Digital Simulation of Catalytic Cyclic Voltammograms for Oxidation of DNA by a Heterobimetallic Dimer: Effects of DNA Binding and Mass Transport

The electrocatalytic oxidation of DNA by a heterodimer, [(bpy)2Ru(tpphz)Os(bpy)2]^4+ (tpphz: tetrapyrido[3,2-a:2ʹ,3ʹ-c:3ʹ ʹ,2ʹ ʹ-h:2ʹ ʹʹ,3ʹ ʹʹ-j]phenazine) (1), was studied using cyclic voltammetry with digital simulation. This dimer was chosen because the Ru(III/II) couple (E1/2 = 1.09 V vs Ag/AgCl) is capable of catalyzing guanine oxidation while the Os(III/II) couple (E1/2 = 0.63 V) provides a convenient reporter on the binding and mass transport of the complex, which can then be determined in the same voltammetric sweep as the electrocatalysis. Proper description of the electrochemical response required careful measurement of the binding constant of 1 to herring testes (HT) DNA, which was (2.0 +- 0.1) × 10^4 M^-1 by both absorption titration and normal pulse voltammetry. Thermal denaturation experiments were consistent with a nonintercalative binding mode and gave a (delta)Tm of only (2.4 +- 0.5) (degree)C. The minor groove binder distamycin did not displace 1 from HT DNA, suggesting that the complex binds in the major groove. As expected, acquisition of the cyclic voltammogram of 1 in the presence of DNA produced catalytic current for the Ru(III/II) couple, while a suppression of current was observed for the Os(III/II) couple. Although the catalytic current for the Ru(III/II) couple initially appeared as a current enhancement, higher concentrations suppressed the catalytic wave as a result of the slower mass transport of the DNAbound complex. The binding studies were used to create a model for digital simulation that reproduced the behavior of 1 with DNA and gave rate constants that were independent of DNA concentration. The apparent second-order rate constant at 25 mV/s for oxidation of guanine in HT DNA (av 1000 bp, 25% guanine) by 1 was 3 +- 1 × 10^4 M^-1 s^-1; similar values were obtained for a 200-bp fragment (7 +- 3 × 10^4 M^-1 s^-1) and a 435-bp fragment (8 +- 2 × 10^4 M^-1 s^-1). As observed in previous studies of these reactions, biphasic kinetics in the catalytic reaction led to a dependence of the rate constant determined by simulation on the sweep rate. Increasing the sweep rate led to a systematic increase in the simulated rate constant, consistent with a fast phase of the homogeneous catalytic reaction.