Mutual Activation of Two Radical Trapping Agents: Unusual “Win–Win Synergy” of Resveratrol and TEMPO during Scavenging of dpph• Radical in Methanol

The reaction of the 2,2′-diphenyl-1-picrylhydrazyl radical (dpph•) with resveratrol in methanol (kMeOH = 192 M–1 s–1) is greatly accelerated in the presence of stable nitroxyl radical TEMPO• (kmixMeOH = 1.4 × 103 M–1 s–1). This synergistic effect is surprising because TEMPO• alone reacts with dpph• relatively slowly (kS = 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•/RSV to the resveratrol (RSV) anion and TEMPOH•+ radical cation, both being extremely fast scavengers of the dpph• radical. The fast initial reaction is followed by a much slower but continuous decay of dpph• because a nitroxyl radical is recovered from the TEMPOnium cation, which is reduced directly by RSV/RSV– to TEMPO• or recovered indirectly via a reaction with methanol, producing TEMPOH subsequently oxidized by dpph• to TEMPO•.


TABLE OF CONTENTS
Title page Figure S1. Table S1 Data and kinetic parameters for reaction of dpph • with excess of TEMPO • in MeOH, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of 4-OH-TEMPO • in MeOH, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess RSV in MeOH, linear fitting. S-5 Figure S4. Table S4.
Data and kinetic parameters for reaction of dpph • with excess of TEMPO • in EtOAc, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of 4-OH-TEMPO • in EtOAc, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of RSV in EtOAc, linear fitting. S-8 Figure S7. Table S7.
Data and kinetic parameters for reaction of dpph • with excess of RSV and TEMPO • 1:1 mol/mol in MeOH, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of RSV and TEMPO • 1:1 mol/mol in EtOAc, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of RSV and 4-OH-TEMPO • 1:1 mol/mol in MeOH, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of RSV and 4-OH-TEMPO • 1:1 mol/mol in EtOAc, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of 3,5-DHA and TEMPO • 1:1 mol/mol in MeOH, linear fitting.
Data and kinetic parameters for reaction of dpph • with excess of RSV in acidified MeOH (10mM AcOH), linear fitting. S-17 Figure S16. Table S16.
Data and kinetic parameters for reaction of dpph • with excess of TEMPO • with 10 mM HFIP, linear fitting. S-18 Figure S17. Data and kinetic parameters for reaction of dpph • with excess of TEMPO-H and RSV 1:1 mol/mol in EtOAc, linear fitting.    Figure S3. Plots of k exp versus concentration of RSV (data taken from Table S3).   Figure S4. Plots of k exp versus concentration of TEMPO • (data taken from Table S4).

Resveratrol/EtOAc
S-9  Figure S7. Plots of k exp versus concentration of RSV (data taken from Table S7).   Figure S8. Plots of k exp versus concentration of RSV (data taken from Table S8).    Figure S10. Plots of k exp versus concentration of RSV (data taken from Table S10).   Figure S11. Plots of k exp versus concentration of 3,5-DHA (data taken from Table S11).   Figure S12. Plots of k exp versus concentration of 3,5-DHA (data taken from Table S12).   Figure S13. Plots of k exp versus concentration of TEMPO • (data taken from Table S13).

4-OH-TEMPO/MeOH, AcOH
S-17  Figure S15. Plots of k exp versus concentration of RSV (data taken from Table S15).   Figure S16. Plots of k exp versus concentration of TEMPO • (data taken from Table S16).

TEMPO-H/MeOH, AcOH
S-21   Figure S20. Plots of k exp versus concentration of RSV (data taken from Table S20).   Figure S21. Plots of k exp versus concentration of RSV (data taken from Table S21).