Theoretical Study of the Antioxidant Activity of Vitamin E: Reactions of α-Tocopherol with the Hydroperoxy Radical

Unveiling the Antioxidative Potential of Galangin: Complete and Detailed Mechanistic Insights through Density Functional Theory Studies

A comprehensive quantum mechanical investigation delved into the antioxidative activity of galangin (Glg). Thermochemical and kinetic data were used to assess antiradical, chelating, and renewal potential under physiological conditions. A brief comparison with reference antioxidants and other flavonoids characterized Glg as a moderate antioxidative agent. The substance showed significantly lower performance in lipid compared to aqueous solvent─the reaction rates for scavenging •OOH in both media were established at 3.77 × 103 M-1 s-1 and 6.21 × 104 M-1 s-1, respectively, accounting for the molar fraction of both interacting molecules at the given pH. The impact of pH value on the kinetics was assessed. Although efficient at chelating Cu(II) ions, the formed complexes can still undergo the Fenton reaction. On the other hand, they persistently scavenge •OH in statu nascendi. The flavonoid effectively repairs oxidatively damaged biomolecules except model lipid acids. All Glg radicals are readily restored by physiologically prevailing O2•-. Given this, the polyphenol is expected to participate in antiradical and regenerating activities multiple times, amplifying its antioxidative potential.

Partial Replacement of Synthetic Vitamin E by Polyphenols in Post-Weaning Piglets

Vitamin E is an essential nutrient usually recommended in post-weaning piglets, when a decline in the serum vitamin E concentration is observed. Selected polyphenols have the potential to partially replace vitamin E in animal feed. The aim of this study was to investigate the effect of the dietary inclusion of some commercial polyphenol products (PPs) on the growth performance, antioxidant status and immunity of post-weaning piglets. A total of 300 piglets (BW 7.18 kg ± 1.18) were randomly assigned to six dietary groups: CON- (40 mg/kg vitamin E); CON+(175.8 mg/kg vitamin E); and PP1, PP2, PP3 and PP4, in which 50% vitamin E of CON+ was replaced with PP with equivalent vitamin E activity. The PP1 group exhibited lower performance (p < 0.05) than the other dietary groups, but a similar performance to that commonly registered in pig farms. Dietary polyphenols did not influence the IgG concentration or the IL-6, IL-10, IFN-γ and TNF-α cytokine concentrations. A lower IL-8 level was found in the PP4 group than in the other groups. The diets that affected the vitamin A content showed the highest value (p < 0.05) in the PP1 group, and a trend was noted for vitamin E with a higher content in PP4 and CON+. The polyphenols-enriched diets, especially the PP3 diet, maintained an antioxidant capacity (whole blood KRL) similar to the CON+ diet. In conclusion, the replacement of vitamin E with all PPs enables partial vitamin E substitution in post-weaning piglets.

On the free radical scavenging and metallic ion chelating activities of pyridoxal - Could the pro-oxidant risk be competitive?

Primary and secondary antioxidant activities of pyridoxal have been investigated by using density functional theory (DFT) at the M05-2X level combined with 6-311++G(d,p) basis set for non-metallic atoms and LanL2DZ for metallic ions. The former has been examined by its free radical scavenging activity towards HOO^●, HO^●, and NO₂^●via different mechanisms including formal hydrogen transfer (FHT), proton transfer (PT), single electron transfer (SET), and radical adduct formation (RAF). The latter has been accomplished through its transition metal-chelating ability with Fe(III)/Fe(II) and Cu(II)/Cu(I) ions. The results show that pyridoxal illustrates as an efficient radical scavenger, especially, for HO^● and NO₂^● in water. The overall rate constants (k_overall) for the reactions with HOO^●, HO^●, and NO₂^● radicals are 1.30 × 10⁴, 5.76 × 10⁹, and 1.43 × 10⁹ M^-1s^-1, respectively. The SET from the anionic state is the most dominant for the HOO^● and NO₂^● scavenging reactions, while both RAF and SET contribute largely to the reaction with highly reactive HO^● radicals. Moreover, the anionic form of pyridoxal demonstrates a better role as a metal chelator than the neutral. However, the pro-oxidant risks of the formed complexes could be observed if there are superoxide radical anion (O₂^●-) and ascorbate (Asc^-) in aqueous media.

