Fig. 3. Characterization of TET2 activity on 5eC. (A) Reaction scheme showing two plausible 5heC positional isomers generated from 5eC. (B and C) Structures of 5-CH₃CD₂-cytidine phosphoramidite 4 and its corresponding DNA. (D) MALDI-MS spectra of TET2-mediated oxidation of deuterated 5eC (top panel). Addition of 15 daltons indicates oxidation of α-carbon carrying deuterium atoms. Oxidation of 5-CH₃CH₂-cytidine (5eC) by TET2 leads to increase of 16 daltons (lower panel).