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The pH Effect on the Naphthoquinone‐Photosensitized Oxidation of 5‐Methylcytosine
Author(s) -
Yamada Hisatsugu,
Tanabe Kazuhito,
Ito Takeo,
Nishimoto Seiichi
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200800840
Subject(s) - chemistry , protonation , deprotonation , aqueous solution , photochemistry , quenching (fluorescence) , fluorescence , medicinal chemistry , reactivity (psychology) , residue (chemistry) , organic chemistry , ion , physics , quantum mechanics , medicine , alternative medicine , pathology
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.