Inhibition of acrylic acid and acrylate autoxidation

Acrylic acid (AA) is a versatile monomer whose high reactivity can present a challenge for transport and storage due to its highly exergonic oligomerization, which can lead to runaway polymerization and explosion. To prevent premature polymerization of acrylic acid, hydroquinone monomethyl ether (MeHQ) and phenothiazine (PTZ) are commonly used as inhibitors/stabilizers. Despite their widespread use, the limited radical-trapping stoichiometry of MeHQ and oxidative consumption of PTZ at process temperatures are clear limitations. Herein, we apply a recently devised spectrophotometric approach employing the autoxidizable STY-BODIPY dye to monitor reaction progress in autoxidations of acrylic acid, n-butyl acrylate and the non-polymerizable 2-ethylhexanol, and the impact of a panel of radical-trapping antioxidants (RTAs, including MeHQ and PTZ) upon them. We find that the radical-trapping stoichiometry is highly substrate-dependent, with nitroxides and aromatic amines that can be converted to nitroxides in situ exhibiting superstoichiometric activities in substrates where hydroperoxyl radicals are formed or in the presence of acid. N-Alkyl derivatives of phenoxazine, the most potent RTA uncovered to date, are found to be particularly excellent inhibitors of AA autoxidation. It is proposed that gradual acid-mediated dealkylation to phenoxazine minimizes accumulation of the phenoxazine-derived nitroxide, which can otherwise undergo acid-catalyzed disproportionation and diminish radical-trapping capacity. These results suggest that N-alkylated phenoxazine derivatives should be explored further as stabilizers of AA.

Controlled anionic polymerization mediated by carbon dioxide

Anionic polymerizations of vinyl monomers are powerful synthetic platforms for making well-defined materials. However, these reactions are extremely sensitive to moisture and oxygen, require the use of highly purified reagents, must be run at low temperatures, and use hazardous and difficult-to-handle alkyl lithium initiators. Together, these drawbacks limit the practicality of these polymerizations and impede their widespread usage. On this basis, the development of a user-friendly anionic polymerization process for methacrylates is a grand challenge. Here we report an anionic polymerization of methacrylates mediated by CO2 that can be run at elevated temperatures and uses an easy-to-handle solid initiator. The reversible addition of CO2 to the enolate chain end efficiently tempers the reactivity of the anion, giving polymers with narrow molar mass distributions and excellent molecular weight targeting at elevated temperatures. Our scalable and more user-friendly CO2-mediated method improves the accessibility and safety of anionic polymerizations and facilitates the production of a variety of polymeric materials.

Nitroxide-Containing Poly(2-oxazoline)s Show Dual-Stimuli-Responsive Behavior and Radical-Trapping Activity

2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) structures possess potent antioxidant activities for biomedical applications. TEMPO immobilization on hydrophilic polymers is a powerful strategy to improve its properties; however, it is mostly limited to reversible-deactivation radical polymerizations or postpolymerization approaches. Here, we immobilized TEMPO units on a hydrophilic poly(2-ethyl-2-oxazoline) (PEtOx) backbone through cationic ring-opening polymerization (CROP) of a new 2-oxazoline monomer bearing a methoxy-protected TEMPO 2-substituent with 2-ethyl-2-oxazoline (EtOx). The ratios of EtOx/TempOx were adjusted to optimize the nitroxide content while maintaining suitable water solubility of the resulting P(EtOxx-stat-TempOx-Oy•) copolymers upon deprotection. P(EtOx40-stat-TempOx-O10•) and P(EtOx33-stat-TempOx-O17•) showed a dual stimuli-responsive behavior and demonstrated significant radical-trapping activities in aqueous media. Particularly, a meaningful augmentation in the activity of TempOx-O• was observed when it was immobilized as P(EtOxx-stat-TempOx-Oy•). The P(EtOx40-stat-TempOx-O10•) system exhibited a longer-lasting activity in water, statistically comparable to that of the antioxidant ferrostatin-1 (Fer-1). Overall, this study introduces a biocompatible polymeric platform for TEMPO immobilization that augments its radical-trapping activity and offers controllable stimuli-responsive properties.

