Autoxidation vs. antioxidants - the fight for forever

Oxidative stress modulating nanomaterials and their biochemical roles in nanomedicine

Many pathological conditions are predominantly associated with oxidative stress, arising from reactive oxygen species (ROS); therefore, the modulation of redox activities has been a key strategy to restore normal tissue functions. Current approaches involve establishing a favorable cellular redox environment through the administration of therapeutic drugs and redox-active nanomaterials (RANs). In particular, RANs not only provide a stable and reliable means of therapeutic delivery but also possess the capacity to finely tune various interconnected components, including radicals, enzymes, proteins, transcription factors, and metabolites. Here, we discuss the roles that engineered RANs play in a spectrum of pathological conditions, such as cancer, neurodegenerative diseases, infections, and inflammation. We visualize the dual functions of RANs as both generator and scavenger of ROS, emphasizing their profound impact on diverse cellular functions. The focus of this review is solely on inorganic redox-active nanomaterials (inorganic RANs). Additionally, we deliberate on the challenges associated with current RANs-based approaches and propose potential research directions for their future clinical translation.

A guide to ferroptosis, the biological rust of cellular membranes

Unprotected iron can rust due to oxygen exposure. Similarly, in our body, oxidative stress can kill cells in an iron-dependent manner, which can give rise to devastating diseases. This type of cell death is referred to as ferroptosis. Generally, ferroptosis is defined as an iron-catalyzed form of regulated necrosis that occurs through excessive peroxidation of polyunsaturated fatty acids within cellular membranes. This review summarizes how ferroptosis is executed by a rather primitive biochemical process, under tight regulation of lipid, iron, and redox metabolic processes. An overview is given of major classes of ferroptosis inducers and inhibitors, and how to detect ferroptosis. Finally, its detrimental role in disease is briefly discussed.

Reactions of lipid hydroperoxides and how they may contribute to ferroptosis sensitivity

The accumulation of lipid hydroperoxides (LOOHs) has long been associated with numerous pathologies and has more recently been shown to drive a specific type of cell death known as ferroptosis. In competition with their detoxification by glutathione peroxidases, LOOHs can react with both one-electron reductants and one-electron oxidants to afford radicals that initiate lipid peroxidation (LPO) chain reactions leading to more LOOH. These radicals can alternatively undergo a variety of (primarily unimolecular) reactions leading to electrophilic species that destabilize the membrane and/or react with cellular nucleophiles. While some reaction mechanisms leading to lipid-derived electrophiles have been known for some time, others have only recently been elucidated. Since LOOH (and related peroxides, LOOL) undergo these various reactions at different rates to afford distinct product distributions specific to their structures, not all LOOHs (and LOOLs) should be equivalently problematic for the cell - be it in their propensity to initiate further LPO or fragment to electrophiles, drive membrane permeabilization and eventual cell death. Herein we briefly review the fates of LOOH and discuss how they may contribute to the modulation of cell sensitivity to ferroptosis by different lipids.

Discovery of Novel Ortho-Aminophenol Derivatives Targeting Lipid Peroxidation with Potent Antiferroptotic Activities

The concept of ferroptosis inhibition has gained growing recognition as a promising therapeutic strategy for addressing a wide range of diseases. Here, we present the discovery of four series of ortho-aminophenol derivatives as potential ferroptosis inhibitors beginning with the endogenous substance 3-hydroxyanthranilic acid (3-HA) by employing quantum chemistry techniques, in vitro and in vivo assays. Our findings reveal that these ortho-aminophenol derivatives exhibit unique intra-H bond interactions, compelling ortho-amines to achieve enhanced alignment with the aromatic π-system, thereby expanding their activity. Notably, compounds from all four series display remarkable activity against RSL3-induced ferroptosis, showcasing an activity 100 times more than that of 3-HA. Furthermore, these compounds also demonstrate robust in vivo efficacy in protecting mice from kidney ischemia-reperfusion injury and acetaminophen-induced hepatotoxicity. In summary, we provide four distinct series of active scaffolds that significantly expand the chemical space of ferroptosis inhibitors, serving as valuable insights for future structural modifications.

Strong Uptake of Gas-Phase Organic Peroxy Radicals (ROO•) by Solid Surfaces Driven by Redox Reactions

Organic peroxy radicals (ROO•) are key oxidants in a wide range of chemical systems such as living organisms, chemical synthesis and polymerization systems, combustion systems, the natural environment, and the Earth's atmosphere. Although surfaces are ubiquitous in all of these systems, the interactions of organic peroxy radicals with these surfaces have not been studied until today because of a lack of adequate detection techniques. In this work, the uptake and reaction of gas-phase organic peroxy radicals (CH3OO• and i-C3H7OO•) with solid surfaces was studied by monitoring each radical specifically and in real-time with mass spectrometry. Our results show that the uptake of organic peroxy radicals varies widely with the surface material. While their uptake by borosilicate glass and perfluoroalkoxy alkanes (PFA) was negligible, it was substantial with metals and even dominated over the gas-phase reactions with stainless steel and aluminum. The results also indicate that these uptakes are controlled by redox reactions at the surfaces for which the products were analyzed. Our results show that the reactions of organic peroxy radicals with metal surfaces have to be carefully considered in all the experimental investigations of these radicals as they could directly impact the kinetic and mechanistic knowledge derived from such studies.

Analysis of the Efficiency of Antioxidants in Inhibiting Lipid Oxidation in Terms of Characteristic Kinetic Parameters

In this work, we aim to find physical evidence demonstrating the crucial role that the effective concentration of antioxidants (AOs) present at the interfacial region of emulsions has in controlling the inhibition of the lipid oxidation reaction. We prepared a series of antioxidants of different hydrophobicities derived from chlorogenic and protocatechuic acids. We first monitored, in intact emulsions, the (sigmoidal) production of conjugated dienes and determined the corresponding induction times, tind. Independently, we determined the effective concentrations of the antioxidants in the same intact emulsions. Results show that both the length of the induction periods and the antioxidant interfacial concentrations parallel each other, with a maximum at the octyl-dodecyl derivatives. The ratio between the interfacial antioxidant concentrations and the induction periods remains constant for all AOs in the same series, so that the rates of initiation of lipid oxidation are the same regardless of the hydrophobicity of the antioxidant employed. The constancy in the rate of initiation provides strong experimental evidence for a direct relationship between interfacial concentrations and antioxidant efficiencies. Results suggest new possibilities to investigate lipid peroxidation under non-forced conditions and are of interest to formulators interested in preparing emulsions with antimicrobial properties.

