Oxidative stress, due to excessive reactive oxygen species such as H2O2, is implicated in the pathogenesis of Parkinson's disease. The electronic behavior of this molecule in the presence of the most common biological electrolyte, sodium chloride, has been investigated by frequency response analysis. H2O2 exhibited negative differential resistance or impedance in the first and second quadrants, a characteristic of tunnel diode behavior. With increasing concentration of H2O2, there was a shift to slightly more negative potentials at which the impedance spectra exhibited negative real impedance. This finding was in agreement with the observations of a slight cathodic shift in the cyclic voltammetric peak. When the potential was changed from the most cathodic to less and less cathodic, the impedance spectral characteristics changed from capacitive loops to impedance in two quadrants and then back to capacitive loops. In the range of potentials at which negative differential resistance was observed, there was a fixed potential at which the frequency was the highest when the impedance became negative. At this potential, the higher the H2O2 concentration, the higher the frequency at which the real part of the impedance became negative. Depending on the potential and concentration of H2O2, negative real impedance was observed at frequencies in the range 260 Hz - 40 mHz. The variations in impedance data were higher in 0.04 M NaOH compared to that in near neutral solutions. Insignificant influence of 1 or 2 mM chloride on the impedance spectra in highly basic solution was observed. Negative differential resistance was also observed for 88 mM H2O2 in solutions of 0.10 M NaClO4 and 0.10 M Na2SO4. The results suggested that the electronic character of H2O2 could be dictated by the potential, bulk electrolyte concentration, pH, and the concentration of H2O2. To explain our admittance data, we have introduced the concept of "potential induced and peroxide-mediated" ion pair formation between sodium and chloride ions in a way similar to our earlier concept, "potential induced and water-structure-enforced" ion pair formation. Our results demonstrate the need for multiple electrodes at multiple locations with tunable frequencies and voltages for possible treatment of Parkinson's disease by deep brain stimulation because of the variations in the concentration of H2O2 at different locations and at different times and consequent subtle changes in its electronic properties.
