Abstract
The pH effect on the one‐electron photooxidation of 5‐methyl‐2′‐deoxycytidine (d^m^C) by sensitization with 2‐methyl‐1,4‐naphthoquinone (NQ) was investigated. Photoirradiation of an aqueous solution containing d^m^C and NQ under slightly acidic conditions of pH 5.0 efficiently produced 5‐formyl‐2′‐deoxycytidine, whereas similar NQ‐photosensitized oxidation of d^m^C proceeded to a lesser extent under more acidic or basic conditions. Fluorescence‐quenching experiments revealed that the less‐efficient photooxidation at pH values below 4.5 is attributed to the decreased rate of one‐electron oxidation of d^m^C owing to protonation at the N(3)‐position. The NQ‐photosensitized oxidation of an N(4)‐dimethyl‐substituted d^m^C derivative under various pH conditions also suggests that the pH change in the range of 5.0 to 8.0 may be responsible for a reversible deprotonation–protonation equilibrium at the N(4)‐exocyclic amino group of the d^m^C radical cation. In accord with the photochemical reactivity of monomeric d^m^C, the 5‐methylcytosine residue in oligodeoxynucleotides was oxidized efficiently by photoexcited NQ‐tethered oligodeoxynucleotides under slightly acidic conditions to form an alkali‐labile 5‐formylcytosine residue.