Oxidative stress in patients with coronavirus disease and end-stage renal disease: a pilot study

BACKGROUND

Oxidative stress, an imbalance between reactive oxygen species production and antioxidant capacity, increases in patients with coronavirus disease (COVID-19) or renal impairment. We investigated whether combined COVID-19 and end-stage renal disease (ESRD) would increase oxidative stress levels compared to each disease alone.

METHODS

Oxidative stress was compared among three groups. Two groups comprised patients with COVID-19 referred to the hospital with or without renal impairment (COVID-ESRD group [n = 18]; COVID group [n = 17]). The third group (ESRD group [n = 18]) comprised patients without COVID-19 on maintenance hemodialysis at a hospital.

RESULTS

The total oxidative stress in the COVID-ESRD group was lower than in the COVID group (p = 0.047). The total antioxidant status was higher in the COVID-ESRD group than in the ESRD (p < 0.001) and COVID (p < 0.001) groups after controlling for covariates. The oxidative stress index was lower in the COVID-ESRD group than in the ESRD (p = 0.001) and COVID (p < 0.001) groups. However, the three oxidative parameters did not differ significantly between the COVID and COVID-ESRD groups.

CONCLUSIONS

The role of reactive oxygen species in the pathophysiology of COVID-19 among patients withESRD appears to be non-critical. Therefore, the provision of supplemental antioxidants may not confer a therapeutic advantage, particularly in cases of mild COVID-19 in ESRD patients receiving hemodialysis. Nonetheless, this area merits further research.

Antioxidant Hydrogels: Antioxidant Mechanisms, Design Strategies, and Applications in the Treatment of Oxidative Stress-Related Diseases

Oxidative stress is a biochemical process that disrupts the redox balance due to an excess of oxidized substances within the cell. Oxidative stress is closely associated with a multitude of diseases and health issues, including cancer, diabetes, cardiovascular diseases, neurodegenerative disorders, inflammatory conditions, and aging. Therefore, the developing of antioxidant treatment strategies has emerged as a pivotal area of medical research. Hydrogels have garnered considerable attention due to their exceptional biocompatibility, adjustable physicochemical properties, and capabilities for drug delivery. Numerous antioxidant hydrogels have been developed and proven effective in alleviating oxidative stress. In the pursuit of more effective treatments for oxidative stress-related diseases, there is an urgent need for advanced strategies for the fabrication of multifunctional antioxidant hydrogels. Consequently, the authors' focus will be on hydrogels that possess exceptional reactive oxygen species and reactive nitrogen species scavenging capabilities, and their role in oxidative stress therapy will be evaluated. Herein, the antioxidant mechanisms and the design strategies of antioxidant hydrogels and their applications in oxidative stress-related diseases are discussed systematically in order to provide critical insights for further advancements in the field.

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Aminic Organoselenium Compounds as Glutathione Peroxidase Mimics and Inhibitors of Ferroptosis

The synthesis of diarylamine-based organoselenium compounds via the nucleophilic substitution reactions has been described. Symmetrical monoselenides and diselenides were conveniently synthesized by the reduction of their corresponding selenocyanates using sodium borohydride. Selenocyanates were obtained from 2-chloro acetamides by the nucleophilic displacement with potassium selenocyanate. Selenides were synthesized by treating the 2-chloro acetamides with in situ generated sodium butyl selenolate as nucleophile. Further, the newly synthesized organoselenium compounds were evaluated for their glutathione peroxidase (GPx)-like activity in thiophenol assay. This study revealed that the methoxy-substituted organoselenium compounds showed significant effect on the GPx-like activity. The catalytic parameters for the most efficient catalysts were also determined. The anti-ferroptotic activity for all GPx-mimics evaluated in a 4-OH-tamoxifen (TAM) inducible GPx4 knockout cell line using liproxstatin as standard.

Interrupted Homolytic Substitution Enables Organoboron Compounds to Inhibit Radical Chain Reactions Rather than Initiate Them

The reactions of organoboranes with peroxyl radicals are key to their use as radical initiators for a vast array of radical chain reactions, particularly at low temperatures where high stereoselectivity or regioselectivity is desired. Whereas these reactions generally proceed via concerted homolytic substitution (SH2) mechanisms, organoboranes that bear groups that can stabilize tetracoordinate boron radical "ate" complexes (e.g., catecholboranes) undergo this reaction via a stepwise addition/fragmentation sequence and serve as useful stoichiometric alkyl radical precursors. Here we show that arylboronic esters and amides derived from catecholborane and diaminonaphthaleneborane, respectively, are potent radical-trapping antioxidants (RTAs). Mechanistic studies reveal that this is because the radical "ate" complexes derived from peroxyl radical addition to boron are sufficiently persistent to trap another radical in an interrupted SH2 reaction. Remarkably, the reactivity of these organoboranes as inhibitors of autoxidation was shown to translate from simple hydrocarbons to the phospholipids of biological membranes such that they can inhibit ferroptosis, the cell death modality driven by lipid autoxidation and relevant in neurodegeneration and other major pathologies. The unique mechanism of these organoboranes is one of only a handful of RTA mechanisms that are not based on H-atom transfer processes and provide a new dimension to boron chemistry and its applications.

