Methylation of DNA bases by methyl free radicals: mechanism of formation of C8-methylguanine

Abstract

The C8-methylguanine (C8mG) lesions are reported to be produced in vivo due to methylation of guanine base of DNA by methyl free (·CH3) radicals derived from the carcinogen 1,2-dimethylhydrazines and tert-butylhydroperoxide. It is believed that C8mG lesions can induce G to T and G to C transversion mutations and deletions. However, the mechanisms of reactions of ·CH3 radicals with DNA bases leading to formation of C8mG and other methylated DNA bases and their biological implications are not properly understood. In the present contribution, we have carried out density functional theory (DFT) calculations to ascertain the various stable methylated derivatives of all the four DNA bases that are formed by the attack of ·CH3 radicals on DNA bases as well as to understand the mechanism of formation of C8mG due to reaction of ·CH3 radicals with the C8 site of guanine. Our calculations reveal that ·CH3 radical would form stable methylated products at the C8 sites of purine bases (guanine and adenine) and at the C5 and C6 sites of pyrimidine bases (cytosine and thymine) by directly attacking to bases. The C8mG is the most stable. This is in agreement with experimental observation. Further, we have found that in absence of any external agents, the C8mG is formed preferably by direct addition of a ·CH3 radical to the C8 site of guanine followed by abstraction of the H8 hydrogen atom by another ·CH3 radical. The barrier energies for these two steps are found to be 18.16 (18.73) and 16.05 (18.54) kcal/mol, respectively, as determined at the M06-2X/6-311+G(d,p) level of theory in gas phase (aqueous media). Thus, the present study explains the mechanism of formation of C8mG.