| Abstract: | The adsorption of deoxyribonucleic acid in the mercury-electrolyte interface was investigated. The effect of this adsorption on the differential capacity of the electrical double layer at the interface between a stationary mercury drop electrode (HMDE) and a buffered aqueous sodium chloride solution was measured. The dependences of this differential capacity on potential, time, and pH was studied in the presence of native and also of denatured DNA. These results were compared with the adsorption of model compounds and with the general theory of the adsorption of polymers. The structure of the adsorbed DNA molecules corresponds to an alternating arrangement of two-dimensional, totally adsorbed sequences and three-dimensional loops extending into the solution. The adsorbed sequences and loops consist of several segments with a specific free-energy change of adsorption. Essentially this energy determines the distribution of the segments between adsorbed sequences and loops. The absolute value of this energy change per segment is fairly large in the case of negatively charged poly-electrolyte DNA at the weakly positively charged interface near the electrocapillary maximum (ECM). The fraction of totally adsorbed segments is relatively large in this potential region. The more negative the potential the lower is the absolute value of free energy change of adsorption per segment. Under the conditions unfavorable for the adsorption, only a few segments can be adsorbed. Most of the segments of the adsorbed DNA molecules extend into the solution and therefore fairly high interface concentrations can be reached. Thus, the arrangement of DNA molecules in the electrode surface is changed when the potential is altered from values near the ECM to more negative ones. This change should produce the wave on the differential capacity curves at a little more negative potential than that of ECM. At a more negative potential, intermolecular interactions between the loops extending into the solution may occur. The adsorption tendency of the resulting associates is higher than that of the isolated molecules. Therefore the isolated molecules desorb at sufficient negatively charged interface producing a round wave while the associates stay adsorbed. At this potential it is impossible for native DNA to generate associates because they are formed from the isolated molecules. This explains the hysteresis loop of the curves of differential capacity vs. potential by using the HMDE. The desorption of the associates is indicated by a sharp wave at much more negative potential. For denatured DNA the associates arise from the very few isolated adsorbed molecules at this potential; therefore, no hysteresis loop occurs. The association constant of denatured DNA must be much higher than that of the native DNA. The reasons for this are discussed. |