Enhancement of Diarylamine Antioxidant Activity by Molybdenum Dithiocarbamates

Molybdenum dithiocarbamates (MDTCs) are indispensable lubricant additives. Although their role as antiwear agents is well established, they have also been attributed antioxidant properties that are not understood. MDTCs do not inhibit autoxidation, but they markedly enhance the capacity of diphenylamines (DPAs)─ubiquitous radical-trapping antioxidants (RTAs)─to do so. We find this synergy to be evident not only at elevated temperatures (160 °C in n-hexadecane) but also at moderate temperatures, where autoxidations can be continuously monitored and kinetics more easily interpreted (100 °C in squalane). Interestingly, the synergy disappeared in an unsaturated hydrocarbon (n-hexadec-1-ene), where the RTA activity of the DPA is known to result from the diarylnitroxide derived therefrom. Autoxidations of squalane carried out in the presence of the diarylnitroxide─wherein it is a poor inhibitor─were much better inhibited in the presence of MDTC, suggesting that it converts the nitroxide to (a) more competent RTA(s). Indeed, preparative experiments revealed two species: DPA and a DPA dimer into which a single oxygen atom had been incorporated. This conversion is accelerated by the oxidation of MDTC to a dioxo molybdenum species. A mechanism is proposed to account for these observations, and the implications of our findings and their interpretation are discussed.

Regulation of ferroptosis by lipid metabolism

Ferroptosis is an iron-dependent lethal mechanism that can be activated in disease and is a proposed target for cancer therapy. Ferroptosis is defined by the overwhelming accumulation of membrane lipid peroxides. Ferroptotic lipid peroxidation is initiated on internal membranes and then appears at the plasma membrane, triggering lethal ion imbalances and membrane permeabilization. Sensitivity to ferroptosis is governed by the levels of peroxidizable polyunsaturated lipids and associated lipid metabolic enzymes. A different network of enzymes and endogenous metabolites restrains lipid peroxidation by interfering with the initiation or propagation of this process. This emerging understanding is informing new approaches to treat disease by modulating lipid metabolism to enhance or inhibit ferroptosis.

Recent advances in lignin antioxidant: Antioxidant mechanism, evaluation methods, influence factors and various applications

Lignin, a by-product of processing lignocellulosic materials, has a polyphenolic structure and can be used as an antioxidant directly or synergistically with synthetic types of antioxidants, leading to different applications. Its antioxidant mechanism is mainly related to the production of ROS, but the details need to be further investigated. The antioxidant property of lignin is mainly related to the content of phenolic hydroxyl group, but methoxy, purity will also have an effect on it. In addition, different methods to detect the antioxidant properties of lignin have different advantages and disadvantages. In this paper, the antioxidant mechanism of lignin, the methods to determine the antioxidant activity and the progress of its application in various fields are reviewed. In addition, the current research on the antioxidant properties of lignin and the hot directions are provided, and an outlook on the research into the antioxidant properties of lignin is provided to broaden its potential application areas.

Tandem Hydroperoxyl-Alkylperoxyl Radical Quenching by an Engineered Nanoporous Cerium Oxide Nanoparticle Macrostructure (NCeONP): Toward Efficient Solid-State Autoxidation Inhibitors

The use of nanomaterials as inhibitors of the autoxidation of organic materials is attracting tremendous interest in petrochemistry, food storage, and biomedical applications. Metal oxide materials and CeO2 in particular represent one of the most investigated inorganic materials with promising radical trapping and antioxidant abilities. However, despite the importance, examples of the CeO2 material's ability to retard the autoxidation of organic substrates are still lacking, together with a plausible chemical mechanism for radical trapping. Herein, we report the synthesis of a new CeO2-derived nanoporous material (NCeONP) with excellent autoxidation inhibiting properties due to its ability to catalyze the cross-dismutation of alkyl peroxyl (ROO•) and hydroperoxyl (HOO•) radicals, generated in the system by the addition of the pro-aromatic hydrocarbon γ-terpinene. The antioxidant ability of NCeONP is superior to that of other nanosized metal oxides, including TiO2, ZnO, ZrO2, and pristine CeO2 nanoparticles. Studies of the reaction with a sacrificial reductant allowed us to propose a mechanism of inhibition consisting of H atom transfer from HOO• to the metal oxides (MOx + HOO• → MOx-H• + O2), followed by the release of the H atom to an ROO• radical (MOx-H• + ROO• → MOx + ROOH). Besides identifying NCeONP as a promising material for developing effective antioxidants, our study provides the first evidence of a radical mechanism that can be exploited to develop novel solid-state autoxidation inhibitors.

Emergence of (hydro)persulfides as suppressors of lipid peroxidation and ferroptotic cell death

Recognition of the prevalence of hydropersulfides (RSSH) and characterization of their enhanced two-electron reactivity relative to thiols have led to their implication in maintaining cellular redox homeostasis, in addition to other potential roles. Recent attention on the one-electron reactivity of RSSH has uncovered their potent radical-trapping antioxidant activity, which enables them to inhibit phospholipid peroxidation and associated cell death by ferroptosis. Herein, we briefly review key aspects of the reactivity and underlying physicochemical properties of RSSH. We emphasize their reactivity to radicals-particularly lipid peroxyl radicals that propagate the lipid peroxidation chain reaction-and the recent recognition that this results in ferroptosis suppression. We highlight open questions related to recent developments in this area and, given that all living organisms possess the ability to synthesize persulfides endogenously, suggest they may be primordial radical scavengers that occurred early in evolution and still play a role today.

Intramolecular H-Atom Transfers in Alkoxyl Radical Intermediates Underlie the Apparent Oxidation of Lipid Hydroperoxides by Fe(II)

The one-electron reduction of lipid hydroperoxides by low-valent iron species is believed to be a driver of cellular lipid peroxidation and associated ferroptotic cell death. We investigated reactions of cholesterol 7α-OOH, the primary cholesterol autoxidation product, with Fe2+ to find that 7-ketocholesterol (7-KC, an oxidation product) is the major product under these (reducing) conditions. Mechanistic studies reveal the intervention of a 1,2-H-atom shift upon formation of the 7-alkoxyl radical to yield a ketyl radical that can be oxidized by either Fe3+ or O2 to give 7-KC, the most abundant oxysterol in vivo. We also investigated the corresponding reduction of the isomeric cholesterol 5α-OOH and again found that an oxidation product (5-hydroxycholesten-3-one) predominates under reducing conditions. An intramolecular H-atom shift (this time 1,4-) in the initially formed 5-alkoxyl radical is suggested to yield a ketyl radical that is oxidized to give the observed product. It would appear that a 1,2-H shift also accounts for the predominance of ketones over alcohols when unsaturated fatty acid hydroperoxides are exposed to iron-based reductants, which had previously been reported with hematin and demonstrated here with Fe2+. The predominance of 7-KC over the corresponding alcohol is maintained when cholesterol 7α-OOH embedded in phospholipid liposomes is treated with Fe2+ or when ferroptosis is induced in mouse embryonic fibroblasts. Our observation that 7-KC accumulates in ferroptotic cells suggests that it may be a good biomarker for ferroptosis.

