Mechanism of direct conversion between C8 adducts and N7 adducts in carcinogenic reactions of arylnitrenium ions with purine nucleosides: a theoretical study.

To validate the mechanism of direct formation of C8 adducts for a series of complicated and controversial carcinogenic reactions, in this study, we examine the key direct conversion process between N7 adducts and C8 adducts in some theoretical models. First, the mechanism of direct conversions between N7 adducts and C8 adducts suggested by some experiments is explored. For the first time, it is approved that direct conversion between N7 and C8 adducts does not proceed because of very high activation energy. Second, the relative stabilities of the N7 adducts and C8 adducts for these model reactions are evaluated by high-level calculations. Our results indicate that C8 adducts are more stable in aqueous solution. Third, on the basis of these findings, experimental phenomenon of rich C8 adducts and rare N7 adducts is well explained theoretically, and the correctness of the three different existing experimental schemes of these reactions is discussed. Finally, the mechanism of formation of C8 adducts by additions of arylnitrenium ions directly to C8 positions of nucleotide bases in DNA is supported.