The antioxidant activity of natural diterpenes: theoretical insights

The antifungal properties of terpenoids from the endophytic fungus Bipolaris eleusines

A series of terpenoids (1-17), comprising six new compounds designated bipolariterpenes A-F (1-6) and eleven recognized compounds (7-17), were isolated from the wheat culture of the potato endophytic fungus Bipolaris eleusines. Their structures and stereochemistry were clarified by HRESIMS, NMR, DP4 + probability analyses, and computations for electronic circular dichroism (ECD). All compounds are made up of six meroterpenoids, four sesterterpenes and seven sesquiterpenes. Among them, four sesterterpenes (4, 5, 10, 11) were investigated for their antifungal, antibacterial and cytotoxic properties, and six meroterpenoids (1-3, 7-9) were evaluated for their antifungal properties. The compounds 7, 9, and 10 had substantial antifungal activity against Epidermophyton floccosum at a concentration of 100 µM. No antibacterial and cytotoxic activities were observed.

Unified short syntheses of oxygenated tricyclic aromatic diterpenes by radical cyclization with a photoredox catalyst

The biomimetic two-phase strategy employing polyene cyclization and subsequent oxidation/substitution is an effective approach for divergent syntheses of [6-6-6]-tricyclic diterpenes. However, this strategy requires lengthy sequences for syntheses of oxygenated tricyclic aromatic abietane/podocarpane diterpenes owing to the many linear oxidation/substitution steps after cyclization. Here, we present a new synthetic route based on a convergent reverse two-phase strategy employing a reverse radical cyclization approach, which enabled the unified short syntheses of four aromatic abietane/podocarpane diterpenes and the divergent short syntheses of other related diterpenes. Oxygenated and substituted precursors for cyclization were convergently prepared through Friedel-Crafts acylation and rhodium-catalyzed site-selective iodination. Radical redox cyclization using an iridium photoredox catalyst involving neophyl rearrangement furnished the thermodynamically favored 6-membered ring preferentially. (±)-5,6-Dehydrosugiol, salvinolone, crossogumerin A, and Δ5-nimbidiol were synthesized in only 8 steps. An oxygenated cyclized intermediate was also useful for divergent derivatization to sugiol, ferruginol, saprorthoquinone, cryptomeriololide, and salvinolone.

Functional Characterization of a 2OGD Involved in Abietane-Type Diterpenoids Biosynthetic Pathway in Salvia miltiorrhiza

Salvia miltiorrhiza is one of the most commonly used Chinese medicinal herbs. Tanshinones, the most abundant lipid-soluble bioactive constituents of S. miltiorrhiza, are a class of structural highly oxidized abietane-type diterpenoids with multiple pharmacological activities. Although several enzymes, including diterpene synthase, cytochrome P450, and Fe(II)/2-oxoglutarate-dependent dioxygenase (2OGD), have been functionally characterized in biosynthesis of abietane-type diterpenoids, the highly oxidized structure and complex secondary metabolic network of tanshinones imply that more oxidases should be characterized. Here, we identified a new 2OGD (Sm2OGD25) from S. miltiorrhiza. Molecular cloning and functional studies in vitro showed that Sm2OGD25 could catalyze the hydroxylation of sugiol at C-15 and C-16 positions to produce hypargenin B and crossogumerin C, respectively. The phylogenetic analysis of the DOXC family demonstrated that Sm2OGD25 belongs to the DOXC54 clade. Furthermore, structural modeling and site-directed mutagenesis characterization revealed the importance of the hydrogen-bonding residue Y339 and the hydrophobic residues (V122, F129, A144, A208, F303, and L344) in substrate binding and enzyme activity. This study will promote further studies on the catalytic characterization of plant 2OGDs and the secondary metabolic biosynthesis network of diterpenoids.

The radical scavenger capacity and mechanism of prenylated coumestan-type compounds: a DFT analysis

The antiradical capacity and mechanisms of two representative coumestan-type compounds, namely isosojagol (Iso) and phaseoul (Pha), were examined using quantum chemistry calculations and computational kinetics methods. From a thermodynamic point of view, the 18CH groups of the prenyl substituent have been found to be the most suitable sites for radical attacks via the formal hydrogen transfer (FHT) mechanism. However, the kinetic study revealed that the reaction at these CH groups is slow and does not contribute to the overall reactivity of these compounds, which the phenolic groups mainly define. The kinetic study also revealed that the studied compounds are good free radical scavengers with overall rate coefficients as high as recognized antioxidants such as carnosic acid, artepillin C, thymol, and rosefuran.

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.

Computational Investigation of the Antioxidant Activity of Dihydroxybenzoic Acids in Aqueous and Lipid Media

Reactive free radicals have both beneficial and destructive effects. Indeed, at physiological levels, free radicals help to preserve homeostasis by acting as signal transducers. However, excessive generation of free radicals can harm and damage membranes, proteins, and DNA, among other cell structures. Dihydroxybenzoic acids (DHBAs) have proven their antioxidant capacity against a large variety of free radicals, as well as their ability to inhibit or restrict reactive species overproduction. In this paper, a computational analysis of the antioxidant activity of a series of DHBAs in polar and nonpolar media was carried out at the DFT/M06-2X/6-[Formula: see text] level of theory. The implicit SMD solvation model was used in order to rationalize the experimental findings and to investigate the solvent effect on the mechanism and the radical scavenging ability. The obtained results put in evidence that HAT is the predominant mechanism in nonpolar media, whereas SPLET is more favored in polar environment. The BDE[Formula: see text], [Formula: see text], and [Formula: see text] descriptors are used to predict the most reactive hydroxyl groups and the antioxidant activity order of the studied DHBAs. Our results are in total agreement with experimental findings (inhibition of lipid peroxidation and scavenging of hydrogen peroxide). Moreover, this study shows that the substitution of the hydrogen atom by strong electron-donating groups, namely NMe2, in the ortho positions of the best experimental DHBAs leads to a significant enhancement of their antioxidant activity.

