were investigated in physiological environments by using DFT calculations. The results suggest high HOO^• scavenging activity in aqueous medium with overall rate constants in the range of k_overall = 1.58 × 10⁸ - 7.59 × 10⁹ M^-1 s^-1, whereas in lipid medium only weak activity is predicted. O₂^•- scavenging is also fast in water with k_app = ∼10⁹ M^-1 s^-1. Like in the case of many other antioxidants that contain acidic moieties, the anion states (H₃A^- and H₂A^2-) and the single electron transfer mechanism play a dominant role in the HOO^• scavenging activity of anthocyanidins in water at pH = 7.4. Analysis of the mechanism suggests that the O₂^•- and HOO^• radical scavenging can occur as a regeneration cycle that might increase the protective efficiency of anthocyanidins against oxidative stress.

The hydroperoxyl radical scavenging activity of sulfuretin: insights from theory

Sulfuretin (SFR), which is isolated from Rhus verniciflua, Toxicodendron vernicifluum, Dahlia, Bidens tripartite and Dipterx lacunifera, is one of the most important natural flavonoids. This compound is known to have numerous biological activities; among these, the antioxidant activity has not been thoroughly studied yet. In this study, the hydroperoxyl scavenging activity of SFR was examined by using density functional theory calculations. SFR is predicted to be an excellent HOO• scavenger in water at pH = 7.40 with k overall = 4.75 × 107 M-1 s-1, principally due to an increase in the activity of the anionic form following the single-electron transfer mechanism. Consistently, the activity of the neutral form is more prominent in the non-polar environment with k overall = 1.79 × 104 M-1 s-1 following the formal hydrogen transfer mechanism. Thus, it is predicted that SFR exhibits better HOO• antiradical activity than typical antioxidants such as resveratrol, ascorbic acid or Trolox in the lipid medium. The hydroperoxyl radical scavenging of SFR in the aqueous solution is approximately 530 times faster than that of Trolox and similar to ascorbic acid or resveratrol. This suggests that SFR is a promising radical scavenger in physiological environments.

Radical Scavenging Activity of Natural Anthraquinones: a Theoretical Insight

Anthraquinones (ANQs) isolated from Paederia plants are known to have antidiarrheal, antitussive, anthelmintic, analgesic, anti-inflammatory, antihyperlipidemic, antihyperglycaemic, and antimicrobial activities. The antioxidant properties were also noted but not confirmed thus far. In this study, the superoxide and hydroperoxide radical scavenging activities of six ANQs were evaluated using a computational approach. The results suggest that the ANQs exhibit low HOO• antiradical activity in all environments, including the gas phase (k < 102 M-1 s-1). In contrast, the ANQs might exert excellent O2 •- radical scavenging activity, particularly in aqueous solution. The rate constants of the superoxide anion scavenging in water (at pH = 7.4) range from 3.42 × 106 to 3.70 × 108 M-1 s-1. Compared with typical antioxidants such as ascorbic acid and quercetin, the superoxide anion scavenging activity of ANQs is significantly higher. Thus, the ANQs are promising O2 •- radical scavengers in polar media.

Is natural fraxin an overlooked radical scavenger?

Fraxin (FX) (7-hydroxy-6-methoxycoumarin 8-glucoside) is a typical natural product of the coumarin family. This compound was shown to protect endothelial cells from oxidative stress; however, the nature of its antioxidant properties is still ambiguous. In this study, we report on a systematic evaluation of the radical scavenging activity of FX using a two-tier protocol based on thermodynamic and kinetic calculations. The results show that FX has moderate activity in the aqueous physiological environment against a range of radicals including HO˙, CCl3O˙, CCl3OO˙, NO2, , and HOO˙. The latter was examined in detail due to the prevalence of HOO˙ as a source of oxidative stress in biological systems. HOO˙ scavenging activity was promising in the gas phase but low in physiological environments with k overall = 1.57 × 106, 3.13 × 102 and 2.68 × 103 M-1 s-1 in the gas phase, pentyl ethanoate and water solvents, respectively. The formal hydrogen transfer mechanism at the O7-H bond dominates the hydroperoxyl radical scavenging of FX in the nonpolar media, whereas, in the polar environment, the activity is exerted by the single electron transfer mechanism of the anion state. This activity falls behind typical antioxidants such as Trolox, ascorbic acid, and trans-resveratrol under the studied conditions. Thus FX may have multiple health benefits, but it is not an outstanding natural antioxidant.

