Molecular structure of benzo(a)pyrene 4,5-oxide.

An X-ray crystallographic study of benzo(a)pyrene 4,5-oxide, a metabolite of the carcinogen benzo(a)pyrene (BP), as given information on the geometry of this molecule. The carbon skeleton of BP itself has been shown by others to be early planar; the planarity of the carbon skeleton has been shown by this work to be perturbed very little by epoxidation of the 4,5-double bond. Epoxidation has, however, increased the double bond character of C-11--C-12, C-9--C-10, and C-7--C-8. The hydrogen atom on C-3 points directly toward the oxygen atom of another molecule. This C--H... O interaction, although weak, suggests that C-3 might be slightly acidic. An analysis of the experimentally determined bond lengths indicates that, after the highly reactive epoxide ring, the most reactive positions are at C-1, C-6, C-7, C-11, and C-12. The oxide ring of BP, unlike that for the K-region oxide of 7,12-dimethylbenz(a)anthracene, is symmetrical (with C--O distances equivalent within experimental error). The C--O distances are longer than those found in most oxides, including those in 7,12-dimethylbenz(a)anthracene-5,6-oxide. Thus it has been shown that the oxide rings of the K-region oxides of the two potent carcinogens BP and 7,12-dimethylbenz(a)anthracene are not similar in dimensions.