Lipid Peroxidation and Antioxidant Protection

Lipid peroxidation (LP) is the most important type of oxidative-radical damage in biological systems, owing to its interplay with ferroptosis and to its role in secondary damage to other biomolecules, such as proteins. The chemistry of LP and its biological consequences are reviewed with focus on the kinetics of the various processes, which helps understand the mechanisms and efficacy of antioxidant strategies. The main types of antioxidants are discussed in terms of structure-activity rationalization, with focus on mechanism and kinetics, as well as on their potential role in modulating ferroptosis. Phenols, pyri(mi)dinols, antioxidants based on heavy chalcogens (Se and Te), diarylamines, ascorbate and others are addressed, along with the latest unconventional antioxidant strategies based on the double-sided role of the superoxide/hydroperoxyl radical system.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Insight into the Antioxidant Activity of 1,8-Dihydroxynaphthalene Allomelanin Nanoparticles

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.

Unexpectedly Efficient Aging of Organic Aerosols Mediated by Autoxidation

Multiphase oxidative aging is a ubiquitous process for atmospheric organic aerosols (OA). But its kinetics was often found to be slow in previous laboratory studies where high hydroxyl radical concentrations ([^•OH]) were used. In this study, we performed heterogeneous oxidation experiments of several model OA systems under varied aging timescales and gas-phase [^•OH]. Our results suggest that OA heterogeneous oxidation may be 2-3 orders of magnitude faster when [^•OH] is decreased from typical laboratory flow tube conditions to atmospheric levels. Direct laboratory mass spectrometry measurements coupled with kinetic simulations suggest that an intermolecular autoxidation mechanism mediated by particle-phase peroxy radicals greatly accelerates OA oxidation, with enhanced formation of organic hydroperoxides, alcohols, and fragmentation products. With autoxidation, we estimate that the OA oxidation timescale in the atmosphere may be from less than a day to several days. Thus, OA oxidative aging can have greater atmospheric impacts than previously expected. Furthermore, our findings reveal the nature of heterogeneous aerosol oxidation chemistry in the atmosphere and help improve the understanding and prediction of atmospheric OA aging and composition evolution.

Differential Contributions of Distinct Free Radical Peroxidation Mechanisms to the Induction of Ferroptosis

Ferroptosis is a form of regulated cell death driven by lipid peroxidation of polyunsaturated fatty acids (PUFAs). Lipid peroxidation can propagate through either the hydrogen-atom transfer (HAT) or peroxyl radical addition (PRA) mechanism. However, the contribution of the PRA mechanism to the induction of ferroptosis has not been studied. In this study, we aim to elucidate the relationship between the reactivity and mechanisms of lipid peroxidation and ferroptosis induction. We found that while some peroxidation-reactive lipids, such as 7-dehydrocholesterol, vitamins D3 and A, and coenzyme Q10, suppress ferroptosis, both nonconjugated and conjugated PUFAs enhanced cell death induced by RSL3, a ferroptosis inducer. Importantly, we found that conjugated PUFAs, including conjugated linolenic acid (CLA 18:3) and conjugated linoleic acid (CLA 18:2), can induce or potentiate ferroptosis much more potently than nonconjugated PUFAs. We next sought to elucidate the mechanism underlying the different ferroptosis-inducing potency of conjugated and nonconjugated PUFAs. Lipidomics revealed that conjugated and nonconjugated PUFAs are incorporated into distinct cellular lipid species. The different peroxidation mechanisms predict the formation of higher levels of reactive electrophilic aldehydes from conjugated PUFAs than nonconjugated PUFAs, which was confirmed by aldehyde-trapping and mass spectrometry. RNA sequencing revealed that protein processing in the endoplasmic reticulum and proteasome are among the most significantly upregulated pathways in cells treated with CLA 18:3, suggesting increased ER stress and activation of unfolded protein response. These results suggest that protein damage by lipid electrophiles is a key step in ferroptosis.

Ferroptosis: A flexible constellation of related biochemical mechanisms

It is common to think about and depict biological processes as being governed by fixed pathways with specific components interconnected by concrete positive and negative interactions. However, these models may fail to effectively capture the regulation of cell biological processes that are driven by chemical mechanisms that do not rely absolutely on specific metabolites or proteins. Here, we discuss how ferroptosis, a non-apoptotic cell death mechanism with emerging links to disease, may be best understood as a highly flexible mechanism that can be executed and regulated by many functionally related metabolites and proteins. The inherent plasticity of ferroptosis has implications for how to define and study this mechanism in healthy and diseased cells and organisms.

Recent Applications of Melanin-like Nanoparticles as Antioxidant Agents

Nanosized antioxidants are highly advantageous in terms of versatility and pharmacokinetics, with respect to conventional molecular ones. Melanin-like materials, artificial species inspired by natural melanin, combine recognized antioxidant (AOX) activity with a unique versatility of preparation and modification. Due to this versatility and documented biocompatibility, artificial melanin has been incorporated into a variety of nanoparticles (NP) in order to give new platforms for nanomedicine with enhanced AOX activity. In this review article, we first discuss the chemical mechanisms behind the AOX activity of materials in the context of the inhibition of the radical chain reaction responsible for the peroxidation of biomolecules. We also focus briefly on the AOX properties of melanin-like NP, considering the effect of parameters such as size, preparation methods and surface functionalization on them. Then, we consider the most recent and relevant applications of AOX melanin-like NPs that are able to counteract ferroptosis and be involved in the treatment of important diseases that affect, e.g., the cardiovascular and nervous systems, as well as the kidneys, liver and articulations. A specific section will be dedicated to cancer treatment, since the role of melanin in this context is still very debated. Finally, we propose future strategies in AOX development for a better chemical understanding of melanin-like materials. In particular, the composition and structure of these materials are still debated, and they present a high level of variability. Thus, a better understanding of the mechanism behind the interaction of melanin-like nanostructures with different radicals and highly reactive species would be highly advantageous for the design of more effective and specific AOX nano-agents.

