Electrochemical behavior of 7-deazaguanine- and 7-deazaadenine-modified DNA at the hanging mercury drop electrode

Abstract

DNA modification with synthetic analogs of natural nucleotides and/or their conjugates with external redox active groups is applied in the development of electrochemical DNA sensors or assay for DNA hybridization, SNP typing, DNA damage and so forth. 7-Deazapurines (Pu*) are analogs of natural purine bases in which N7 atom is replaced by CH group. The Pu* bases retain Watson–Crick base pairing of their parent purines (and the ability to form duplex DNA) but are incapable of Hoogsteen pairing (and thus cannot be involved in triplex or quadruples DNA structures). Previously, we studied electrochemical oxidation of Pu* residues in DNA fragments (prepared by PCR in the presence of Pu* deoxynucleoside triphosphates) at a carbon electrode and reported on significantly lower potentials of oxidation of both 7-deazaguanine (G*) and 7-deazaadenine (A*), compared to natural guanine (G) and adenine (A), respectively. In this work, we studied faradaic and tensammetric responses of G*- or A*-modified DNA on the hanging mercury drop electrode (HMDE). While A* was reduced at the HMDE, giving rise to a similar irreversible cathodic peak as the natural A, G* did not yield any peak analogous to the peak G due to guanine, in agreement with a loss of corresponding redox site in G*. Responses of DNA modified with A* were relatively similar to those of unmodified DNA (albeit we observed certain differences in tensammetric peak currents). Effects of G substitution by G* were more pronounced, being reflected in diminution of peak due to guanine, decrease of the peak CA (due to cytosine and adenine reduction) and in significantly changed shape of tensammetric DNA signals, indicating altered adsorption/desorption processes. While substitution of A by A* resulted in certain destabilization of the DNA duplex at the negatively charged HMDE surface (in qualitative agreement with significantly decreased melting temperature of the same DNA duplexes in solution), G*-modified duplex DNA displayed apparently lower susceptibility to surface denaturation.