A computational methodology for accurate predictions of rate constants in solution: Application to the assessment of primary antioxidant activity

Antioxidant Activity of Natural Samwirin A: Theoretical and Experimental Insights

Samwirin A (SW), a natural compound isolated from Sambucus williamsii or Rourea harmandiana, is known to exhibit potent antiosteoporosis activity and promote cell proliferation in rat osteoblast-like UMR 106 cells. Antiosteoporosis activity suggests that the compound must also exhibit antioxidant activity but this has not been studied thus far. In the present study, the antioxidant activity of SW was examined by experimental and computational studies. It was found that SW exhibits good hydroperoxyl scavenging activity, particularly in water at physiological pH (k overall = 1.01 × 107 M-1 s-1). The single-electron transfer mechanism defines the HOO• + SW reaction in water, while the activity in the lipid medium is moderate and it follows the formal hydrogen transfer mechanism. The rate constant of the HOO• scavenging reaction in the aqueous solution is about 78 times higher than the reference compound Trolox. The computational results are in line with experimental data underscoring that SW is a promising radical scavenger in aqueous media at physiological pH.

A detailed DFT-based study of the free radical scavenging activity and mechanism of daphnetin in physiological environments

Daphnetin, a biologically active coumarin derivative found in plants of the genus Daphne, is a potent antioxidant phenolic compound. The present work describes the mechanisms and kinetics of the HO, NO, HOO, and NO₂ scavenging activities of daphnetin in physiological environments using quantum chemistry calculations. The main antiradical mechanisms have been studied: formal hydrogen transfer (FHT), sequential electron transfer proton transfer (SETPT), sequential proton loss electron transfer (SPLET), and radical adduct formation (RAF). Besides its good HO scavenging activity in physiological environments, daphnetin is expected to exhibit good HOO and NO₂ scavenging activities in water with k_overall = 1.51 × 10⁷ and 4.79 × 10⁸ M^-1s^-1, respectively. The FHT mechanism decides the HO scavenging activity in aqueous solution, as well as HO, HOO, and NO₂ scavenging activities in lipid media, while SPLET is the primary mechanism in water for HOO and NO₂ scavenging activities. The theoretical predictions were found to be in good agreement with the available experimental data, which supports the reliability of the calculations.

Encapsulation of Polyphenols from Lycium barbarum Leaves into Liposomes as a Strategy to Improve Their Delivery

This study is focused on the encapsulation of polyphenols from Lycium barbarum leaves into liposomes as a strategy to improve their delivery. Liposomes loaded with Lycium barbarum leaves extract were obtained and characterized for particle size, polydispersity, entrapment efficiency, and stability. Liposomes presented entrapment efficiency higher than 75%, nanometric particle size, narrow polydispersity, and good stability over three months at 4 °C. The liposomes containing Lycium barbarum offered a slower release of polyphenols with attenuated burst effect compared with the dissolution of free Lycium barbarum extract in phosphate buffer solution at pH 7.4. Moreover, an in vitro pretreatment of 24 h with loaded liposomes showed a cytoprotective effect against H₂O₂-induced cytotoxicity on L-929 mouse fibroblasts cells. These preliminary findings imply that liposomes could be successfully employed as carriers for polyphenols in pharmaceutical applications.

A Boron-Containing Compound Acting on Multiple Targets Against Alzheimer's Disease. Insights from Ab Initio and Molecular Dynamics Simulations

Given the multifactorial nature and pathogenesis of Alzheimer's disease, therapeutic strategies are addressed to combine the benefits of every single-target drug into a sole molecule. Quantum mechanics and molecular dynamics (MD) methods were employed here to investigate the multitarget action of a boron-containing compound against Alzheimer's disease. The antioxidant activity as a radical scavenger and metal chelator was explored by means of density functional theory. The most plausible radical scavenger mechanisms, which are hydrogen transfer, radical adduct formation, and single-electron transfer in aqueous and lipid environments, were fully examined. Metal chelation ability was investigated by considering the complexation of Cu(II) ion, one of the metals that in excess can even catalyze the β-amyloid (Aβ) aggregation. The most probable complexes in the physiological environment were identified by considering both the stabilization energy and the shift of the λ^max induced by the complexation. The excellent capability to counteract Aβ aggregation was explored by performing MD simulations on protein-ligand adducts, and the activity was compared with that of curcumin, chosen as a reference.