A Computational Analysis of the Proton Affinity and the Hydration of TEMPO and Its Piperidine Analogs

The study investigated the impact of protonation and hydration on the geometry of nitroxide radicals using B3LYP and M06-2X methods. Results indicated that TEMPO exhibited the highest proton affinity in comparison to TEMPOL and TEMPONE. Two pathways contribute to hydrated protonated molecules. TEMPO shows lower first enthalpies of hydration (ΔH1-M), indicating stronger H-bonding interactions, while TEMPONE shows higher values, indicating weaker interactions with H2O. Solvent effects affect charge distribution by decreasing their atomic charge. Spin density (SD) is primarily concentrated in the NO segment, with minimal water molecule contamination. Protonation increases SD on N-atom, while hydration causes a more pronounced redistribution for water molecules. The stability of the dipolar structure (>N⋅+-O-) is evident in SD redistributions. The frontier molecular orbital (FMO) analysis of TEMPONE reveals a minimum EHOMO-LUMO gap (EH-L), enhancing the piperidine ring's reactivity. TEMPO is the most nucleophilic species, while TEMPONE exhibits strong electrophilicity. Transitioning from NO radicals to protonated forms increases the EH-L gap, indicating protonation stabilizes FMOs. Increased water molecules make the molecule less reactive, while increasing hydration decreases this energy gap, making the molecule more reactive. A smaller EH-L gap indicates the compound becomes softer and more prone to electron density and reactivity changes.

Computational analysis of substituent effects on proton affinity and gas-phase basicity of TEMPO derivatives and their hydrogen bonding interactions with water molecules

The study investigates the molecular structure of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives in the gas phase using B3LYP and M06-2X functional methods. Intermolecular interactions are analyzed using natural bond orbital (NBO) and atoms in molecules (AIM) techniques. NO2-substituted TEMPO displays high reactivity, less stability, and softer properties. The study reveals that the stability of TEMPO derivatives is mainly influenced by LP(e) → σ∗ electronic delocalization effects, with the highest stabilization observed on the oxygen atom of the nitroxide moiety. This work also considers electron density, atomic charges, and energetic and thermodynamic properties of the studied NO radicals, and their relative stability. The proton affinity and gas-phase basicity of the studied compounds were computed at T = 298 K for O-protonation and N-protonation, respectively. The studied DFT method calculations show that O-protonation is more stable than N-protonation, with an energy difference of 16.64-20.77 kcal/mol (22.80-25.68 kcal/mol) at the B3LYP (M06-2X) method. The AIM analysis reveals that the N-O…H interaction in H2O complexes has the most favorable hydrogen bond energy computed at bond critical points (3, - 1), and the planar configurations of TEMPO derivatives exhibit the highest EHB values. This indicates stronger hydrogen bonding interactions between the N-O group and water molecules.

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.

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.

Mutual Activation of Two Radical Trapping Agents: Unusual "Win-Win Synergy" of Resveratrol and TEMPO during Scavenging of dpph• Radical in Methanol

The reaction of the 2,2'-diphenyl-1-picrylhydrazyl radical (dpph^•) with resveratrol in methanol (k^MeOH = 192 M^-1 s^-1) is greatly accelerated in the presence of stable nitroxyl radical TEMPO^• (k_mix^MeOH = 1.4 × 10³ M^-1 s^-1). This synergistic effect is surprising because TEMPO^• alone reacts with dpph^• relatively slowly (k^S = 31 M^-1 s^-1 in methanol and 0.03 M^-1 s^-1 in nonpolar ethyl acetate). We propose a putative mechanism in which a mutual activation occurs within the acid-base pair TEMPO^•/RSV to the resveratrol (RSV) anion and TEMPOH^•+ radical cation, both being extremely fast scavengers of the dpph^• radical. The fast initial reaction is followed by a much slower but continuous decay of dpph^• because a nitroxyl radical is recovered from the TEMPOnium cation, which is reduced directly by RSV/RSV^- to TEMPO^• or recovered indirectly via a reaction with methanol, producing TEMPOH subsequently oxidized by dpph^• to TEMPO^•.

