Abstract
Abstract —The near‐UV photosensitization of thymine by 2‐methyl‐1,4‐naphthoquinone in oxygenated aqueous solution leads to the formation of nine major thymine‐derived products. Six of the photoproducts, representing 70% of the initial thymine loss, are also known to be formed by γ‐ray or X‐ray irradiation of oxygenated water solutions of thymine. They include the cis and trans isomers of 5‐hydroperoxy‐6‐hydroxy‐5,6‐dihydrothymine, the cis and trans thymine glycols, 5‐hydroxy‐5‐methylhydantoin, and N‐formyl‐N′‐pyruvylurea. The three remaining major products, accounting for a further 25% of the initial thymine loss, are thymine di‐adducts of a type not previously reported. Preliminary characterization of these novel photoproducts, based on their UV absorption, mass spectra, and proton and carbon‐13 nuclear magnetic resonance spectra, suggests that in each of the three, the thymine moieties are linked by an N(1)‐C(5) or N(1)‐C(6) bond. A thymine cation radical is implicated as the common precursor of all 9 products. Accordingly. pyrimidine photosensitization by 2‐methyl‐1,4‐naphthoquinone serves as a model system for studying the chemical consequences of radiation‐induced ionization of the DNA bases.
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