Electrochemical behaviour of mono- and oligonucleotides: IV. The adsorption of adenine oligonucleotides at the mercury-solution interface

The adsorption behaviour of some adenine oligonucleotides was studied at the hanging mercury drop electrode by phase sensitive a.c.-voltammetry and sweep voltammetry. These substances are adsorbed in the whole potential range from most positive potentials adjustable up to the potential of their reduction. The adsorbed layer is transformed at a fresh drop rather rapidly to a condensed film at all concentrations due to significant stacking forces acting between adjacent adenine base rings. The time dependence of the voltammetric reduction response under conditions where only the contribution due to the adsorbed species is measured shows that diffusion controlled adsorption takes place. Differences in adsorption behaviour for adsorption at a fresh surface and at a surface partially covered by slowly desorbing blocking reduction products have been elucidated. From the limiting value of the surface concentration an average surface area per adsozbed mononucleotide unit of about 40 Ų was found which is comparable to the value resulting under the same conditions for poly-A. It was concluded that in both cases the adsorption is occurring via the adenine base residues of the nucleotide chain. As the distance between adjacent bases in the chain is fixed by its backbone formed by sugar and phosphate residues the average adsorption area is essentially the same in both cases. The possible orientation of the bases attached to the electrode is discussed.