DNA Conformational Transitions at Different Concentrations and Temperatures Monitored by EIS

Abstract

We present an EIS study and its analysis to determine the conformational behavior of DNA calf-thymus (13 kbp). The study is carried out at open circuit potential at a pH of 7.3. The results are interpreted in terms of the adsorption impedance theory by using an equivalent circuit approach similar to the one proposed by Frumkin-Melik-Gaikazyan-Randles. All equivalent circuit parameters are inspected as a function of DNA concentrations where it is possible to identify two transitions on the double-layer structure at DNA ca. 0.33 ± 0.07 and 1.50 mg/mL related to the overlap, C*, and entanglement, C e , concentrations. Electrochemical Impedance Spectroscopy (EIS) is an outstanding and non-destructive technique suitable for investigating charged surfaces interacting with molecules such as DNA. Recent reports in the literature have demonstrated its potential to investigate DNA hybridization events that allow the development of DNA biossensing platforms, inmunosensors, and gene therapy.1-3 The last one requires a full understanding of DNA conformational changes before attempting a transfection process, which involves the collapsing of extended DNA chains into a more compact, arranged particles containing one or a few molecules before entering a cell. 4,5 Since DNA is a complex charged molecule, it becomes evident that most of the conformational changes perceived for DNA strains in solution or interacting with charged electrode surfaces may depend on several parameters, such as concentration, pH, temperature and polarization potential. 6One of the alternatives currently applied to investigate transitions in polymer science consists of measuring rheological properties at a frequency range (0.0016-16 Hz).7-9 Since DNA is a biopolymer it can be analyzed in a reometer with a cone-plate geometry, applying an oscillatory mode, while measuring the viscoelastic properties. 8 Thus, it is possible by this technique to derive two critical parameters sensible to DNA conformation, i.e. the overlap (C*) and the entanglement concentration (C e ). The first one consists of a dilute polymer solution without any interaction between the molecules until reaching C*, where molecules start interacting each other and getting packed, then reaching the overlap concentration C e , where polymer chains are entangled. 7,8 By applying rheological techniques a calf-thymus DNA solutions at a pH 8.0 have been analyzed identifying both transitions at 0.35 and 2.0 mg/mL for C* and C e respectively. 8 Although rheological methods provide macroscopic properties of DNA chains in solution a microscopic overview of the systems is still needed. 7,8 In this direction a method that has contributed to the understanding of DNA conformations at a microscopic scale is dielectric relaxation.10 By applying this technique it has been possible to investigate the dynamics and structure of water bound to DNA molecules at high frequency domain (10 6 -10 −2 Hz). 11,12 A different frequency window can be also explored by EIS (10 3 -10 −3 Hz), in conjunction with an analysis...

DNA Transitions by an Adsorption Impedance Study

“…26 Recently, an EIS investigation of the interface formed by a platinum electrode and calf-thymus DNA molecule at a temperature of 30 • C has been presented as a new alternative method for detecting biopolymer critical concentrations, C* and C e . 30 Both transitions have been explained in terms of a double-layer structure rearrangement as a function of the DNA molecule conformation at the interface.…”