title: The photochemistry of 5-methylcytosine and 5-methyl-2'-deoxycytidine in aqueous solution.
abstract: The nucleobase 5-methylcytosine (I) is a minor component of eukaryotic DNA thought to be important in regulation of gene expression. The photochemical reactions of this nucleobase and its 2'-deoxyribonucleoside, 5-methyl-2'-deoxycytidine (II), in water have been studied. These reactions lead, respectively, to 3-amino-2-methylacrylamidine (Ib) and 3-(2-erythro-D-pentopyranos-1-yl)amino-2-methylacrylamidine (IIb) as the main photoproducts. The structure of the photoproducts was established by spectroscopic methods (1H and 13C NMR, UV spectroscopy, electron impact and liquid secondary ion mass spectrometry); in the case of Ib, confirmatory evidence was obtained by chemical methods (photolysis of 5-methyl[2-13C]cytosine, hydrolysis of N-carbomethoxy-3-amino-2-methylacrylamidine and reaction of Ib with 1,1'-carbonyldiimidazole to give I). The quantum yield for formation of Ib was determined to be 1.8 x 10(-3) at pH 7.5 while the quantum yield for formation of IIb has a lower value of 0.2 x 10(-3) at pH 7.5. These quantum yields depend strongly on pH and reach maximum values of 2.0 x 10(-3) at pH 7.0 (Ib) and 0.6 x 10(-3) at pH 5.0 (IIb). The mechanism of formation of Ib (or IIb) is proposed to involve nucleophilic attack of water on the C-2 position of photoexcited I (or II), followed by ring opening and decarboxylation of an intermediate carbamic acid.
authors.affiliation: Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco 94143.
authors.forename: L
authors.initials: L
authors.lastname: Celewicz
authors.forename: M D
authors.initials: MD
authors.lastname: Shetlar
mesh_terms: D044503:5-Methylcytosine; D000970:Antineoplastic Agents / Q000528:radiation effects*; D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000528:radiation effects; D003841:Deoxycytidine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000528:radiation effects; D010777:Photochemistry; D014466:Ultraviolet Rays
chemical_list: D000970:Antineoplastic Agents; D003841:Deoxycytidine; D044503:5-Methylcytosine; D003596:Cytosine; C056034:2'-deoxy-2'-methylenecytidine