Impact of the phenolic O–H vs. C-ring C–H bond cleavage on the antioxidant potency of dihydrokaempferol

In order to correctly estimate hydrogen atom abstraction from polyphenols, kinetic analysis including suitable tunneling effects should be mandatory.

Substituent Effects on the N-H Bond Dissociation Enthalpies, Ionization Energies, Acidities, and Radical Scavenging Behavior of 3,7-Disubstituted Phenoxazines and 3,7-Disubstituted Phenothiazines

The substituent effects on the N-H bond dissociation enthalpies (BDE), ionization energies (IE), acidities (proton affinity, PA), and radical scavenging behavior of 3,7-disubstituted phenoxazines (PhozNHs) and 3,7-disubstituted phenothiazines (PhtzNHs) were determined using density functional theory, with the M05-2X functional in conjunction with the 6-311++G(d,p) basis set. These thermochemical parameters calculated in both gas phase and benzene solution with respect to the changes in several different substituents including halogen, electron-withdrawing, and electron-donating groups at both 3 and 7 positions in both PhozNHs and PhtzNHs systems were analyzed in terms of the inherent relationships between them with some quantitative substituent effect parameters. The kinetic rate constants of hydrogen-atom exchange reactions between PhozNH and PhtzNH derivatives with the HOO• radical were also calculated, and the effects of the substituents on the kinetic behaviors of these reactions were thereby quantitatively evaluated.

Effect of vitamin E on low density lipoprotein oxidation at lysosomal pH

Many cholesterol-laden foam cells in atherosclerotic lesions are macrophages and much of their cholesterol is present in their lysosomes and derived from low density lipoprotein (LDL). LDL oxidation has been proposed to be involved in the pathogenesis of atherosclerosis. We have shown previously that LDL can be oxidised in the lysosomes of macrophages. α-Tocopherol has been shown to inhibit LDL oxidation in vitro, but did not protect against cardiovascular disease in large clinical trials. We have therefore investigated the effect of α-tocopherol on LDL oxidation at lysosomal pH (about pH 4.5). LDL was enriched with α-tocopherol by incubating human plasma with α-tocopherol followed by LDL isolation by ultracentrifugation. The α-tocopherol content of LDL was increased from 14.4 ± 0.2 to 24.3 ± 0.3 nmol/mg protein. LDL oxidation was assessed by measuring the formation of conjugated dienes at 234 nm and oxidised lipids (cholesteryl linoleate hydroperoxide and 7-ketocholesterol) by HPLC. As expected, LDL enriched with α-tocopherol was oxidised more slowly than control LDL by Cu²⁺ at pH 7.4, but was not protected against oxidation by Cu²⁺ or Fe³⁺ or a low concentration of Fe²⁺ at pH 4.5 (it was sometimes oxidised faster by α-tocopherol with Cu²⁺ or Fe³⁺ at pH 4.5). α-Tocopherol-enriched LDL reduced Cu²⁺ and Fe³⁺ into the more pro-oxidant Cu⁺ and Fe²⁺ faster than did control LDL at pH 4.5. These findings might help to explain why the large clinical trials of α-tocopherol did not protect against cardiovascular disease.

Synthesis, DFT Calculations, and In Vitro Antioxidant Study on Novel Carba-Analogs of Vitamin E

Vitamin E is the most active natural lipophilic antioxidant with a broad spectrum of biological activity. α-Tocopherol (α-T), the main representative of the vitamin E family, is a strong inhibitor of lipid peroxidation as a chain-breaking antioxidant. Antioxidant and antiradical properties of vitamin E result from the presence of a phenolic hydroxyl group at the C-6 position. Due to stereoelectronic effects in the dihydropyranyl ring, the dissociation enthalpy for phenolic O–H bond (BDE_OH) is reduced. The high chain-breaking reactivity of α-T is mainly attributed to orbital overlapping of the 2p-type lone pair on the oxygen atom (O1) in para position to the phenolic group, and the aromatic π-electron system. The influence of the O1 atom on the antioxidant activity of vitamin E was estimated quantitatively. The all-rac-1-carba-α-tocopherol was synthesized for the first time. Along with model compounds, 1-carba-analog of Trolox and its methyl ester were screened for their in vitro antioxidant activity by inhibition of styrene oxidation, and for the radical-reducing properties by means of 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH) scavenging assay. To study the antioxidant activity, density functional theory (DFT) was also applied. Reaction enthalpies related to HAT (hydrogen atom transfer), SET–PT (sequential electron transfer—proton transfer), and SPLET (sequential proton loss—electron transfer) mechanisms were calculated.

