The electrochemical behaviour of DNA at electrically charged interfaces

Abstract

The adsorption and the reduction of native and denatured DNA has been studied at the hanging mercury drop electrode (HMDE) by triangular sweep polarography. It has been established from the dependence of the reduction peak of denatured DNA (Fig. 2) on the waiting time at the starting potential E₈ of the sweep (Fig. 3) where the adsorption takes place and on pH (Fig. 4) that a protonated form of denatured DNA is reduced from the adsorbed state in a totally irreversible electrode process at a pH lower than 7.5. From the logarithmic analysis of the beginning part of the reduction peak the charge transfer rate constant and the charge transfer coefficient were determined (Fig. 5).

The reduction products are interconnected to a high molecular network strongly adhering to the surface of the electrode and blocking it for a repeated reduction on the same drop. In neutral media the protonation of DNA becomes rate-determining. The adsorption is diffusion controlled and from the time dependence of the coverage the diffusion coefficient of DNA could be estimated (Fig. 6) to 5.10⁻⁷ cm² s⁻¹ in 1 M KCl at 25 ‡C and at pH 6.7 for denatured DNA of the mean molecular weight 10⁶ g. The protonated molecules of DNA are adsorbed via strong induced dipole interactions of theπ-electron system in the heteroaromatic ring of the bases with electrons of the metal while the other parts of the DNA molecule including the solvated sugar and phosphate groups are orientated to the solution.

From the time integral of the reduction peak (Fig. 6) the maximal surface concentration of the electrochemically active monomeric units in DNA was estimated to 1.4·10⁻¹⁰ mol cm⁻² with an average area per mononucleotide of about 60 å². Both values vary only within 10% in the temperature range 5 to 25 ‡C, in the concentration range of DNA 10 to 360 Μg/ml and for starting potentials E_s −0.2 to −1.3 V vs. SCE.

Native DNA is also adsorbed at the electrode and reduced in acid solutions in the same irreversible electrode reaction as denatured DNA. However, before a partial opening of the helix is induced by the strong electric field at the interface leading with respect to helix-coil transition to an effect equivalent to partial melting which has to precede the electron transfer step. The extent of unscreened reducible groups and their orientation with the electrochemically reactive sites towards the electrode depends on pH (Fig. 8), on electrical parameters of the interface as the starting potential E_s (Fig. 7) and on the portion of the DNA molecule which comes into the region where the electric field is acting. The maximal extents of unscreened reducible groups are 85% for native DNA of the mean molecular weight of 2.5·10⁶ g and 46% for native DNA of the mean molecular weight 10⁷ g (Fig. 6). The results have with respect to the physicochemical properties of DNA general significance also for the behaviour of nucleic acids at charged biological interfaces as for instance the membrane of living cells.