1,721,004 research outputs found
Methods to Determine Chain-Breaking Antioxidant Activity of Nanomaterials beyond DPPH<sup>•</sup>. A Review
Full text linkThis review highlights the progress made in recent years in understanding the mechanism of action of nanomaterials with antioxidant activity and in the chemical methods used to evaluate their activity. Nanomaterials represent one of the most recent frontiers in the research for improved antioxidants, but further development is hampered by a poor characterization of the ‘‘antioxidant activity’’ property and by using oversimplified chemical methods. Inhibited autoxidation experiments provide valuable information about the interaction with the most important radicals involved in the lipid oxidation, namely alkylperoxyl and hydroperoxyl radicals, and demonstrate unambiguously the ability to stop the oxidation of organic materials. It is proposed that autoxidation methods should always complement (and possibly replace) the use of assays based on the quenching of stable radicals (such as DPPH• and ABTS•+). The mechanisms leading to the inhibition of the autoxidation (sacrificial and catalytic radical trapping antioxidant activity) are described in the context of nanoantioxidants. Guidelines for the selection of the appropriate testing conditions and of meaningful kinetic analysis are also given
Synergic Antioxidant Activity of γ-Terpinene with Phenols and Polyphenols Enabled by Hydroperoxyl Radicals
Full text linkAntioxidant interactions of γ-terpinene with α-tocopherol mimic 2,2,5,7,8-pentamethyl-6-chromanol (PMHC)
and caffeic acid phenethyl ester (CAPE), used as models, respectively, of mono- and poly-phenols were
demonstrated by differential oximetry during the inhibited autoxidation of model substrates: stripped sunflower
oil, squalene, and styrene. With all substrates, γ-terpinene acts synergistically regenerating the chain-breaking
antioxidants PMHC and CAPE from their radicals, via the formation of hydroperoxyl radicals. The inhibition
duration for mixtures PMHC/γ-terpinene and CAPE/γ-terpinene increased with γ-terpinene concentration, while
rate constants for radical-trapping were unchanged by γ-terpinene, being 3.1 × 106 and 4.8 × 105 M 1s 1 for
PMHC and CAPE in chlorobenzene (30 ◦C). Using 3,5-di-tert-butylcatechol and 3,5-di-tert-butyl-1,2-bezoquinone
we demonstrate that γ-terpinene can reduce quinones to catechols enabling their antioxidant activity. The
different synergy mechanism of γ-terpinene with mono- and poly-phenolic antioxidants is discussed and its
relevance is proven in homogenous lipids using natural α-tocopherol and hydroxytyrosol as antioxidants, calling
for further studies in heterogenous food products
Singlet oxygen quenching- and chain-breaking antioxidant-properties of a quercetin dimer able to prevent age-related macular degeneration
No full textA dimer of quercetin prepared through a Mannich reaction protects pyridinium bisretinoid A2E from photooxidation at 430 nm in aqueous medium at pH 7.4. In the presence of light and O2, A2E is quickly attacked by 1O2 produced in situ (by excited A2E) to give nonaoxirane and other oxygenated compounds which can be involved in diseases of the macula. Peroxyl radicals might have only a marginal role on the photooxidation of A2E. The dimer is actually a potent quencher of 1O2 with a rate constant kQ of 8.5 × 108 M−1 s−1 in methanol at room temperature. On the other hand, its antioxidant abilities against ROO· radicals are quite limited since kROO· = 7.3 × 105 M−1 s−1 (in buffer solution at pH 7.4), the value being essentially identical to that of quercetin. The quenching of 1O2 by the dimer is therefore the main reason for the A2E protection and prevention of age-related macular degeneration
Hydroperoxyl Radicals (HOO.): Vitamin E Regeneration and H-Bond Effects on the Hydrogen Atom Transfer
No full textHydroperoxyl (HOO.) and alkylperoxyl (ROO.) radicals show a different behavior in H-atom-transfer processes. Both radicals react with an analogue of α-tocopherol (TOH), but HOO., unlike ROO., is able to regenerate TOH by a fast H-atom transfer: TO.+HOO.→TOH+O2. The kinetic solvent effect on the H-atom transfer from TOH to HOO.is much stronger than that observed for ROO.because noncovalent interactions with polar solvents (Solv⋅⋅⋅HOO.) destabilize the transition state
From catechol-tocopherol to catechol-hydroquinone polyphenolic antioxidant hybrids
No full textMultidefence antioxidants represent a valuable solution for the protection against oxidative stress. From the planned synthesis of a catechol-tocopherol hybrid, we isolated a catechol-hydroquinone hybrid through a BBr3-mediated benzochromene-fluoren-1-ol transposition. The compound prepared showed a remarkable chain-breaking antioxidant in the catechol portion, while the very sensitive hydroquinone moiety revealed to be an efficient generator of hydroperoxyl radicals
The role of sulfur and heavier chalcogens in the chemistry of antioxidants
