Stable nitroxyl radicals and hydroxylamines as inhibitors of methyl linoleate oxidation in micelles

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^•.

Amphiphilic chitosan-polyaminoxyls loaded with daunorubicin: Synthesis, antioxidant activity, and drug delivery capacity

The binding of aminoxyls to polymers extends their potential use as antioxidants and EPR-reporting groups and opens up new horizons for tailoring new smart materials. In this work, we synthesized and characterized non-sulfated and N-sulfated water-soluble amphiphilic chitosans with a critical micelle concentration of 0.02-0.05 mg/mL that contain 13-18% of aminoglycosides bound with various aminoxyls. Chitosan-polyaminoxyls (CPAs) formed micelles with hydrodynamic radii R_h of ca. 100 nm. The EPR spectra of CPAs were found to depend on the rigidity of the aminoxyl-polymer bond and structural changes caused by sulfation. CPAs demonstrated antioxidant capacity/activity in three tests against reactive oxygen species (ROS) of various nature. The charge of micelles and structure of aminoxyls significantly affected their antioxidant properties. CPAs were low toxic against tumor (HepG2, HeLa, A-172) and non-cancerous (Vero) cells (IC₅₀ > 0.8 mM of aminoglycosides). Sulfated CPAs showed better water solubility and the ability of binding and retaining the anti-tumor antibiotic daunorubicin (DAU). DAU-loaded micelles of CPAs (CPAs-DAU) demonstrated a 1.5-4-fold potentiation of DAU cytotoxicity against several cell lines. CPAs-DAU micelles were found to affect the cell cycle in a manner markedly different from that of free DAU. Our results demonstrated the ability of CPAs to act as bioactive drug delivery vehicles.

Kinetic model of polyunsaturated fatty acids oxidation in micelles

A kinetic model of polyunsaturated fatty acids (PUFAs) radical chain oxidation in micelles is presented, taking into account the diffusion of active intermediates between aqueous and organic phases, and its effect on the detailed mechanism of the process. The model made it possible to indirectly involve the structural changes of micelles and their kinetic characteristics by varying the actual values of the reactions rate constants. The modeling results are in good agreement with experimental data for the oxidation of methyl linoleate and linoleic acid.

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.

Iminoxyl radicalsvs. tert-butylperoxyl radical in competitive oxidative C–O coupling with β-dicarbonyl compounds. Oxime ether formation prevails over Kharasch peroxidation

Oxidative coupling of oxime and β-dicarbonyl compounds dominates in a β-dicarbonyl compound/oxime/Cu(ii)/t-BuOOH system; in the absence of oxime, oxidative coupling of t-BuOOH and a β-dicarbonyl compound (Kharasch peroxidation) takes place. The proposed conditions for oxidative coupling of oximes with dicarbonyl compounds require only catalytic amounts of copper salt and t-BuOOH serves as a terminal oxidant. The C–O coupling reaction proceeds via the formation of tert-butoxyl, tert-butylperoxyl and iminoxyl radicals. Apparently, tert-butylperoxyl radicals oxidize oxime into iminoxyl radical faster than they react with β-dicarbonyl compounds forming the Kharasch peroxidation product. Iminoxyl radicals are responsible for the formation of the target C–O coupling products; the yields are up to 77%.

The Kharasch peroxidation system Cu(ii)_cat./t-BuOOH, the source of t-BuOO˙ radicals, can be switched to generate iminoxyl radicals by adding various oximes.