Indirect Detection of HO• Radical Produced by a Copper Complex under Electrolysis Conditions: Ascorbic Acid as Redox Probe

Despite the nonexistence of free metal ions in healthy biological systems, under cellular acidosis or ischemic conditions, such ions are released and catalyze the production of HO• through Fenton or Haber-Weiss reactions. Copper compounds have been proposed as exogeneous sources of reactive oxygen species (ROS). In this work, we have used ascorbic acid (AA) as redox probe to indirectly prove the generation of ROS by [Cu(4-mbpy-Bz-SMe)2]2+ adsorbed on gold (4-mbpy-Bz-SMe = (E)-1-(4'-methyl-[2,2'-bipyridine]-4-yl)-N-(4(methylthio)phenyl)methanimine). Using the modified gold surface as working electrode in solution containing AA, the voltammogram shows a one-electron wave centered at 0.23V whose current is dependent on the concentration of AA. Accounting that the CuII/I redox wave is observed at 0.32 V, we suggest a ROS produced by the complex during the negative potential scan (0.5 to 0.1V) oxidizes AA to ascorbyl radical (AA+•) in a chemical step previous to the electrochemical one (AA+•  DAA + 1e + H+). The EPR spectrum of the electrolyzed solution (0.2V) was acquired in the presence of DMPO and shows hyperfine splittings at AN = 15.0 and 15.2 G, consistent with the adduct OH-DMPO. We hypothesize that a dimeric copper compound formed on surface, Au/[CuI-O-O-CuI], is oxidized back to Au/[2CuII] during the reverse scan. In acidic and aerated medium, this oxidative step produces H2O2 which, in turn, reacts with the reduced adsorbed complex generating HO• and recovering the starting Cu(II) complex in a catalytic mechanism involving a Fenton like reaction.