Melanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO^., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO^.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO^.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO^. (k=1.5×10⁷ and 1.1×10⁵ M⁻¹ s⁻¹ in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.

Bio-oils employed for various industrial purposes, such as biodiesel production, undergo extensive oxidation and degradation during transformation processes. Therefore, it is extremely important to predict their stability at high temperature. We report herein a new procedure based on the optically detected profile of headspace O₂ concentration during isotherms at 130 °C for evaluating the oxidation kinetic parameters of several bio-oil feedstocks. The slope of O₂ consumption and the induction period duration were related to the oil characteristics (molecular structure, acidity, and presence of intrinsic antioxidants or metals). The increase of the induction time caused by a standardized propyl gallate addition yielded a semiquantitative value of radical generation rate. Investigated oils included used cooking oils; mono-, di-, and triglycerides from natural sources; free fatty acids; transesterified oils; and their blends. With respect to other methods, this characterization presents the advantage of disentangling and evaluating the role of both fatty acids composition and naturally occurring antioxidants, and allows the development of rational strategies for antioxidant protection of oils and of their blends.

Melanins are stable and non-toxic pigments with great potential as chemopreventive agents against oxidative stress for medical and cosmetic applications. Allomelanin is a class of nitrogen-free melanin often found in fungi. The artificial allomelanin obtained by the polymerization of 1,8-dihydroxynaphthalene (DHN), poly-DHN (PDHN), has been recently indicated as a better radical quencher than polydopamine (PDA), a melanin model obtained by the polymerization of dopamine (DA); however, the chemical mechanisms underlying this difference are unclear. Here we investigate, by experimental and theoretical methods, the ability of PDHN nanoparticles (PDHN-NP), in comparison to PDA-NP, to trap alkylperoxyl (ROO^•) and hydroperoxyl (HOO^•) radicals that are involved in the propagation of peroxidation in real conditions. Our results demonstrate that PDHN-NP present a higher antioxidant efficiency with respect to PDA-NP against ROO^• in water at pH 7.4 and against mixed ROO^• and HOO^• in acetonitrile, showing catalytic cross-termination activity. The antioxidant capacity of PDHN-NP in water is 0.8 mmol/g (ROO^• radicals quenched by 1 g of PDHN-NP), with a rate constant of 3 × 10⁵ M⁻¹ s⁻¹ for each reactive moiety. Quantum-mechanical calculations revealed that, thanks to the formation of a H-bond network, the quinones in PDHN-NP have a high affinity for H-atoms, thus justifying the high reactivity of PDHN-NP with HOO^• observed experimentally.

Peroxidation of vegetable oils represents a major problem for the food and biodiesel industries, and it is greatly accelerated by oil degree of unsaturation and by temperature increase. Phenols represent the most common additives used to counteract oil peroxidation, however clear structure-activity relationships at high temperatures are not available. We report, herein, a kinetic study of O₂ consumption during spontaneous peroxidation of sunflower oil at 130 °C in the presence of 18 antioxidants belonging to the main families of natural and synthetic phenols, including α-tocopherol, alkylphenols (BHT, BHA), hydroquinones (TBHD), catechols (quercetin, catechin) and gallates. Results show that TBHQ provide the best protection in terms of induction period (IP) duration and O₂ consumption rate. EPR spectroscopy demonstrated that the inhibition activity is negatively correlated to the stability of the phenoxyl radical of the antioxidant (A^•), suggesting that chain propagation with linoleate (RH) moieties A^• + RH → AH + R^• decreases the efficacy of those antioxidants forming persistent A^• radicals. These results provide important information to optimize the antioxidant activity of phenols and of novel phenol-based materials.

Superparamagnetic iron oxide nanoparticles (SPION) are important materials for biomedical applications, and phenol capping is a common procedure to passivate their surface. As phenol capped SPION have been reported to behave as antioxidants, herein, we investigate the mechanism underlying this activity by studying the reaction with alkyl peroxyl (ROO^•) radicals. SPION were prepared by coprecipitation of Fe(II) and Fe(III), using phenolic antioxidants (gallic acid, Trolox and nordihydroguaiaretic acid) as post-synthesis capping agents and by different purification procedures. The reactivity of ROO^• was investigated by inhibited autoxidation studies, using styrene as an oxidizable substrate (solvent MeCN, 30 °C) and azo-bis(isobutyronitrile) as a radical initiator. While unprotected, bare SPION behaved as prooxidant, accelerating the O₂ consumption of styrene autoxidation, phenol capping provided a variable antioxidant effect that was dependent upon the purification degree of the material. Thoroughly washed SPION, containing from 7% to 14% (w/w) of phenols, had a low reactivity toward peroxyl radicals, while SPION with a higher phenol content (46% to 55%) showed a strong radical trapping activity. Our results indicate that the antioxidant activity of phenol-capped SPION can be caused by its release in a solution of weakly bound phenols, and that purification plays a major role in determining the properties of these materials.

Melanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO^., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO^.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO^.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO^. (k=1.5×10⁷ and 1.1×10⁵ M⁻¹ s⁻¹ in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.