Introduction of Methyl Group in Substituted Isoselenazolones: Catalytic and Mechanistic Study

Copper-catalyzed direct selenation of substituted 2-bromo-N-phenylbenzamide substrates with elemental selenium powder provided a series of methoxy-substituted isoselenazolones via the C-Se and Se-N bond formations. Phenolic substituted isoselenazolones have been obtained by O-demethylation of the corresponding methoxy-substituted analogues using boron tribromide. Some isoselenazolones have been structurally characterized by X-ray single-crystal analysis. The glutathione peroxidase (GPx)-like antioxidant activity of isoselenazolones has been evaluated both in thiophenol and coupled-reductase assays. All isoselenazolones showed good GPx-like activities in the coupled-reductase assay. The ferric-reducing antioxidant power of phenolic antioxidants has also been evaluated. The best phenolic antioxidants were found to be good ferric-reducing antioxidant power agents. The single electron transfer, hydrogen atom transfer, and proton-coupled electron transfer mechanisms for the antioxidant properties of all catalysts have been supported by density functional theory calculations. The catalytic cycle was proposed for one of the phenolic isoselenazolones involving diselenide, selenenyl sulfide, selenol, and selenenic acid as intermediates using ⁷⁷Se{¹H} NMR spectroscopy.

General Approach to Identify, Assess, and Characterize Inhibitors of Lipid Peroxidation and Associated Cell Death

Lipid peroxidation (LPO) is associated with a variety of pathologies and drives a form of regulated necrosis called ferroptosis. There is much interest in small-molecule inhibitors of LPO as potential leads for therapeutic development for neurodegeneration, stroke, and acute organ failure, but this has been hampered by the lack of a universal high-throughput assay that can identify and assess candidates. Herein, we describe the development and validation of such an approach. Phosphatidylcholine liposomes loaded with ∼10% phospholipid hydroperoxide and STY-BODIPY, a fluorescent signal carrier that co-autoxidizes with polyunsaturated phospholipids, are shown to autoxidize at convenient and constant rates when subjected to an optimized Fe²⁺-based initiation cocktail. The use of this initiation system enables the identification of each of the various classes of LPO inhibitors which have been shown to rescue from cell death in ferroptosis: radical-trapping antioxidants (RTAs), peroxidase mimics, and iron chelators. Furthermore, a limited dose-response profile of inhibitors enables the resolution of RTA and non-RTA inhibitors─thereby providing not only relative efficacy but mechanistic information in the same microplate-based experiment. Despite this versatility, the approach can still be used to estimate rate constants for the reaction of RTAs with chain-propagating peroxyl radicals, as demonstrated for a representative panel of RTAs. To illustrate the utility of this assay, we carried out a preliminary investigation of the 'off-target' activity of several ferroptosis suppressors that have been proposed to act independently of inhibition of LPO, including lipoxygenase inhibitors, cannabinoids, and necrostatins, the archetype inhibitors of necroptosis.

Synergistic or antagonistic antioxidant combinations - a case study exploring flavonoid-nitroxide hybrids

Neurodegenerative diseases impose a considerable medical and public health burden on populations throughout the world. Oxidative stress, an imbalance in pro-oxidant/antioxidant homeostasis that leads to the generation of reactive oxygen species (ROS), has been implicated in the progression of a number of neurodegenerative diseases. The manipulation of ROS levels may represent a promising treatment option to slow down neurodegeneration, although adequate potency of treatments has not yet been achieved. Using a hybrid pharmacology approach, free radical nitroxide antioxidants were hybridised with a class of natural antioxidants, flavonoids, to form a potential multitargeted antioxidant. Modification of the Baker-Venkataraman reaction achieved the flavonoid-nitroxide hybrids (6-9) in modest yields. Antioxidant evaluation of the hybrids by cyclic voltammetry showed both redox functionalities were still active, with little influence on oxidation potential. Assessment of the peroxyl radical scavenging ability through an ORAC assay showed reduced antioxidant activity of the hybrids compared to their individual components. It was hypothesized that the presence of the phenol in the hybrids creates a more acidic medium which does not favour regeneration of the nitroxide from the corresponding oxammonium cation, disturbing the typical catalytic cycle of peroxyl radical scavenging by nitroxides. This work highlights the potential intricacies involved with drug hybridization as a strategy for new therapeutic development.

Modulation of the chain-breaking antioxidant activity of phenolic organochalcogens with various co-antioxidants at various pH values

Phenolic organochalcogen chain-breaking antioxidants, i.e. 6-bromo-8 (hexadecyltellanyl)-3,3-dimethyl-1,5-dihydro-[1,3]dioxepino[5,6-c]pyridin-9-ol and 2-methyl-2,3-dihydrobenzo[b]selenophene-5-ol, have been investigated in a two-phase (chlorobenzene/water) lipid peroxidation model system as potent inhibitors of lipid peroxyl radicals with various co-antioxidants at various pH values. The pH has a significant effect on the chain-breaking antioxidant activities of phenolic organochalcogens. The key chain-breaking mechanism profile was attributed to the first oxygen atom transfer from the lipid peroxyl radicals to the Se/Te atom, followed by hydrogen atom transfer in a solvent cage from the nearby phenolic group to the resulting alkoxyl radical. Finally, regeneration of organochalcogen antioxidants could take place in the presence of aqueous-soluble co-antioxidants. Also, in the presence of aqueous soluble N-acetylcysteine at pH 1-7, both antioxidants behaved as very good inhibitors of lipid peroxyl radicals. The role of aqueous soluble mild co-antioxidants in the regeneration studies of organochalcogen antioxidants has been investigated in a two-phase lipid peroxidation model system. The importance of the phase transfer catalyst has been explored in the inhibition studies of selenium containing antioxidants using an Fe(II) source. The overall pH-dependent antioxidant activities of organochalcogens depend on their hydrogen atom transfer ability, relative stability, and distribution in the aqueous/lipid phase.

