BENZO[ALPHA]PYRENE AND ITS ANALOGS - STRUCTURAL STUDIES OF MOLECULAR STRAIN

The molecular geometry of benzo[a]pyrene, its 4-methyl- and 3,11-dimet hyl derivatives, benzo[e]pyrene, and two azabenzo[a]pyrenes are descri bed, Results of these three-dimensional crystal structure determinatio ns, together with those from previous studies in this laboratory of 11 -methylbenzo[a]pyrene, indicate the extent to which nonbonded interact ions between hydrogen atoms contribute to molecular distortions, parti cularly 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 addi tion, the effect of a nitrogen atom replacing one of the C-H groups in the aromatic system is shown, Molecules stack in planes similar to 3. 5 Angstrom apart, In benzo[a]pyrene, 5-azabenzo[a]pyrene and 3,11-dime thylbenzo[a]pyrene crystals the stacking is similar to that in graphit e, 4-Methylbenzo[a]pyrene molecules stack with less molecular overlap, The packing in 4-aza-5-methylbenzo[a]pyrene consists of modules of fo ur stacked molecules, packed in a 'the-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 inte ractions involving PAHs in the crystalline state, described here, can also be expected to be found when the PAHs bind to hydrophobic areas o f biological macromolecules such as proteins, nucleic acids and membra nes.