Mechanism of C8 alkylation of guanine residues by activated arylamines: evidence for initial adduct formation at the N7 position.

Aromatic amines are bioactivated to electrophilic compounds that react with DNA, predominantly at the C8 position of guanine bases. This site is weakly nucleophilic and it has been proposed that the C8 adduct is the final product after initial N7-adduct formation. To consider this possibility, we reacted several C8-substituted guanine derivatives with N-acetoxy-2-aminofluorene, prepared in situ from 2-acetylsalicylic acid and N-hydroxy-2-aminofluorene. With C8,N9-dimethylguanine, an adduct was isolated in good yield that was consistent, by NMR and mass spectral characterization, with a structure involving carcinogen substitution at the N7 position of guanine and linked through the 2-aminofluorenyl nitrogen--N-(C8,N9- dimethylguanin-N7-yl)-2-aminofluorene. This adduct could be easily reduced with NaBH4, consistent with the proposed N7-adduct structure. The same reaction was also carried out with C8-methylguanosine and C8-methyldeoxyguanosine and similar adducts were isolated. In contrast, C8-bromoguanosine reacted with N-acetoxy-2-aminofluorene to yield the C8-substituted arylamine adduct N-(guanosin-C8-yl)-2-aminofluorene directly. These products are uniquely consistent with a scheme in which C8-adduct formation is preceded by an initial electrophilic substitution on the N7 atom, which is postulated to be a general reaction for activated arylamines and heterocyclic amines.