Clinical Evaluation Based on a New Approach to Improve the Accuracy of 4β-Hydroxycholesterol Measurement as a Biomarker of CYP3A4 Activity

This study examines 4β-Hydroxycholesterol (4β-HC), which is considered to be a potential marker for the CYP3A4 induction of new chemical entities (NCEs) in drug development. To ensure the use of 4β-HC as a practical biomarker, it is necessary to accurately measure 4β-HC and demonstrate that CYP3A4 induction can be appropriately assessed, even for weak inducers. In clinical trials of NCEs, plasma is often collected with various anticoagulants, in some cases, the plasma is acidified, then stored for an extended period. In this study, we examined the effects of these manipulations on the measurement of 4β-HC, and based on the results, we optimized the plasma collection and storage protocols. We also found that a cholesterol oxidation product is formed when plasma is stored, and by monitoring the compound, we were able to identify when plasma was stored inappropriately. After evaluating the above, clinical drug-drug interaction (DDI) studies were conducted using two NCEs (novel retinoid-related orphan receptor γ antagonists). The weak CYP3A4 induction by the NCEs (which were determined based on a slight decline in the systemic exposure of a probe substrate (midazolam)), was detected by the significant increase in 4β-HC levels (more specifically, 4β-HC/total cholesterol ratios). Our new approach, based on monitoring a cholesterol oxidation product to identify plasma that is stored inappropriately, allowed for the accurate measurement of 4β-HC, and thus, it enabled the evaluation of weak CYP3A4 inducers in clinical studies without using a probe substrate.

Nanoantioxidant Materials: Nanoengineering Inspired by Nature

Oxidants are very active compounds that can cause damage to biological systems under specific environmental conditions. One effective way to counterbalance these adverse effects is the use of anti-oxidants. At low concentrations, an antioxidant is defined as a compound that can delay, control, or prevent an oxidative process. Antioxidants exist in plants, soil, and minerals; therefore, nature is a rich source of natural antioxidants, such as tocopherols and polyphenols. In nature, antioxidants perform in tandem with their bio-environment, which may tune their activity and protect them from degradation. In vitro use of antioxidants, i.e., out of their biomatrix, may encounter several drawbacks, such as auto-oxidation and polymerization. Artificial nanoantioxidants can be developed via surface modification of a nanoparticle with an antioxidant that can be either natural or synthetic, directly mimicking a natural antioxidant system. In this direction, state-of-the-art nanotechnology has been extensively incorporated to overcome inherent drawbacks encountered in vitro use of antioxidants, i.e., out of their biomatrix, and facilitate the production and use of antioxidants on a larger scale. Biomimetic nanoengineering has been adopted to optimize bio-medical antioxidant systems to improve stability, control release, enhance targeted administration, and overcome toxicity and biocompatibility issues. Focusing on biotechnological sciences, this review highlights the importance of nanoengineering in developing effective antioxidant structures and comparing the effectiveness of different nanoengineering methods. Additionally, this study gathers and clarifies the different antioxidant mechanisms reported in the literature and provides a clear picture of the existing evaluation methods, which can provide vital insights into bio-medical applications.

Novel Indane Derivatives with Antioxidant Activity from the Roots of Anisodus tanguticus

Four novel indane derivatives, anisotindans A-D (1-4), were isolated from the roots of Anisodus tanguticus. Their structures were established using comprehensive spectroscopic analyses, and their absolute configurations were determined by electronic circular dichroism (ECD) calculations and single-crystal X-ray diffraction analyses. Anisotindans C and D (3 and 4) are two unusual indenofuran analogs. ABTS^•+ and DPPH^•+ assays of radical scavenging activity reveal that all compounds (1-4) are active. Specifically, the ABTS^•+ assay results show that anisotindan A (1) exhibits the best antioxidant activity with an IC₅₀ value of 15.62 ± 1.85 μM (vitamin C, IC₅₀ = 22.54 ± 5.18 μM).

Antioxidant conjugated metal complexes and their medicinal applications

Antioxidants are naturally available and man-made substances have the ability to protect cells from damage due to a number of intracellular redox activities. Moreover, Antioxidants such as α-lipoic acid, curcumin and catechin are good anticancer agents. In recent years, the usage of metal complexes as therapeutic agents is gaining importance due to their useful biological properties. Most of the metal ions act as the essential components in building drug molecules that serve as medicines for cancer and neurodegenerative diseases. In particular, metals like copper, gold, ruthenium, and platinum have adequate anticancer properties at both micro- and nano-levels. Hence, conjugation of antioxidants with metals and metal-based compounds results in hybrid bioactive materials with improved anticancer properties. In this chapter, medicinal applications of antioxidant conjugated metal complexes are reviewed and discussed.

Reaction with ROO• and HOO• Radicals of Honokiol-Related Neolignan Antioxidants

Honokiol is a natural bisphenol neolignan present in the bark of Magnolia officinalis, whose extracts have been employed in oriental medicine to treat several disorders, showing a variety of biological properties, including antitumor activity, potentially related to radical scavenging. Six bisphenol neolignans with structural motifs related to the natural bioactive honokiol were synthesized. Their chain-breaking antioxidant activity was evaluated in the presence of peroxyl (ROO•) and hydroperoxyl (HOO•) radicals by both experimental and computational methods. Depending on the number and position of the hydroxyl and alkyl groups present on the molecules, these derivatives are more or less effective than the reference natural compound. The rate constant of the reaction with ROO• radicals for compound 7 is two orders of magnitude greater than that of honokiol. Moreover, for compounds displaying quinonic oxidized forms, we demonstrate that the addition of 1,4 cyclohexadiene, able to generate HOO• radicals, restores their antioxidant activity, because of the reducing capability of the HOO• radicals. The antioxidant activity of the oxidized compounds in combination with 1,4-cyclohexadiene is, in some cases, greater than that found for the starting compounds towards the peroxyl radicals. This synergy can be applied to maximize the performances of these new bisphenol neolignans.

