Electrocatalysis of Guanine Electron Transfer:  New Insights from Submillimeter Carbon Electrodes

Electrocatalytic oxidation of guanines in DNA by Ru(bpy) 3 3+/2+ (bpy = 2,2‘-bipyridine) has been detected at 86 μm-radius carbon electrodes. The electrocatalysis, which is apparent as a current enhancement of the Ru(bpy) 3 3+/2+ couple in the presence of DNA, is dependent on the ionic strength. The ionic strength effect is due not to a change in the mediator−electrode surface interaction, but rather to a change in the mediator−DNA interaction. The small size of the electrode allows examination of the contribution of the time-independent current, which is a function of the diffusion coefficients of the electroactive species, to the total current generated by guanine oxidation by Ru(bpy) 3 3+ . The second-order rate constants from digital simulation of the experimental cyclic voltammograms collected at low ionic strength are (in terms of moles of guanine) (5.6 ± 1.4) × 10 4 M -1 s -1 at 5 mV/s and (8.8 ± 2.2) × 10 4 M -1 s -1 at 250 mV/s. At high ionic strength, the second-order rate constants for voltammograms collected at 5 mV/s or 250 mV/s are both 2 × 10 4 M -1 s -1 . Digital simulation of the time-independent current reveals that weak electrostatic association of Ru(bpy) 3 2+ or Ru(bpy) 3 3+ with DNA produces maximal currents; extensive association of either the 2+ or the 3+ form inhibits the electrocatalysis and depresses the catalytic currents. Therefore, Ru(bpy) 3 2+, which binds to DNA with a weaker affinity than does Ru(bpy) 3 3+, is a near-ideal catalyst for electrocatalytic guanine oxidation in DNA.