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THE PHOTOCHEMISTRY OF 5‐METHYLCYTOSINE AND 5‐METHYL‐2′‐DEOXYCYTIDINE IN AQUEOUS SOLUTION
Author(s) -
Celewicz Lech,
Sheilar Martin D.
Publication year - 1992
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1992.tb08530.x
Subject(s) - deoxycytidine , aqueous solution , chemistry , 5 methylcytosine , photochemistry , organic chemistry , biochemistry , biology , dna methylation , genetics , gene expression , chemotherapy , gemcitabine , gene
—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 ( 1 H and 13 C 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– 13 C]cytosine, hydrolysis of N ‐carbomethoxy‐3‐amino‐2‐methylacryl‐amidine and reaction of Ib with 1,1′‐carbonyldiimidazole to give I ). The quantum yield for formation of Ib was determined to be 1.8 × 10 ‐3 at pH 7.5 while the quantum yield for formation of IIb has a lower value of 0.2 × 10 ‐3 at pH 7.5. These quantum yields depend strongly on pH and reach maximum values of 2.0 × 10 ‐3 at pH 7.0 ( Ib ) and 0.6 × 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.