Direct Voltammetric Analysis of DNA Modified with Enzymatically Incorporated 7-Deazapurines
Nucleic acids studies use 7-deazaguanine (G(star)) and 7-deazaadenine (A(star)) as analogues of natural purine bases incapable of forming Hoogsteen base pairs, which prevents them from being involved in DNA triplexes and tetraplexes. Reduced propensity of the G(star)- and/or A(star)-modified DNA to form alternative DNA structures is utilized, for example, in PCR amplification of guanine-rich sequences. Both G(star) and A(star) exhibit significantly lower potentials of their oxidation, compared to the respective natural nucleobases. At carbon electrodes, A(star) yields an oxidation peak which is by about 200-250 mV less positive than the peak due to adenine, but coincides with oxidation peak produced by natural guanine residues. On the other hand, oxidation signal of G(star) occurs at a potential by about 300 mV less positive than the peak due to guanine, being well separated from electrochemical signals of any natural DNA component. We show that enzymatic incorporation of G(star) and A(star) can easily be monitored by simple ex situ voltammetric analysis of the modified DNA at carbon electrodes. Particularly G(star) is shown as an attractive electroactive marker for DNA, efficiently incorporable by PCR. While densely G(star)-modified DNA fragments exhibit strong quenching of fluorescence of SYBR dyes, commonly used as fluorescent indicators in both gel staining and real time PCR applications, the electrochemical detection provides G(star)-specific signal suitable for the quantitation of the amplified DNA as well as for the determination of the DNA modification extent. Determination of DNA amplicons based on the measurement of peak G(star ox) is not affected by signals produced by residual oligonucleotide primers or primary templates containing natural purines.