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Advisor(s)
Abstract(s)
The flavonoid quercetin is known to reduce the α-tocopheroxyl radical (˙TocO) and reconstitute
α-tocopherol (TocOH). Structurally related polyphenolic compounds, hydroxy-2,3-diarylxanthones (XH),
exhibit antioxidant activity which exceeds that of quercetin in biological systems. In the present study
repair of ˙TocO by a series of these XH has been evaluated using pulse radiolysis. It has been shown that,
among the studied XH, only 2,3-bis(3,4-dihydroxyphenyl)-9H-xanthen-9-one (XH9) reduces ˙TocO,
though repair depends strongly on the micro-environment. In cationic cetyltrimethylammonium bromide
(CTAB) micelles, 30% of ˙TocO radicals are repaired at a rate constant of ∼7.4 × 106 M−1 s−1 by XH9
compared to 1.7 × 107 M−1 s−1 by ascorbate. Water-soluble Trolox (TrOH) radicals (˙TrO) are restored by
XH9 in CTAB (rate constant ∼3 × 104 M−1 s−1) but not in neutral TX100 micelles where only 15% of
˙TocO are repaired (rate constant ∼4.5 × 105 M−1 s−1). In basic aqueous solutions ˙TrO is readily reduced
by deprotonated XH9 species leading to ionized XH9 radical species (radical pKa ∼10). An equilibrium is
observed (K = 130) yielding an estimate of 130 mV for the reduction potential of the [˙X9,H+/XH9]
couple at pH 11, lower than the 250 mV for the [˙TrO,H+/TrOH] couple. A comparable value (100 mV)
has been determined by cyclic voltammetry measurements.
Description
Keywords
Pulse-radiolysis Redox potentials Vitamin-E Electron Micelles Fluorescence
Citation
Morlière, Patrice; Patterson, Larry K.; Santos, Clementina M.M.; Silva, Artur; Marzière, Jean-Claude; Filipe, Paulo; Gomes, Ana; Fernandes, Eduarda; Garcia, Beatriz; Santus, René (2012). The dependence of alpha-tocopheroxyl radical reduction by hydroxy-2,3-diarylxanthones on structure and micro-environment. Organic & Biomolecular Chemistry. ISSN 1477-0539. 10, p. 2068-2076
Publisher
Royal Society of Chemistry