Conformations of depurinating adducts from dibenzo[a,l]pyrene diolepoxide

The extremely potent carcinogen dibenzo[a,l]pyrene (DB[a,l]P) is known to form a multitude of stable and unstable (depurinating) adducts, the latter via both the one-electron oxidation and the monooxygenation pathway. Laser induced fluorescence spectra of the depurinating adducts from DB[a,l]P diolepoxide show either a blueshift or a redshift, and in some solvent systems a double origin band is observed. These effects are believed to be due to subtle conformational equilibria and were further investigated for syn-DB[a,l]PDE-N7Gua, syn-DB[a,l]PDE-N7Ade, and syn-DB[a,l]PDE-N3Ade. Using molecular mechanics, molecular dynamics, and semi-empirical quantum mechanical calculations we found that these adducts may indeed exist in two conformations: one in which the saturated ring adopts a half-chair conformation (allowing for π-π stacking), and one in which a boat conformation is preferred. In the case of synDB[a,l]PDE-N7Ade the calculations predict, in agreement with experimental results, a strong preference for the latter conformation. This unusual conformation explains the experimentally observed ¹H NMR coupling constants for the protons of the saturated ring.