Towards deeper understanding of DNA electrochemical oxidation on carbon electrodes

Abstract

To better understand the electrochemical behavior of deoxyribonucleic acid (DNA) on carbon electrodes, the oxidation of synthetic oligonucleotides of 4 to 72 nucleotides long, their thermally stable duplexes, polymerase chain reaction amplicons of 141 base pairs long, and samples of double- and single-stranded DNA (dsDNA and ssDNA) of natural origin was studied on carbon screen printed electrodes by square wave and cycling voltammetry within the potential range of 0.5–1.5 V (vs. Ag/AgCl). In square wave voltammograms, ssDNA of natural origin produced two distinct signals at approximately 0.75 V and 1.05 V (at the nucleotide concentration of 1 mM; phosphate buffer, pH 7.4), attributed to the oxidation of guanine (Gua) and adenine (Ade) residues, respectively. The oxidation reactions of both residues of ssDNA were found diffusion-controlled. Similar to that, synthetic DNA oligonucleotides exhibited the oxidation peaks at potentials of either around 0.85 V or 1.15 V (at the nucleotide concentration of 1 mM), or both, depending on their sequence. In contrast to ssDNA, dsDNA molecules showed no sign of oxidation on carbon electrodes. The low molecular weight ssDNA fragments were found as the main contributors to the overall electrooxidation currents generated via oxidation of Gua and Ade residues of DNA from a natural source. Oxidation signals of Gua and Ade residues exponentially decreased with the oligonucleotide length. In the thermally stable duplex DNA molecules even as short as those of 24 base pairs long, Gua and Ade restudies were shown to be inaccessible for oxidation on carbon electrodes.