Benzo[a]pyrene and its analogues: structural studies of molecular strain

The molecular geometry of benzo[a]pyrene, its 4-methyl-and 3,11-dimethyl derivatives, benzo[e]pyrene, and two azabenzo[a]pyrenes are described. Results of these three-dimensional crystal structure determinations, together with those from previous studies in this laboratory of 11-methylbenzo[a]pyrene, indicate the extent to which nonbonded interactions between hydrogen atoms contribute to molecular distortions, particularly in the bay-region. This strain is high if a bay-region methyl group is present. The major effect is an increase in the C-C-C angles in that area of the molecule, rather than torsion about bonds. In addition, the effect of a nitrogen atom replacing one of the C-H groups in the aromatic system is shown. Molecules stack in planes approximately 3.5 A apart. In benzo[a]pyrene, 5-azabenzo[a]pyrene and 3,11-dimethylbenzo[a]pyrene crystals the stacking is similar to that in graphite. 4-Methylbenzo[a]pyrene molecules stack with less molecular overlap. The packing in 4-aza-5-methylbenzo[a]pyrene consists of modules of four stacked molecules, packed in a 'tile-like' arrangement. Nonbonded C....H interactions between adjacent molecules lead to a herring-bone arrangement between these stacks. The types of C....H and pi-pi interactions involving PAHs in the crystalline state, described here, can also be expected to be found when the PAHs bind to hydrophobic areas of biological macromolecules such as proteins, nucleic acids and membranes.