No full textSulfur chemistry plays a central role in cellular redox homeostasis and cysteine-derived antioxidants like glutathione are the most abundant in biological systems. Inspired by Nature, the insertion of divalent sulfur as a substituent in phenolic antioxidants, e.g in thiatocopherol, seeking for improved antioxidant performance, has been an important strategy for long time. Replacement of sulfur with heavier chalcogens like Selenium and Tellurium has brought to even more performing antioxidants, able to quench peroxyl radical in a catalytic fashion and to express unusually high reactivity. On the other hand, natural bioactive compounds like plant-derived thiosulfinates (R-S(O)S-R) own their exceptional antioxidant properties to the ability of releasing sulfenic acids, whose antioxidant behavior has only recently been clarified. The chemistry and redox properties of unstable sulfenic acids (R-SOH), and analogous selenenic acids (R-SeOH) have also recently been elucidated, to better understand the properties of chalcogen-based natural antioxidants, and to develop novel bio-inspired compounds. This fascinating chemistry will be reviewed and the most significant achievement will be presented
) as a reducing agent: unexpected synergy with antioxidants. A review
Full text linkThis review highlights the progress made in recent years in harnessing the peculiar chemistry of the hydroperoxyl, or perhydroxyl, radical (HOO center dot) during lipid peroxidation, particularly with regard to its interaction with antioxidants. The HOO center dot radical, the protonated form of superoxide, plays an important role in the propagation and termination of lipid peroxidation in nonaqueous systems. However, differently from alkylperoxyl (ROO center dot) radicals that have only oxidizing ability, HOO center dot has a two-faced oxidizing and reducing activity. The HOO center dot radical can reduce the radical of the antioxidant (phenols and aromatic amines) by H-atom transfer (A(center dot) + HOO center dot -> AH + O-2) thus increasing the length of the inhibition period and the effectiveness of the antioxidant. The simultaneous presence of HOO center dot and ROO center dot radicals triggers the catalytic antioxidant activity of quinones and nitroxides and explains the antioxidant activity of melanin-like polymers. The HOO center dot radical can be formed by fragmentation of ROO center dot radicals deriving from amines, alcohols, substituted alkenes and may be present at low concentrations in many oxidizing systems. Pro-aromatic compounds, like the natural essential oil component gamma-terpinene, are the most effective sources of HOO center dot and behave as co-antioxidants in the presence of nitroxides or quinones. The future developments and applications of HOO center dot chemistry in the context of the inhibition of autoxidation are also discussed
Antioxidant activity of nanomaterials
Full text linkNanomaterials represent one of the most promising frontiers in the research for improved antioxidants. Some nanomaterials, including organic (i.e. melanin, lignin) metal oxides (i.e. cerium oxide) or metal (i.e. gold, platinum) based nanoparticles, exhibit intrinsic redox activity that is often associated with radical trapping and/or with superoxide dismutase-like and catalase-like activities. Redox inactive nanomaterials can be transformed into antioxidants by grafting low molecular weight antioxidants on them. Herein, we propose a classification of nanoantioxidants based on their mechanism of action, and we review the chemical methods used to measure antioxidant activity by providing a rationale of the chemistry behind them
Measuring Antioxidant Activity in Bioorganic Samples by the Differential Oxygen Uptake Apparatus: Recent Advances
Full text linkThe measure of O2 consumption during the inhibited autoxidation of an easily oxidizable substrate is one of the most reliable and predictive methods to assess antioxidant activity, especially for structure-activity relationship studies, for food and industrial applications. The differential oxygen uptake apparatus described herein represents a powerful and cost-effective way to obtain antioxidant activity from inhibited autoxidation studies. These experiments provide the rate constant and the stoichiometry of the reaction between antioxidants and peroxyl radicals (ROO∙), which are involved in the propagation of radical damage. We show the operation principles and the utility of this instrumentation in the bioorganic laboratory, with regard to the recent advances in this field, ranging from the study of natural antioxidants in biomimetic system, to the use of substrates generating hydroperoxyl radicals, and to the evaluation of novel nanoantioxidants
The Antioxidant Activity of Quercetin in Water Solution
Full text linkAbstract: Despite its importance, little is known about the absolute performance and the mechanism for quercetin’s antioxidant activity in water solution. We have investigated this aspect by combining differential oxygen-uptake kinetic measurements and B3LYP/6311+g (d,p) calculations. At pH = 2.1 (30 °C), quercetin had modest activity (kinh = 4.0 × 103 M−1 s−1), superimposable to catechol. On raising the pH to 7.4, reactivity was boosted 40-fold, trapping two peroxyl radicals in the chromen-4-one core and two in the catechol with kinh of 1.6 × 105 and 7.0 × 104 M−1 s−1. Reaction occurs from the equilibrating mono-anions in positions 4′ and 7 and involves firstly the OH in position 3, having bond dissociation enthalpies of 75.0 and 78.7 kcal/mol, respectively, for the two anions. Reaction proceeds by a combination of proton-coupled electron-transfer mechanisms: electron–proton transfer (EPT) and sequential proton loss electron transfer (SPLET). Our results help rationalize quercetin’s reactivity with peroxyl radicals and its importance under biomimetic settings, to act as a nutritional antioxidant
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