One-electron reduction of superoxide radical-anions by 3-alkylpolyhydroxyflavones in micelles. Effect of antioxidant alkyl chain length on micellar structure and reactivity

In micellar solutions, one-electron reduction of •O2− radical-anions by 3-alkylpolyhydroxyflavones (FnH) with alkyl chains of n = 1, 4, 6, 10 carbons produces phenoxyl radicals (•Fn) identical to those obtained by one-electron oxidation by •Br2− radical-anions or by repair of tryptophan radicals. In cetyltrimethylammonium bromide (CTAB), F1H localizes in the Stern layer, and alkyl chains of other FnH solubilize in the hydrophobic interior, interacting with cetyl tails. This interaction produces more compact micelles with lower intramicellar fluidity, as suggested by the increase in the pseudo-first-order rate constant of •Fn formation (k1) from ∼390 s−1 for n = 1 to 610 s−1 for n = 10, leading to an intramicellar bimolecular rate constant of 1 × 105 M−1 s−1. Additionally, •F1 and •F4 decay by intermicellar bimolecular reaction (2k = 20 and 2 × 105 M−1 s−1, respectively) whereas other •Fn radicals are stable over seconds due to increased localization with regards to the Stern layer. In contrast, the thick uncharged hydrophilic palisade layer and the compact hydrophobic core of Triton X100 micelles are responsible for a much higher microviscosity resulting in a decrease in k1 from ∼15.6 s−1 for n = 1 to 9.6 s−1 for n = 10.