DFT/B3LYP Study of the Substituent Effects on the Reaction Enthalpies of the Antioxidant Mechanisms of Sesamol Derivatives in the Gas phase and water

In this paper, the study of various ortho and meta–substituted Sesamol derivatives is presented. The reaction enthalpies related to three antioxidant action mechanisms HAT, SET–PT, and SPLET for substituted Sesamols, have been calculated using the DFT/B3LYP method in gas phase and water. Calculated results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP), and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In the ortho position, substituents show a larger effect on reaction enthalpies than in the meta position. In comparison with the gas phase, water attenuates the substituent effect on all reaction enthalpies. In the gas phase, BDEs are lower than PAs and IPs, i.e., HAT represents the thermodynamically preferred pathway. On the other hand, the SPLET mechanism represents the thermodynamically favored process in water. Results show that calculated enthalpies can be successfully correlated with Hammett constants (σm) of the substituted Sesamols. Furthermore, calculated IP and PA values for substituted Sesamols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO).

Detailed Investigation of the OH Radical Quenching by Natural Antioxidant Caffeic Acid Studied by Quantum Mechanical Models

The effectiveness of naturally occurring antioxidant caffeic acid in the inactivation of the very damaging hydroxyl radical has been theoretically investigated by means of hybrid density functional theory. Three possible pathways by which caffeic acid may inactivate free radicals were analyzed: hydrogen abstraction from all available hydrogen atoms, hydroxyl radical addition to all carbon atoms in the molecule, and single electron transfer. The reaction paths were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory including the contribution of tunneling. The more reactive sites in caffeic acid are the C4OH phenolic group and the C4 carbon atom, for the hydrogen abstraction and radical addition, respectively. The single electron transfer process seems to be thermodynamically unfavored, in both polar and nonpolar media. Both hydrogen abstraction and radical addition are very feasible, with a slight preference for the latter, with a rate constant of 7.29 × 10(10) M(-1) s(-1) at 300 K. Tunnel effects are found to be quite unimportant in both cases. Results indicate caffeic acid as a potent natural antioxidant in trapping and scavenging hydroxyl radicals.

The inactivation of lipid peroxide radical by quercetin. A theoretical insight

The effectiveness of naturally occurring antioxidant quercetin in the inactivation of the damaging lipid peroxide radical was investigated by means of hybrid density functional based approach, using the direct dynamics method, where the thermal rate constants were calculated using variational transition-state theory with multidimensional tunneling. H-atom abstraction in quercetin by CH(3)OO peroxide occurs preferentially at the 4'OH phenolic site, from both kinetic and thermodynamic points of view. In principle, the narrowness of the obtained adiabatic potential-energy profile makes the occurrence of a significant tunnelling contribution possible. In fact, this contribution enhances the value of the computed rate constant at 300 K from 1.94 x 10(1) to 9.63 x 10(3) M(-1) s(-1) indicating that quercetin is a potent natural antioxidant in trapping and scavenging free radicals.

Donator-acceptor map and work function for linear polyene-conjugated molecules. A density functional approximation study

Carotenoids are one particular type of conjugated chromophores with a great capacity for accepting electrons. The question posed here is how the capacity to accept electrons is related to extension of the conjugation. If there is a connection, any chromophore should represent a good antiradical, a point of interest for those investigating the biological effects of antioxidants. In order to analyze the relationship between the extension of the conjugation and the absorbance and electron-donor properties described in this paper, full geometry optimizations at the BPW91/D5DVZ level of theory are reported for a number of linear conjugated polyene systems. Maximum wavelengths were obtained using the TDDFT methodology. From these results, it is possible to conclude that large conjugated chromophores have a great capacity for accepting electrons but diminished power for donating electrons. Apparently, any chromophore should be a good antiradical, but various mechanisms exist for scavenging free radicals. In the case of linear polyene-conjugated molecules, indigo, blue, and green chromophores represent good antiradicals because they are also good antioxidants (effective electron donors). Yellow and red chromophores represent good antiradicals because they are good antireductants (effective electron acceptors). In the case of the molecules reported in this paper, the ionization energy and the electron affinity come close to the work function of graphite. This may be important for future applications, where the movement of the electrons is crucial.

