Stereochemical origin of opposite orientations in DNA adducts derived fromenantiomeric anti-benzo[a]pyrene diol epoxides with different tumorigenic potentials

When covalently linked to DNA, enantiomeric pairs of mirror image aromatic diol epoxides with differing tumorigenic potencies adopt opposite orientati ons along the DNA helix. This phenomenon has been observed by high-resoluti on NMR solution studies in a number of systems, preliminary modeling effort s [Geacintov et al. (1997) Chern. Res. Toxicol. 10, 111-146) had suggested that the origin of the opposite orientation effect may be manifested even a t the level of the carcinogen-modified nucleoside due to primary steric hin drance effects between the aromatic moiety and the attached base and sugar. Such a small system can be computationally investigated extensively, since a very thorough survey of the potential energy surface is feasible. Conseq uently, in an effort to understand the underlying origins of the opposite o rientations in (+)-trans and (-)-trans-anti adduct pairs, we have undertake n an extensive investigation of the paradigm 10S (+) and 10R (-)-trans-anti -[BP]-N-2-dG mononucleoside adduct pair, derived from the binding of the ()-7R,8S,9S,10R and (-)-7S,8R,9R,10S enantiomers of 7,8-dihydro-9, 10-epoxy- 7,8,9,10-tetrahydrobenzo[a]pyrene (BP) to the exocyclic amino group of 2'-d eoxyguanosine. In the present work we created 373248 different conformers f or each adduct, which uniformly sampled the possible rotamers about the thr ee flexible torsion angles governing the orientation of the base (chi) and its covalently linked BP residue (alpha', beta') at 5 degrees intervals, an d computed each of their energies with AMBER 4.0. The extensive results per mitted us to map the potential energy surface of the molecule. Only four la w-energy structural domains are found for the (+)-trans adduct and four for the (-)-trans adduct; the (+)/(-) pairs of each structural domain are mino r images, with the minor image symmetry broken by the sugar and its attache d C4'-C5' group. The most favored of these four is observed experimentally in the duplexes containing the same (+) and (-)-trans-anti-[BP]-N-2-dG addu cts (Cosman et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1914-1918; de l os Santos et al. (1992) Biochemistry 31, 5245-5252). The origin of the oppo site orientations resides in steric hindrance effects resulting from the mi rror image relationship of the BP benzylic rings in the adduct pair, such t hat rotation of one stereoisomer into the conformational domain preferred b y the other causes crowding between the base and the BP benzylic ring. Limi ted conformational, flexibility in the torsion angle beta', the one closest to the bulky BP moiety at the linkage site to guanine, plays a key role in governing the orientations in each adduct. The opposite orientation phenom enon is likely to manifest itself when the adducts are processed by cellula r enzymes involved in replication, repair, and transcription and thus play a role in the differing biological outcomes stemming from the (+) and (-)-t rans-anti adducts.