A Photocatalytic System Composed of Benzimidazolium Aryloxide and Tetramethylpiperidine 1-Oxyl to Promote Desulfonylative α-Oxyamination Reactions of α-Sulfonylketones

A new photocatalytic system was developed for carrying out desulfonylative α-oxyamination reactions of α-sulfonylketones in which α-ketoalkyl radicals are generated. The catalytic system is composed of benzimidazolium aryloxide betaines (BI⁺-ArO^-), serving as visible light-absorbing electron donor photocatalysts, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), playing dual roles as an electron donor for catalyst recycling and a reagent to capture the generated radical intermediates. Information about the detailed nature of BI⁺-ArO^- and the photocatalytic processes with TEMPO was gained using absorption spectroscopy, electrochemical measurements, and density functional theory calculations.

Absolute Antioxidant Activity of Five Phenol-Rich Essential Oils

Essential oils (EOs) have promising antioxidant activities which are gaining interest as natural alternatives to synthetic antioxidants in the food and cosmetic industries. However, quantitative data on chain-breaking activity and on the kinetics of peroxyl radical trapping are missing. Five phenol-rich EOs were analyzed by GC-MS and studied by oxygen-uptake kinetics in inhibited controlled autoxidations of reference substrates (cumene and squalene). Terpene-rich Thymus vulgaris (thymol 4%; carvacrol 33.9%), Origanum vulgare, (thymol 0.4%; carvacrol 66.2%) and Satureja hortensis, (thymol 1.7%; carvacrol 46.6%), had apparent kinh (30 °C, PhCl) of (1.5 ± 0.3) × 104, (1.3 ± 0.1) × 104 and (1.1 ± 0.3) × 104 M-1s-1, respectively, while phenylpropanoid-rich Eugenia caryophyllus (eugenol 80.8%) and Cinnamomum zeylanicum, (eugenol 81.4%) showed apparent kinh (30 °C, PhCl) of (5.0 ± 0.1) × 103 and (4.9 ± 0.3) × 103 M-1s-1, respectively. All EOs already granted good antioxidant protection of cumene at a concentration of 1 ppm (1 mg/L), the duration being proportional to their phenolic content, which dictated their antioxidant behavior. They also afforded excellent protection of squalene after adjusting their concentration (100 mg/L) to account for the much higher oxidizability of this substrate. All investigated EOs had kinh comparable to synthetic butylated hydroxytoluene (BHT) were are eligible to replace it in the protection of food or cosmetic products.

SET and HAT/PCET acid-mediated oxidation processes in helical shaped fused bis-phenothiazines

Helical shaped fused bis-phenothiazines 1-9 have been prepared and their red-ox behaviour quantitatively studied. Helicene radical cations (Hel.+ ) can be obtained either by UV-irradiation in the presence of PhCl or by chemical oxidation. The latter process is extremely sensitive to the presence of acids in the medium with molecular oxygen becoming a good single electron transfer (SET) oxidant. The reaction of hydroxy substituted helicenes 5-9 with peroxyl radicals (ROO. ) occurs with a 'classical' HAT process giving HelO. radicals with kinetics depending upon the substitution pattern of the aromatic rings. In the presence of acetic acid, a fast medium-promoted proton-coupled electron transfer (PCET) process takes place with formation of HelO. radicals possibly also via a helicene radical cation intermediate. Remarkably, also helicenes 1-4, lacking phenoxyl groups, in the presence of acetic acid react with peroxyl radicals through a medium-promoted PCET mechanism with formation of the radical cations Hel.+ . Along with the synthesis, EPR studies of radicals and radical cations, BDE of Hel-OH group (BDEOH ), and kinetic constants (kinh ) of the reactions with ROO. species of helicenes 1-9 have been measured and calculated to afford a complete rationalization of the redox behaviour of these appealing chiral compounds.