A comparative study of the antioxidant power of flavonoid catechin and its planar analogue

The antioxidant ability of the flavanol catechin and its planar derivative, catechin 1 (PC1), was explored using the DF/B3LYP theoretical approach. Their potentiality in the hydrogen abstraction and electron transfer reactions, the main working mechanisms of antioxidants, was evaluated by computing the values of two key parameters, which are the OH bond dissociation energy and the ionization potential. Results indicated that the effect of a planar arrangement in the catechin molecule is small in the case of the hydrogen abstraction but greater for the electron transfer, since the in vacuo ionization potential value decreases by about 3 kcal/mol. The reaction of these molecules with the hydroperoxyl radical (*)OOH indicated that the H(*) abstraction is faster with the planar catechin.

Computation of relative bond dissociation enthalpies (DeltaBDE) of phenolic antioxidants from quantum topological molecular similarity (QTMS)

A recently proposed method called quantitative topological molecular similarity (QTMS) generated a model for the computation of the relative substituent effects on the bond dissociation enthalpies (DeltaBDEs) for a set of 39 phenols. The data set includes a diverse set of substituents with monosubstituted and poly-substituted derivatives that exhibit different electronic and steric effects. Many share common structural features with already well-established antioxidants. QTMS reveals the active region of the substituted phenols and identifies the electronic descriptors that best explain the range of DeltaBDEs observed. For substituents in the 4-X position (para) we find that our model requires a correction for radical stabilization enthalpy (RSE). Application of the QTMS methodology yields an unrivalled QSAR with r(2) = 0.98 and q(2) = 0.85 for the bond dissociation enthalpies of this phenolic antioxidant data set.

Tunneling in Green Tea: Understanding the Antioxidant Activity of Catechol-Containing Compounds. A Variational Transition-State Theory Study

The catechol functionality present in the catechins is responsible for the protective effects exerted by green tea against a wide range of human diseases. High-level electronic structure calculations and canonical variational transition-state theory including multidimensional tunneling corrections have allowed us to understand the key factors of the antioxidant effectiveness of the catechol group. This catechol group forms two hydrogen bonds with the two oxygen atoms of the lipid peroxyl radical, leading to a very compact reactant complex. This fact produces an extremely narrow adiabatic potential-energy profile corresponding to the hydrogen abstraction by the peroxyl radical, which makes it possible for a huge tunneling contribution to take place. So, quantum-mechanical tunneling highly increases the corresponding rate constant value, in such a way that catechins become able to trap the lipid peroxyl radicals in a dominant competition with the very damaging free-radical chain-lipid peroxidation reaction.

Trapping of the OH radical by alpha-tocopherol: a theoretical study

The antioxidant activity of alpha-tocopherol against the damaging hydroxyl radical was analyzed theoretically by hybrid density functional theory, following the direct dynamics method, where the thermal rate constants were calculated using variational transition-state theory with multidimensional tunneling. We found that the OH radical is only slightly or not at all selective, attacking by different mechanisms at several positions of the alpha-tocopherol molecule, giving competitive reactions. The most favorable pathways are the hydrogen abstraction reaction from the phenolic hydrogen and the electrophilic addition onto the aromatic ring. We propose a final rate constant, the sum of the competitive hydrogen abstraction and addition reactions, > or =2.7 x 10(8) M(-1) s(-1) at 298 K, where the hydrogen abstraction reaction represents only 20% of the total OH radical reaction. This result indicates that, molecule by molecule, in an apolar environment, alpha-tocopherol is less effective than coenzyme Q (which presents a rate constant of 6.2 x 10(10) M(-1) s(-1) at 298 K) as a scavenger of OH radicals. It was also found that both mechanisms are not direct but pass through intermediates in the entry channel, with little or no influence on the dynamics of the reactions. The hydrogen abstraction reaction also presents another intermediate in the exit channel, which may have a significant role in preventing the pro-oxidant effects of alpha-tocopherol, although less important than with free radicals other than OH.