Mutation in p53 tumor suppressor gene is a hallmark of human cancers. Six major mutational hotspots in p53 contain methylated CpG (mCpG) sites, and C -->T transition is the most common mutation at these sites. It was hypothesized that the formation of 5-methylcytosine glycol induced by reactive oxygen species, its spontaneous deamination to thymine glycol and the miscoding property of the latter may account, in part, for the ubiquitous C -->T mutation at CpG site. Here, we assessed the kinetics of deamination for two diastereomers of 5-methylcytosine glycol in duplex DNA. Our results revealed that the half-lives for the deamination of the (5S,6S) and (5R,6R) diastereomers of 5-methylcytosine glycol in duplex DNA at 37 degrees C were 37.4 +/- 1.6 and 27.4 +/- 1.0 h, respectively. The deamination rates were only slightly lower than those for the two diastereomers in mononucleosides. Next, we assessed the formation of 5-methyl-2'-deoxycytidine glycol in the form of its deaminated product, namely, thymidine glycol (Tg), in methyl-CpG-bearing duplex DNA treated with Cu(II)/H(2)O(2)/ascorbate. LC-MS/MS quantification results showed that the yield of Tg is similar as that of 5-(hydroxymethyl)-2'-deoxycytidine. Together, our data support that the formation and deamination of 5-methylcytosine glycol may contribute significantly to the C -->T transition mutation at mCpG dinucleotide site.