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.

Antioxidant and UV-radiation absorption activity of aaptamine derivatives - potential application for natural organic sunscreens

Antioxidant and UV absorption activities of three aaptamine derivatives including piperidine[3,2-b]demethyl(oxy)aaptamine (C1), 9-amino-2-ethoxy-8-methoxy-3H-benzo[de][1,6]naphthyridine-3-one (C2), and 2-(sec-butyl)-7,8-dimethoxybenzo[de]imidazo[4,5,1-ij][1,6]-naphthyridin-10(9H)-one (C3) were theoretically studied by density functional theory (DFT). Direct antioxidant activities of C1-C3 were firstly evaluated via their intrinsic thermochemical properties and the radical scavenging activity of the potential antioxidants with the HOO˙/HO˙ radicals via four mechanisms, including: hydrogen atom transfer (HAT), single electron transfer (SET), proton loss (PL) and radical adduct formation (RAF). Kinetic calculation reveals that HOO˙ scavenging in water occurs via HAT mechanism with C1 (k app, 7.13 × 106 M-1 s-1) while RAF is more dominant with C2 (k app, 1.40 × 105 M-1 s-1) and C3 (k app, 2.90 × 105 M-1 s-1). Antioxidant activity of aaptamine derivatives can be classified as C1 > C3 > C2. Indirect antioxidant properties based on Cu(i) and Cu(ii) ions chelating activity were also investigated in aqueous phase. All three studied compounds show spontaneous and favorable Cu(i) ion chelating activity with ΔG 0 being -15.4, -13.7, and -15.7 kcal mol-1, whereas ΔG 0 for Cu(ii) chelation are -10.4, -10.8, and -2.2 kcal mol-1 for C1, C2 and C3, respectively. In addition, all compounds show UVA and UVB absorption; in which the excitations are determined mostly as π-π* transition. Overall, the results suggest the potential applications of the aaptamines in pharmaceutics and cosmetics, i.e. as a sunscreen and antioxidant ingredient.

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.

Quantum Mechanical Predictions of the Antioxidant Capability of Moracin C Isomers

The antioxidant capability of moracin C and iso-moracin C isomers against the OOH free radical was studied by applying density functional theory (DFT) and choosing the M05-2X exchange-correlation functional coupled with the all electron basis set, 6-311++G(d,p), for computations. Different reaction mechanisms [hydrogen atom transfer (HAT), single electron transfer (SET), and radical adduct formation (RAF)] were taken into account when considering water- and lipid-like environments. Rate constants were obtained by applying the conventional transition state theory (TST). The results show that, in water, scavenging activity mainly occurs through a radical addition mechanism for both isomers, while, in the lipid-like environment, the radical addition process is favored for iso-moracin C, while, redox- and non-redox-type reactions can equally occur for moracin C. The values of pKa relative to the deprotonation paths at physiological pH were predicted in aqueous solution.

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.

Are thymol, rosefuran, terpinolene and umbelliferone good scavengers of peroxyl radicals?

DFT-based computational calculations have been used to investigate the hydroperoxyl radical scavenging activity of four essential oil constituents namely thymol (Thy), rosefuran (Ros), terpinolene (Ter), and umbelliferone (Umb). Different reaction mechanisms including formal hydrogen transfer (FHT), radical adduct formation (RAF), sequential proton loss electron transfer (SPLET), and sequential electron transfer proton transfer (SETPT) have been examined in the gas phase and physiological environments. It was found that the HOO radical scavenging activity of these compounds is strongly influenced by the environment, which becomes more important in water than pentyl ethanoate. According to the overall reaction rate constants, the phenolic compounds Thy and Umb are predicted to exhibit excellent activity in aqueous solution. Umb with an overall rate constant of 1.44 × 10⁸M^-1s^-1 at physiological pH is among the best HOO radical scavengers in water with activity comparable to that of caffeic acid, higher than those of ascorbic acid, guaiacol and eugenol, and much higher than that of Trolox.

