Indirect evidence strongly suggests that oxidation reactions of
cytosine and its minor derivative
5-methylcytosine play a major role in mutagenesis and
cancer. Therefore, there is an emerging necessity to identify the final oxidation products of these reactions, to search for their formation in cellular
DNA, and to assess their mutagenic features. In this Account, we report and discuss the main *
OH and one-electron-mediated oxidation reactions, two of the most potent sources of DNA damage, of
cytosine and
5-methylcytosine nucleosides that have been recently characterized. The addition of *
OH to the 5,6-unsaturated double bond of
cytosine and
5-methylcytosine generates final degradation products that resemble those observed for
uracil and
thymine. The main product from the oxidation of
cytosine,
cytosine glycol, has been shown to undergo
dehydration at a much faster rate as a free
nucleoside than when inserted into
double-stranded DNA. On the other hand, the predominant *
OH addition at C5 of
cytosine or
5-methylcytosine leads to the formation of 5-hydroxy-5,6-dihydro radicals that give rise to novel products with an
imidazolidine structure. The mechanism of the formation of
imidazolidine products is accounted for by rearrangement reactions that in the presence of molecular
oxygen likely involve an intermediate
pyrimidine endoperoxide. The reactions of the radical
cations of
cytosine and
5-methylcytosine are governed by competitive hydration, mainly at C6 of the
pyrimidine ring, and deprotonation from the exocyclic amino and methyl group, leading in most cases to products similar to those generated by *
OH. 5-Hydroxypyrimidines, the
dehydration products of
cytosine and
uracil glycols, have a low oxidation potential, and their one-electron oxidation results in a cascade of decomposition reactions involving the formation of
isodialuric acid,
dialuric acid, 5-hydroxyhydantoin, and its hydroxyketone isomer. In biology, GC --> AT transitions are the most common mutations in the genome of aerobic organisms, including the lacI gene in bacteria, lacI transgenes in rodents, and the
HPRT gene in rodents and humans, so a more complete understanding of
cytosine oxidation is an essential research goal. The data and insights presented here shed new light on oxidation reactions of
cytosine and
5-methylcytosine and should facilitate their validation in cellular
DNA.