Reduction of Adenine and Cytosine Residues

One well-established observation is that, under conditions where single-stranded polynucleotides give rise to a d.c. polarographic reduction wave, both native DNA and other double-helical natural and synthetic polynucleotides are inactive 22 23,46-47, 58,59,61) Tjjjs js rea(ji]y interpretable in that, in such helical structures, the adenine and cytosine residues are located in the interior of the helix, and hydrogen bonded in complementary base pairs (see below). Z-DNA, in which cytosine residues are at the surface of the helix, is of obvious interest in this regard, and the B - Z transition in the synthetic poly(dG dC) has been investigated with the aid of differential pulse polarography and UV spectroscopy 60). 

Berg et al. 711 proposed that the adenine and cytosine residues in native DNA are reduced by a so-called electron hopping mechanism, the only condition for this being adsorption of protonated DNA at the electrode surface at the reduction potential of these bases. It was also assumed that the DNA is adsorbed in its A-form, exhibiting semi-conducting properties. There is consequently no surface denaturation of the DNA. 

The ability of adenine and cytosine to undergo reduction in aqueous medium is at least partially retained at the level of oligo- and polynucleotides. In both acid and neutral media, such residues in single-stranded RNA, DNA and synthetic polynucleotides, when subjected to d.c. polarography, undergo irreversible reduction in a protonated, adsorbed, state with the transfer of four electrons to adenine... 

Electrochemical reduction of natural and biosynthetic nucleic acids at a dropping mercury electrode [1, 3, 73] showed that adenine and cytosine residues, as well as guanine residues in a polynucleotide chain, are reducible. The CV of DNA at a hanging mercury drop electrode showed a cathodic peak due to irreversible reduction of cytosine and adenine moieties. The reduction of the guanine moiety occurs at very negative potentials, but a peak due to the oxidation of the reduction product of the guanine moiety (7,8-dihydroguanine moiety) could be detected in the reverse scan [3]. 

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