Structural Behavior of Au-Calf Thymus DNA Interface Estimated Through an Electrochemical Impedance Spectroscopy and Surface Plasmon Resonance Study

This work presents an original adsorption study of calf-thymus DNA molecules monitored by Electrochemical Impedance Spectroscopy (EIS) and Surface Plasmon Resonance (SPR) as a function of DNA concentration at a constant pH of 7.3 and room temperature (25 ± 1 °C). Since EIS measurements were carried out at open circuit potential, there was an adsorption process of DNA molecules at the electrode surface. This enabled Cdl values to be obtained by using an equivalent circuit similar to the one proposed by Frumkin-Melik-Gaikazyan-Randles (FMGR). Values of optical film thickness (dopt) of adsorbed DNA molecules were estimated by using SPR measurements. The variations of the dopt as a function of DNA concentration (cDNA) can be related to transitions in the structural arrangement of the electrochemical double-layer, presumably caused by DNA conformational changes observed at two critical cDNA, i.e. c* and ce, known as the overlap and entanglement concentrations, respectively. A linear behavior was observed in each regime, in which the relation between dop and cDNA followed the power law according to: dilute regime (cDNA < c*): dopt ∼ cDNA0.20, semi-dilute regime without entanglements (c* < cDNA < ce): dopt ∼ cDNA0.79and semi-dilute regime with entanglements (ce < cDNA): dopt ∼ cDNA0.57. Through dopt and Cdl values, a dielectric constant behavior as a function of DNA concentration was obtained, showing DNA transitions passing from diluted, semi-diluted unentangled and semi-diluted entangled solutions.