Direct Organocatalytic Reductive Alkylation of Syncarpic Acid: Scope and Applications

Biologically important 4-alkylsyncarpic acids, which resemble the core structure of many natural products, were synthesized in one-pot through the organocatalytic three-component reductive alkylation with excellent yields and C-selectivity. Synthetic applications of 4-alkylsyncarpic acids were demonstrated by converting into the functionally rich molecules through different reactions like Michael, retro-Michael, reduction, and oxidation reactions. In a continuation, formal total synthesis of (±)-triumphalone, (±)-isotriumphalone, and monomeric phloroglucinol derivatives was reported in a few steps starting from 4-alkylsyncarpic acids in overall very good yields. Further showcasing the importance of C-alkylated products, 4-benzylsyncarpic acid and its Michael adduct with methyl vinyl ketone were synthesized in a gram scale without compromising rate/yields.

Kinetic Analysis of High-Temperature Sunflower Oil Peroxidation Inhibited by the Major Families of Phenolic Antioxidants Unveils the Extraordinary Activity of 1,4-Hydroquinones

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.

Hydropersulfides Inhibit Lipid Peroxidation and Protect Cells from Ferroptosis

Hydropersulfides (RSSH) are believed to serve important roles in vivo, including as scavengers of damaging oxidants and electrophiles. The α-effect makes RSSH not only much better nucleophiles than thiols (RSH), but also much more potent H-atom transfer agents. Since HAT is the mechanism of action of the most potent small-molecule inhibitors of phospholipid peroxidation and associated ferroptotic cell death, we have investigated their reactivity in this context. Using the fluorescence-enabled inhibited autoxidation (FENIX) approach, we have found RSSH to be highly reactive toward phospholipid-derived peroxyl radicals (k_inh = 2 × 10⁵ M^-1 s^-1), equaling the most potent ferroptosis inhibitors identified to date. Related (poly)sulfide products resulting from the rapid self-reaction of RSSH under physiological conditions (e.g., disulfide, trisulfide, H₂S) are essentially unreactive, but combinations from which RSSH can be produced in situ (i.e., polysulfides with H₂S or thiols with H₂S₂) are effective. In situ generation of RSSH from designed precursors which release RSSH via intramolecular substitution or hydrolysis improve the radical-trapping efficiency of RSSH by minimizing deleterious self-reactions. A brief survey of structure-reactivity relationships enabled the design of new precursors that are more efficient. The reactivity of RSSH and their precursors translates from (phospho)lipid bilayers to cell culture (mouse embryonic fibroblasts), where they were found to inhibit ferroptosis induced by inactivation of glutathione peroxidase-4 (GPX4) or deletion of the gene encoding it. These results suggest that RSSH and the pathways responsible for their biosynthesis may act as a ferroptosis suppression system alongside the recently discovered FSP1/ubiquinone and GCH1/BH₄/DHFR systems.

Intrinsic and Extrinsic Limitations to the Design and Optimization of Inhibitors of Lipid Peroxidation and Associated Cell Death

The archetype inhibitors of ferroptosis, ferrostatin-1 and liproxstatin-1, were identified via high-throughput screening of compound libraries for cytoprotective activity. These compounds have been shown to inhibit ferroptosis by suppressing propagation of lipid peroxidation, the radical chain reaction that drives cell death. Herein, we present the first rational design and optimization of ferroptosis inhibitors targeting this mechanism of action. Engaging the most potent radical-trapping antioxidant (RTA) scaffold known (phenoxazine, PNX), and its less reactive chalcogen cousin (phenothiazine, PTZ), we explored structure-reactivity-potency relationships to elucidate the intrinsic and extrinsic limitations of this approach. The results delineate the roles of inherent RTA activity, H-bonding interactions with phospholipid headgroups, and lipid solubility in determining activity/potency. We show that modifications which increase inherent RTA activity beyond that of the parent compounds do not substantially improve RTA kinetics in phospholipids or potency in cells, while modifications that decrease intrinsic RTA activity lead to corresponding erosions to both. The apparent "plateau" of RTA activity in phospholipid bilayers (k_inh ∼ 2 × 10⁵ M^-1 s^-1) and cell potency (EC₅₀ ∼ 4 nM) may be the result of diffusion-controlled reactivity between the RTA and lipid-peroxyl radicals and/or the potential limitations on RTA turnover/regeneration by endogenous reductants. The metabolic stability of selected derivatives was assessed to identify a candidate for in vivo experimentation as a proof-of-concept. This PNX-derivative demonstrated stability in mouse liver microsomes comparable to liproxstatin-1 and was successfully used to suppress acute renal failure in mice brought on by tissue-specific inactivation of the ferroptosis regulator GPX4.

Polyolefin Innovations toward Circularity and Sustainable Alternatives

The unprecedented growth and socioeconomic impacts of polyolefins clearly outline a major success story in the world of polymer science. Polyolefins revolutionizes industries such as health care, construction, and food packaging. Despite the benefits of polyolefins, there is a rising concern for the environment due to high production volume (i.e., fossil fuel consumption), often short usage time, and problems related to waste management and accumulation in the natural environment. Creating a circular economy for polyolefins through effective recycling technologies has the potential to decrease the environmental impact of these materials. This perspective discusses polyolefins and their impact, existing and emerging recycling/upcycling solutions, and recycle-by-design alternatives that are challenging the status quo.

Chain-Breaking Antioxidant and Peroxyl Radical Trapping Activity of Phenol-Coated Magnetic Iron Oxide Nanoparticles

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.

Remote Stereoelectronic Effects in Pyrrolidone- and Caprolactam-Substituted Phenols: Discrepancies in Antioxidant Properties Evaluated by Electrochemical Oxidation and H-Atom Transfer Reactivity

New antioxidants are commonly evaluated via two main approaches, i.e., the ability to donate an electron and the ability to intercept free radicals. We compared these approaches by evaluating the properties of 11 compounds containing both antioxidant moieties (mono- and polyphenols) and auxiliary pharmacophores (pyrrolidone and caprolactam). Several common antioxidants, such as butylated hydroxytoluene (BHT), 2,3,5-trimethylphenol (TMP), quercetin, and dihydroquercetin, were added for comparison. The antioxidant properties of these compounds were determined by their rates of reaction with 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and their oxidation potentials from cyclic voltammetry. Although these methods test different chemical properties, their results correlate reasonably well. However, several exceptions exist where the two methods give opposite predictions! One of them is the different behavior of mono- and polyphenols: polyphenols can react with DPPH more than an order of magnitude faster than monophenols of a similar oxidation potential. The second exception stems from the size of a "bystander" lactam ring at the benzylic position. Although the phenols with a seven-membered lactam ring are harder to oxidize, the sterically nonhindered compounds react with DPPH about 2× faster than the analogous five-membered lactams. The limitations of computational methods, especially those based on a single parameter, are also evaluated and discussed.

