Location of TEMPO derivatives in micelles: subtle effect of the probe orientation

Physical evidence that the variations in the efficiency of homologous series of antioxidants in emulsions are a result of differences in their distribution

“…Many investigators have aimed to understand the role of the hydrophobicity of antioxidants (AOs) when attempting to modulate their efficiency and activity in emulsions and cell‐based models …”

The effect of micellization on the EPR spectra and reactivity of 2,2,4,4-tetramethylpiperidinoxyl (TEMPO) radicals

“…Their persistency is determined by their intrinsic reactivity and by their environment in solution. Thus, antioxidant activities in microheterogeneous systems, measured by their hydrogen‐abstraction ability, must take into account both the partitionings of the antioxidant and of the nitroxide probe in these media …”

Partitioning of aryl radicals in micellar systems

“…However, the second partner in the anti‐oxidation reactions, the radicals, may also distribute between the different regions of the emulsions, but their distributions have been, by far, much less investigated in spite that they also need to be considered to fully understand the lipid oxidation reactions and the antioxidant efficiency. Determining their distributions is particularly important because their local concentrations in the different regions of the multiphasic systems may change significantly between the different regions or domains of the system, as does that of the antioxidants, and because reactivity of radicals in the different regions of the system does not need necessarily to be the same …”

“…Experimental work in the laboratory with radicals is not easy because radicals are chemical species having a single unpaired electron in their outermost orbitals, and this electronic configuration is highly energetic, making them, in general, to be chemically unstable and very reactive. Usually, radicals are generated “in‐situ” by different methods, but the inherent difficulties in their preparation and the necessary control of their concentrations in the reaction vessel make that most studies on the effects of radicals and antioxidants in lipid oxidation reactions are carried out by employing stable, commercially available radicals, galvinoxyl • , TEMPO •− , and DPPH • . Results from these tests, however, are not frequently comparable because different radical species may well differ in their hydrogen atom abstracting ability, and so measurements using different radical centers might lead to different reactivity scales of antioxidants.…”

“…Moreover, recent results on the reactivity of series of radicals having the same reactive moieties but different hydrophobicity with the same antioxidant show that their relative reactivities are different and are not directly comparable because of their differential location and orientation at the interfaces. For example, Aliaga et al prepared a series of TEMPO • derivatives and evaluated the effects of the hydrophobicity of the radicals on the antioxidant efficiency of selected antioxidants in micelles and in oil‐in‐water emulsions, finding that the partitioning of TEMPO derivatives in micellar systems attained a maximum value and then decreased with increasing probe hydrophobicity, thus affecting their reactivity.…”