Elementary reactions of the antioxidant action of trans-stilbene derivatives: resveratrol, pinosylvin and 4-hydroxystilbene

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Abstract
Using pulse radiolysis, elementary reactions of oxidizing radicals such as OH˙, N3˙ and Tl2+ with three trans-stilbene derivatives (4-hydroxy-, 3,5-dihydroxy- and 3,5,4′-trihydroxystilbene) were studied in detail. Depending on the reaction conditions (pH, oxidizing radical), hydroxycyclohexadienyl and phenoxyl radicals of all three polyphenols could beidentified from the transient absorption spectra. The hydroxycyclohexadienyl radicals decay by water- or OH−-elimination forming phenoxyl radicals in a delayed manner. For trans-3,5-dihydroxystilbene it was found that its phenoxyl radicals were mesomeric with keto C-centered radical forms which could explain their sensitivity towards oxygen. From comparison of the spectral and kinetic properties of the transients derived from trans-resveratrol (trans-3,5,4′-trihydroxystilbene) and its analogues, it could be concluded that in the neutral and acidic solution the para-hydroxy group of trans-resveratrol is more reactive than its meta-hydroxy groups, whereas the reactivity of meta-hydroxy groups increases in alkaline solution. This occurrence enables trans-resveratrol to effectively scavenge free radicals in the pH range 2–12.

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