Synthesis and antioxidant activities of N-thiophenyl ebselenamines: a 77Se{1H} NMR mechanistic study

Synthesis of N-thiophenyl ebselenamines and selenenyl sulphides as efficient radical-trapping and hydroperoxide-decomposing antioxidants, respectively has been described.

Antiradical Activity of Dopamine, L-DOPA, Adrenaline, and Noradrenaline in Water/Methanol and in Liposomal Systems

Catecholamines play a crucial role in signal transduction and are also expected to act as endogeneous antioxidants, but the mechanism of their antioxidant action is not fully understood. Here, we describe the impact of pH on the kinetics of reaction of four catecholamines (L-DOPA, dopamine, adrenaline, and noradrenaline) with model 2,2-diphenyl-1-picrylhydrazyl radical (dpph^•) in methanol/water. The increase in pH from 5.5 to 7.4 is followed by a 2 order of magnitude increase in the rate constant, e.g., for dopamine (DA) k^pH5.5 = 1,200 M^–1 s^–1 versus k^pH7.4 = 170,000 M^–1 s^–1, and such rate acceleration is attributed to a fast electron transfer from the DA anion to dpph^•. We also proved that at pH 7.0 DA breaks the peroxidation chain of methyl linoleate in liposomes assembled from neutral and negatively charged phospholipids. In contrast to no inhibitory effect during peroxidation in non-ionic emulsions, in bilayers one molecule of DA traps approximately four peroxyl radicals, with a rate constant k_inh >10³ M^–1 s^–1. Our results from a homogeneous system and bilayers prove that catecholamines act as effective, radical trapping antioxidants with activity depending on the ionization status of the catechol moiety, as well as microenvironment: organization of the lipid system (emulsions vs bilayers) and interactions of catecholamines with the biomembrane.

Radical-Trapping Antioxidant Activity of Copper and Nickel Bis(Thiosemicarbazone) Complexes Underlies Their Potency as Inhibitors of Ferroptotic Cell Death

Herein we demonstrate that copper(II)-diacetyl-bis(N⁴-methylthiosemicarbazone)(CuATSM), clinical candidate for the treatment of ALS and Parkinson's disease, is a highly potent radical-trapping antioxidant (RTA) and inhibitor of (phospho)lipid peroxidation. In THF autoxidations, CuATSM reacts with THF-derived peroxyl radicals with k_inh = 2.2 × 10⁶ M^-1 s^-1─roughly 10-fold greater than α-tocopherol (α-TOH), Nature's best RTA. Mechanistic studies reveal no H/D kinetic isotope effects and a lack of rate-suppressing effects from H-bonding interactions, implying a different mechanism from α-TOH and other canonical RTAs, which react by H-atom transfer (HAT). Similar reactivity was observed for the corresponding Ni²⁺ complex and complexes of both Cu²⁺ and Ni²⁺ with other bis(thiosemicarbazone) ligands. Computations corroborate the experimental finding that rate-limiting HAT cannot account for the observed RTA activity and instead suggest that the reversible addition of a peroxyl radical to the bis(thiosemicarbazone) ligand is responsible. Subsequent HAT or combination with another peroxyl radical drives the reaction forward, such that a maximum of four radicals are trapped per molecule of CuATSM. This sequence is supported by spectroscopic and mass spectrometric experiments on isolated intermediates. Importantly, the RTA activity of CuATSM (and its analogues) translates from organic solution to phospholipid bilayers, thereby accounting for its (their) ability to inhibit ferroptosis. Experiments in mouse embryonic fibroblasts and hippocampal cells reveal that lipophilicity as well as inherent RTA activity contribute to the potency of ferroptosis rescue, and that one compound (CuATSP) is almost 20-fold more potent than CuATSM and among the most potent ferroptosis inhibitors reported to date.

Methods to Determine Chain-Breaking Antioxidant Activity of Nanomaterials beyond DPPH•. A Review

This 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.

Temperature-dependence of radical-trapping activity of phenoxazine, phenothiazine and their aza-analogues clarifies the way forward for new antioxidant design

The prediction and/or rationalization of diarylamine radical-trapping antioxidant (RTA) activity at the elevated temperatures where they are most useful presents a significant challenge, precluding the development of new technology. Whilst structure-activity relationships at ambient temperatures are well established, their predictive capacity at elevated temperatures is poor due to competing reactions. A striking example involves phenoxazine, which is a superior RTA relative to its sulfur analog phenothiazine at ambient temperature (e.g. k = 3.9 × 107 vs. 7.6 × 106 M-1 s-1 at 37 °C, respectively), but is demonstrably inferior at elevated temperatures. Despite being inherently less oxidizable in electrochemical experiments and high-accuracy computations, phenoxazine is more rapidly consumed than phenothiazine in autoxidations at 160 °C - a result which we attribute to a lower reorganization energy barrier to oxidation. Given these observations, we surmised that incorporation of an electronegative N-atom into the phenoxazine ring system would decrease the driving force for oxidation and 'rescue' its activity. Indeed, this was found to be the case for nitrogen incorporation at any position, regardless of the impact on the inherent RTA activity. Analogous experiments were carried out on phenothiazines into which nitrogen atoms were incorporated, revealing little benefit at 160 °C. These results suggest that for highly reactive diarylamines (i.e. those with k > 106 M-1 s-1), further enhancements in reactivity do not materially improve their ability to inhibit hydrocarbon autoxidation at elevated temperatures. Instead, their stability to one-electron oxidation determines their efficacy.