Abstract
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• (kinh) 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 (kinh) 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 kinh = 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.
Highlights
Nanomaterials with antioxidant properties represent an emerging strategy to counteract oxidative spoilage of organic materials and to modulate redox reactions in biological systems.[1]
Core−shell silica-PEG nanoparticles were prepared by hydrolysis/condensation of tetraethoxysilane (TEOS) under acidic conditions in a micellar solution of Pluronic F127, a triblock polyethylene oxide, i.e., the poly(ethylene glycol))−polypropylene oxide copolymer as already reported.[33]
We have investigated the antioxidant activity in water of nitroxides 2−7 for the first time and compared them with the well-known TEMPO and the reference antioxidant Trolox
Summary
Nanomaterials with antioxidant properties (nanoantioxidants) represent an emerging strategy to counteract oxidative spoilage of organic materials and to modulate redox reactions in biological systems (see for instance lipid peroxidation in Figure 1A).[1] They can provide large local concentration,[2] stabilization, and controlled release of labile antioxidants,[3] and the possibility to target specific cells or organs.[4] The antioxidant activity can be displayed by intrinsically redox-active nanomaterials (i.e., metal oxides, melanins, lignins)[1] or can be obtained by anchoring small-molecule antioxidants to the surface of inert scaffolds.[1] Surface functionalization is typically performed by exploiting natural and synthetic antioxidants including glutathione,[5] carotenoids,[6] gallic acid,[7] curcumin,[8] α-tocopherol analogues,[9,10] and butylated hydroxytoluene (BHT).[11] phenols represent the most common surface-active antioxidant agents, their efficacy is drastically diminished by their instability under air.[12,13] In water, phenols typically degrade by the deprotonation of ArOH groups, followed by the reaction with O2 generating superoxide (O2−/HOO) and phenoxyl radicals.[12] In the context of our ongoing research in the field of nanoantioxidants, we envisaged that these shortcomings could be overcome using hindered nitroxides as surface-bound antioxidants
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