Synthesis and evaluation of the antioxidant activity of 3-pyrroline-2-ones: experimental and theoretical insights

The heterocyclic γ-lactam ring 2-pyrrolidinone has four carbon atoms and one nitrogen atom. Among the group of derivatives of 2-pyrrolidinones, 1,5-dihydro-2H-pyrrol-2-ones, also known as 3-pyrroline-2-ones, play a significant structural role in a variety of bioactive natural compounds. In this study, three-component reactions were used to successfully synthesize six polysubstituted 3-hydroxy-3-pyrroline-2-one derivatives. The antioxidant activity of the compounds was tested by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, identifying 4-ethoxycarbonyl-3-hydroxy-5-(4-methylphenyl)-1-phenyl-3-pyrroline-2-one (4b) as the most promising radical scavenger. Quantum chemistry calculations of the thermodynamics and kinetics of the radical scavenging activity also suggest that 4b is an effective HO˙ radical scavenger, with k_overall values of 2.05 × 10⁹ and 1.54 × 10¹⁰ M⁻¹ s⁻¹ in pentyl ethanoate and water, respectively. On the other hand, 4b could not scavenge hydroperoxyl radicals in either media. The ability of 4b to scavenge hydroxyl radicals in polar and non-polar environments is comparable to that of conventional antioxidants such as melatonin, gallic acid, indole-3-carbinol, ramalin, or Trolox. Thus 4b may be classed as a promising HO˙ radical scavenger in the physiological environment.

Derivatives of 3-hydroxy-3-pyrroline-2-one were effectively synthesized via multicomponent reactions and exhibited potential HO˙ radical scavenging activity.

Erectcyanthins A-C from marine sponge Hyrtios erectus: anti-dyslipidemic agents attenuate hydroxymethylglutaryl coenzyme A reductase

Bioactivity-steered chromatographic purification of the solvent extract of marine sponge Hyrtios erectus (Thorectidae) led to the isolation of three undescribed cyanthiwigin-type diterpenoids, erectcyanthins A-C. Erectcyanthin B exhibited comparable attenuation activity against 3-hydroxy-3-methylglutaryl-coenzyme A reductase (IC₅₀ 0.07 mM) with that displayed by anti-dyslipidemic agent atorvastatin (IC₅₀ 0.08 mM). Comparatively greater antioxidant properties of erectcyanthin B (IC₅₀ ∼ 0.4 mM) than that displayed by erectcyanthin A (IC₅₀ ∼ 0.5 mM), erectcyanthin C and the standard α-tocopherol (IC₅₀ 1.5-1.7 mM) against oxidants also corroborated its promising bioactivity. Erectcyanthin B exhibited considerably greater anti-inflammatory activities (IC₅₀ 0.88-1.09 mM) than other erectcyanthin analogues in the series. The potential anti-dyslipidemic activity of erectcyanthins was linearly correlated with electronic parameter (topological polar surface area ∼ 74.6) along with balanced hydrophilic-lipophilic properties (logarithmic octanol-water partition coefficient 1.76). This study recognized the anti-dyslipidemic property of erectcyanthin B as a promising pharmaceutical lead.

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 significance of long-range correction to the hydroperoxyl radical-scavenging reaction of trans-resveratrol and gnetin C

Density functional theory has been gaining popularity for studying the radical scavenging activity of antioxidants. However, only a few studies investigate the importance of calculation methods on the radical-scavenging reactions. In this study, we examined the significance of (i) the long-range correction on the coulombic interaction and (ii) the London dispersion correction to the hydroperoxyl radical-scavenging reaction of trans-resveratrol and gnetin C. We employed B3LYP, CAM-B3LYP, M06-2X exchange-correlation functionals and B3LYP with the D3 version of Grimme's dispersion in the calculations. The results showed that long-range correction on the coulombic interaction had a significant effect on the increase of reaction and activation energies. The increase was in line with the change of hydroperoxyl radical's orientation in the transition state structure. Meanwhile, the London dispersion correction only had a minor effect on the transition state structure, reaction energy and activation energy. Overall, long-range correction on the coulombic interaction had a significant impact on the radical-scavenging reaction.

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.

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.

Concept, mechanism, and applications of phenolic antioxidants in foods

In this review, the concept of phenolic antioxidants, mechanisms of action, and applications have been reviewed. Phenolic compounds (PCs) acts as an antioxidant by reacting with a variety of free radicals. The mechanism of antioxidant actions involved either by hydrogen atom transfer, transfer of a single electron, sequential proton loss electron transfer, and chelation of transition metals. In foods, the PCs act as antioxidants which are measured with several in vitro spectroscopic methods. The PCs have been found in milk and a wide range of dairy products with sole purposes of color, taste, storage stability, and quality enhancement. The role of PCs in three types of food additives, that is, antimicrobial, antioxidant, and flavoring agents have been critically reviewed. The literature revealed that PCs present in a variety of foods possess several health benefits such as antibacterial, antihyperlipidemic, anticancer, antioxidants, cardioprotective, neuroprotective, and antidiabetic properties. PRACTICAL APPLICATIONS: Phenolic compounds are strong antioxidants and are safer than synthetic antioxidants. The wide occurrence in plant foods warranted continuous review applications. This review, therefore, presented an updated comprehensive overview of the concept, mechanism, and applications of phenolic antioxidants in foods.

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.