Hexachloroiridate(IV) as a redox probe for the electrochemical discrimination of B-DNA and M-DNA monolayers on gold

This paper demonstrates the effectiveness of using the IrCl62-/3- redox couple to investigate DNA monolayers, and compares the potential advantages of this system to the standard Fe(CN)63-/4- redox couple. B-DNA monolayers were converted to M-DNA by incubation in buffer containing 0.4 mM Zn2+ at pH 8.6 and studied by cyclic voltammetry (CV), impedance spectroscopy (IS) and chronoamperometry (CA) with IrCl62-/3-. Compared to B-DNA, M-DNA showed significant changes in CV, IS and CA spectra. However, only small changes were observed when the monolayers were incubated in Mg2+ at pH 8.6 or in Zn2+ at pH 6.0. The heterorgeneous electron-transfer rate (kET) between the redox probe and the surface of a bare gold electrode was determined to be 5.7 × 10−3 cm/s. For a B-DNA modified electrode, the kET through the monolayer was too slow to be measured. However, under M-DNA conditions, a kET of 1.5 × 10−3 cm/s was reached. As well, the percent change in resistance to charge transfer, measured by IS, was used to illustrate the dependence of M-DNA formation on pH. This result is consistent with Zn2+ ions replacing the imino protons on thymine and guanine residues. The IrCl62-/3- redox couple was also effective in differentiating between single-stranded and double-stranded DNA during de-hybridization and rehybridization experiments.