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.

Inhibitory activity of quercetin, its metabolite, and standard antiviral drugs towards enzymes essential for SARS-CoV-2: the role of acid-base equilibria

The recently declared global pandemic of a new human coronavirus called SARS-CoV-2, which causes respiratory tract disease COVID-19, has reached worldwide resonance and global efforts are being made to look for possible cures. Sophisticated molecular docking software, as well as available protein sequence and structure information, offer the ability to test the inhibition of two important targets of SARS-CoV-2, furin (FUR) enzyme, and spike glycoprotein, or spike protein (SP), that are key to host cell adhesion and hijacking. The potential inhibitory effect and mechanism of action of acid-base forms of different antiviral drugs, dominant at physiological pH, chloroquine (CQ), hydroxychloroquine (HCQ), and cinanserin (CIN), which have been shown to be effective in the treatment of SARS-CoV-2 virus, is reported with the special emphasis on their relative abundances. On the other hand, the potential inhibitory effect of the dominant acid-base forms of quercetin (Q) and its oxidative metabolite 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H) benzofuranone (BZF), which are constituents of traditional food products believed to exhibit antiviral effects, was also examined. The undertaken study includes the determination of the major energy contributions to the binding energy as well as in-depth analysis of amino acid residues at the active pocket and possible interactions. The approach that we propose here may be an additional strategy for combating the deadly virus by preventing the first step of the virus replication cycle. Preliminary research has shown that the investigated compounds exert an inhibitory effect against the SARS-CoV-2 furin enzyme and spiked glycoprotein through different acid-base forms. These investigations may be helpful in creating potential therapeutic agents in the fight against the SARS-CoV-2 virus. On the other hand, the results we predicted in this computational study may be the basis for new experimental in vitro and in vivo studies.

Insights into the mechanisms and kinetics of the hydroperoxyl radical scavenging activity of Artepillin C

Through the single electron transfer mechanism, Artepillin C scavenges the hydroperoxyl radical in water approximately 572 times faster than Trolox.

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.

Computationally designed p-coumaric acid analogs: searching for neuroprotective antioxidants

Newly designed p-coumaric acid derivatives are promising candidates as multifunctional antioxidants with neuroprotective effects.

Structure–antioxidant activity relationships in boldine and glaucine: a DFT study

DFT calculations indicate that boldine and glaucine exhibit direct antioxidant activity through the HAT and SPLET (at high pH values) mechanisms.

Free radical scavenging activity of newly designed sesamol derivatives

Recently proposed derivatives of sesamol as better oxidants than the parent molecule are predicted to react faster, with several orders larger rate constants than sesamol itself.

Antioxidant and Anticancer Properties of Functionalized Ferrocene with Hydroxycinnamate Derivatives-An Integrated Experimental and Theoretical Study