The radical scavenging activity of muriolide in physiological environments: mechanistic and kinetic insights into double processes

Muriolide, which is a natural lactone that was isolated from Ranunculus muricatus, is a promising natural radical scavenger in the physiological environment.

Another look at reactions of 4-hydroxycoumarin with hydroxyl radical in the environment: deprotonation and diffusion effects

Quantum chemistry calculations suggest that chemical fate of the anticoagulant rodenticide 4-hydroxycoumarin in the environment is crucially dependent on deprotonation in aqueous medium.

Comparison of the scavenging capacities of phloroglucinol and 2,4,6-trihydroxypyridine towards HO˙ radical: a computational study

In this work the scavenging capacities of biologically active phloroglucinol (1,3,5-trihydroxybenzene, THB-OH) and structurally similar 2,4,6-trihydroxypyridine (THP-OH) towards HO˙ were examined. This task was realized by means of density functional theory, through investigation of all favorable antioxidative pathways in two solvents of different polarity: benzene and water. It was found that in benzene both compounds conform to the hydrogen atom transfer (HAT) and radical adduct formation (RAF) mechanisms. In water, the mechanisms of antioxidative action of the investigated compounds are far more complex, especially those of THB-OH. This compound and HO˙ undergo all four investigated mechanisms: HAT, RAF, sequential proton loss electron transfer (SPLET), and single electron transfer-proton transfer (SET-PT). HAT, RAF and SPLET are operative mechanisms in the case of THP-OH. Independently of solvent polarity, both investigated compounds are more reactive towards HO˙ in comparison to Trolox. Our final remark is as follows: the electron-withdrawing effect of the nitrogen is stronger than the electron-donating effect of the OH groups in the molecule of THP-OH. As a consequence, THB-OH is more powerful antioxidant than THP-OH, thus implying that the presence of nitrogen decreases the scavenging capacity of the respective compound.

Theoretical and Experimental Studies of the Antioxidant and Antinitrosant Activity of Syringic Acid

Syringic acid (SA) is a natural phenolic acid found in vegetables, fruits, and other plant-based foods. A range of biological activities were proposed for this compound including anticancer, antimicrobial, anti-inflammation, and anti-diabetic activities, as well as antioxidant and antinitrosant properties. In this study, the focus is on the latter two. The HO^•, HOO^•, NO, and NO₂ scavenging activities of SA were evaluated in physiological environments by kinetic and thermodynamic calculations. The computed rate constants of the HO^• radical scavenging of SA were 4.63 × 10⁹ and 9.77 × 10⁷ M^-1 s^-1 in polar and nonpolar solvents, respectively. A comparison with the experimentally determined rate constant in aqueous solution yields a k_calculated/k_experimental ratio of 0.3, thus the computed kinetic data are reasonably accurate. SA exhibited excellent HOO^• and NO₂ scavenging activity in water (k_overall(HOO^•) = 1.53 × 10⁸ M^-1 s^-1 and k_overall(NO₂) = 1.98 × 10⁸ M^-1 s^-1), whereas it did not show NO scavenging activity in any of the studied environments. In lipid medium, SA exhibited weak activity. Thus, in polar environments, the HOO^• radical scavenging of SA is 1.53 times higher than that of ascorbic acid. Consistently, SA is a promising antioxidant and antinitrosant agent in polar environments.

The radical scavenging activity of moracins: theoretical insights

Moracins are natural products that have been isolated from different plants such as Artocarpus heterophyllus, Cassia fistula, Morus alba, and Morus mesozygia. Studies showed that moracins may have various advantageous physiological effects such as anticancer, anti-inflammatory, anticholinesterase and particularly antioxidant activities. Most of these bioactivities have not been studied systematically. In this study, the radical scavenging of a typical moracin (moracin M, MM) against HO˙ and HOO˙ radicals was evaluated by thermodynamic and kinetic calculations in the gas phase as well as in water and pentyl ethanoate solvents. It was found that the overall rate constants for the HO˙ radical scavenging in the gas phase and the physiological environments are in the range of 1011 to 1010 M-1 s-1, respectively. For the HOO˙ + MM reaction the rate constants are 4.10 × 107 and 3.80 × 104 M-1 s-1 in the polar and lipid media, respectively. It is important to notice that the single electron transfer pathway of the anion state (MM-O6'-) dominated the HOO˙ radical scavenging in the aqueous solution, whereas in lipid medium the neutral MM exerted its activity by the formal hydrogen transfer mechanism. The HOO˙ radical scavenging of MM is comparable to that of Trolox in lipid medium, whereas it is 315.4 times more active in the polar environment.

