title: The pH effect on the naphthoquinone-photosensitized oxidation of 5-methylcytosine.
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.
authors.affiliation: Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto, Japan.
authors.forename: Hisatsugu
authors.initials: H
authors.lastname: Yamada
authors.forename: Kazuhito
authors.initials: K
authors.lastname: Tanabe
authors.forename: Takeo
authors.initials: T
authors.lastname: Ito
authors.forename: Sei-Ichi
authors.initials: S
authors.lastname: Nishimoto
mesh_terms: D044503:5-Methylcytosine / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D009285:Naphthoquinones / Q000737:chemistry*; D010084:Oxidation-Reduction; D017319:Photosensitizing Agents / Q000737:chemistry*
chemical_list: D009285:Naphthoquinones; D017319:Photosensitizing Agents; D044503:5-Methylcytosine