Two ferrocenyl derivatives, Fc-CA and Fc-FA, were synthesized by a condensation reaction between the amino ferrocene and hydroxycinnamic acids, that is, caffeic acid (CA) and ferulic acid (FA). The structures and purity of all compounds were characterized by ¹H- and ¹³C NMR spectroscopies, Mass spectrometry (MS), and elemental analysis. The antioxidant properties of Fc-CA and Fc-FA and of its ligand were studied for free radical scavenging activity toward DPPH^•, superoxide anion (O₂^•-), NO^•, and ABTS^•+ by UV-vis and electron spin resonance spectroscopies. The cytotoxicity of Fc-CA and Fc-FA against MCF-7 and MDA-MB-231 breast cancer cells and MRC-5 human lung fibroblasts cell was higher than that of cisplatin. The geometry and electronic structures of all compounds were then simulated using density functional theory at M05-2X/6-311+G(d,p) level of theory. Thermodynamics of the free radical quenching reactions by common mechanisms reveal the higher antioxidant properties of the Fc-CA and Fc-FA in comparison to their ligands. An in-depth study of the free radical scavenging activity against HOO^• and HO^• radicals was performed for two of the most favorable and competitive mechanisms, the hydrogen transfer (either hydrogen atom transfer or proton-coupled electron transfer mechanisms) and the radical adduct formation. The in silico studies indicated that ferrocenyl derivatives exhibited prominent binding affinity to protein models in comparison to CA and FA. Their dock scores were notable at ligand binding sites of ERα, Erβ, and JAK2 proteins. Dock pose analysis also shed light into the possible mechanism of action for the studied compounds.

Capsaicin, a Powerful •OH-Inactivating Ligand

Oxidative conditions are frequently enhanced by the presence of redox metal ions. In this study, the role of capsaicin (8-methyl-N-vanillyl-6-nonenamide, CAP) in copper-induced oxidative stress was investigated using density functional theory simulations. It was found that CAP has the capability to chelate Cu(II), leading to complexes that are harder to reduce than free Cu(II). CAP fully turns off the Cu(II) reduction by Asc⁻, and slows down the reduction in this cation by O₂^•−. Therefore, CAP is proposed as an ^•OH-inactivating ligand by impeding the reduction in metal ions (OIL-1), hindering the production of ^•OH via Fenton-like reactions, at physiological pH. CAP is also predicted to be an excellent antioxidant as a scavenger of ^•OH, yielded through Fenton-like reactions (OIL-2). The reactions between CAP-Cu(II) chelates and ^•OH were estimated to be diffusion-limited. Thus, these chelates are capable of deactivating this dangerous radical immediately after being formed by Fenton-like reactions.

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 antioxidant capacity of an imidazole alkaloids family through single-electron transfer reactions

The single-electron transfer (SET) reactions from the neutral and mono-anion species of five imidazole alkaloids (lepidines A, B, C, D, and E) against hydroperoxyl radicals have been studied using the density functional theory and the Marcus theory. The deprotonated species of three alkaloids were found to have free radical scavenging activity. The antioxidant activity was studied via single-electron transfer (SET) under physiological conditions. The SET reactions for lepidines B, D, and E were found to have rate constants ranging from 10⁵ to 10⁶ M^-1 s^-1. Therefore, they are predicted to be able to deactivate hydroperoxyl radicals and therefore the damage caused by them to polyunsaturated fatty acids. It is important to mention that the acid-base equilibrium plays an important role in their free radical scavenging activity. Graphical abstract Lepidines are predicted to be able to deactivate hydroperoxyl radicals and the damage caused by them to polyunsaturated fatty acids.

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.

Radical scavenging activity of hydroxycinnamic acids in polar and nonpolar solvents: A computational investigation

The aim of this work is to perform a computational study of the radical scavenging activity of a series of common hydroxycinnamic acids (HCAs) in polar and nonpolar solvents in order to rationalize the experimental order obtained in ethanol and to analyze the solvent effect on mechanism and radical scavenging capacity. The thermodynamics of the main mechanisms, namely, hydrogen atom transfer (HAT), sequential proton loss followed by electron transfer (SPLET), and single electron transfer followed by proton transfer (SET-PT) were investigated at the M05-2X/6-31[Formula: see text]G([Formula: see text]) level of theory using the SMD solvation model. This study shows that the SET-PT mechanism is disfavored in all media, whereas HAT is the most thermodynamically favored mechanism in gas phase and SPLET is the preferred reaction pathway in pentyl ethanoate, ethanol and water. The thermodynamically preferred site of antioxidant action and the radical scavenging order are predicted using the BDE[Formula: see text] and (PA[Formula: see text]ETE)[Formula: see text] descriptors corresponding to the HAT and SPLET mechanisms, respectively. The obtained results point out that the mechanism and the radical scavenging potency are influenced by solvent polarity and our predictions are in agreement with the experimental measurements performed in ethanol giving the following descending order: caffeic [Formula: see text] [Formula: see text] [Formula: see text]-coumaric acid. Our results also show that the ortho substitution of caffeic acid by strong electron donating groups leads to a notable increase of their radical scavenging activity and new potent HCA derivatives are designed.

