Ascorbyl and hydroxyl radical generation mediated by a copper complex adsorbed on gold

This work presents the results obtained for a thioether derivative of bipyridine, (E,Z)-1-(4'-methyl-[2,2'-bipyridine]-4-yl)-N-(4(methylthio)phenyl)methanimine (4-mbpy-Bz-SMe), and its copper complex [Cu^II(4-mbpy-Bz-SMe)₂]²⁺. Electronic spectra acquired at 183 K of the cuprous complex [Cu^I(4-mbpy-Bz-SMe)₂]⁺ generated in situ indicated the formation of the peroxodicopper compound {[Cu^II(4-mbpy-Bz-SMe)₂]₂(μ-O₂^2-)}²⁺. A gold electrode modified with [Cu^II(4-mbpy-Bz-SMe)₂]²⁺ (Au/[Cu]) was fully characterized by SERS spectroscopy, electrochemistry and impedance spectroscopy thus showing adsorption occurs through the sulfur atom of the 4-mbpy-Bz-SMe moieties. DNA cleavage assays showed the copper complex, in solution and adsorbed on gold, degrades DNA if reducing conditions are maintained, i.e. ascorbic acid (H₂AA) in solution or applied potentials more negative than 0.12 V vs. Ag/AgCl (Cu^I form). The electron paramagnetic resonance (EPR) spectra obtained for the electrolyzed solution (E_apl = -0.2 V, no H₂O₂) and for the solution containing [Cu^II(4-mbpy-Bz-SMe)₂]²⁺ and H₂O₂ showed hydroxyl radical, HO˙, generation had occurred. The cyclic voltammograms obtained with H₂AA in solution at Au/[Cu^II(4-mbpy-Bz-SMe)₂]²⁺ as the working electrode showed a one-electron reaction leading to the ascorbyl radical (HA˙), which was detected by EPR. The current assigned to the electrode oxidation of HA˙ to AA decreased with the addition of catalase, a scavenger of H₂O₂, meaning peroxide is involved in the mechanism.