Adsorptive transfer stripping voltammetry: effect of electrode potential on the structure of DNA adsorbed at the mercury surface

Abstract

The effect of electrode potential on the structure of DNA adsorbed at a hanging mercury drop electrode was studied by adsorptive stripping cyclic voltammetry (CV). The cathodic CV peak (due to adenine and cytosine residues) and the anodic peak (due to guanine residues) were measured to discover changes in DNA structure at the electrode surface. In addition to conventional CV, three variants of adsorptive transfer stripping CV (AdTSCV) were applied which made it possible to prepare a DNA-modified mercury electrode and to study the influence of electrode potential on the adsorbed DNA layer in the absence of DNA molecules in the bulk of the solution. DNA was absorbed at the electrode from solutions of native double-stranded and thermally denatured single-stranded DNA to obtain full electrode coverage.The DNA adsorbed at the electrode from denatured DNA solution exhibited CV peaks with heights independent of the electrode potential to which the DNA was exposed prior to the CV measurements, and practically no differences were observed between the results of conventional CV and the variants of AdTSCV. With DNA adsorbed from native DNA solution, both CV peaks increased owing to exposure of the adsorbed DNA to potentials around −1.25 V (against a saturated calomel electrode, region U) and marked differences between the results of conventional CV and AdTSCV variants were found. If DNA was adsorbed at the electrode at −0.1 V (i.e. at the potential where no extensive surface denaturation was observed) and exposed to potentials of region U in the absence of DNA in the bulk of the solution, opening of the DNA double helix occurred in a narrow potential region (its half-width corresponding to less than 200 mV). If DNA was adsorbed at potentials more negative than −0.6 V this region was more than twice as wide. The results suggest that (i) opening of DNA at the electrode surface can occur both in the presence and in the absence of DNA molecules in the bulk of the solution, (ii) the potential at which DNA is adsorbed can influence the DNA opening process at the electrode, and (iii) DNA opening is faster at more negative potentials of the U region, this being in agreement with the mechanism of the opening process proposed earlier.