Cytosine, polarography

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). 

The foregoing proposed mechanism of reduction of cytosine, cytidine and their derivatives in acid and moderately alkaline media are further supported by the results of a recent study, by means of a.c. polarography, of the orientation of cytosine and cytidine at the mercury electrode. It was shown87) that, whereas at acid pH (about 5) both of these are adsorbed via the positive charge on the ring N3, so that the N3 = C4 bond is adjacent to the electrode surface, the orientation in alkaline medium (pH 9) is modified so that the C5 = C6 bond is located at the surface. This is consistent with the proposed mechanisms of reduction. 

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... 

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