In Silico Evaluation of the Radical Scavenging Mechanism of Mactanamide

Mactanamide (MA) is a diketopiperazine isolated from marine fungi of the genus Aspergillus. This compound is known as a natural antioxidant from experimental studies, yet this activity has not been successfully modeled thus far. In this work, the hydroperoxyl radical scavenging activity of MA was evaluated in the gas phase and physiological environments by thermodynamic and kinetic calculations. The results revealed that the HOO• radical scavenging of MA in the lipid media follows the formal hydrogen transfer mechanism via hydrogen abstraction at the O11-H bond. In the aqueous solution, however, the antioxidant activity follows the sequential proton loss electron transfer mechanism. The rate constant of the HOO• scavenging of MA in the polar environment is about 1045 times (k overall = 2.23 × 106 M-1 s-1) higher than that in the lipid medium (k overall = 2.20 × 103 M-1 s-1). In polar media, the HOO• radical scavenging activity of MA is therefore 18 times higher than that of Trolox, the reference compound when assessing antioxidant activity. The results presented here align well with the experimental data, validating the mechanistic pathways and thus providing useful insights into the antioxidant activity of MA.

A thermodynamic and kinetic study of the antioxidant activity of natural hydroanthraquinones

Novel hydroanthraquinones isolated from marine algal-derived endophytic fungus Talaromyces islandicus EN-501 exhibited promising antioxidant properties in preliminary studies, raising the prospect of adapting these compounds for therapeutic use in diseases caused by oxidative stress. For medicinal applications it is beneficial to develop a full understanding of the antioxidant activity of these compounds. In this study, the hydroperoxide radical scavenging activity of five natural hydroanthraquinones was evaluated by kinetic and thermodynamic calculations. The results showed that the radical scavenging of these hydroanthraquinones in the gas phase and in lipid solvents was defined by the formal hydrogen transfer mechanism, that for the polar environments was decided by the sequential proton loss electron transfer pathway. The hydroanthraquinones exhibited good hydroperoxide scavenging activity in both polar and non-polar media. The overall rate constant values for the radical scavenging reaction were in the range of 3.42 × 101 to 2.60 × 105 M-1 s-1 and 3.80 × 106 to 5.87 × 107 M-1 s-1 in pentyl ethanoate and water solvents, respectively. Thus the activity of 8-hydroxyconiothyrinone B (1) is about 2.6 and 444.6 times higher than that of Trolox in the studied solvents, identifying 8-hydroxyconiothyrinone B as a promising antioxidant.

Thermodynamic and kinetic studies of the antiradical activity of 5-hydroxymethylfurfural: computational insights

The antiradical properties of 5-HMF in the gas-phase and in physiological environments were examined by thermodynamic and kinetic calculations.

The antioxidant activity of natural diterpenes: theoretical insights

Diterpenes that were isolated from Crossopetalum gaumeri (Loes.) Lundell (Celastraceae) plants are reported to exhibit a range of biological activities, in particular as radical scavengers. Thus further insight into the antioxidant activity of diterpenes in physiological environments is much needed but not studied yet. In this study, the antioxidant activity of nine natural diterpenes was evaluated using kinetic and thermodynamic calculations. It was found that the sequential proton loss electron transfer (SPLET) mechanism is favored in polar environments, whereas formal hydrogen transfer (FHT) is the main pathway for the radical scavenging of these diterpenes in the gas phase as well as in lipid media. The rate constants for the HOO˙ radical scavenging of these compounds in the gas phase, polar and nonpolar solvents are in the range of 2.29 × 10⁻² to 4.58 × 10⁷, 9.74 × 10⁻³ to 1.67 × 10⁸ and 3.54 × 10⁻⁵ to 1.31 × 10⁵ M⁻¹ s⁻¹, respectively. 7-Deoxynimbidiol (6), exhibits the highest HOO˙ radical scavenging with k_overall = 1.69 × 10⁸ M⁻¹ s⁻¹ and 9.10 × 10⁴ M⁻¹ s⁻¹ in water and pentyl ethanoate solvents, respectively, that is about 1300 times higher than that of Trolox in polar environments. It is thus a promising natural antioxidant in physiological environments.

Diterpenes that were isolated from Crossopetalum gaumeri (Loes.) Lundell (Celastraceae) plants are reported to exhibit a range of biological activities, in particular as radical scavengers.