Theoretical Study of the Iron Complexes with Aminoguanidine: Investigating Secondary Antioxidant Activity

A thorough analysis of the thermodynamic stability of various complexes of aminoguanidine (AG) with Fe(III) at a physiological pH is presented. Moreover, the secondary antioxidant activity of AG is studied with respect to its kinetic role in the Fe(III) reduction to Fe(II) when reacting with the superoxide radical anion or ascorbate. Calculations are performed at the M05(SMD)/6-311+G(d,p) level of theory. Solvent effects (water) are taken into account in both geometry optimizations and frequency calculations employing the SMD solvation method. Even though the results of this study show that AG can form an extensive number of stable complexes with Fe(III), none of these can reduce the rate constant of the initial step of the Haber-Weiss cycle when the reducing agent is O2•-. However, when the reductant is the ascorbate anion, AG is capable of reducing the rate constant of this reaction significantly, to the point of inhibiting the production of •OH radicals. In fact, the most stable complex of Fe(III) with AG, having a ∆Gf° of -37.9 kcal/mol, can reduce the rate constant of this reaction by 7.9 × 105 times. Thus, AG possesses secondary antioxidant activity relative to the Fe(III)/Fe(II) reduction with ascorbate, but not with O2•-. Similar results have also been found for AG relative to the Cu(II)/Cu(I) reduction, in agreement with experimental results.

Antioxidant Potential of Psychotropic Drugs: From Clinical Evidence to In Vitro and In Vivo Assessment and toward a New Challenge for in Silico Molecular Design

Due to high oxygen consumption, the brain is particularly vulnerable to oxidative stress, which is considered an important element in the etiopathogenesis of several mental disorders, including schizophrenia, depression and dependencies. Despite the fact that it is not established yet whether oxidative stress is a cause or a consequence of clinic manifestations, the intake of antioxidant supplements in combination with the psychotropic therapy constitutes a valuable solution in patients' treatment. Anyway, some drugs possess antioxidant capacity themselves and this aspect is discussed in this review, focusing on antipsychotics and antidepressants. In the context of a collection of clinical observations, in vitro and in vivo results are critically reported, often highlighting controversial aspects. Finally, a new challenge is discussed, i.e., the possibility of assessing in silico the antioxidant potential of these drugs, exploiting computational chemistry methodologies and machine learning. Despite the physiological environment being incredibly complex and the detection of meaningful oxidative stress biomarkers being all but an easy task, a rigorous and systematic analysis of the structural and reactivity properties of antioxidant drugs seems to be a promising route to better interpret therapeutic outcomes and provide elements for the rational design of novel drugs.

Insights into the Mechanism of Hydroxyl Radical Mediated Oxidations of 2-Aminopurine: A Computational and Sonochemical Product Analysis Study

Mechanistic details of hydroxyl radical (^•OH) mediated oxidations of 2-aminopurine (2AP) in the aqueous phase have been established in this study via a combination of DFT calculations (at the M05-2X/6-311+G(d,p) level with SMD solvation) and sonochemical end product analyses by the LC-Q-TOF-MS/MS method. Rate constants and branching ratios for single electron transfer (SET), two H-abstractions (HA), and seven radical adduct formation (RAF) reactions of ^•OH with 2AP were evaluated using transition state theory (TST). The RAF at the C8-position of 2AP is noted as the dominant process, which constitutes almost 46.1% of overall reaction routes. The SET mechanism accounts for the second major pathway (39.6%) followed by RAF at the C6-position (14.3%). Formations of 14 transformation products (TPs, i.e., the nonradical end products) in the sonochemical reactions of ^•OH with 2AP have been identified by means of the LC-Q-TOF-MS/MS technique. Among the 14 TPs (designated as TP1 to TP14), the lowest and highest mass to charge ratio (m/z) were respectively observed at 129 and 269 in ESI-MS positive ionization mode. The identities of all TPs have been proposed on the basis of elemental composition of [M + H]⁺ ions and their respective MS-MS fragmentation pattern. Four TPs (including guanine) are considered as obtained directly from primary transients by radical elimination, radical-radical combination/disproportionation reactions. The remaining 10 TPs are postulated as a result of successive self- and/or cross-reactions of primary transients/four first generation TPs with reagents such as ^•OH, O₂, and solvent H₂O molecules.