Nitroxides as Building Blocks for Nanoantioxidants

Nitroxides are an important class of radical trapping antioxidants whose promising biological activities are connected to their ability to scavenge peroxyl (ROO^•) radicals. We have measured the rate constants of the reaction with ROO^• (k_inh) for a series of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) derivatives as 5.1 × 10⁶, 1.1 × 10⁶, 5.4 × 10⁵, 3.7 × 10⁵, 1.1 × 10⁵, 1.9 × 10⁵, and 5.6 × 10⁴ M^-1 s^-1 for -H, -OH, -NH₂, -COOH, -NHCOCH₃, -CONH(CH₂)₃CH₃, and ═O substituents in the 4 position, with a good Marcus relationship between log (k_inh) and E° for the R₂NO^•/R₂NO⁺ couple. Newly synthesized Pluronic-silica nanoparticles (PluS) having nitroxide moieties covalently bound to the silica surface (PluS-NO) through a TEMPO-CONH-R link and coumarin dyes embedded in the silica core, has k_inh = 1.5 × 10⁵ M^-1 s^-1. Each PluS-bound nitroxide displays an inhibition duration nearly double that of a structurally related "free" nitroxide. As each PluS-NO particle bears an average of 30 nitroxide units, this yields an overall ≈60-fold larger inhibition of the PluS-NO nanoantioxidant compared to the molecular analogue. The implications of these results for the development of novel nanoantioxidants based on nitroxide derivatives are discussed, such as the choice of the best linkage group and the importance of the regeneration cycle in determining the duration of inhibition.

Autoxidation vs. antioxidants - the fight for forever

Autoxidation limits the longevity of essentially all hydrocarbons and materials made therefrom - including us. The radical chain reaction responsible often leads to complex mixtures of hydroperoxides, alkyl peroxides, alcohols, carbonyls and carboxylic acids, which change the physical properties of the material - be it a lubricating oil or biological membrane. Autoxidation is inhibited by addtitives such as radical-trapping antioxidants, which intervene directly in the chain reaction. Herein we review the most salient features of autoxidation and its inhibition, emphasizing concepts and mechanistic considerations important in understanding this chemistry across the wide range of contexts in which it is relevant.

Theoretical study of hydrogen bonding interactions in substituted nitroxide radicals

Interaction energy (E_int) of hydrogen bonded complexes of nitroxide radicals can be assessed in terms of the deepest minimum of molecular electrostatic potential (V_min).

Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Three novel coordination compounds, [Cu(ca)2(Hbae)2] (1), [Cu(va)2(Hbae)2] (2) and [Cu4(va)4(bae)4]·H2O (3), have been prepared by self-assembly reactions of copper(ii) chloride (1 and 2) or tetrafluoroborate (3) and CH3OH (1 and 3) or CH3CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu4(μ3-O)4} core, wherein the metal atoms are bound together by μ3 oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1-3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1-3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10-7, 2.5 × 10-7 and 2.1 × 10-7 M s-1 for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic 18O-labelling experiments suggested that o-aminophenol coordinates to CuII species with the formation of reactive intermediates, while the oxygen from 18O2 is not incorporated into the phenoxazinone chromophore.

A Supported Bismuth Halide Perovskite Photocatalyst for Selective Aliphatic and Aromatic C-H Bond Activation

Direct selective oxidation of hydrocarbons to oxygenates by O2 is challenging. Catalysts are limited by the low activity and narrow application scope, and the main focus is on active C-H bonds at benzylic positions. In this work, stable, lead-free, Cs3 Bi2 Br9 halide perovskites are integrated within the pore channels of mesoporous SBA-15 silica and demonstrate their photocatalytic potentials for C-H bond activation. The composite photocatalysts can effectively oxidize hydrocarbons (C5 to C16 including aromatic and aliphatic alkanes) with a conversion rate up to 32900 μmol gcat -1  h-1 and excellent selectivity (>99 %) towards aldehydes and ketones under visible-light irradiation. Isotopic labeling, in situ spectroscopic studies, and DFT calculations reveal that well-dispersed small perovskite nanoparticles (2-5 nm) possess enhanced electron-hole separation and a close contact with hydrocarbons that facilitates C(sp3 )-H bond activation by photoinduced charges.

Computational and experimental approach to evaluate the effect of initiator concentration, solvents, and enes on the TEMPO driven thiol–ene reaction

The fundamental mechanism and reaction kinetics of the TEMPO initiated thiol–ene reaction between benzyl mercaptan and variable enes in the presence of varying initiator concentration and varying solvents has been studied experimentally and computationally.

