Anthraquinone-Sensitized Photooxidation of 5-Methylcytosine in DNA Leading to Piperidine-Induced Efficient Strand Cleavage

摘要

One-electron photooxidations of 5-methyl-2′-deoxycytidine (dmC) and 5-trideuteriomethyl-2′-deoxycytidine ([D₃]dmC) by sensitization with anthraquinone (AQ) derivatives were investigated. Photoirradiation of an aerated aqueous solution containing dmC and anthraquinone 2-sulfonate (AQS) afforded 5-formyl-2′-deoxycytidine (dfC) and 5-hydroxymethyl-2′-deoxycytidine (dhmC) in good yield through an initial one-electron oxidation process. The deuterium isotope effect on the AQS-sensitized photooxidation of dmC suggests that the rate-determining step in the photosensitized oxidation of dmC involves internal transfer of the C5-hydrogen atom of a dmC-tetroxide intermediate to produce dfC and dhmC. In the case of a 5-methylcytosine (mC)-containing duplex DNA with an AQ chromophore that is incorporated into the backbone of the DNA strand so as to be immobilized at a specific position, mC underwent efficient direct one-electron oxidation by the photoexcited AQ, which resulted in an exclusive DNA strand cleavage at the target mC site upon hot piperidine treatment. In accordance with the suppression of the strand cleavage at 5-trideuterio-methylcytosine observed in a similar AQ photosensitization, it is suggested that deprotonation at the C5-methyl group of an intermediate mC radical cation may occur as a key elementary reaction in the photooxidative strand cleavage at the mC site. Incorporation of an AQ sensitizer into the interior of a strand of the duplex enhanced the one-electron photooxidation of mC, presumably because of an increased intersystem crossing efficiency that may lead to efficient piperidine-induced strand cleavage at an mC site in a DNA duplex.