Advances in Radical-Trapping Antioxidant Chemistry in the 21st Century: A Kinetics and Mechanisms Perspective

Abstract

Bonded Complex 9026 2.3. Pushing to the Limit: Pyridinols and Pyrimidinols 9026 2.4. Effects of Intramolecular Hydrogen Bonds on the RTA Activities of Phenols 9028 2.5. Enhancing Reactivity: Effects of Acids and Bases 9029 2.6. Synergy and Regeneration of Phenolic RTAs 9029 2.6.1. Synergy in Phenolic RTAs 9030 2.6.2. Regeneration of Alkyl-Chalcogen-Substituted Phenols by Sacrificial Reductants 9030 3. Peroxyl Radical Trapping by Diarylamines and Closely Related Compounds 9033 4. Di-tert-alkyl Amine, Nitroxide, and Alkoxyamine RTAs 9035 4.1. Hindered Amine Light Stabilizers, HALS 9035 4.1.1. HALS Activation 9035 4.1.2. Mechanism of Catalytic Inhibition by HALS at Ambient Temperatures 9036 4.2. Effect of Organic Acids on the RTA Activity of TEMPO 9036 5. Sulfenic Acid RTAs 9037 5.1. Organosulfur Compounds from Garlic and Other Alliums 9037 5.2. Insights from Reactions of Selenenic Acids 9040 6. Carbon-Centered Radicals as RTAs 9040 7. Conclusions and Outlook 9042 Author Information 9043 Corresponding Authors 9043 Notes 9043 Biographies 9043 Acknowledgments 9043 References 9043

On the vasoprotective mechanisms underlying novel β-phosphorylated nitrones: Focus on free radical characterization, scavenging and NO-donation in a biological model of oxidative stress

“…Our results are in agreement with the reported weak activity of caffeic, gallic and sinapic acids (at concentrations up to 100 µM) toward xanthine oxidase [50]. and/or (ii) electron transfer from the phenolate groups [10,25,45], in the case of antioxidant-based PPNs.…”

“…The mechanism involves reduction of DPPH by a formal H-atom transfer reaction from the aromatic hydroxy groups [10,41,44]. Accordingly, mixing DPPH (0.13 mM in methanol) for 3 min with PBN, PPN or its non-phenolic derivatives 4j−q (5 µM−1 mM) did not result in a significant absorption decrease, giving EC 50 values > 1 mM (defined as the concentration necessary to halve DPPH concentration).…”

“…radical formation during the decomposition of 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH, 150 mM) at 37 °C. If OH groups in phenolic antioxidants are privileged targets for AAPH-derived peroxyl radicals [10,25,41,44], additional trapping at the nitrone site of PPNs may synergistically participate in the TRAP. However, PBN, PPN and all non-phenolic PPNs (with the exception of the ester nitrone 4o) had virtually no TRAP, a result also reported for PBN and some non phenolic N-alkyl analogues [47].…”

“…In most investigations, the privileged targets are natural or synthetic substances having improved ability to (i) interrupt free radical-mediated chains of cell lipid peroxidation by scavenging free radicals or inactivating metal ions catalysts such as Fe 2+ or (ii) retard or prevent new free radical chains to form, e.g., by decreasing local oxygen concentration [7][8][9][10].…”

Acid Is Key to the Radical-Trapping Antioxidant Activity of Nitroxides

“…1,2 Phenols and diphenylamines are the archetype RTAs (represented by A-H in eqs 1 and 2); they undergo efficient formal H atom transfer to peroxyl radicals to yield stable RTA-derived radicals that do not propagate the chain reaction (A•, eq 1). Instead, A• reacts with another peroxyl to yield nonradical products (eq 2).…”