Calibration of Squalene, p-Cymene, and Sunflower Oil as Standard Oxidizable Substrates for Quantitative Antioxidant Testing

The autoxidation kinetics of stripped sunflower oil (SSO), squalene (SQ), and p-cymene ( p-C) initiated by 2,2'-azobis(isobutyronitrile) at 303 K were investigated under controlled conditions by differential oximetry in order to build reference model systems that are representative of the natural variability of oxidizable materials, for quantitative antioxidant testing. Rate constants for oxidative chain propagation ( k_p) and chain termination (2 k_t) and the oxidizability ( k_p/√2 k_t) were measured using 2,6-di- tert-butyl-4-methoxyphenol, 2,2,5,7,8-pentamethyl-6-chromanol, BHT, and 4-methoxyphenol as reference antioxidants. Measured values of k_p (M^-1 s^-1)/2 k_t (M^-1 s^-1)/oxidizability (M^-1/2 s^-1/2) at 303 K in chlorobenzene were 66.9/3.45 × 10⁶/3.6 × 10^-2, 68.0/7.40 × 10⁶/2.5 × 10^-2, and 0.83/2.87 × 10⁶/4.9 × 10^-4, respectively, for SSO, SQ, and p-C. Quercetin, magnolol, caffeic acid phenethyl ester, and 2,4,6-trimethylphenol were investigated to validate calibrations. The distinctive usefulness of the three substrates in testing antioxidants is discussed.

A Robust Fungal Allomelanin Mimic: An Antioxidant and Potent π-Electron Donor with Free-Radical Properties that can be Tuned by Ionic Liquids

Developing effective strategies to increase the chemical stability and to fine-tune the physico-chemical properties of melanin biopolymers by rational control of π-electron conjugation is an important goal in materials science for biomedical and technological applications. Herein we report that poly-1,8-dihydroxynaphthalene (pDHN), a non-nitrogenous, catechol-free fungal melanin mimic, displays a high degree of structural integrity (from MALDI-MS and CP/MAS ¹³ C NMR analysis), a strong radical scavenging capacity (DPPH and FRAP assays), and an unusually intense EPR signal (g=2.0030). Morphological and spectral characterization of pDHN, along with deassembly experiments in ionic liquids, indicated amorphous aggregates of small globular structures with an estimated stacking distance of 3.9 Å and broadband absorption throughout the visible range. These results indicate that DHN-based melanins exhibit a high structural integrity and enhanced antioxidant and free-radical properties of potentially greater biomedical and technological relevance than for typical indole-based eumelanins.

Nanoscale PDA disassembly in ionic liquids: structure–property relationships underpinning redox tuning

An integrated EPR and electrical impedance spectroscopy approach to predict ionic liquid-mediated tuning of the redox properties of polydopamine nanoparticles.

Efficacy of a new delivery system based on solid lipid microparticles for the oral administration of the non-conventional antioxidant IAC on a diabetes mouse model

PURPOSE

We previously showed the positive effects of the new antioxidant molecule bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl)-decandioate (IAC) in reducing basal hyperglycaemia and relieving glucose intolerance in a diabetes model. However, the chemical properties of IAC did not allow an efficient oral administration, thus representing the main failing of that study. Here, we tested the effect of a new oral delivery system based on solid lipid microparticles (SLMs) in a diabetes mouse model.

METHODS

The diabetes model was induced in C57B1/6J mice using streptozotocin and nicotinamide. Only the animals that overcame the glycaemic threshold of 180 mg/dL were enrolled in the study. Diabetic animals were then randomly assigned to 4 groups (n = 9) and treated once a day for 5 consecutive weeks with IAC (50, 100, and 150 mg/kg b.w.). The control group was composed of (n = 7) healthy mice that received only the vehicle. Glucose level was weekly monitored during the treatment period and up to 3 weeks after the suspension of the treatment. Glucose tolerance and insulin-resistance test were carried out.

RESULTS

Our results showed that SLMs maintained the IAC effect in reducing basal hyperglycaemia as well as improving the insulin sensitivity and glucose tolerance.

CONCLUSION

The present study confirms that SLMs are promising drug carriers, which allow the oral administration of IAC ensuring its therapeutic efficacy. The concrete possibility to administer IAC per os represents a significant breakthrough in the putative consideration of this multi-radical scavenger in the diabetes therapeutic approach.

