Interaction of nucleic acids with electrically charged surfaces. VIII. Dependence of changes in DNA double helical structure on pH and on potential of the mercury electrode

Abstract

Native double-stranded (ds) and thermally denatured DNAs were studied by means of linear sweep voltammetry, (LSV) in connection with h.m.d.e. and a.c. polarography with d.m.e. LSV measurements at various initial potentials ( U i ) in dependence on pH in acid and neutral regions showed that the potential of peak III of denatured and ds DNA was shifted to negative values with increasing pH. It was concluded that the reduction of ds DNA (conditioned by the previous interaction of ds DNA with the mercury electrode charged to potentials more positive than the reduction potential) takes place in the protonated state (similar to the case with thermally denatured DNA). LSV behaviour of thermally denatured and ds DNA was studied in alkaline media in dependence on U i , pH and ionic strength. It was shown that both DNA forms yielded, over a wide range of pHs and ionic strengths, capacitive LSV peaks which depend on U i in a way similar to that of reduction peaks in neutral media. Alkaline denaturation of DNA was studied by means of LSV at various U i and with the aid of a.c. polarography and UV spectrophotometry. It was demonstrated that in accordance with the measurements in neutral media methods working with small voltage excursion during the drop lifetime (represented here by a.c. polarography in connection with d.m.e.) reflected in alkaline media the helix coil transition in the bulk of the solution. Conversely the results of LSV at U i -1.2 V ( i.e. a method working with large voltage excursion during the drop lifetime) did not show the alkaline helix-coil transition. These results agree with our conception of conformational changes in ds DNA at the mercury electrode surface. According to this conception in the narrow potential range around −1.2 V (region u ) changes in the DNA double-helical structure occur which result in the release of bases for the interaction with the electrode (surface denaturation). On the other hand at potentials more negative than the region u (i.e. those corresponding to the potential of the reduction or capacitive peak III) no changes in DNA double-helical structure occur. The socalled model A proposed by Berg et al. [1] is in contradiction with the main experimental data presented in this paper or is not suitable for their interpretation.