Mutual Activation of Two Radical Trapping Agents: Unusual Win-Win Synergy of Resveratrol and TEMPO during Scavenging of dpph(center dot) Radical in Methanol

The reaction of the 2,2'-diphenyl-1-picrylhydrazyl radical (dpph(center dot)) with resveratrol in methanol (k(MeOH) = 192 M-1 s(-1)) is greatly accelerated in the presence of stable nitroxyl radical TEMPO center dot (k(m)(ix)(MeOH) = 1.4 x 10(3) M-1 s(-1)). This synergistic effect is surprising because TEMPO center dot alone reacts with dpph(center dot) relatively slowly (k(S) = 31 M-1 s(-1) in methanol and 0.03 M-1 s(-1) in nonpolar ethyl acetate). We propose a putative mechanism in which a mutual activation occurs within the acid-base pair TEMPO center dot/RSV to the resveratrol (RSV) anion and TEMPOH center dot+ radical cation, both being extremely fast scavengers of the dpph(center dot) radical. The fast initial reaction is followed by a much slower but continuous decay of dpph(center dot) because a nitroxyl radical is recovered from the TEMPOnium cation, which is reduced directly by RSV/RSV- to TEMPO center dot or recovered indirectly via a reaction with methanol, producing TEMPOH subsequently oxidized by dpph(center dot) to TEMPO center dot.

Automated summary

The reaction between the stable nitroxyl radical TEMPO center dot and the 2,2'-diphenyl-1-picrylhydrazyl radical (dpph(center dot)) is greatly accelerated in the presence of resveratrol. This surprising synergistic effect is proposed to occur through a mutual activation mechanism between TEMPO center dot/RSV, resulting in extremely fast scavenging of dpph(center dot) radical.