Recent Insights on Hydrogen Atom Transfer in the Inhibition of Hydrocarbon Autoxidation

Autoxidation, the free radical chain reaction that nominally inserts O₂ into hydrocarbons to give peroxides, is primarily responsible for the degradation of all organic materials. Peroxyl radicals propagate autoxidation mainly by abstraction of labile H-atoms from the hydrocarbons, whereas radical-trapping antioxidants (RTAs) inhibit autoxidation by donating an H-atom to the peroxyl radical to give a nonpropagating radical. As such, a detailed understanding of the kinetics and thermodynamics of H-atom transfer (HAT) reactions to peroxyl radicals, and the effects of sterics, electronics, and medium thereupon, is key to understanding the mechanisms and products of autoxidation and the ability of RTAs to inhibit it. Due to their relatively weak O-H and N-H bonds, phenols and aromatic amines have long been utilized as RTAs, but only phenols have been extensively optimized to maximize their reactivity. Amines offer greater structural variability owing to their trivalent central nitrogen atom. Simply linking the two aromatic rings of a diarylamine to afford a phenoxazine offers profound differences in HAT reactivity: 1000-times greater than diphenylamine and 10-fold more reactive than α-tocopherol, Nature's optimized phenolic RTA. Thus, phenoxazines are an exciting scaffold for RTA development. Indeed, we have recently shown that ring substitution of phenoxazine or 2,4-diazaphenoxazine can yield compounds that undergo barrierless HAT reactions with peroxyl radicals. Amines also have the distinct advantage that they can react with peroxyl radicals to yield nitroxides, which can inhibit autoxidation in a catalytic manner utilizing the substrate itself as the stoichiometric reductant. Herein we provide an account of our recent efforts to understand how they manage this feat, which have revealed at least four mechanisms depending on the specific reaction conditions (i.e., saturated hydrocarbons at elevated temperatures, unsaturated hydrocarbons, acidic media, aqueous media/lipid dispersions). We also reiterate how their impressive RTA activity translates from solution to mammalian cell culture, wherein we have demonstrated that diarylamines and their derived nitroxides are potent inhibitors of ferroptosis, a recently characterized form of cell death associated with lipid peroxidation (autoxidation). In addition to phenols and amines, organosulfur compounds have long been used as antioxidants. The prevailing view has been that they undergo ionic reactions with product peroxides, preventing the initiation of further chain reactions. In recent years, we have found that many organosulfur compounds exhibit very good RTA activity. In particular, sulfenic acids (RSOH) and hydropersulfides (RSSH) are found to be among the best HAT agents known, particularly to peroxyl radicals where secondary orbital interactions are found to play a significant role. Consequently, oxidation of the sulfenic acid to a sulfinic acid greatly diminishes its HAT reactivity to peroxyls. Polysulfides and their oxides also undergo direct reactions with peroxyl radicals, thereby inhibiting autoxidation, but do so by homolytic substitution reactions. These insights suggest that the RTA activity of organosulfur compounds may be as important to the inhibition of hydrocarbon autoxidation, if not more so, than their ionic reactions.

Inhibition of hydrocarbon autoxidation by nitroxide-catalyzed cross-dismutation of hydroperoxyl and alkylperoxyl radicals

Nitroxides are putative intermediates in the accepted reaction mechanisms of the diarylamine and hindered amine antioxidants that are universally added to preserve synthetic and natural hydrocarbon-based materials. New methodology which enables monitoring of hydrocarbon autoxidations at low rates of radical generation has revealed that diarylnitroxides and hindered nitroxides are far better inhibitors of unsaturated hydrocarbon autoxidation than their precursor amines, implying intervention of a different mechanism. Experimental and computational investigations suggest that the nitroxides catalyze the cross-dismutation of hydroperoxyl and alkylperoxyl radicals to yield O2 and a hydroperoxide, thereby halting the autoxidation chain reaction. The hydroperoxyl radicals - key players in hydrocarbon combustion, but essentially unknown in autoxidation - are proposed to derive from a tunneling-enhanced intramolecular (1,4-) hydrogen-atom transfer/elimination sequence from oxygenated radical addition intermediates. These insights suggest that nitroxides are preferred additives for the protection of hydrocarbon-based materials from autoxidation since they exhibit catalytic activity under conditions where their precursor amines are less effective and/or inefficiently converted to nitroxides in situ.

