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 tile IrCl(6)(2-/3-) redox couple to investigate DNA monolayers, and compares tile potential advantages of this system to the standard Fe(CN)(6)(3-/4)- redox couple. B-DNA monolayers were converted to M-DNA by incu bation in buffer containing 0.4 mM Zn(2+) at pH 8.6 and studied by cyclic voltammetry (CV), impedance spectroscopy (IS) and chrono-amperometry (CA) with IrCl(6)(2-/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 Mg(2+) at pH 8.6 or in Zn(2+) at pH 6.0. The heterorgeneous electron-transfer rate (k(ET)) between the redox probe and the surface of a bare gold electrode was determined to be 5.7 x 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 k(ET) Of 1.5 x 10(-3) cm/s was reached. As well, tile 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 Zn(2+) ions replacing the imino protons on thymine and guanine residues. The IrCl(6)(2-/3-) redox couple was also effective in differentiating between single-stranded and double-stranded DNA during dehybridization and rehybridization experiments. (c) 2008 Elsevier B.V. All rights reserved.