Is Usnic Acid a Promising Radical Scavenger?

Usnic acid (UA) is a natural product found in the lichen genera. Because of the phenolic groups in its structure, UA is suspected to be an antioxidant. Therefore, in this study, the radical scavenging of UA was investigated in physiological environments in silico by using kinetic calculations. It was found that the overall rate constant for the hydroxyl radical scavenging activity was approximately 10⁹ M^-1 s^-1 in all environments, whereas the HOO^• and CH₃OO^• radical scavenging activities were only significant in the polar environments with k in the range of 10³-10⁴ M^-1 s^-1. The results also revealed that the HO^• scavenging activity followed the single electron transfer (SET) and radical adduct formation mechanisms; however, the SET pathway (for the dianion HU^2-) played a dominant role in the scavenging of other studied radicals, including CH₃O^•, CCl₃O^•, CCl₃OO^•, NO₂, SO₄ ^•-, and N₃ ^•. The activity of UA against these radicals was as high as that of typical phenolic acids such as ferulic acid, p-coumaric acid, caffeic acid, dihydrocaffeic acid, and sinapinic acid (k _f ∼ 10⁸ M^-1 s^-1) in polar solvents. Thus, UA is a promising natural antioxidant in aqueous environments.

Antioxidative Action of Ellagic Acid-A Kinetic DFT Study

Although one can find numerous studies devoted to the investigation of antioxidative activity of ellagic acid (EA) in the scientific literature, the mechanisms of its action have not yet been fully clarified. Therefore, further kinetic studies are needed to understand its antioxidative capacity completely. This work aims to reveal the underlying molecular mechanisms responsible for the antioxidative action of EA. For this purpose, its reactions with HO^• and CCl₃OO^• radicals were simulated at physiological conditions using the quantum mechanics-based test for overall free-radical scavenging activity. The density functional theory in combination with the conductor-like polarizable continuum solvation model was utilized. With HO^• radical EA conforms to the hydrogen atom transfer and radical adduct formation mechanisms, whereas sequential proton loss electron transfer mechanism is responsible for scavenging of CCl₃OO^• radical. In addition, compared to trolox, EA was found more reactive toward HO^•, but less reactive toward CCl₃OO^•. The calculated rate constants for the reactions of EA with both free radicals are in a very good agreement with the corresponding experimental values.

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.

The Antioxidant Capability of Higenamine: Insights from Theory

Density functional theory was employed to highlight the antioxidant working mechanism of higenamine in aqueous and lipid-like environments. Different reaction mechanisms were considered for the reaction of higenamine with the •OOH radical. The pH values and the molar fraction at physiological pH were determined in aqueous solution. The results show that the preferred reaction mechanism was the hydrogen atom transfer from the catecholic ring. The computed kinetic constants revealed that, in order to obtain reliable results, it is important to consider all the species present in water solution derived from acid-base equilibria. From the present investigation, it emerges that at physiological pH (7.4), the scavenging activity of higenamine against the •OOH radical is higher than that of Trolox, chosen as a reference antioxidant. Furthermore, higenamine results to be more efficient for that purpose than melatonin and caffeine, whose protective action against oxidative stress is frequently associated with their reactive oxygen species (ROS) scavenging activity.