PHOTOSENSITIZED OXIDATION OF 5-METHYL-2'-DEOXYCYTIDINE BY 2-METHYL-1,4-NAPHTHOQUINONE - CHARACTERIZATION OF 5-(HYDROPEROXYMETHYL)-2'-DEOXYCYTIDINE AND STABLE METHYL-GROUP OXIDATION-PRODUCTS

5-Methylcytosine is a minor nucleobase of eukaryotic DNA which plays a central role in the regulation of gene expression. UV-A irradiation o f an aerated aqueous solution of 5-methyl-2'-deoxycytidine in the pres ence of menadione (MQ) as a type I photosensitizer leads to the format ion of several stable oxidation products. Emphasis was placed in this study on the isolation and the characterization of the major class of decomposition products whose formation involves the oxidation of the m ethyl group. These include 5-(hydroperoxymethyl)-2'-deoxycytidine and two stable decomposition products namely, 5-(hydroxymethyl)-2'-deoxycy tidine and 5-formyl-2'-deoxycytidine. Structural assignment df the lat ter modified nucleosides was inferred from extensive spectroscopic mea surements (H-1 and C-13 NMR, UV spectroscopy, and mass spectrometry). In addition, conformational analysis of the oxidized nucleosides was i nferred from detailed H-1 NMR analysis. All of the above photooxidatio n products appear to arise from the deprotonation of the 5-methyl-2'-d eoxycytidine radical cation which is generated by menadione photosensi tization.