Antioxidant activity of nanomaterials

Nanomaterials 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.

The Catalytic Reaction of Nitroxides with Peroxyl Radicals and Its Relevance to Their Cytoprotective Properties

Sterically-hindered nitroxides such as 2,2,6,6-tetramethylpiperidin- N-oxyl (TEMPO) have long been ascribed antioxidant activity that is thought to underlie their chemopreventive and anti-aging properties. However, the most commonly invoked reactions in this context-combination with an alkyl radical to give a redox inactive alkoxyamine or catalysis of superoxide dismutation-are unlikely to be relevant under (most) physiological conditions. Herein, we characterize the kinetics and mechanisms of the reactions of TEMPO, as well as an N-arylnitroxide and an N, N-diarylnitroxide, with alkylperoxyl radicals, the propagating species in lipid peroxidation. In each of aqueous solution and lipid bilayers, they are found to be significantly more reactive than Vitamin E, Nature's premier radical-trapping antioxidant (RTA). Inhibited autoxidations of THF in aqueous buffers reveal that nitroxides reduce peroxyl radicals by electron transfer with rate constants ( k ≈ 10⁶ to >10⁷ M^-1 s^-1) that correlate with the standard potentials of the nitroxides ( E° ≈ 0.75-0.95 V vs NHE) and that this activity is catalytic in nitroxide. Regeneration of the nitroxide occurs by a two-step process involving hydride transfer from the substrate to the nitroxide-derived oxoammonium ion followed by H-atom transfer from the resultant hydroxylamine to a peroxyl radical. This reactivity extends from aqueous solution to phosphatidylcholine liposomes, where added NADPH can be used as a hydride donor to promote nitroxide recycling, as well as to cell culture, where the nitroxides are shown to be potent inhibitors of lipid peroxidation-associated cell death (ferroptosis). These insights have enabled the identification of the most potent nitroxide RTA and anti-ferroptotic agent yet described: phenoxazine- N-oxyl.

Phenoxazine: A Privileged Scaffold for Radical-Trapping Antioxidants

Diphenylamines are widely used to protect petroleum-derived products from autoxidation. Their efficacy as radical-trapping antioxidants (RTAs) relies on a balance of fast H-atom transfer kinetics and stability to one-electron oxidation by peroxidic species. Both H-atom transfer and one-electron oxidation are enhanced by substitution with electron-donating substituents, such as the S-atom in phenothiazines, another important class of RTA. Herein we report the results of our investigations of the RTA activity of the structurally related, but essentially ignored, phenoxazines. We find that the H-atom transfer reactivity of substituted phenoxazines follows an excellent Evans-Polanyi correlation spanning k_inh = 4.5 × 10⁶ M^-1 s^-1 and N-H BDE = 77.4 kcal mol^-1 for 3-CN,7-NO₂-phenoxazine to k_inh = 6.6 × 10⁸ M^-1 s^-1 and N-H BDE = 71.8 kcal mol^-1 for 3,7-(OMe)₂-phenoxazine (37 °C). The reactivity of the latter compound is the greatest of any RTA ever reported and is likely to represent a reaction without an enthalpic barrier since log A for this reaction is likely ∼8.5. The very high reactivity of most of the phenoxazines studied required the determination of their kinetic parameters by inhibited autoxidations in the presence of a very strong H-bonding cosolvent (DMSO), which slowed the observed rates by up to 2 orders of magnitude by dynamically reducing the equilibrium concentration of (free) phenoxazine as an H-atom donor. Despite their remarkably high reactivity toward peroxyl radicals, the phenoxazines were found to be comparatively stable to one-electron oxidation relative to diphenylamines and phenothiazines (E° ranging from 0.59 to 1.38 V vs NHE). Thus, phenoxazines with comparable oxidative stability to commonly used diphenylamine and phenothiazine RTAs had significantly greater reactivity (by up to 2 orders of magnitude). Computations suggest that this remarkable balance in H-atom transfer kinetics and stability to one-electron oxidation results from the ability of the bridging oxygen atom in phenoxazine to serve as both a π-electron donor to stabilize the aminyl radical and σ-electron acceptor to destabilize the aminyl radical cation. Perhaps most excitingly, phenoxazines have "non-classical" RTA activity, where they trap >2 peroxyl radicals each, at ambient temperatures.

