Oxidation kinetics of guanine in DNA molecules adsorbed onto indium tin oxide electrodes

Oligonucleotides containing the guanine nucleobase were adsorbed onto ffO e lectrodes from mixtures of DMF and acetate buffer. Chronocoulometry and chr onoamperometry were performed on the modified electrodes in both phosphate buffer and buffer containing low concentrations of the inorganic complex Ru (bpy)(3)(2+) (bpy = 2,2' bipyridine), which catalyzes guanine oxidation, Th e charge and current evolution with and without the catalyst were compared to the charge and current evolution for electrodes that were treated with i dentical oligonucleotides that were substituted at every guanine with the e lectrochemically inert nucleobase hypoxanthine. Chronocoulometry over 2.5 s shows that roughly 2 electrons per guanine were transferred to the electro de in both the presence and absence of Ru(bpy)(3)(2+), although at a slower rate for the uncatalyzed process. Chronoamperograms measured over 250 ms c an be fit to a double exponential decay, with the intensity of the fast com ponent roughly 6-20 times greater than that of the slow component. First- a nd second-order rate constants for catalytic and direct guanine oxidation w ere determined from the fast component. The maximum catalytic enhancement f or immobilized guanine was found to be i(cat)/i(d) = 4 at 25 muM Ru(bpy)32, The second-order rate constant for the catalyzed reaction was 1.3 x 10(7) M-1 s(-1), with an apparent dissociation constant of 8.8 muM. When compare d to parallel studies in solution, a smaller value of the dissociation cons tant and a larger value of the second-order rate constant are observed, pro bably due to distortion of the immobilized DNA, an increase in the local ne gative charge due to the oxygen sites on the ITO surface, and redox cycling of the catalyst, which maintains the surface concentration of the active f orm.