EFFECT OF RING SIZE ON CONFORMATIONS OF AROMATIC AMINE-DNA ADDUCTS - THE ANILINE-C8 GUANINE ADDUCT RESIDES IN THE B-DNA MAJOR GROOVE

While the one-ring amine aniline (AN) has only slight genetic activity , the polycyclic aromatic amines 2-aminofluorene (AF) and 1-aminopyren e (AP) are significant mutagens and carcinogens. Moreover, the bulkier AP is more mutagenic per adduct than AF in the tetracycline-resistanc e gene of plasmid pBR322 [Melchior et al. (1994) Carcinogenesis 15, 88 9]. To elucidate possible conformational origins of the differing muta genic effects of these three adducts, which may stem from their differ ing ring sizes, we have examined their conformations in two mutation-s usceptible sequences from the above gene: TTGAGGCCG (sequence I) and GAATGGTGC (sequence II), where G* = C8-modified guanine. No experimen tal high-resolution NMR data are yet available for the aniline adduct in a DNA duplex. Minimized potential energy calculations were carried out, using the molecular mechanics program DUPLEX to explore the confo rmation space of these adducts. In the case of AN, a relatively unpert urbed B-DNA helix with the amine in the major groove was strongly favo red in both sequences. In the case of AF- and AP-modified DNA, however , several differing conformations were competitive in energy. They inc luded major groove structures, as well as conformations with syn-modif ied guanine and the polycyclic amine in the minor groove, or the amine rings intercalated into the helix with displacement of the modified g uanine, in overall harmony with high-resolution NMR solution structure s. Thus, aniline distorts DNA structure to a lesser extent than larger aromatic amine ring systems, since a number of different conformation s are energetically feasible and have been observed for the larger sys tems. This result may be relevant to their enhanced mutagenicity and t heir repair propensity, in contrast to aniline's low mutagenic effect.