The complementary pentadecamers d(5'-TACTCTTCTTGACCT) (strand A) and d(5'-AGGTCAAGAAGAGTA) (strand B), which span a portion of the mouse c-Ha-ras protooncogene centered around codon 61, were synthesized by using standard beta-cyanoethyl phosphoramidite chemistry and characterized by sequence analysis. Strand A, containing a sole guanine at the position corresponding to the first base of codon 61, was modified with N-acetoxy-N-(trifluoroacetyl)-2-aminofluorene or its 4-aminobiphenyl analogue. In both cases only the corresponding N-(deoxyguanosin-8-yl)arylamine adduct was formed, as judged from HPLC and UV analyses conducted after enzymatic hydrolysis of the modified oligomers. Nonmodified and modified pentadecamers were annealed with strand B. Cooperative melting transitions were observed with all samples, thus indicating the formation of stable duplexes. Melting temperatures decreased in the order nonmodified duplex greater than 2-aminofluorene-modified duplex greater than 4-aminobiphenyl-modified duplex, which indicated destabilization of the helical structure upon incorporation of the adducts, with 4-aminobiphenyl having the greatest effect. Circular dichroism spectra of all duplexes were characteristic of an overall right-handed B-type conformation, with no major conformational differences being detected between the two arylamine-modified oligomers.