5-methylcytosine attack by free radicals arising from bromotrichloromethane in a model system: Structures of reaction products

Abstract

The interaction between free radicals derived from the catalytic decomposition of bromotrichloromethane and 5-methylcytosine (5MC) under different conditions were studied. The structures of the reaction products formed was established by the GC/MS analysis of their trimethylsilyl derivatives. Under anaerobic conditions, the formation of the following products was found: (1) thymine; (2) 5-hydroxymethyl uracil. Under aerobic conditions, the following reaction products were identified: (1) The same two products formed under anerobic conditions. (2) Monohydroxylated thymine. Precise location of the hydroxyl group was not established but probably corresponds to the six position isomer. (3) Two monochloro monohydroxy thymines. It is suggested that they are cis-trans isomers whose substituents are located at the 5-methyl and six positions of the base. (4) The trimethylsilyl derivative of thymine glycol. (5) Two monobromo monohydroxy adducts of thymine. One of them was detected as its underivatized form in the hydroxyl group position. (6) A partially silylated dihydroxythymine. When benzoyl peroxide was omitted from aerobic incubation mixtures, the compounds formed changed. No longer observable were: thymine; the two monochloro monohydroxy derivatives of thymine; thymine glycol, and one monobromo monohydroxythymine. On the other hand, two new reaction products were formed instead: a partially silylated monochloro-monohydroxy thymine and 5-hydroxy-methyl-cytosine. If similar or equivalent reaction products were formed in DNA during CBrCl 3 or CCl 4 poisoning, results might be of relevance, because the 5MC content in DNA from eukaryotes is related to differentiation, gene control, and to carcinogenesis.