Formation and reactions of N(7)-aminoguanosine and derivatives

Arylamines are mutagens and carcinogens and are thought to initiate tumors by forming adducts with DNA. The major adducts are C(8)-guanyl, and we have previously suggested a role for guanyl-N(7) intermediates in the formation process. N(7)-Aminoguanosine (Guo) was synthesized and characterized, with the position of the NH(2) at N7 established by two-dimensional rotating frame Overhauser enhancement NMR spectroscopy. In DMF, N(7)-NH(2)Guo formed C(8)-NH(2)Guo and the cyclic product C(8):5'-O-cycloGuo. In aqueous media, these products were formed along with 8-oxo-7,8-dihydroGuo, N(7)-NH(2)guanine, and a product characterized as a purine 8, 9-ring-opened derivative (N-aminoformamidopyrimidine). The rate of aqueous decomposition of N(7)-NH(2)Guo increased with pH, with a t(1/2) of 10 h at pH 7 and a t(1/2) of 2 h at pH 9. The rate of migration of NH(2) from N7 to C8 is fast enough to explain the formation of C(8)-NH(2)Guo from the reaction of 2, 4-dinitrophenoxyamine with Guo but not the formation of C(8)-(arylamino)Guo in the reaction of Guo with aryl hydroxylamine esters; however, the fluorenyl moiety may facilitate the proposed rearrangement by stabilizing an incipient negative charge in the transfer. In the reaction of Guo with N-hydroxy-2-aminofluorene and acetylsalicylic acid, a peak with the mass spectrum expected for N(7)-(2-aminofluorenyl)Guo was detected early in the reaction and was distinguished from C(8)-(2-aminofluorenyl)Guo. NMR experiments with [8-(13)C]Guo also provided some additional support for transient formation of N(7)-(2-aminofluorenyl)Guo. We conclude that a guanyl-N(7) intermediate is reasonable in the reaction of activated arylamines with nucleic acids, although an exact rate of transfer of an N(7)-arylamine group to the C8 position has not yet been quantified. The results provide an explanation for the numerous products associated with modification of DNA by activated arylamines. However, the contribution of "direct" reaction at the guanine C8 atom cannot be excluded.