Distribution of electron trapping in DNA: protonation of one-electron reduced cytosine.

Electron spin resonance was employed to study one-electron reduced cytosine stabilized in glasses at low temperatures. In a LiCl/H2O glass, deoxycytidine gives an extra approximately 1 mT splitting that is not observed in oligomers. To better understand the source of the extra splitting, 1-methylcytosine (1mC) and N,N-dimethyldeoxycytidine (dmC) were examined in an HCl/H2O glass. The spectrum of 1mC is a quartet and the spectrum of dmC is a triplet. A probable explanation for this is that in both cases N4 is fully protonated prior to electron addition. In the LiCl/H2O glass, monomeric cytosine, after one-electron reduction, appears to protonate at N4. However, oligomeric cytosine, after one-electron reduction, does not protonate at N4 and therefore must protonate at N3. This could be due to the exclusion of Li+ coordination at N3 and/or the constraining of N4 to a planar configuration via stacking interactions. These findings provide additional insight into why cytosine is the major site of electron capture in DNA. Proton transfer across the N1-H...N3 hydrogen bond is expected to stabilize electron addition to cytosine preferentially.