Electrochemical behaviour of natural and biosynthetic polynucleotides at the mercury electrode Part IV. Reduction and adsorption of polyriboadenylic acid at the hanging mercury drop electrode

Abstract

In continuation of our preceding study of native and denatured DNA 4–7 poly-A has been studied at the HMDE by the triangular sweep technique under potentiostatic control in the pH range 4–8. Both helical conformations of poly-A, the single-stranded one at neutral pH and the double-stranded one at acid pH, are reduced at the HMDE totally irreversibly, by the uptake of four electrons provided the adenine bases are protonised. The reduction potential almost coincides with that of denatured DNA. Poly-A is adsorbed at the electrode in the whole potential range attainable. The T afel plot for small currents at the foot of the reduction peak indicates a two-electron rate determining step and a charge transfer rate constant of the same order as for the reduction of denatured DNA results. The adsorption at low concentrations of poly-A is controlled by diffusion. From the time integral of the reduction peak at full coverage, an average area per mononucleotide unit of the neutral form of poly-A has been calculated to be 46 Å 2. This indicates that the molecular arrangement in the surface layer is on a rather perpendicular position of the heteroaromatic ring with respect to the electrode surface. The adsorbed acid form of poly-A undergoes an opening of the double helix due to the electric field at the charged interface rendering some of the adenine moieities accessible for reduction. In general the results support the views that adenine is one of the reducible bases in DNA and our finding that the electric field induces an opening of the double helix in native DNA.