Enabling Efficient Positron Emission Tomography (PET) Imaging of Synaptic Vesicle Glycoprotein 2A (SV2A) with a Robust and One-Step Radiosynthesis of a Highly Potent 18F-Labeled Ligand ([18F]UCB-H)

We herein describe the straightforward synthesis of a stable pyridyl(4-methoxyphenyl)iodonium salt and its [¹⁸F] radiolabeling within a one-step, fully automated and cGMP compliant radiosynthesis of [¹⁸F]UCB-H ([¹⁸F]7), a PET tracer for the imaging of synaptic vesicle glycoprotein 2A (SV2A). Over the course of 1 year, 50 automated productions provided 34 ± 2% of injectable [¹⁸F]7 from up to 285 GBq (7.7 Ci) of [¹⁸F]fluoride in 50 min (uncorrected radiochemical yield, specific activity of 815 ± 185 GBq/μmol). The successful implementation of our synthetic strategy within routine, high-activity, and cGMP productions attests to its practicality and reliability for the production of large doses of [¹⁸F]7. In addition to enabling efficient and cost-effective clinical research on a range of neurological pathologies through the imaging of SV2A, this work further demonstrates the real value of iodonium salts for the cGMP ¹⁸F-PET tracer manufacturing industry, and their ability to fulfill practical and regulatory requirements in that field.

From the dual function lead AP2238 to AP2469, a multi-target-directed ligand for the treatment of Alzheimer's disease

The development of drugs with different pharmacological properties appears to be an innovative therapeutic approach for Alzheimer's disease. In this article, we describe a simple structural modification of AP2238, a first dual function lead, in particular the introduction of the catechol moiety performed in order to search for multi-target ligands. The new compound AP2469 retains anti-acetylcholinesterase (AChE) and beta-site amyloid precursor protein cleaving enzyme (BACE)1 activities compared to the reference, and is also able to inhibit Aβ₄₂ self-aggregation, Aβ₄₂ oligomer-binding to cell membrane and subsequently reactive oxygen species formation in both neuronal and microglial cells. The ability of AP2469 to interfere with Aβ₄₂ oligomer-binding to neuron and microglial cell membrane gives this molecule both neuroprotective and anti-inflammatory properties. These findings, together with its strong chain-breaking antioxidant performance, make AP2469 a potential drug able to modify the course of the disease.

3-Pyridinols and 5-pyrimidinols: Tailor-made for use in synergistic radical-trapping co-antioxidant systems

The incorporation of nitrogen atoms into the aromatic ring of phenolic compounds has enabled the development of some of the most potent radical-trapping antioxidants ever reported. These compounds, 3-pyridinols and 5-pyrimidinols, have stronger O-H bonds than equivalently substituted phenols, but possess similar reactivities toward autoxidation chain-carrying peroxyl radicals. These attributes suggest that 3-pyridinols and 5-pyrimidinols will be particularly effectiveco-antioxidants when used in combination with more common, but less reactive, phenolic antioxidants such as 2,6-di-tert-butyl-4-methylphenol (BHT), which we demonstrate herein. The antioxidants function in a synergistic manner to inhibit autoxidation; taking advantage of the higher reactivity of the 3-pyridinols/5-pyrimidinols to trap peroxyl radicals and using the less reactive phenols to regenerate them from their corresponding aryloxyl radicals. The present investigations were carried out in chlorobenzene and acetonitrile in order to provide some insight into the medium dependence of the synergism and the results, considered with some from our earlier work, prompt a revision of the H-bonding basicity value of acetonitrile to β2 (H) of 0.39. Overall, the thermodynamic and kinetic data presented here enable the design of co-antioxidant systems comprising lower loadings of the more expensive 3-pyridinol/5-pyrimidinol antioxidants and higher loadings of the less expensive phenolic antioxidants, but which are equally efficacious as the 3-pyridinol/5-pyrimidinol antioxidants alone at higher loadings.