Spectroscopic characterization of the initial C8 intermediate in the reaction of the 2-fluorenylnitrenium ion with 2 '-deoxyguanosine

Irradiation of 2-azidofluorene in an aqueous solution containing 2 '-deoxyg uanosine (dG) gives good yields of 8-(2-fluorenylamino)dG. This is the C8 a dduct implicated in the carcinogenicity of 2-aminofluorene, and formed in v ivo from the reaction of DNA-guanine and an ester of N-hydroxy-2-aminofluor ene. Flash photolysis of the azide reveals two intermediates on the pathway that forms this adduct, the 2-fluorenylnitrenium ion, and a subsequent lon ger-lived species formed in the reaction of this ion with de. A number of p ieces of evidence identify this later intermediate as the initial Cs adduct derived from addition of the nitrenium ion to the C8 carbon prior to loss of the C8 proton. Both spectroscopic and kinetic analyses show that the lat ter actually exists in both a cationic acid form and a base form, with a pK (a) for the acid of 3.9. The base form is a tautomer of the final product a nd is the species present at pH 7. There is also evidence that the reaction of the nitrenium ion and dG that forms this intermediate proceeds directly by addition at C8. In other words, the substitution of ArNH+ for H+ at the C8 position of 2 '-deoxyguanosine is a straightforward electrophilic aroma tic substitution.